#1647812 - 04/20/04 03:30 PM
Bass Trapping at Different Volumes
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caseyLA
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Since insulation traps use 1/4 wavelength velocities to convert the oncoming sound waves into heat, I'm assuming that the effectiveness of these traps won't change with varying degrees of volume. If a room showed a flatter response at lower volumes, it would not be because the traps were working better, but because of differences in the total amount of comb filtering going on. Is this correct?
Since panel traps work on pressure, will their effectiveness change with changing volume?
curious,
caseLA
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#1647813 - 04/20/04 06:04 PM
Re: Bass Trapping at Different Volumes
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Ethan Winer
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Casey,
Bass traps work exactly the same at different levels - unless they're defective. It doesn't matter by what mechanism they operate. If they didn't work linearly with level, they would introduce distortion into the room!
--Ethan
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#1647814 - 04/20/04 09:30 PM
Re: Bass Trapping at Different Volumes
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caseyLA
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I guess a better question would be, does a room's response ever become any flatter at different volumes? Or is it completely indepedent of how loud you are mixing?
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#1647815 - 04/21/04 01:55 AM
Re: Bass Trapping at Different Volumes
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Andre Vare
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I guess a better question would be, does a room's response ever become any flatter at different volumes? Or is it completely indepedent of how loud you are mixing?
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#1647816 - 04/22/04 02:19 PM
Re: Bass Trapping at Different Volumes
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Jabbaz
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I always thought there was a critical volume at which standing waves occur. If you take a rope tied to a wall and oscillate up and down with a constant frequency but with very little amplitude, a wave doesn't develope. With higher amplitudes it does.
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#1647817 - 04/22/04 04:32 PM
Re: Bass Trapping at Different Volumes
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MurMan
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Creation of standing waves, like most things acoustical, is linear. Ethan replied earlier that rooms behave linearly with level, but I think that you may not understand the importance of linear.
Linear is an engineering term. Simply stated, a system is linear if the output doubles when you double the input. Linear systems are a thing of beauty because if you know how they behave at one level, you can calculate how they behave at higher/lower levels.
Our hearing, on the other hand as Andre pointed out, is non-linear. (it's logarithmic) Don't let this concern you. Just assume that acoustics is linear.
As for the rope example, there are lots of explanations. First, it's difficult to see small amplitude standing waves, especially when the exciting amplitude (your hand going up and down) is larger than the resonant amplitude (the standing waves).
Also, it's difficult to maintain rope tension control with your hand, especially as you lower the amplitude of your shaking. But, the resonant freq. of rope is related to the tension. As the tension goes lower, at some point the rope no longer behaves like an oscillator, i.e. it starts becoming non-linear. This a rope issue, not an acoustics issue.
Yeah, if you look at extreme cases, acoustics goes non-linear too. But for the practical applications discussed here, take it on faith that room acoustics are linear.
Murray
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#1647818 - 04/22/04 05:20 PM
Re: Bass Trapping at Different Volumes
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caseyLA
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I understand that the effects of room modes are linear, but what about boundary-induced comb filtering which, as we've seen, causes most of the peaks and dips within a small room?
At higher volumes, sound waves travel further
before dissipating. So wouldn't the waves that hit a wall bounce further back into the room at a higher lever of volume? As the volume is turned down when mixing, wouldn't there be a point (depending on the distance of the walls from the mix position) at which the reflected waves fully dissipate before getting back to the mix position and interfere with the direct signal?
The low volume levels required to avoid comb filtering altogether might be impractically low, but it seems volume would have an effect on the level of interference.
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#1647819 - 04/22/04 07:28 PM
Re: Bass Trapping at Different Volumes
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Andre Vare
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At higher volumes, sound waves travel further before dissipating
It is a bit of a niggly, but when writing about sound level, could you not use "volume"? When I first read your post, I thought you referring to room volume, not sound pressure level.
Thanks.
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#1647820 - 04/22/04 08:12 PM
Re: Bass Trapping at Different Volumes
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caseyLA
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Ok, good point. "Volume" is a horrible term for sound pressure level. How the hell did that word come to be used for that purpose? Anyway....
I keep imagining sound waves in a small room acting as waves in a kiddie pool. If small enough stones are dropped in the middle, the radiating waves die out before they have a chance to bounce back to the middle.
I guess that isn't really very similar to bass frequencies reflecting off walls though.
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#1647821 - 04/22/04 08:37 PM
Re: Bass Trapping at Different Volumes
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MurMan
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Originally posted by caseyLA:
I understand that the effects of room modes are linear, but what about boundary-induced comb filtering which, as we've seen, causes most of the peaks and dips within a small room?
The instanteous acoustic amplitude at any point in a room is the sum of the direct sound waves plus all of the reflected sound waves.
Comb filtering occurs at points in a room where the amplitude of the reflected sound is large enough to affect the direct sound. (Technical detail: Amplitude here means the instanteous algebraic amplitude, so that it incorporates the concept of phase.)
Because comb filters are based on summing, and summing is a linear process, then
comb filters are linear.
As Martha would say, "Linear. It's a good thing."
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#1647822 - 04/24/04 01:50 PM
Re: Bass Trapping at Different Volumes
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Ethan Winer
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Folks,
> Comb filters are linear. Here's why <
I just submitted an article to EQ magazine explaining comb filtering and related issues in great detail. Though I didn't think to mention that it's always linear! 
--Ethan
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