NOT TO SCALE

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Implementing the active bass trap at my listening position, doesn’t work.

The bass from the front subwoofer is cancelled, as there’s no room behind me to place the active trap.

If the active trap sits in front of me, it can’ logically work, as it’ll cancel the sound at my listening position!

It needs to be behind the listening position or tucked in a corner to do its job. Placing the active bass trap on the side was not possible either, as both my rear corners have a door in the way.

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Therefore, in order to test the concept, I had to improvise.

I ran a test simulating the listening position at "mic 1" and used "mic 2" to send signal to both A and B "rear" subwoofers

This way, my rear subs were acting as active dual mono bass trap.

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Because "mic 1" was positioned roughly in the middle of the room, it had a big dip @ 60 Hz.

As a result of rear wall reflection, cancellation occurred creating the null.

That was with my makeshift ABT turned off.

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"Mic 2"  reads the front sub-woofer energy about to hit the rear wall, reverse the phase, process some delay and sends a proper cancellation signal to both 12" A and B subwoofers.

I was amazed as sub A and B effectively canceled the wave and killed the null seen at "mic 1" position.

​It worked! Whilst the 60 Hz null didn’t totally disappear, it was damn close :)

​Because it was a quick n' dirty experiment, I won’t post any measurements.

Sorry. The 20 Hz – 70 Hz decays also cleaned up nicely. An amazing result, considering I didn't spend hours to optimize the system.

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But the trap didn’t work as great as I had hoped, when the volume was cranked to louder levels.

The problem being that when you have six 18" drivers pushing air, generating bis pressure, you need almost the same energy at the other end to cancel it.

Six 18" drivers can move much more air than four 12" can. In order to work, my rear trap would need to beefed up as four 12" do not have enough capacity to keep up with the 18".

 

Obviously, you lose a few dB as the sound attenuates with increased distance, while traveling through air.

My room is ~15' long. From the front sub-woofer array, I calculated that I loose about 5 dB before the sound reaches the rear wall.

So, the rear sound cancelling sub-woofer has to be able to provide the same SPL as the front sub minus 5 dB.

​This suggests that if the front sub-woofer pushes 90 dB, the rear active sub-woofer must also push ~85 dB to fully cancel the energy.

My theoretical maximum output from six 18" drivers is about 132 dB (before room gain)

Maximum theoretical output from four 12" drivers is only ~115 dB. Which is 12 dB less than required. (132-115-5=12)

Houston… we have a problem!

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I need more output capacity from my active bass trap to cancel at high SPL.  Let alone distortion from the 12" drivers seriously limits the maximum output to much less than the theoretical 115 dB capacity.

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Obviously, the longer the room, the more sound will attenuate as it travels the distance. This translates to much less energy requirements for the active rear sub-woofer to cancel out the incoming wave. Let’s say we have a room that is 40' deep. The rear subwoofer output requirement needs to be ~18 dB’s less than front  one. As a hypothetical example for this 40' long room, my four 12" drivers would be adequate to cancel my six 18" drivers in the front. The bigger the room, the least energy is needed, and the easier it’ll be to actively treat it. This suggests that bigger rooms are in fact better. Seems to be a common theme on this website.

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I can imagine the AVAA and Bagend Trap working like a charm between 85 dB - 90 dB, but with their single 12" driver, the maximum absorption is severely limited. This is why they recommend people to treat the room, as you need more woofers. This is especially true @ 20 Hz – 30 Hz. Multiple drivers would be required to get proper results, hence costing lot more than anticipated. Additionally, their website suggests multiple units are necessary for optimum results. This isn’t because they want to sell more products… well maybe it is… but the reality is that one unit won't be enough.

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Also, the active trap is not working @ 150 Hz and above. 

Since my 12" subwoofers are not very happy above 70 Hz, the test trap only worked best below 70 Hz.

Subsequently, tweeter reflection to the rear wall is still present, as well as the mid-range energy and mid-bass.

The trap does nothing in that regard, and those reflections must be treated as well, by applying conventional passive absorption!

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This suggests that passive absorption down to ~70 Hz is still required if using active bass traps.

In order to absorb 70 Hz and above, 5-inch-thick fiberglass with a 1" airgap behind works perfectly to attenuate >90% of the soundwave.

The active bass trap simply acts as a band aid below 70 Hz.

Absorbing 90% of the 20 Hz – 70 Hz energy would require 5-foot-deep traditional fiberglass absorption* so the ABT is really helping to save floor space.

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The moral of this story is… you always need fiberglass :)

Active bass traps only save fiberglass depth. 

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* Diaphragm based bass absorber (limp membrane) and Helmholtz resonator can absorb 20 Hz - 70 Hz without 5 foot depth requirement of regular conventional fiberglass panel.  But that’s are another discussion altogether.