Dustcover Blues

@mijostyn  : At some time we should just seat and enjoy the MUSIC in each one room/audio system.

That " time " is different for all of us and I can see that it's not yet your time. You are still looking for the best way to arrive at the top quality level in your system.

All of us live years and years of room/system up-dates/up-grades/tweaks and the like.

Today my "time " let me enjoy more and more listening MUSIC hours and I think that my " time " is arriving to its end. Can I be sure about? well more or less.

What am I  doing on my system this weekend? something that's a necessity but not really and up-date or tweack job: my Levinson 20.6 Reference monoblocks amplifiers have around 33 years that were made and these running pure class A design over those years work with no single fault but the power supply filter capacitors  I think " needs " to be changed and that's what I'm doing. The cap´s already arrives from Mouser: Vyshay_Sprega and that's it.

Sooner or latter you will done on your room system with out missing the " fun " of those years of up-dates.

R.

I have custom acrylic dustcovers for my 2 turntables(that are on Townshend platforms) to cover when not playing.

While I wouldn't mind, I thought it wasn't a good idea to use them during playing. 

There's one way to use a dustcover when playing that will actually work in terms of both dust and vibration control.

Raul, I think you are right. The material a dust cover is made of is important. It can't be flimsey but it can't weight too much either or hinges won't be able to hold it open. The Sota cover is much heavier than it use to be. It is made of butte joined 1/8" Laxan with all the edges chamfered. It is acutally quite a nice cover. There is a lot more going on here besides the cover. Read on!

 @lewm , Your speakers should be able to go down a few Hz lower than mine because they are 4" wider. You do not listen to the music I do. As I remember you are into old jazz and singers? If you want to know how your system would handle what I listen too sometimes get a copy of Soundgarden's Badmotorfinger and play the 1st cut at 100 dB peaks, 95 dB is probaly enough. Measure it with a meter. I do not think you will like what you hear. You'll be OK at 85 dB. That kind of bass is going to distort the heck out of everything. The voice will actually modulate with the bass. Like you I have had one ESL after another since 1978. I set up one pair of Beverages in a big system in Miami. ESLs will make bass but they will not like doing it. You listen to relatively polite music. Going down to 40 Hz is probably enough but even in jazz the bass drum can go down lower. Even on very old recordings from the early 60s the bass drums can go quite low. With modern recording, drum and synthesizers there is a lot of information below 40 Hz.

I use 4 12" subwoofers each one with an X-max of over 2 cm. They are in a linear array so acoustically they function as one driver with useful output down to 10 Hz. Each driver gets 2000 watts. I cross over to them at 100 Hz and use an 8th orde slope or 48 dB/oct.  Look at those frequency response curves I put up on my system page. That is the frequency response of my system and room at the left lateral side wall with the microphone sitting on the plater. The response at the listening position is much flatter with the bass up 5 dB at 20 Hz and the treble down 3 dB at 20 kHz. The bass and treble curves are intensional. I programmed the system to do that because I like it that way. At the wall it is very bass heavy which certainly does not help but I am now stuck with that position so, I have to work with it.

Now for the juicey part. I put felt strips under the dust cover to seal it when closed and..... the feedback got worse. I hate when that happens:-(   But, lift the cover and it goes right away. I do not know what made me do it but I put two fingers under the turntable where you can feel the underside of the subchassis and it was boucing at 24 Hz! Lift the dust cover and it stops. Put it down and the subchassis starts bouncing at 24 Hz. This is way above the chassis's resonance frequency. Somehow I have created a 24 kHz musical instrument. When I am listening loud it will have to be with the cover up until I figure this one out. There are three spaces. One under the dust cover, one above the subchassis and below the plinth the one below the subchassis and above the granite the table sits on. There is an open port at the tonearm board between chambers 1 and two. Also, I do not have to be playing the turntable. If I play a CD with a heavy bassline the subchassis will start bouncing to the music. There is no feedback path so it stops immediately. Lift the dust cover and it all stops. My brother has a PhD in aquadic acoustics. His response was to tell me to think about sound waves like water waves. "Now you can figure it out for yourself."  Wondeful. 

Dear @mijostyn  : I used the Denon hinges and the really heavy glass dust cover thickness were made it especially for that application as how the hinges were fixed to the onyx/marble plynths.

The glass dust covers where made by 5 sides joined each one with special silicon like.

R.

what did Donna say ? 

Yes Raul, like a fish tank. You could have one made up here easily. Not sure about the hinges. They would have to be very hardy to hold the weight. My old Transcriptors had a glass plinth and cover. I only kept it for a few months. It was awful. I got an Oracle after that and it was almost as bad. After, I went back to the Linn LP 12 until I found the Sota Sapphire. Breath of fresh air.

@tomic601 , Donna really does the business end. Christan is the one to talk to about the technical aspects. I have not brought it up with him yet but, I will as soon as I have it figured out. I am making a"skirt" to go around the bottom of the plinth closing off the cavity under the turntable. Switching cartridges does nothing.

i use Donna as a point of contact but i get you are plugged in. Is your new TT the mag lev platter ? there's  some strange energy at work to get the subchassis to move at 24 hz. i would check to see if your feet are limiting sub chassis travel, and recheck subchassis level. The old Sapphire i have rebuilt 3 years ago does fine in my room not that it sees 100 db often but i know w certainty that bass is flat to 20 hz at the listening position....dustcover up, down or removed....

AFAIK, it's been common knowledge that you never have the cover on when playing a record.

You say this whole problem came to light while you were listening to Herbie Hancock’s "Secrets". How does that square with your inference that my own preference for jazz ("old" and new) limits the demands I place on my systems? I also listen to classical orchestral and chamber music, big band jazz, latin or afro-cuban jazz which is usually big band with lots of drums of all types, and I love R&B, which I do like to play "loud", also to certain pop artists where the sonics can be very demanding at low frequencies if you want to feel satisfied. I would also say that dynamics of any kind of music, not just heavy duty bass notes, are what places stress on our audio systems and is a real determinant of how close the system can come to sounding real. But I admit, I will never ever want to listen to "Soundgarden’s Badmotorfinger" at any SPL, let alone 100db.

You also wrote, "Now for the juicey part. I put felt strips under the dust cover to seal it when closed and..... the feedback got worse. I hate when that happens:-( " So much for the Helmholtz Resonator theory, which I couldn’t buy in the first place.

My speakers are 4 inches wider according to you; I don’t know the dimensions of your SLs, so I cannot comment. But if the difference in size between your panels and mine is only 4 inches in width, that would equate to a nearly 400 sq in per channel difference in surface radiating area, since my panels are 8 feet tall (96 inches). If your panels are also that tall, I was not aware of that. If they are less than 8 feet tall, then the difference in surface area between mine and yours would be greater than 400 sq in by that height difference factor. I also use tube traps and wall-mounted absorbent panels tuned to low frequencies behind each speaker, in order to absorb as much as possible of the rear radiation, so as to minimize bass cancellation. Furthermore, I have 100 lbs of lead weight sitting on each SL backplate, so as to minimize the tendency of the panel to sway when asked to reproduce low bass. The positive effect of those weights was immediately noticeable in terms of bass definition. (This is why I completely disagree with anyone who wants to put soft spongy feet or springs under any speaker.) Anyway, you don’t need huge panels because of your predilection for electromagnetic woofers (not "subwoofers" if used up to 120Hz). I am not for one second claiming that my SL system can compete with yours for bass SPLs that can blow your head off below say 40-50Hz, but I do claim that my SL speakers get all the music at low frequencies.

@lewm , my SLs are also 8 feet tall. They are a custon Job because the full width 845's were just too wide for my 16 foot wall with the theater screen. According to Roger West bass performance is the same but I am not so sure about that. If you want to "stabilize" the speakers you have to add weight to the top of the speaker. The bottom is fixed by the floor and the interface. Sound Labs speakers will make bass. I never said they did not. The problem is at higher volumes vigourous bass will distort everything else. It is a phenomenon I have noticed will every single ESL I have owned. Taking the bass out of them cleans things up and I am sure you would notice the improvement if you would try it. Unfortunately, analog crossovers do not work near as well as digital ones. 18 dB/oct is too slow. You will be getting subwoofer up into your midrange where it certainly does not belong. 

I had forgotten when I switched to the Sound Labs I changed the crossover to 100 Hz 48dB/octave. That really does not matter. The drivers being used are specifically subwoofer drivers. There is a marked difference in parameters between subwoofers and woofers given the much higher x max of subwoofer drivers.

I hate to be a stick in the mud Lew but, you have no idea what your system is doing unless you measure it. Throwing tube traps and other room mods in an empirical fashion depending on what you think you are hearing is a sure fire way of screwing things up. I know, it is your system and you are entitle to screw things up any way you want.

The helmholtz resonator theory is in no way shape or form dead. What would you venture is boucing the sub chassis at 24 Hz? That is not it's natural resonance frequency nor that of the tonearm. Something else is resonating at 24 Hz and it certainly is not the granite the turntable is sitting on. Look at the design of the RH Labs subwoofer. The enclosure is described as a helmholtz resonator. At any rate I think I know how to stop it. My brother said to think of sound waves as waves in the ocean. What would happen if the turntable was rhythmically immersed in water? What he was getting at is you have to keep the water (air pressure) from getting into the turntable. 

The easy thing to do here is just keep the cover open during play. Never take the easy way out. It is not sporting and you never learn anything. I do not think there can be any question that a closed dustcover keeps dust off the record. I have also demonstrated that a closed dust cover does attenuate sound by as much as 10 dB (not 15 dB) at certain frequencies. 

@secretguy , that is common mythology. It was perpetrated by manufacturers that can't or won't add a dust covers to their tables. It is very much like the tube vs solid state and  Analog vs Digital arguements. Most people with dust covers will tell you that they can not hear any difference dust cover up or down. I do not know for a fact whether or not I can. Once I get this straightened out I'll see if I can set up a blinded AB comparison. My own mentality is to protect my records first and work on the sound second. 

@tomic601 , very interesting point tomic. Yes, it has the magnetic bearing. But, it is the whole subchassis moving. There is no give that  can feel in the thrust bearing. If it has a resonance it would be way up high. 

@rauliruegas , whenever you get new equipment you can run into teething problems like burning up the high frequency balance control in your brand new $45,000 speakers and this one with the turntable. This is where the hobbiest approach comes into play. What would life be like without the occasional challenge? 

Thinking about time can get very depressing. Keep yourself busy having fun. This is just one way to have fun and a good one at that.

Golly! I am so glad to be instructed by you after only 50 years of messing about with audio.  First, I wouldn't argue for a moment that my SL experience would be improved if I were to add SUBwoofers to the Sound Lab system.  (In other words, woofers that come into play at below 50-60Hz and never above those frequencies.) You are absolutely right about that. I already explained to you privately that I have made a choice not to do it, partly because the system is in our living room, which is already too dominated by audio equipment.  But if I did do it, I would never add a woofer that comes in at 120Hz; there is too much real music at that frequency and down below to 40-50Hz. Your 12-inch electromagnetic woofers, impressive and loud though they may be, can never match the speed and articulation of an ESL at 100Hz.  But that's just my opinion based on some prior experience.  The only woofer I would even think of pairing with an ESL at those frequencies would be one based on a transmission line enclosure using a much faster woofer than average.  With the Beveridge system (in my basement), I am not constrained by decor, and the 2SWs do require woofer supplementation; that's how they were originally marketed.  There I do use KEF B139 woofers in a large and ungainly TL cabinet that I built myself decades ago.  But even there, the crossover, as determined by Beveridge and not me, is at 80Hz.  Likewise, the slope of the hi-pass filter is Beveridge, 18db/octave.  Consequently, I use an outboard electronic crossover for the woofers that has a complementary 18db/octave slope. If I had my druthers, I would use the Linkwitz-Riley slope of 24db, because that maintains phase between the high an low frequency reproducers. The woofer x-over has controls for level and frequency, but I keep it around 80Hz.  Finally, there is no free lunch in audio. Every electronic x-over I have ever heard has a "sound".  I've never heard a digital x-over, but I am sure it has a sound too, not to mention the extensive amount of digital room correction that you have chosen to incorporate.  I admit to being almost too pure a purist, but I like to avoid x-overs, both passive and electronic, if it makes sense. This is all fine. The goal is to satisfy onesself.  I would never be so bold as to sit here and tell you what you are hearing over there. Perhaps you should adopt the same approach.  Sorry for the very long digression, but you raised my hackles.  I didn't know I had hackles.

Helmholtz Resonator.  Every definition I can find, including the one on Wikipedia, states in one way or another that you need a closed container with a large hole at one end and a small outlet at the end of a narrow neck at the other end.  Apparently Helmholtz built several of different sizes to demonstrate how size of the enclosed volume of air, neck length, and aperture area all determine the frequency heard at the small outlet. Based on what you wrote, it seemed to me that your problem persists after you seal the dust cover to the plinth surface.  Thus there is no pathway for air to go in or out. So, you cannot have a "Helmholtz Resonator".  For sure, you do have a resonance problem, just not the one envisioned by Dr Helmholtz.  Where have I gone wrong in this reasoning?  I think you may have the forme fruste of "dust cover blues", as you so aptly put it in the first place.  In other words, you have an extreme and unusual problem that nevertheless falls under the category of why some of us eschew the use of a dust cover when playing LPs.

@lewm , thank you Lewm. It is a pleasure instructing you:-)

As I mentioned before I dropped the crossover frquency from 120 to 100 when I got the Sound Labs as they have so much more surface area than my old Acoustats. Given you have even more you could try a little lower maybe 80 Hz but I would never go lower than that. If you have a digital crossover you can cross up higher and still keep the sub out of your midrange with steeper slopes. There is no comparison between digital and analog bass management. They are totally different worlds. If you replaced the crossover in the Beverages with a digital one you could cross up a little higher using a much steeper slope without phase issues. Would be fun to try. 

Lew, the speed of a driver will dictate it's upper frequency limit. Most subwoofer drivers, even the 18" ones are capable of reaching 1000 Hz before cone breakup.The electromagnetic forces generated by ESLs are several orders of magnetude weaker than a modern subwoofer motor. It is not a speed or transient response issue. The problem is matching the subwoofer up to a dipole ESL. The very muddy midrange that a sub produces clashes with the incredibly detailed midrange of the ESL. If you turn off the main speakers and just listen to most subwoofers you will clearly hear some midrange leaking through. This is made even worse by timing errors. This is why early on we were keeping crossover points so low, to keep the sub out of the midrange. The trade off was decreasing distortion in the ESL. The higher you cross, the less distortion you get out of the ESL. It was digital bass management that finally solved this problem.

I still have some issues with the subs I have now. I think there is some distortion added by the enclosures and although they will go very loud I think they could do it with less distortion from the drivers themselves. I am working on new enclosures and will use 8 instead of 4 drivers which will halve their excursions. 

There are different forms of helmholtz resonators. With the dust cover closed you have a sealed system with a large opening at the bottom (under the plinth) leading to a constriction at the opening for the tonearm board into the blind cavity under the dustcover. This is complicated by the springloaded platform hanging underneath. Regardless, as you suggest,  I have a resonating system and I need to figure out how to make it a non resonant system. My next approach will be to make a tight fitting skirt that closes off the bottom of the plinth so that air cannot enter from below. I'll use cheap poplar just to see if it works. If it works I will make a new plinth cover using the old one as a jig. 

You definitely have hackles. 

Mike

Whether its Helmholz or not isn't important. Resonance is.

This isn't the first time I've seen this. Since most high end machines aren't meant to be used with the dust cover down (if they even have one) the dust cover may well not have been included in the design of the turntable insofar as its resonance control methods are concerned.

IOW its the luck of the draw that it works or not.

If you simply grab the dust cover with both hands and try to squeeze it can you make the oscillation stop?

@atmasphere , The original Sapphire came with a dust cover. It was not an option. Later it became an option, extra charge. This design is not all that much different from the original Sapphire. The early Cosmos had a Corian plinth cover. Later they reverted to wood. As far as I know this problem has never been reported before. If I had to guess it is because few people reach bass output below 30 Hz that would excite the resonance. I have to go almost to max volume to excite it and this is with very boosted bass. Look at the curves I put up on my system page. There are two pictures of the table with a microphone sitting on the platter. "UP" is with the dust cover up (the "UP" curve) and the other with the dust cover down (the "DOWN" curve) You can see that up against the side wall the bass is way up, at least 20 dB from where it is in the center of the room. It may be worse than usual because the table is in an alcove. If you read my posts you'll get a better feeling for what is going on. If I press down hard on the cover the feedback will slowly diminish. If I open the cover it stops immediately. The reason it feeds back is the sub chassis is bouncing at 24 Hz which gets the subs going at 24 Hz which keeps the sub chassis boucing. The sub chassis resonance frequency is 2-3 Hz. The tonearm is 8-10 Hz. 

I have three cavities. The one below the sub chassis, the one above the sub chassis and the one under the dustcover. The last two communicate through the tonearm board hole and the 1/8" space around the platter. The last two communicate through the slot that goes all the way around the sub chassis. It is also about 1/8" 

I am going to build a skirt to go around the plinth to seal off the space under the turntable. I'm hoping that will stop it.  I plan on making a new plinth cover out of a more exotic wood than walnut. Depends on what I find. It is like shopping for vegetables. You buy whatever looks good.

Look at those curves again. The dust cover diminishes higher frequencies up to 10 dB. Except in the bass it is 10 dB quieter under the dustcover. Does this mean anything? Who knows? I can not tell the difference between up or down until the feedback starts. I keep it down to protect the records. Am I actually doing that? Who knows? 

You can see that up against the side wall the bass is way up

My surmise is that is your room boundary effect and may also be complicated by a standing wave (looks like you have cancellation in the center of the room). A distributed bass array might be really helpful. You'd only need a pair of subs to break up the standing wave. IIRC sounds like you already have them. If they are located by the Sound Labs  they are probably exacerbating the problem- they need to be elsewhere in the room.

Once the bass is evenly distributed you may not have this problem anymore.

I never had a dust cover for my Cosmos (it was serial number 0).

I suspect your alcove is a bass node. Have you tried putting the 'table in a different spot?

Yep.  Audiokinesis and Duke Lejeune.  Woofer array.

The question of whether the resonance has anything to do with Helmholtz is somewhat relevant, because if it were due to the Helmholtz phenomenon, then you could live on with your prior conviction about the unalloyed benefits of the dust cover.  You point out that there is a pathway for air to exit the space enclosed by the sealed dust cover and plinth, downward around the platter evidently.  But if that constitutes a Helmholtz Resonator, then sealing the space between the edge of the dust cover and the plinth ought to have changed the resonant frequency. Sealing the dust cover to the plinth changes both the area of the aperture and the net path length across the aperture, and those two parameters are in the equation for resonant frequency of a Helmholtz Resonator.  Face it; dust covers are not all good.  If you admit that, I will admit they are not all bad.

Works for me Lew. Obviously, I am having trouble involving the dust cover. It is a first for me and my music habits have not changed. Once I fix the problem the dust cover will be all benefit again:-)

@atmasphere , Actually, because there is no rear wall and I use dipole linear arrays my room is very well behaved. The turntable is adjacent to the listening position. The bass is due to the boundary effect. This happens in almost every room. The table handles this just fine with the dust cover open. I am not putting it in the middle of the room.

I already have four subwoofers soon to be 8. Although they meet the definition of a "swarm" system they are arrayed in a specific pattern for a reason. At any rate the problem is not with the subs. The problem is a resonating turntable and it only happens with a few records cranked to the max. Next weekend I will make that skirt to see if closing off the space under the turntable will stop the resonance. The dustcover is now effectively sealed. Sealing off the bottom will prevent air pressure from having immediate access to the interior of the table.

The bass is due to the boundary effect.

Yes. But it appears you have a standing wave; if you were to break that up the boundary effect might be a bit reduced (that's how it worked out in my room anyway...). From how you've written about this, it appears that if you make just a bit of an improvement the problem would be gone since it only happens with certain cuts at higher volumes. I forgot to ask- what sort of stand are you using for the turntable?

Look at my system page and you can see where the turntable is. It is on a granite slab sitting on a built in walnt equipment/record cabinet (which I made) which rests on a concrete slab. The uprights are 1 7/16" plywood. All the shelves are braced. The entire affair sits in an alcove in the side wall. 

On my system page you can also see graphs of the frequency response at the turntable dustcover up and down. There is a definite rise in the bass but no really big peak or trough. Over 1 kHz the response drops 10 dB with the cover down. 

@atmasphere , see above!

Dear @mijostyn : "" much steeper slope without phase issues. ""

I use first order high-pass and the blend between main speakers and the Velodynes are in " heaven ".

"" It was digital bass management that finally solved this problem ""

not really must be digital IMD goes really low if you cross over 80hz to 100hz and does not needs to be digital.

"" I am working on new enclosures and will use 8 instead of 4 drivers which will halve their excursions. ""

Look, contrary of what you think my take is not more drivers or more subs. What we and you need is proved bass quality performance developed by each driver and that means low bass driver it self.

Velodyne subs are the lower distortions levels in the world. Eacg bass driver in the Velodyne is monitored over 16K times at each second and when the driver distortion goes near the Velodyne limit of distortions impedes that the distortion levels goes up and no matters if the subs are developing 120db SPL.

Quality is the name of the game in overall bass management too.

I understand that you designed and builded your four subs, well maybe needs not more drivers but a different design but I can’t know for sure.

In the mean time enjoy the MUSIC with out dust cover because life is to short and you have not bougth it.

R.

"" On my system page you can also see graphs of the frequency response at the turntable dustcover up and down. There is a definite rise in the bass but no really big peak or trough. Over 1 kHz the response drops 10 dB with the cover down ""

Maybe you are not measuring what you need to measure that can tell you what is going own down the dust cover. You like science then use it in the rigth way.

Look at my system page and you can see where the turntable is. It is on a granite slab sitting on a built in walnt equipment/record cabinet (which I made) which rests on a concrete slab. The uprights are 1 7/16" plywood. All the shelves are braced.

@mijostyn Equipment stands are really important. I'm using a Sound Anchors stand with anti-vibration platforms (Ultra Resolution Technologies, long defunct but highly effective; the stand was custom built to accomodate them) for both the preamp and turntable, but I found out 20 years ago that it still had problems- I had to place the whole thing on a set of bearings rather than use its points into the floor. This made a profound difference. Although my 'table (Atma-Sphere 208, which is quite 'dead'; you can thwack the platter as its playing and not hear anything in the speakers) has no suspension, after the bearings were installed I could run the volume up to 110dB (measured) without any ill effects from vibration affecting the turntable in any way.

Have you done any measurements to verify the effectiveness of your stand? I say this because I've seen stands from established manufacturers that actually made things worse!

@atmasphere , Thanx for your input. The cabinet is not really a "stand." It is a very large built in cabinet weighing hundreds if not over 1000 lbs with the granite and records installed. Look at the picture. The only thing that will affect it is ground rumble from something like a dump truck traveling down the street. Having said this, the idea behind an isolated turnable is that it should perform well regardless of what it is planted on. In this case only frequencies below 3 Hz should be able to get to it. But, I have a situation where airborn vibration, sound is able to shake the sub chassis at 24 Hz creating a positive feedback loop. The dust cover is certainly part of the resonating system because when I open it the feedback stops. As you suggest I am sure the acoustic situation around the turntable is in part responsible for getting the sound pressure levels at 24 Hz high enough to get things started. However, with the dustcover up the table sounds great and I can thump on it with the record playing, volume wide open and you won't here a thing. I can also throw myself against the cabinet and you won't hear a thing. Try that with any stand!  Yes, I have now demonstrated this to non beleivers on several occasions. I have the bruises to prove it.

Lets look just at the facts that we have so far. With very loud volumes using program with high output at low frequency's the subchassis shakes at 24 Hz. If I raise the dust cover the shaking stops immediatedly. If I press down hard on the dust cover it extinguishes over a period of a few seconds. There are two cavities above the sub chassis separated by plate that has a hole the size of the tonearm board in it along with the 1/8" space around the platter. I think it is safe to assume that air is resonating in and out of this structure shaking the sub chassis. My best guess is closing off the space under the turntable will stop the resonance by preventing pressure (or vacuum) from building up under the sub chassis. I forgot to mention that the only structure I can feel vibrating is the sub chassis and I would guess that it is shaking up and down at lease 1/32" 

@rauliruegas , I owned two 12" Veldyne subs for years until the foam surrounds disintegrated. I also had the main amps rolled off at 6dB/oct. Once I had digital bass management I was set free to design whatever I wanted for subwoofers. Distortion in subwooder drivers is dependent on the motor design and the size of the excursions the subwoofer is taking. The Velodyne uses a sensor on the cone in a negative feedback loop to help control the driver at long excursions. It allows you to get more clean volume out of a smaller driver. However another path to low distortion is, do not let the driver take long excursions. You can do this by increasing the size of the driver or by going to multiple drivers. It is all about the surface area of cone you have working for you. The design of my current subwoofers was based on the thought that using four 12" subs with very heavy, ultra stiff enclosures with the mass focused in line with the driver would lead to a low distortion subwoofer line array that would match well with line array main speakers. The subwoofers are made with Corian laminated to MDF with a layer of glass microspheres in epoxy between the two. Each one weights 200 Lb. They turned out to be excellent subwoofers but I still detect some coloration due to cabinet resonance in spite of the insane construction. They also are taking longer excursions than I want at high volume. The new ones are a balanced force design with a 12" driver in both ends of a cylindrical enclosure that has 1.5 to 2" walls. They vary in thickness!  Cylinders are inherently stiff structures. Only a sphere is stiffer. The forces of the opposed drivers will cancel out leaving an encosure that will not shake at all. 8 drivers will decrease excursions decreasing distortion.

1st order crossovers are not steep enough for subwoofer especially when trying to match them to ESLs. To decrease the distortion and increase the headroom of ESLs it is vital to remove from 100 Hz down from them. The subwoofers also have to match the radiation pattern of ESLs to mate well. In this case the ESLs are line sources over their entire range. You have to keep the subs out of the midrange but you want to run them up to 100 Hz which requires a slope of at least 48 dB/oct. That is 8th order! Plus, you have to match the subs the ESLs in time and phase. If you use a turntable you have to add a steep sub sonic filter or your subwoofer drivers will jump out of the enclosures at volume. The only way you can accomplish all of this without "F"ing things up is with digital bass management. 

I hope to have the new subs finished by next Summer. I have to finish the wife's new kitchen on the way. I will show you the results. As you can see from the frequency response curves that I put up the current system has no trouble reaching 20 Hz with power, enough to get my turntable bouncing:-) Gotta love those subwoofers!

Dear @mijostyn  : I prefert pulp/paper as material for subs instead what you use it or even Kevlar.

The sound of natural fabrics is just that " natural ". I have the last Velodyne designs using " paper/pulp ": HGS series.

R.

For anyone following along, now one week in, this is the price you pay for being a, well this guy. Simple enough problem yet no one has figured it out let alone provided the solution. Don't be that guy. 

What guy?

This guy:

Don't listen to me. My hearing sucks. 

AND your point is???

Mijo, I took a look at your system page.  You do have a beautiful and comfortable looking listening room, and you are obviously a highly skilled woodworker. It's always good when one hobby supports the other.  Seeing where your TT is located, on a shelf set into a wall with a shelf above, I do have to wonder whether moving your TT, just experimentally, would be worth a try. I get it that you don't want to radically reposition the TT to a permanent new location, but just for diagnostic purposes, it would be worth moving it temporarily, so that it is not against a wall and in a shelf.  In lieu of moving the TT, you could also remove the dust cover when playing LPs.

@rauliruegas , The subs I am using now do have paper cones. The ones I am thinking of using in the new subs have aluminum cones which I am not crazy about because they are easy to damage but I am space limited and I need a driver that will operate in a 1 cubic foot enclosure, These will. Subs you do not hear as much as feel even though I run mine up to 100 Hz. People shy away from running subs up high because everyone is used to using just low pass filters and not full two way crossovers. Plus, they have no ability to match them in time with the main speakers. They use very low low pass filters so they can not hear how bad the match up really is. If you try to cross this way at 100 Hz it becomes a real mess.

@lewm , thank you for the compliment Lew. Right now if I am going to listen to rock loud I keep the dust cover open. This weekend I am going to make a skirt to seal off the bottom of the turntable. Hopefully, it will work. Moving the turntable to the center or back of the room would certainly work as the bass is much lower in those locations. I doubt moving the turntable along the shelf it is on would do anything as the bass response is pretty much the same all along that shelf. As you may notice the turntable is right above the phonostage. There are holes drilled in the granite in strategic places to lead the wires through keeping AC wires away from signal wires. Moving the phonostage is not a nightmare but close. I can get to all the wiring by removing the records and equipment but it was originally put in place before the granite went down when it was much easier to deal with.

The turntable can handle the location fine except for this one fluke. The cabinet is not the problem. It vibrates very little if at all. The only direct connection between the cabinet and the wall is the trim around the upper cabinet and there are triple 2 X 6 studs under it. It is also not just planted on concrete. It is right on top of the foundation which goes down 12 feet to footing. 

If the skirt works I will make a new plinth cover and bring it down to within 1/8". Just enough room to be able to level the platter. If the skirt does not work then I will just have to keep the dust cover up when playing at volume which is not all that often, I'd wind up divorced. 

Rumble filter?

@rwwear , I already use one, a digital one! It is 3 dB down at 18 Hz and rolls off at 80 dB/oct. The way I run my subwoofers if I did not use this type of rumble filter the cones would wind up across the room with the first glitch:-) My problem is occuring just above the cutoff frequency. I could program the filter up higher but that would start interfering with the sound. The solution is in controling the resonance problem and there is a solution to this. I just have to figure it out. 

Ok I have played my dust cover up....down with the turntable on.and I have played it off.....i believe good balance of my turntable. Keeps me from ever hearing any strange sounds .The cabinet the turntable is on is well balanced. I hear all of these problems, but I never had any....I have had a number of turn tables.. .Since 1969......

My problem is occuring just above the cutoff frequency.

@mijostyn Filter theory says that the greater the filter slope, the greater the hump is prior to cutoff- and the closer to the cutoff frequency. A digital filter might behave differently (some do, some don't) but a simple solution might be to move the cutoff down a few Hz so as to get the turntable resonance out of the filter bump that occurs just above cutoff.

That should be easy- worth a try.

@atmasphere , I can do that easily. I'll give it a go tomorrow. 

@limomangus , I'm not quite sure what you are saying. You have had a number of turntables and you have used them with the dust cover on and off and do not have any problems with either?

Dear @mijostyn :  If you are using paper cone I think that you have to stay that way ( btw, I cross too at around 100hz. My Velodyne HGS are heavy modified by me because its electronics design: input and crossover are very poor but great all other design/build. ). Stay away of other cone material on subs.

Now, I think that you are not measuring what you need to measure ( I don't know what you need to measure. ) or you are doing not in the rigth way.

In theory everything can be measured " problem " is what to measure and you have to " figure " out about instead to use your time in this thread.

R.

Mike--you say the bass frequency response is pretty much the same all along the TT shelf--is the bass response pretty much the same all along the left wall?  or could you possibly slide the entire cabinet one way or the other to find a bass null at 20-30 Hz---or would that be a real PITA?

@wyoboy , several issues. The first is that the cabinet is built into an alcove so it is definitely not moving. The second is that I designed the room so that it has no back wall. It is broken up between the kitchen and dinning room, the nearest solid wall being about 75 feet away. I also use line arrays which limit dispersion so as to minimize room effects. There is some modal behavior but it is very weak in comparison to the usual situation. The bass at both side walls is very much determined by the boundry effect. In order to get the turntable to a balanced frequency response environment I would have to put it out in the middle of the room. 

@rauliruegas , I owned Velodynes for perhaps a decade in the 90's and I got them working tolerably well but I was using an outboard crossover.  IMHO the major problem with subwoofers is not the cone material but the enclosure and positioning of the drivers. Any good subwoofer driver 12" and up is capable of beating 0.5% distortion it not over driven. Everything else comes down to enclosure resonance and shaking. For commercial subs Your Velodynes are not bad but (and I really mean this politely) they are not near as good as what I am using now never mind. what I have in store. I aim to build the most accurate subwoofers possible with current drivers and I will do it. (or go broke trying)  As for measuring. How do you know what and how I am measuring except the tools I have mentioned in these threads? The only part of a HiFi system that is not measurable is the human mind. Everything else is easily measured with the right tools. The bane of speaker design has for decades been the room they are placed in and even that is measurable. Any problem you identify can be fixed. If you can't identify it it becomes a mystery and something you "can't measure."  

As for my dustcover problem. That has been fixed by sealing off the space under the turntable with a skirt. I can now bury the volume with the dustciver down and it will not feed back. I put a picture of the solution up on my system page. The final solution is a new plinth which I will get around to when I find that special plank.

Mike, You sure get up early in the morning.  More important or at least as important in conceiving a supplementary woofer or subwoofer is the cabinet design. The choice of cabinet type would affect the choice of woofer, I would think.  So what are you going to build, acoustic suspension, bass reflex or other ported design, open baffle, or what?  (I can't be sure from your photos what type of cabinet you are presently using, either, and I don't see 4 woofers.  Where are the other 3?) For me, I don't care much about frequencies below 30Hz.  I much prefer a "fast" woofer that can mate well with an ESL around the crossover point.  A seamless blend is hard to come by, even if using an 80db/octave slope, or especially so if using such a very steep slope.  I have no dog in this fight; I'm just curious to learn. I long ago decided on a fast, smallish woofer in a Transmission Line enclosure as my ideal for mating with a full-range ESL or an ESL that needs help at low frequencies.  If I wanted more oomph (as in SPLs) I would use two or more of such agile woofers per channel.

By the way, I agree with you, everything can be measured.  But sadly, once you've done that and technically perfected the frequency response in room, I have yet to hear a system that was obviously benefited by such contouring. (On the other hand, in the past several years, I have not auditioned that many systems other than my own.)

Dear @mijostyn  : Thia is what I posted:

"" 

Now, I think that you are not measuring what you need to measure ( I don't know what you need to measure. ) or you are doing not in the rigth way.

In theory everything can be measured " problem " is what to measure and you have to " figure " out about in ""

You posted: " sealing off the space under the turntable with a skirt. "

You with all your measure tools still does not know what happened down there with out the skirt.

" The final solution is a new plinth... ", yes that's the easy road to go that not necessary warranty success: maybe yes or maybe not.

R.

Dear @lewm   : "" I don't care much about frequencies below 30Hz.  I much prefer a "fast" woofer that can mate well with an ESL around the crossover point.  A seamless blend is hard to come by..""

Who told you all that including that no sense " fast " woofer or your statement came from your first hand experiences and I said " first hand experiences " because you are totally sure about. Could you explain it?

Btw, please read this link:

http://www.soundstagenetwork.com/maxdb/maxdb061999.htm

R.

@lewm , It can work either way. I think the majority of people building subwoofers come up with a design concept, then choose an appropriate driver and finally given the parameters of the specific driver tweak the dimensions of the enclosure to suit. 

If you look at the picture of my system on the floor you will see a woofer to the outside of each panel then in the center those two boxes on the floor are subwoofers. They are pointed at eachother. These woofers have 200 lb sealed enclosures with a Q of 7.6. 

The new design is a decagon cylinder with 1.5 to 2" side walls (they vary). There will be a 12" driver mounted in each end. They will also be sealed. There is no reason to resort to ports when you have advanced "room control" which is really speaker control. With enough power you can make a subwoofer do just about anything. I would have liked to use larger drivers but space will not allow.

The tonality of a system is debatable and we all have our own preferences. What is not is image and detail. DSP allows you the adjust the frequency response of the channels individually so that they are exactly the same. This gives the best image and with it detail. Every audiophile that has heard it goes out and gets a processor and I am not kidding, every single one. 

Lastly, there is no such thing as a "fast subwoofer" when a woofer is not fast enough it's high frequencies roll off. Usually, the cone breaks up first.  Even 18" drivers can make it to 500 Hz. I think what people really mean by this is muddy vs well defined. Mud is coming primarily from the enclosure. Then there is the amp's ability to control the driver. Some amps are good at it, others not so hot. You need a powerful amp with a very low impedance output stage. Transmission lines are a way of effectively doubling the size of the driver at certain frequencies. They are very difficult to make and require a lot of trial and error tuning. It is much easier to use large drivers or a multitude of small ones.

@rauliruegas , I know exactly what is going on. The two chambers above the subchassis are resonating at 24 Hz. This causes the subchassis to bounce at 24 Hz. This is picked up by the cartridge completing the feedback loop. The sub chassis is a solid 1" thick aluminum plate that extends to within 3/32" of all sides of the plinth. Your middle ear has a vent tube to release pressure. It is called the Eustacean Tube. When it gets block your hearing gets damped by up to 10 dB at some frequencies because pressure in the middle ear will not allow the ear drum to move as well. On a hunch I decided to block my turntable's Eustacean Slot with the skirt you see on my system page effectively giving it a hearing problem. It was very easy to test. The first track on Soundgarden's Badmotorfinger gets the feedback going throughout the entire song. With the skirt in place there is none. It is a bit hard to miss. When a 24 Hz feedback loop gets going the entire house shakes.

Right now the skirt works perfectly I just have to dress it up a bit. Yes, I can and will build a new plinth that won't do this when I run across that special plank of wood. If I am going to put in all that effort it has to be very special.

Dear @mijostyn  : Good, I applaud your attitude to achieve the best sound that puts you nearer to the recording and nearer to the live MUSIC.

I'm sure that your new subs will be an improvement as the new plinth however anything you do can't  carry to full success till you change the item that proccess the cartridge signal and that's is the Phonolinepreamp.

No matters what the unit you own rigth now is way inferior of what you are looking for..

Yes, it's up to you.

R.

Dear Raul, That’s a very interesting article. The author makes a good point that using the term "fast" when describing a woofer is specious, because the "speed" required to reproduce very low frequencies accurately is within reach of any well designed woofer. But then he goes on to say: "There are reasons to use lighter, lower-mass woofer cones. They just happen to be different reasons than the ones you’ve read in print. Smaller woofers don’t make faster bass, but they do reproduce higher frequencies than larger woofers can reproduce, and this is all important when it comes to speaker design. You want the midrange driver and the woofer to integrate with sublime symmetry, with perfection and with nary a single problematic interaction throughout their overlap zone. This is why you want smaller, lighter, "faster" woofer cones -- not because they lead to faster bass. That overlap zone is so amazingly critical to your perception of bass speed that there is little or no tolerance for error." Note also that your expert does allow for the idea that some woofers are "faster" than others; he has only re-defined the term in a sense with which I do not disagree. By the way, no one "told me" anything about this. My conclusions are based on real world experiences that I had maybe 35-40 years ago when I was playing around with woofers to supplement ESLs that I then owned. So, I am expressing my personal independently arrived at opinion.

So perhaps I can be faulted for my choice of the word "fast", but what your authority wrote above is what I had in mind. Now if Mijostyn is using an 80db/octave crossover, on both high and low pass filters, then perhaps the capacity of his woofers in his system to produce higher frequencies is moot. On the other hand, all of his audio is passing through the digital domain afforded by his digital filter. That does not appeal to me.

Mijostyn, Along the same line of reasoning, you wrote, "Lastly, there is no such thing as a "fast subwoofer" when a woofer is not fast enough it’s high frequencies roll off." I hope you see the internal contradiction there. If there is no such thing as a fast woofer, then there is no such thing as a woofer that is not fast enough. But to both you and Raul, I would concede that I was guilty of sloppy semantics. When I say "fast woofer", I am thinking of the woofer and its enclosure as a whole. And I did not make that clear. You could put a small, i.e, "fast", woofer in an enclosure that limited its speed by virtue of what happens to the back wave, and it wouldn’t sound so fast, which we can define here as able to integrate well with an ESL panel. Like I responded to Raul, perhaps with an 80db/octave slope you need not worry about the capacity of your woofer enclosures to deal with frequencies above your crossover chosen point.

@rauliruegas , what would you suggest?

On December 3rd I am traveling to New York City and on the way we are stopping at Soundsmith to audition the strain gauge cartridge. If I decide to go with that it comes with a dedicated phono stage. For a moving coil phonostage I was looking at the Channel D Lino C, the transimpedance unit. I would probably try a My Sonic Lab cartridge to go with it. 

@lewm , Actually, right now I am using 48 dB/oct slopes with a cut off of 100 Hz. Yes, smaller drivers are capable of higher freuquencies.  The "speed" of a woofer is indicted by it's frequency response as I said before. Otherwise a subwoofer's sound is not dictated by speed as long as you stay away from it's limits. 12" woofers generally are good to 1000 Hz and we go nowhere near that. With the right crossover and time correction you can match subwoofers to any loudspeaker. 

I can get you over your digital phobia in a heart beat. Who knows. Maybe next time you wander towards Vermont :-) 

Dear @mijostyn  : I hope you don't pull the trigger on that kind of SG cartridge because make no sense to play LP recorded with the RIAA eq. curve through a dedicated phono stage with no inverse RIAA eq...

Just saying,

R.

I have the Soundsmith Strain Gauge, got one used about a month ago.  SG does not use RIAA or any kind of normal phono stage because it is not any kind of normal cartridge.

MM and MC both are generators that generate a voltage by moving. The faster they move the greater the voltage. SG does not work that way at all. What it does, the cantilever is connected to strain sensors that detect deflection. These are used to modulate a voltage coming in from the SG preamp. So the preamp generates the voltage, the cartridge modulates it (similar to the way a tube acts as a valve) and the resulting signal comes back into the preamp where it is amplified. 

Because of the way this works it requires no RIAA equalization. One of its many advantages over the conventional approach.

Overall it is a nice step up from Koetsu Black Goldline/Herron VTPH2A. With one exception, the low end is a bit weak in comparison. In terms of tracking though it is darn near seamless. Recordings that seemed to have hopelessly large amounts of sibilance now sound like the normal level of sibilance you would expect from a real live person. Percussion in particular is tracked exceptionally well. Listening to SG confirms Koetsu has a real talent for tailoring resonance. The Black Goldline may be entry level Koetsu but it truly is a wonderful cartridge. The SG on the other hand gives the distinct impression of listening to the master tape.

SG also has some nice features like auto-mute and a built-in 12Hz filter. Its main weakness would seem to be the power supply, which can be upgraded with SBooster, Farad, etc. 

@millercarbon , Correct MC, when the gauge is stressed it changes resistance which modulates the supplied voltage which is subtracted afterwards leaving only the signal. Messing with the power supply might not be a good thing. If the voltage subtracted is not exactly equal to the voltage added you will get a dc offset. If your amps go down to DC you could have fried woofer for dinner. If run through a preamp this might filter the DC out.

I do not think it is weak bass but rather exaggerated highs supposedly a hangover from the RIAA curve. Several reviews have noticed this. I can correct for it so it is no worry for me. Weak bass I won't tolerate. If I can't feel the bass drum beat that would kill it for me.

@rauliruegas , you did not make a suggestion for a phono stage or comment on the Lino C. I'll be taking 5 records I know very well down with me. I should be able to pick up any major problem depending on the system Mr Ledermann is using. I won't spend that kind of money if I am not completely comfortable with it. I am also very captivated by the transimpedance approach to amplifying a moving coil cartridge. The only problem is that it limits you to very low impedance cartridges. I have no problem with this as there are many excellent low impedance cartridges. I read uniformly good things about the MySonicLab cartridges then there is Lyra and Ortofon. The new Verismo looks very interesting. The My Sonic Ultra Eminant Ex with an impedance of 0.6 ohms is perfect for a transimpedance stage. 

Dear @mijostyn  : Years ago I listened the SS SG mounted in a Schroeder tonearm and in first rate room/system and there I had the opportunity to compare it against the Lyra Olympos and at least for me no contest against that Olympos set up.

Years ago too in the SS site was a link and the SG real curve and unfortunatelly SS decided to take out/off the site.

" The Italian article did a very good job of explaining to a general audience the difference between "velocity" devices and "displacement" devices.."

In a very hot dialogue where I posted PL him self posted:

" I have measured the SG in many arms, and recently in my Schroder Reference SQ, the new SG design (which you did not hear) it was +/- 1dB from 50 Hz to 12K in conformation with RIAA. ..."

That measure tell us a swing of 2db where our ears are more sensitive but the deviation from the RIAA below 50hz and over 12khz are even higher.

No problem with if what you listen like it.

Now, I understand you owned or listen to the Lino that could be better performer that your today phono stage.

I know very well the overall design of the Lino and other current mode phono stages and for personal reasons I'm not with the kind of overall designs/parts used to build those phono stages.

But that " preference "/captivated you have with and that you ""  exaggerated highs supposedly a hangover from the RIAA curve. Several reviews have noticed this. I can correct for it "" in the digital domain precludes my recomendation to you to really  arrive nearer to the recording and live MUSIC event.

My path/road is totally different from yours.

R.