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Free air resonance

Hello all,

Is a speaker with a free air resonance of 25 hz meaningfully different from one with a free air resonance of 38 hz?

Specifically: is the one at 25 hz low enough to be in a sealed enclosure, as opposed to the one at 38 hz which most likely/definitely should be in a ported enclosure? And why?

Thank you in advance …

unreceivedogma

Showing 7 responses by unreceivedogma

unreceivedogma's avatar
unreceivedogma OP
1,082 posts
 

Thank you @mijostyn ​​​​@mahgister


This is not about a subwoofer.


The reason I ask is that I have a pair of Altec Lansing 604Cs and I now, as of last night, was able to get a 604D to match the 604D that I already have.

I am currently using the 604Cs. The D was a backup in case one blew. Now that I have two D’s, should I switch?

As far as I can tell, however, the ONLY difference in the specs is the free air resonance.

I will be putting the cabinets on rubber feet designed to isolate them from the wood plank floor so that the building doesn’t become a de-facto loudspeaker. I’d like to close the port so as to make the midrange more detailed. This entails losing some bass, but …

I use a Velodyne ULD15, with an active crossover at 60hz for the bottom.

I’m thinking that a speaker with lower resonant frequency will sound better than the one with a higher resonant frequency.

But is the difference too small to even matter, or are the numbers a logarithmic progression instead of arithmetic and thus the difference bigger than one might think? And, does having an active sub crossover at 60hz make this question moot?

one other question: the cabinets are DIY, made of 3/4 ply with a veneer, geometry and materials to Altec specs standard in the day (1950s/60s). Would the bass benefit by stiffening them with another 3/4 layer of ply or solid wood on the interior? If so, do I stiffen all 6 sides, or would just the 4 perimeter sides (not front and back) suffice?

Also, someone told me that one mechanical difference between the two is that the stock C used a paper surround and the stock D used an accordion surround. As Cs are today almost always reconed with the accordion surround (mine are), and as the materials of the cone help determine the free air resonance, simply reconing the C allegedly makes it a D.

And yes, while I did once have a high math aptitude I’m these days inclined to look for layman’s answers to these questions, or as simple as possible.  

Thoughts?

Thank you again.

 

unreceivedogma's avatar
unreceivedogma OP
1,082 posts
 

@erik_squires

 

thank you Eric.

I am working on finishing the room.
All the walls are 5” rock wool.
The gable ceiling is 12” rock wool.
All covered with fire resistant burlap instead of sheet rock.
The floor is hard wood.

Essentially, a semi-anechoic room. Very dead: the difference in sound quality is immediately perceptible as you climb the stairs into the room.

I was told by the guy who built my Futterman OTL3s that stiffening lowers the resonant frequency, but he was also quick to offer that speaker cabinet design is not his area of expertise. What you said about stiffening the baffle only makes good sense, as I would like to decrease the energy flowing through the cabinet walls. 

unreceivedogma's avatar
unreceivedogma OP
1,082 posts
 

@mijostyn @erik_squires

 

Thank you both, bunches. I’m learning a lot already.

Eric, the room is semi-anechoic, not anechoic. The floor would have to be treated as well, which it most certainly is not.

In my last home, I did the room this way and it worked very well. The room doubled as a library, and as I added books to the walls, the room became brighter: the books were the way to control room acoustics.

I started out building my own Dyna 70 and Dyna PAS when I was 14 years old, but since then I’ve otherwise never given too much technical thought to how to design a better system other than to substitute new components.

I’ve had the components I have now for 30 years on average, and with the new home (the wife and I leave this home feet first only: I came close 8 weeks ago, I got double bypass surgery after the discovery of a completely blocked Widowmaker, 😂. The surgeon says that after I complete recovery, I will be better than new! 🙂) and new listening room, and with my amp engineer who has worked on many home audio systems saying that he has heard only one other system that sounds as good as this one, I’m not gonna change the components, my thoughts have turned to how to get more out of what I have, so that means returning to thinking more about the science of these things. The weakness of the Altecs and of the Futtermans are convincing extension at the bottom. They are really good right now, don’t get me wrong but something does nag at me in this area. The cabinets are solid for their day, but there is some resonance there. They do allow for port size adjustments, but it’s just unscrewing a block of wood and sliding it up or down! Maybe painting the interior with a material that will stiffen the walls? I don’t know, and then it has to be reversible because what if it doesn’t work. A friend built beautiful new cabs for his Tannoys but he is top shelf carpenter, works for Broadway, I can’t afford him.

Maybe I need to change the subwoofer. But I don’t want to spend a lot of money either. The current sub crosses at 70. And maybe I just never found the right spot for that sub. In the new room, there will be lots of room to fool with placement.

configuration is

Cartridge: Koetsu Onyx

Tonearm: Sumiko MMT

Table: VPI HW MK IV with SAM

Step-Up Transformer: SHURE A86A

Preamp: Beard P505

Phase Alignment: BBE282ri sonic maximizer

Subwoofer: Velodyne ULD-15

Amps: Julius Futterman OTL3s, converted to triode by Jon Specter

Crossovers: Mastering Lab

Speakers: Altec Lansing 604C coaxial studio monitors

Semi-anechoic room design and treatment: by architect C.B. Wayne

cables: whatever

https://www.theaudioatticvinylsundays.com/about


Oh, and as for rock wool, it is used in a lot of commercial and audio acoustic panels. I use it to insulate my home because

- it’s the best thermal barrier

- it’s far and away the most fire resistant. Fiberglass, fire-resistant cellulose, etc burns by the time you hit 375 degrees This stuff doesn’t melt until you hit 2,000 degrees. An important consideration in historic buildings, of which all the buildings we have restored to date are

- it has excellent sound absorbing properties

My architect had done a few recording studios. He said that in my audio room, don’t sheet rock the walls Just cover the rock wool with burlap and then I will have the equivalent of a high quality recording studio for 1/20 of the cost, for the amount I was spending to make the thermal barrier anyway. Safe health wise also.

 

I don’t like using headphones  

Thank you again.

unreceivedogma's avatar
unreceivedogma OP
1,082 posts
 

@mijostyn

- there will be three Moroccan rugs. 🙂

- I use Doug Sax’s Mastering Lab crossovers. http://www.rintelen.ch/download/604xover_SP.pdf I had Jon Specter replace all the caps with Jensen audio grade caps

- as for the current enclosures, they ARE custom DIY enclosures, built to Altec specs of the 1960s. https://www.hifiengine.com/manual_library/altec-lansing/620a.shtml I think there is room for improvement, but I don’t (yet) know the first thing about it, engineering-wise.

- Altec claims that the 604C and D go down to 30HZ

- As for the Velodyne UHL15, it seems to crossover at 85HZ, see https://www.stereophile.com/content/velodyne-uld-18-amp-uld-15-subwoofers-specifications and https://www.manualslib.com/manual/533050/Velodyne-Uld-15.html?page=10#manual I don’t know that it can be adjusted to 100.

- T-S parameters?

- I had Lafayette speakers to go with the Dyna. I couldn’t afford AR speakers.

 

see what J Frum says here:

https://hifihaven.org/index.php?threads/embiggening-the-altec-620-thoughts-experiences-with-9-cu-ft-cabs-for-altec-604-8g.7649/
 

Y’all need to get religion with regard to enclosure simulation. With T/S parameters, modeling a basic bass-reflex cabinet is easy and accurate. It’s a brave new world.

You’ll learn all about the delicate balancing act between the driver’s electromechanical properties, enclosure volume, and tuning. It’s a lot more complicated than “bigger = better”, and you’ll see the effect changes have on frequency response, power handling, and group delay.

I’ve modeled the 604-8G every which-way, and I can’t come up with anything that’s better overall than a 9 cu. ft. cabinet tuned to about 40 Hz. - essentially, a 620.

I’ve said it before, and I’ll say it again: It’s almost like those Altec engineers knew what they were doing.

unreceivedogma's avatar
unreceivedogma OP
1,082 posts
 

@mijostyn 

 

"....In order for me to get a handle on your enclosures I need the interior dimensions and the material they are made of such as 3/4" plywood or 1" MDF."

I am certain that they are plywood but I can tell you tomorrow. Altec specs from their brochure below: I will check to see if mine in actuality do match exactly.

 

SPECIFICATIONS

Type: Bass reflex enclosure

Dimensions:  40" (1 01 .6 em) H  26" (66.0 em) W  18" (45.7 em) D

Pressure Sensitivity: 100 dB SPL at 4' with ALTEC 604-8G loudspeaker when measured on axis with 1 watt input of band-limited pink noise from 100 to 10,000 Hz and calculated to 4' equivalent (Ref.: 0 dB == 0.0002 dyne/cm2)

Speaker Accommodations: 15", front-mounted

Internal Volume 9.0 cubic  ft

Weight: 104 pounds (47.2 kg), enclosure only

Finish: Hand-rubbed oil on rift-cut oak, charcoal brown grille cloth

Recommended ALTEC Speaker: 604-8G Duplex Loudspeaker

 

ARCHITECT'S AND ENGINEER'S SPECIFICATIONS

The speaker cabinet shall be the bass reflex type, and shall provide front mounting for 15" speakers only. The cabinet shall meet the following criteria. Grille assembly,snap-on. Grille cloth, charcoal brown. Internal volume, 9.0 fe. Dimensions, 40" H x 26"W x 18"0. Weight, 104 pounds. Finish, hand-rubbed oil on rift-cut oak.

The speaker cabinet shall be the ALTEC Model 620A.

unreceivedogma's avatar
unreceivedogma OP
1,082 posts
 

Good morning @mijostyn

- plywood

- the “port” is just a rectangular hole about 2.5” or 3” x 12” cut into the baffle in front, below the driver. A wooden block is affixed inside the cabinet that I slide up and down to open or close the port, or place in between.

- the Altec crossover is 1600

- The Mastering Lab crossover that I use is 2000. It’s shelved (I’m not sure what the term means, but I know what they are for, 😆) at 8000 also.


- the novel cabinet shape that you use sounds like an interesting approach but it begs the question: all that weight being supported so high off the floor must be supported by something that will have its own resonance issues. ? Might be a bridge too far for me 😬

- I would send pictures but this forum does not seem to allow for attachments.


- I have noticed that some cabinets have a piece of wood at an angle inside the cabinet. This does not. ?

 

@timlub  thanks for that link. I see that this topic has a history here!

As always, thanks again. I’m really learning something here.

unreceivedogma's avatar
unreceivedogma OP
1,082 posts
 

@curiousjim 
— I am not aware that I have any control over the Velodyne crossover setting. It comes at 85HZ. I don't know where you get over 100. The Velodyne frequency response is 18-85hz. The resonant frequency is <4hz

— Yes they are great! This is a fun conversation.

@mijostyn @erik_squires 

— Altec's 620 cabinet is 18" x 26" x 40", for 9 cubic ft. Mine is 18 x 26 x 38, with a 2" base

— The diameter of the driver frame is 15.2", so 18" leaves little room. Otherwise, I am trying to visualize your cabinet design. I guess I would have to draw it out first, but it seems quite ambitious: not easy to execute. 

— My latest thinking is to start at the beginning. I've had the 604Cs for 40 years, the D for 25 years. They haven't been re-coned in almost 10 years. They are all over 60 years old. It might be wise to send them to Great Plains to get them re-magnetized and refurbished. Then fool with the cabinet.

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