Art's Blog: Remove a corner bass trap and the “suck-out” went away?

It happened to Bob Hodas once and it was so unusual that he mentioned it in a MIX article on room acoustics. Actually, what he ran into could easily have been a live example of a technique we use in the design and set up of listening room and control room acoustics.

It happened to Bob Hodas once and it was so unusual that he mentioned it in a MIX article on room acoustics. Actually, what he ran into could easily have been a live example of a technique we use in the design and set up of listening room and control room acoustics. Here’s how it goes:

Usually people put bass traps in corners of the room to damp the room modes, reducing the Q or sharpness of resonance from 30 down to about 10. This lowers the strength of the resonant peaks a little, around 1 dB and at the same time significantly reduces the amount of “suck-out” by as much as +5 dB. Unfortunately, no matter how many bass traps you use, you’ll still have the same room mode being stimulated. All you can do is tweak the strength of the peaks and valleys, using corner loaded bass traps.

But what is going on when you inadvertently pull a bass trap out of the corner and the suck-out you were measuring disappears. Clearly we are not fixing a suck-out by adding bass traps, as is the standard procedure. Something weird happened. We put the bass trap back into the corner, where it belongs and again, the suck-out reappears. This may be a mode problem or it may well not be a mode problem at all. Here we are going to ignore modes aspect of this and just look at how reflections can cause this effect.

What if the listening position is 5 feet off the back wall and the room is wide enough that the distance to the back corner is 10 feet. The round trip distance for the back wall bounce is 10 feet and the round trip distance for the diagonal bounce out of the back corner is 20 feet.

The rear wall bounce produces a cancel of the direct signal for a frequency whose half wavelength equals the round trip distance, 10 feet. This is the classic 55 Hz suck-out that everyone hates. But notice the back corner. It is producing a strong reflection that has a 20’ time delay. At 55 Hz, that amounts to a whole wavelength time delay.

Wait a minute, the rear wall bounce produces a half cycle delayed out-of-phase pressure point at the listening position while the back corner bounce produces a one cycle delayed, in-phase pressure point at the listening position. These two reflections, the – of one is cancelled by the + of the other. The corner bounce cancels the rear wall bounce at the listening position at 55 Hz and the rear wall bounce caused 55 Hz suck-out just goes away.

But what happens when you put a big, high efficiency deep-bass trap in the back corner? The reflection of the bass wave out of that corner is significantly diminished. And along with that is its sound canceling power at the listening position. The net effect is that the rear wall bounce is stronger at the listening position than it was when the empty corner bounced a strong reflection towards the listener The stronger the bass trap is at 55 Hz, the weaker the reflection out of the rear corner becomes and the stronger the rear wall bounce sound-cancel effect becomes.

And so, it is true that under certain circumstances, removing the bass trap from a nearby corner can result in the “suck-out” going away.

Of course removing bass traps from the room is not the answer to good acoustics. Adding bass traps to the room is the answer. What we need to do is to leave the corner trap in place and add a big bass trap behind the listener to absorb the rear wall bounce, or just open the door behind the listener to vent the rear wall bounce. Either way, we have minimized our rear-wall bounce. Combine that with the minimized rear corner bounce and we end up with minimal phase cancel or add problems at the listening position.

Sometimes in acoustics, things seem to happen which make no sense. But in reality, it makes perfect sense. The problem was only in how we were looking at it. Something similar also takes place when the direct signal combines with room modes at the listening position. But that adventure is for another day…

Art Noxon is a fully accredited Acoustical Engineer with Master of Science degrees in Mechanical Engineering/Acoustics and Physics. A Professional Engineer since 1982, he is licensed in Oregon to practice engineering in the public domain with the specialty area of acoustics. A prolific inventor, he developed and patented the iconic TubeTrap, the original corner-loaded bass trap/treble diffuser, 150 other acoustic devices, and counting. Lecturer, writer, and teacher of acoustics, he has presented 7 AES papers, numerous magazine articles, white papers and blogs. He is president of Acoustic Sciences Corporation, the company he founded in 1984.

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