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Speaker Asylum General speaker questions for audio and home theater. |
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Speaker Asylum General speaker questions for audio and home theater. |
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In Reply to: 100hz to 200hz Dip posted by kana813 on April 19, 2010 at 10:17:32:
Link you posted:
http://stereophile.com/standloudspeakers/pmc_db1iii_loudspeaker/index4.html
I agree with John Atkinson and Duke that the sharp dip at 220 Hz is caused by a line resonance out of phase with the output of the woofer. These kinds of resonances can be damped (usually by lengthening the transmission line or using some kind of damping material in the line).
However, the broad 100 Hz to 500 Hz suckout in the speaker's in-room response does not appear to be line related but rather caused by the Allison Effect, which is a 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 affected 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.
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.
The frequency of these nulls can be determined using the formula: Frequency of Null = 1130 (velocity of sound)/Distance in feet x .3 (quarter wave coefficient).
The Allison effect also explains how boundaries boost bass frequencies below the frequency of the quarter-wave cancellation. The bass boost is basically the inverse of the quarter-wave cancellation. Direct sound from the woofers and reflected output (from multiple room boundaries) add together in phase.
It's easy to see that the positioning of the speaker in the room with respect to boundaries will have a large effect on the quality of the speaker's bass. The trick is to make sure that the woofers are a difference distance from all room boundaries and that none of the distances are multiples of any other.
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Follow Ups
- Allison Effect - layman 19:10:04 04/21/10 (1)
- Your post confirms my rule-of-thumb speaker positioning - kurt s 13:31:11 05/07/10 (0)
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