Speaker Asylum General speaker questions for audio and home theater.

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Allison Effect

216.78.101.79

Posted by layman on January 6, 2002 at 00:15:54

In Reply to: Formula for the floor reflection dip posted by timbo on January 5, 2002 at 22:41:08:

Allison Effect: “Predictable dip, or suckout, in the low-frequency response that is determined by the distance from the center of the driver to each room boundary. This effect is noticeable only for the woofer, since for the distances normally involved, the frequencies effected are usually around 150 Hz to 200 Hz. For a given distance from the speaker to one surface, there is a 1dB dip at the maximally affected frequency. For a speaker that is the same distance from each of two surfaces, such as the floor and the wall behind the speaker, the dip is 3dB. If, by chance, the speaker is the same distance from each of the nearest three surfaces, the suckout is approximately 11 dB.”

I have posted extensively concerning Boundary Effects in the past year. Do a search under layman. Look particularly for a post entitled “Power Response vs. Power Distribution.”

The distance from the listener to the speaker does not come into play in determining the Allison Effect. The Allison Effect is a power-response effect and will influence the sound no matter where in the room you sit.

“If boundaries are far enough away, the suckout moves downward in frequency and begins to affect the lower bass range. A large boundary 3 feet away from a speaker will cause a power response dip at 113 Hz everywhere in the room. At 4 feet, the suckout will occur at 84.75 Hz; at 5 feet the null drops to 67.8 Hz; at 6 feet it is 56.5 Hz; at 7, 48.4Hz; at 8, 42.4 Hz; at 9, 37.66 Hz; at 10, 33.9 Hz; and so forth.”

Don’t forget to include the distance between the center of the woofer in one speaker to the center of the woofer in the other (of a stereo pair) divided by 2 as one of your boundary measurements. Woofer to woofer interactions create the equivalent of another boundary at half the distance between two speakers.

To minimize the unevenness caused by boundary interactions, it is important to make sure the speaker is a different distance from each room boundary and none of the distances are multiples of any other.

You can see that from the way the Allison Effect works, that floorstanding loudspeakers are particularly vulnerable to deep floor-bounce suckouts, as the distance from the center of the woofer to the floor is always a constant and the standard height of these speakers (around 34 inches) always places the woofer (24 inches from the floor) in a place where it will be maximally affected by boundary nulls.

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