How Bass Traps (Acoustic Absorbers) Complement EQ

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MartinC

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Inspired by another thread today, I've made some quick measurements to compare the effects of both EQ and acoustic absorbers on specifically decay times. The comparison used Dirac Live (but just a single measurement point) and two Rockwool corner bass traps (one 1200 x 600 x 600 mm and the other 1200 x 600 x 300 mm). The responses shown is using a Power Sound Audio S1510 subwoofer crossed over to a pair of Edifier S300 Pro main speakers at 110 Hz.

With no EQ or bass traps, I measured the following response at the listening position. (For those not familar, this is a waterfall plot, showing frequency left-to-right and time front-to-back. At each frequency, the further forward the signal still shows, the slower the decay.)

Waterfall - R Combo No EQ No BTs.jpg

Applying EQ just up to 180 Hz and re-measureing at the exact same position gave the following result:

Waterfall - R Combo +EQ No BTs.jpg

The effect of the EQ is seen not only in terms of the initial bass response (back of the graph) being much smoother but also how the bass notes don't persist at high levels for so long compared to the higher frequencies. The rate of decay of bass notes hasn't actually decreased but they get to lower levels sooner purely by virtue of being lower to begin with.

What is needed to genuinely increase the rate of decay is absorption. Putting my bass traps back into position, making a measurement to generate a different EQ correction filter specific to this situation but with the same target curve, I got the following:

Waterfall - R Combo +EQ +BTs.jpg

Comparing this to the blue plot above, the added benefit of the bass traps on the decay is seen most clearly between 50 and 100 Hz, although there are smaller reductions at higher frequencies too. The initial response is also slightly smoother with the bass traps in place due to there being a little less for the EQ to try to compensate for.

For completeness, here's what the resposne looks like with the bass traps in place but without any EQ:

Waterfall - R Combo No EQ +BTs.jpg

In this particular example it's pretty clear the EQ has the bigger effect, but the bass traps I have offer a further, and different, improvement. More money and space for acoustic treatments could I'm sure make this a closer 'fight' though. The above is all just at a single point as well, whereas another benefit of bass traps is to reduce the variation in response with position.
 
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MartinC

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As I posted this photo in another thread I thought I'd add it here, since it gives a better idea of the size and location of the acoustic absorbers used. I plan to cover them in some grey fabric once I've finalised what will be staying long term.

View attachment 102205

 
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MartinC

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Is the bass trap having any effect whatsoever?
On what specifically? On decay times? Yes, a small one as the graphs show. If you mean more generally then the data later on in the following thread demonstrates better what they do - both in frequency response and spatial variation terms. 

 
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MartinC

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Actually I responded there from the perspective of when using EQ as well. If you meant in isolation then just compare the first and last waterfall plots to see what the effect is.

 

MartinC

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By what 1-2dB 
If you're interested in the effect on the frequency response peaks then the graphs in the thread I linked to above will show this better. The point of this thread was to focus on decay time effects - hence the waterfall plots.

 

tuga

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Inspired by another thread today, I've made some quick measurements to compare the effects of both EQ and acoustic absorbers on specifically decay times. The comparison used Dirac Live (but just a single measurement point) and two Rockwool corner bass traps (one 1200 x 600 x 600 mm and the other 1200 x 600 x 300 mm). The responses shown is using a Power Sound Audio S1510 subwoofer crossed over to a pair of Edifier S300 Pro main speakers at 110 Hz. 

With no EQ or bass traps, I measured the following response at the listening postion. (For those not familar, this is a waterfall plot, showing frequency left-to-right and time front-to-back. At each frequency, the further forward the signal still shows, the slower the decay.)

View attachment 99355

Applying EQ just up to 180 Hz and re-measureing at the exact same position gave the following result:

View attachment 99347

The effect of the EQ is seen not only in terms of the initial bass response (back of the graph) being much smoother but also how the bass notes don't persist at high levels for so long compared to the higher frequencies. The rate of decay of bass notes hasn't actually decreased but they get to lower levels sooner purely by virtue of being lower to begin with.

What is needed to genuinely increase the rate of decay is absorption. Putting my bass traps back into position, making a measurement to generate a different EQ correction filter specific to this situation but with the same target curve, I got the following:

View attachment 99346

Comparing this to the blue plot above, the added benefit of the bass traps on the decay is seen most clearly between 50 and 100 Hz, although there are smaller reductions at higher frequencies too. The initial response is also slightly smoother with the bass traps in place due to there being a little less for the EQ to try to compensate for.

For completeness, here's what the resposne looks like with the bass traps in place but without any EQ:

View attachment 99348

In this particular example it's pretty clear the EQ has the bigger effect, but the bass traps I have offer a further, and different, improvement. More money and space for acoustic treatments could I'm sure make this a closer 'fight' though. The above is all just at a single point as well, whereas another benefit of bass traps is to reduce the variation in response with position.
Good job! As I expected the combination of EQ and traps gives the best results.

Perhaps you could reduce the frequency (X) scale to 20-300Hz or 2-500Hz on your plots for a clearer picture.

.

I think that the biggest challenge with bass traps is getting the bandwith right, amplitude also.

 
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tuga

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Interestingly the traps have made the decay slightly worse at ~34Hz:

swwEJbS.png