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Speaker Placement?


shinerman

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If you're still in frame walls, try some random blocking between studs. My old 1950's home has two fireblocks in each stud space. Couple that with the double 1/2" plaster board and you have walls that don't 'ping' or resonate although they are quite live.

To my ears, one of the most objectionable acoustic problems is all that drywall, only 1/2" thick, and suspended in evenly sized panels (16" oc x 8' or whatever). Think of that as a bunch of very similar sized drum heads that vibrate sympathetically with the music. The dreaded 'hand clap' test in large drywall rooms is a good demonstration of this. I'm a believer in dense, well braced walls as not only good construction technique, but a first step in a better sounding room. And glue and screw the drywall if you can.

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Thanks for your suggestions, I do appreciate it. Helmholtz Resonators inbetween the studs? How would one do this?

The tuning of a HR is based upon the area of the mouth, and the volume of trapped air behind the mouth (think tuned pipe). What you do is put a vent in the wall at a location where you've got a peak from a standing wave. Behind the vent you would create an airtight volume of air inbetween the studs. The only catch is that you need to know what frequency the HR needs to be tuned at, and where the peaks of the standing waves are in your room. If you're off by even a bit, the HR will pretty much do nothing.

Speaking of which - I wonder how that would play into problems shifting around as people and furniture changed in the room...[^o)]

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FWIW, I am going to try to sporadically start posting material on room modes and standing waves, and then move into the small room analysis bit on the Small Acoustics thread. So may some of the issues being addressed here can be addressed in a little more general manner, if thats OK... Oh, and if its not, a myriad Monty Python retorts immediately spring to mind...

[;)]

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  • 1 month later...

Thanks for your suggestions, I do appreciate it. Helmholtz Resonators inbetween the studs? How would one do this?

The tuning of a HR is based upon the area of the mouth, and the volume of trapped air behind the mouth (think tuned pipe). What you do is put a vent in the wall at a location where you've got a peak from a standing wave. Behind the vent you would create an airtight volume of air inbetween the studs. The only catch is that you need to know what frequency the HR needs to be tuned at, and where the peaks of the standing waves are in your room. If you're off by even a bit, the HR will pretty much do nothing.

Speaking of which - I wonder how that would play into problems shifting around as people and furniture changed in the room...[^o)]

The idea in your last sentence is something that I have been worried about for some time now.

If I do measurements and room treatments for my room, then what? After doing this what if I decide to rearrange the room or experiment with different speaker locations, won't I have to remeasure and re-treat the room? This would be very frustrating, every time I make a change in the room. The realities for me are that most likely I will want to try different speaker locations over time and most assuredly my wife Janet, eventually will want to rearrange some things is the room? And how well do room treatments work for just me in the room vs. 5 people in the room? Isn't this also a factor?

How would I prevent obsolesence of my room treatments once the changes mentioned above begin to occur?

Please advise Mike.

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LF standing waves are not greatly modified by room contents. And the degree of change tends to be in gain/intensity and not frequency. The room dimensions and total volume is determinate regarding the frequency. This aspect greatly overshadows any smaller object - especially as most smaller contents are invisible to the larger waveforms.

Contents will have more effect on smaller wavelengths- higher frequencies - where the object size and the wavelength become near equal and where the object is larger than the wavelength. This situation will tend to provide still more 'arbitrary'/residual absorption, something that will tend to run counter to what most will be trying to achieve, which is the sense of space which is achieved by 'redistributing' the acoustic energy in a well-behaved diffuse manner rather than absorbing, deadening and making the space feel very claustrophobic.

Also, to reinforce an understanding that folks need to make regarding standing waves. Below 300 Hz we often look at frequency plots . But the problem we are having is not so much a frequency and gain problem. It is a persistence problem. It is a TIME problem. We have a resonance at a particular frequency. In a waterfall plot this time element becomes very easy to spot. It is the resonance that fundamentally causes the boominess, NOT the gain. The resonance and persistence in time is what makes the room sound boomy.

By the way. perhaps the easiest way to measure the problematic frequencies is to stimulate the room in one corner and to measure the results in the corner diagonal from it, as in a closed room, all of the modes will be present in the corner (and this is also the reason corner bass traps are optimal as well.

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That's not going to tell you very much - you'll just find that the SPL changes drastically as you move your meter around...like +-10dB. You really need to measure in the time domain so that you can see the reflections that are contributing to the weird issues....so even though the mic moves and the frequency response changes, the reflections points are still in the same spot.

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It would seem that a change in some of the dimensions would/could minimize the standing waves. I did not do the math. What dimension chages would do the most good? A false wall. A false ceiling. BTW is the ceiling closed in or still open to rafters?

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