Allison Effect

by Linnin, Alex Claber, and billfitzmaurice

September 7, 2015

original post on TalkBass.com

billfitzmaurice said:

it’s called Boundary Reinforcement, which boosts bass sensitivity by 6dB in the low end when you have the cab close enough to one wall, and by 12dB in a corner. The distance required to get these free dBs is approximately 2.8 feet or less from the front of the cab to the boundary/boundaries. With placement further away you get room cancellation modes that actually suck out the bass. Boundary reinforcement and cancellation modes affect every type of speaker equally, be it sealed or ported or horn, and in the case of ported front or rear makes no difference.

and this from Alex Claber designer and maker of Barefaced speaker cabinets

Thanks to Bill Fitzmaurice (horn speaker guru) for mentioning this and getting me Googling...

The Allison Effect was named after Roy Allison who realized that room boundaries seriously affect bass reproduction, not only in terms of boundary reinforcement (generally good for us) but also boundary cancellation.

“The best bass response is achieved by having the bass source either very close to or very far away from reflective surfaces. This generally means close to the rear and side walls and floor and far from the far wall and ceiling.”

Why is this?

If you place your speaker ½ a wavelength away from a boundary then, at the frequency that corresponds to that wavelength, the sound will be reinforced significantly. We're talking the kind of boost you get from cranking a parametric EQ with Q narrow. If this particular frequency is a boomy/muddy one, it will not be fun. And if you’re playing an acoustic instrument, feedback city...

The flip side: If you place your speaker ¼ wavelength away from a boundary, then at the frequency that corresponds to that wavelength (twice the previous frequency if the speaker hasn't moved) the sound will be cancelled significantly. A typical amount would be -15dB at the null frequency. Nasty. Truly tone sucking.

The solution? Place your cab so it’s less than ¼ wavelength (of the upper bass range i.e. 100Hz) away from any boundaries OR more than ½ wavelength (of your lowest frequency) from any boundary.

How does this relate to the real bass playing world? A 41Hz (low E) sound has a wavelength of 27.5 feet (I'm sorry, I'm still hanging onto the Imperial system). A 31Hz (low B) sound has a wavelength of 36.5 feet. So, fellow four stringers, you need to place your cab at least 13′9″ away from any walls (fivers, 18′3″). So that's how to avoid peaky boundary reinforcement.

To avoid notching boundary cancellation you need to be within ¼ wavelength of a boundary in the range of 31 to 100 Hz. So really you need to get your cab within 2′10″ of a boundary. Which part of the cab, you may ask? The bit that's making the sound, i.e. the bass speakers themselves and the ports. So given that most bass cabs are 14″-20″ deep, you need to get the cab really close to the back wall, but not so close that the ports’ operation (if rear-ported) is affected. Think 6″-12″ away.

So what's the best solution on the gig? Place your cab on the floor or on a short stand. Place it close to the rear wall. Place it close to one side wall. The far wall and the other side wall should be far enough away unless you’re playing a box-room. The ceiling will often be a pain, but one boundary causing nulls and peaks is a lot less worse than all six getting in on the action.

Alex

Follow-up

eric_B:

So how does this translate when you push a speaker against a wall or in a corner to gain extra low?

Linnin:

See, that's exactly what you don't want to do, Eric. You want to stay at least a foot away from any wall, and not shoved in a corner. So you would be at least at foot away from the back and side wall that forms the corner. If you get too much boost, you're just going to have to cut the bottom end.

Alex Claber:

I completely disagree with this. I would always start with your bass rig hard into a corner wherever possible. If it’s too boomy then move it. But as soon as you pull the rig away from the wall you start causing uneven response from boundary reflections causing sharp notches in output.