Thoughts on attenuating 1/4 wave cancellation


Among other things, I would like to address the 1/4 wave cancellation coming from the wall behind my speakers. I would like to avoid soffit mounting, as that would require building a new wall, and after all of that I would end up with a fixed speaker position.

My room is solely for audio, so I need not worry about aesthetic issues arising out of bass trap placement. If I were to place a big (e.g., 24" diameter) ASC tube trap against each side of the speaker, and maybe one more behind and to the outside of each of these, it seems to me that the bass waves heading backwards would be significantly attenuated and the cancellation would be significantly reduced. This might help reduce edge diffraction, too, but I am far out of my element with most of these acoustic phenomena.

I am hoping that someone can debunk this notion before I buy the extra tube traps and discover the hard way that it was a boneheaded idea.
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Showing 2 responses by zargon

Before offering advice, can you describe the room and speaker placement and explain how the 1/4 wave cancellation is affecting the listening position?

What is the speaker distance off the front wall? Where is the listening position wrt the front and rear walls? What frequency are we talking about?

This would be helpful in understanding the situation.
The phenomina you are describing is correctly referred to as "1/4 wave cancellation" in acoustic circles. Your understanding as it occurs on the front wall is accurate as described. The 1/4 wave cancellation also applies to sound waves reflected off the back wall to the listening position. So in your example, if the listening position is 5 ft off the back wall, you can expect another cancellation at 56 hz.

Anything you can do to absorb the incident wave on the wall at the cancellation frequency will help. Of course, at these fairly low frequencies that can be a challenge. Bass traps, even 20" in diameter do not absorb much at 50-60 hz. I believe a Real Trap is also ineffective at this frequency.

One solution is a helmholtz resonator which can be tuned to a specific frequency. F. Dalton Everest in "Master Handbook of Acoustics" has quite a good discussion of how to do this. They can be boxes, corner traps or policylindrical absorbers. If you don't have this reference it would be worth while to buy one (new or used on Amazon).

I built a helmholtz resonator behind the rear wall of my room with good success (see my system). Of course, I was starting from scratch and you may not have that option. I also built in corner traps, however, while they help reduce excess low frequency energy, these are primarily aimed at reducing the RT60 in the room.

You also need to look at the natural room modes and take into account their interaction with the speaker location. A simple free tool like Room Response Calculator, by Yavuz Aksan, is quite useful for this purpose.

I am curious how you did the measurements to confirm the cancellation? Remember, you are measuring the accumulation of all effects, not just 1/4 wave off the front wall.