Corner Bass Traps - Always beneficial?
- Thread starter edorr
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Aside: Deep nulls are usually caused by signal cancellations. For example, at some frequencies and places in your room the reflected and direct signals add in phase, doubling their magnitude (+6 dB). At other points (frequency and place) they are out of phase and cancel, resulting is (theoretically) -infinity dB. In practice the walls are not perfect reflectors and other things reduce the null depth, but it is not at all uncommon IME to achieve 20 to 40+ dB nulls in a room. And, because the nulls are due to cancellation, they are very difficult to equalize -- it's like dumping energy into a black hole. Your speaker can put out all sorts of power and it won't help since the signal still cancels at the listening spot. You either need lots of bass traps or other means to kill reflections, very challenging at bass frequencies, or need to move the listening position. Since the worst nulls almost always occur for the first few room modes, there are only a few frequencies (often just 2 or 3) and moving out of the null is not unreasonable.
HTH - Don
HTH - Don
Question: and this may extend beyond treatment of bass issues- Let's assume I have my next room designed, measured and built with some built- in acoustic treatment. Then, i make a change in loudspeaker. Now what? If the room is 'right,' it is right for all purposes? i'm getting the sense that different playback equipment will excite different frequencies that need to be addressed. Does this auger for having moveable/tuneable acoustic treatement in the room?
PS Amir: i appreciate that you advocate digital room correction. My hesitation here has to do with processing the analog signal (sorry, i'm not trying to open up a debate over analog v digital). I suppose if it were from 100hz down, that would make me far less nervous than running the midrange through processing and perhaps, in your view, I wouldn't have to (although adjusting mids relative to bass may then pose a problem); right now, i have no crossover whatsoever between the amps and midrange horn.
PS Amir: i appreciate that you advocate digital room correction. My hesitation here has to do with processing the analog signal (sorry, i'm not trying to open up a debate over analog v digital). I suppose if it were from 100hz down, that would make me far less nervous than running the midrange through processing and perhaps, in your view, I wouldn't have to (although adjusting mids relative to bass may then pose a problem); right now, i have no crossover whatsoever between the amps and midrange horn.
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I would think optimal treatment with speaker A is still optimal with speaker B. The reason being the treatment addresses modes which show up at certain frequencies specific to the room, and irrespective of the source of the sound (i.e. speaker).
Unless you're doing vinyl, this does not have to be an issue. Just get a processor with digital input and feed it a digital source (CD transport or PC audio). You can always elect not to touch higher frequencies, but in my experience the concern about messing up the mid/highs is unnfounded (I had the same concerns), at least if you use a very good processor.
I am doing A/D conversion with my SACD sources and the benefits of DRC easily offset any loss associated with A/D conversion. However, this outcome will vary depending on system and room.
I will shortly try a modded Oppo that digitally outputs SACD as high rez LPCM over S/PDIF, so I can assess how much I am really losing through A/D conversion. Of course, going the digital output route on SACD through the Oppo involves DSD -> PCM conversion, so this is not a complete analog to digital apples comparison either.
I will however also spin a 2 channel BR digitally and analog into the trinnov, which will be as close to a fair comparison of digital source to A/D conversion as I can think of. Not sure if any of these expiriments are of any help for you to assess if DRC is worth your vinyl while though...
Don,
I think this would be true if your listening position is close to the back wall or the speakers close to the front wall. In my room I am fighting with two nulls - 24 and 64 Hz. None of them seems to be related with the room length - 9.3 m (30.5 feet) or other room modes. The listening position is at 38% of the distance from the back wall.
As I have always used Soundlabs in this room I did not have this problem before.
@microstrip: What are the dimensions of your room? Any openings/bays or large furniture that would perturb the modes?
There is usually (always unless there is some strange mitigating factor) a node at the center of the room and that was my case. Unfortunately it was very difficult to arrange my set-up to get out of the null, partly because two dimensions are very close. Originally it was primed, but we took out room to add a bedroom and hallway to it, and a duct soffit was added, thus my media room got significantly smaller than I had planned.
There is usually (always unless there is some strange mitigating factor) a node at the center of the room and that was my case. Unfortunately it was very difficult to arrange my set-up to get out of the null, partly because two dimensions are very close. Originally it was primed, but we took out room to add a bedroom and hallway to it, and a duct soffit was added, thus my media room got significantly smaller than I had planned.
In the case of the subject at hand, bass frequencies, the room is absolute control. You can see that in this measurement:
So there, changing speakers other than absolutely power, will make little difference otherwise.
In frequencies above transition, as the chart shows, the speaker response dominates. At the risk of stating the obvious, changing speakers will most definitely change the sound in the room. That aside, there are design considerations that are specific to speakers. For example, for surround channels, the room may have been designed around a speaker with wide directivity. Changing that to narrow may somewhat change the response. As someone noted already though, in general a well designed room will work most speakers.
---...So the room is basically the loudspeaker's bass extension (below 200 Hz or so).
...Heck, make that 400 Hz and below. ...Or the four lower octaves (20 Hz to 320 Hz).
...Heck, make that 400 Hz and below. ...Or the four lower octaves (20 Hz to 320 Hz).
Not as much extension but imparting its own frequency response on top of the speaker with incredible force such that changing speakers does not materially change what is going on there.
One way to think about it is to consider the wavelengths, how long a full cycle of a sound takes at various frequencies. Interactions at half- and quarter-wave points cause peaks and valleys in the response. When the wavelength is so much longer than the speaker, the room dominates the sonic picture.
10 Hz => 112.7 feet = 34.35 meters
100 Hz => 11.27 ft = 3.435 m
1000 Hz => 1.127 ft = 0.3435 m
You can see room dimensions start entering the picture in a big way around 100 Hz or so (remember length, width, and height all matter). At 200 Hz, the wavelength is about 5.6 ft, and only 2.8 ft at 400 Hz. As the wavelength shrinks the sound waves interact with more things, breaking up modes, and the speaker becomes more of a point source as the wavelengths approach the speaker's (driver's, plus some interaction with the box) dimensions.
10 Hz => 112.7 feet = 34.35 meters
100 Hz => 11.27 ft = 3.435 m
1000 Hz => 1.127 ft = 0.3435 m
You can see room dimensions start entering the picture in a big way around 100 Hz or so (remember length, width, and height all matter). At 200 Hz, the wavelength is about 5.6 ft, and only 2.8 ft at 400 Hz. As the wavelength shrinks the sound waves interact with more things, breaking up modes, and the speaker becomes more of a point source as the wavelengths approach the speaker's (driver's, plus some interaction with the box) dimensions.
The opposite of that is also interesting. As wavelengths get much smaller relative to the size of your head, then what the two ears hear is different. Your head also then presents a 0.4 millisecond delay to the ear further from the source (e.g. a reflection). Translating, nothing will seem like it is anymore! Psychoacoustics gets involved and the picture gets far less intuitive. I will post longer articles on this but for now, this is a good teaser
.Don,
The room dimensions are 3.8x9.35x2.4 meters (13 x 30.5 x 8 feet). The walls are 2 feet stone - the worst possible in terms of bass damping. I have borrowed a Martin Logan Descent I subwoofer and I am going to "scan" my room with it using REW this weekend. The unfiltered sub is linear up to 200 Hz. From previous measurements I have found that the peaks are dominated by the main longitudinal axial modes, but could not relate mathematically the nulls with the dimensions or placement, something that bothers me.
As the room is very long, I am building a full wall membrane bass trap at the rear wall, but would like to understand what is going on before tuning it.
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