Thoughts on attenuating 1/4 wave cancellation

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.

Good questions from Zargon. Also, are the speakers dipolar, front-firing, other? And how far down does their low frequency response go?

Also, I could be wrong but I believe it is more correctly referred to as 1/2 wave cancellation. The path from the speaker to the wall behind it would be 1/4 wavelength at the affected frequency, then another 1/4 wavelength for the reflection to return to the plane of the speaker, where it would be a total of 1/2 wavelength or 180 degrees out of phase with the forward-radiated waveform, resulting in cancellation.

-- Al

The deployment of the speakers and listening position is constantly varying, so I hope to keep the discussion general and related to the possibility of soaking up low frequency bass backwash. Will that generally work, or is there some flaw in the concept? A few general parameters follow:

The room is 18' deep, 23' wide, and 8' high. Speakers along the long wall, placed out from that wall from 3 feet to about 8 feet.

Listening position varies, naturally, but I try to keep it far enough from the back wall so as to avoid bass enhancement from proximity to the wall.

20" tube traps in corners. Various Realtraps and Echobusters here and there.

For present purposes, let's assume speakers about 4.5 feet from the wall, 10 feet apart, listening position about 5 feet from the back wall. The 1/4 wave cancellation centers at about 63 Hz (86Hz/distance from wall in meters), and corresponding hump is at about 126Hz. If I deploy like this, the measurements with an SPL meter are uncannily precisely just like the calculation. Indeed, no matter what the distance to the wall, the cancellation is as calculated.

I've had very good luck attenuating floor and ceiling reflection cancellations with Realtraps, so I would hope to be able to extend the concept to the front wall reflection, despite the fact that it is a much lower frequency.

Thanks for giving it some thought.

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.