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Suck out at 165 hz...can't find the cause

I have been focusing on improving my room and dealing with its effects on sound quality. I have been able to deal with some peaks in the 40-100 Hz region, and my room/speakers now measure generally quite well.

But I have a fairly narrow band suck out centered at 165 Hz, that is fairly deep (around 10 db). I have tried moving the speakers, putting up soft stuff (blankets, pillows, human beings) along the axes of the room, etc. Nothing seems to affect it at all.

Could this be something related to the speakers themselves (e.g. internal speaker cross-overs)?

This is an analog two-channel system with a Jeff Rowland Capri pre-amp, Kharma MP150 ampifiers and Wilson Benesch Curve speakers. Unfortunately, the reviews of these speakers do not have any measurements with them, so I don't have an external reference.

The amps are wonderful but are not super powerful, so I'm thinking that an RCS system won't help because trying to get rid of a dip of that degree will force the amps to work too hard and they probably won't be able to do it anyway.

Your thoughts as to what might be causing this and any advice on dealing with it are most appreciated.

Thanks!

--dan
dgaylin
9 responses Add your response
darkmoebius
900 posts
 
What's the dimensions of your room?

Have you used a graphical room mode calculator to see what modes your room dimensions create?

If not, you can just enter your room measurements into this webpage (for Axial, Tangential, & Oblique room modes), this one or this webpage(if using Internet Explorer & MS Silverlight plugin)

Or, just download and run ModeCalc if you are using Windows
dgaylin OP
255 posts
 
Darkmoebius --

thanks for the response. I have tried a few of these sites before. They are useful, but I've never fully understood what I'm looking at, and I definitely don't understand how you interpret the results to see which frequencies will be emphasized versus those that will be de-emphasized. Any tutoring here would be most appreciated!

Thanks.

--dan
palasr
519 posts
 
I suspect it's something in your room, or as others have noted, something in the speaker positioning. Also, while you confess some degree of confusion regarding room-mode calculations, I'm curious how you arrived at the exact frequency you cite? Do you have access to a spectrum analyzer?

I had a similar problem around 132Hz, with a -12dB dip. As it turned out, it was my built in record shelves which have very deep cavities and a small 2" opening along the top of each shelf of records. Once I placed an airtight backing behind the records sealing this cavity, my problem was solved as I had inadvertently created a perfect Helmholtz resonator at this exact frequency.

I recommend the Master Handbook of Acoustics by F. Alton Everest as a must read for anyone interested in maximizing their listening environment.

-Richard
shadorne's avatar
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shadorne
7,467 posts
 
If it is fairly consistent across the room then it could be woofer to floor and ceiling height effect (quarter wave cancellation) - a down firing subwoofer crossed over at 90 Hz might help.

FWIW a null/suckout at 165 Hz is not too bad provided it is narrow.
dgaylin OP
255 posts
 
Thanks all. These are very helpful suggestions and thoughts. Richard, I do indeed have a spectrum analyzer -- I have the PC based one from XTZ, as well as the Velodyne SMS-1. You point about built-ins shelves is very interesting -- my room has built in-shelves (housing CDs) all along the longer axis and this is the axis and the side of the room that the speakers are placed with.

Shadorne, that's very interesting about the subwoofer. Mine is side firing unfortunatley, but I could try angling it down a bit. I guess I am lucky that it the null is definitely narrow bandwidth -- I don't have it in front of me here to report back but it is definitely not a wide band problem.

One question: when you look at the results of the websites that analyze the room, how do you know what will be a null and what will be a node?

Thanks again, all.
shadorne's avatar
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shadorne
7,467 posts
 
Depending on the order of the mode and type (Axial, Tangential or oblique) you will get peaks and troughs at different places across the room. The worst effects are when these combine at th elistening position.

Some things go across the entire room. The quarter wave cancellation reflection of the bass frequencies from the wall behind the speakers generally affects the entire room.
darkmoebius
900 posts
 
Hi Dgaylin,

Give us the dimensions of your room so I can post some graphs and data from the room mode calculator. Then, maybe we all can walk you through the most obvious possible culprits.

Also, a picture or two of your listening room would help.
dgaylin OP
255 posts
 
Okay, thanks again guys, and I think based on the additional websites you have sent me that Shadorne is probably correct and this is the quarter wave cancellation -- main listening seating (couch) is right against the rear wall, and so my ears are about 18 inches from the wall.

Darkmoebius, thank you for the willingness to help. I am in a hotel room and can't send pictures, but here are the basic room dimensions:

L: 19' 9"
W: 12' 6"
H: 8'

But it is a bit more complicated than that with the built in bookshelves and cabinets and a couple of small alcoves I can e-mail you a diagram of the room that I have worked up -- it doesn't transfer well as an forum post. Please let me know.

THANKS!!!

--dan
hornguys
180 posts
 
In the boundary dependent region - below 300 Hz - moving the listening location - even slightly - nearly always produces a much more noticeable difference than moving speakers or room treatments.

Best,

Jim
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