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HI,
surely to betsy ther is aformula for clacualting the frequency cntre of this dip for a given spkr and listener set-up.Parameters would be.
a. Spkr Height - to centre of main bass or mid bass driver or notional centre for >1 driver.
b.listener distance.
c. Listener ear height -usually greater.
This would be nice to know, I would then know the cause of just one of my mid-bass and upper bass dips!
All 2-wy stand mounters!
Timbo
Follow Ups:
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.
layman> > > “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.” < < <
Do you have an on-line reference for this?
I found this through a Google Search. I am sure that you can find a lot more on the subject.Best,
layman
Very interesting. Do you know how this applies to planar speakers? In addition to having a large radiating area the speakers may not be perpendicular to the floor.Thanks,
Nick
I would not expect planars to be immune as the Allison Effect is created as bass frequencies are reflected back towards the driver and null it's output at precise frequencies (quarter-wave cancellations).
Sure. The interesting thing though is that a planar will have an essentially infinite number of "quarter wave lengths" since the bass frequencies are radiated over a continuous surface.It seems like the over-all effect would be a very slight reduction in a range of frquencies rather than a higher reduction confined to a single fequency.
Also, planars don't radiate from the sides so there wouldn't be the same interaction with the side walls.
And they radiate from the back so there is an additional sound source to factor in.
An interesting puzzle to solve!
“Sure. The interesting thing though is that a planar will have an essentially infinite number of "quarter wave lengths" since the bass frequencies are radiated over a continuous surface. It seems like the over-all effect would be a very slight reduction in a range of frequencies rather than a higher reduction confined to a single frequency. Also, planars don't radiate from the sides so there wouldn't be the same interaction with the sidewalls.”In every frequency response graph of planars that I have ever seen, I have spotted boundary effects in the bass (unless the speakers were measured in an an-echoic chamber). The dipolar nature of planar radiation does not change the fact that the bass that planars do produce behaves like all bass and radiates omni-directionally. Boundary interactions will still come into play in the bass then, even if they do not come into play in the mid-range and treble. Planars do radiate from the sides at bass frequencies, or rather, at those long wavelengths, the “side” cannot be distinguished from the “front” or “rear.” Planar produced bass will interact with boundaries just like that produced by moving coil drive units.
“And they radiate from the back so there is an additional sound source to factor in.”
The back-wave is the reason unenclosed planars do not delve very deep into the bass, as the back and front waves cancel each other out at low frequencies, but (big) planars can produce bass down to the 150 Hz to 200 Hz range, and this is where the majority of boundary induced suckouts occur.
Thanks for the info. Some of it makes sense to me.The back-wave is the reason unenclosed planars do not delve very deep into the bass, as the back and front waves cancel each other out at low frequencies, but (big) planars can produce bass down to the 150 Hz to 200 Hz range, and this is where the majority of boundary induced suckouts occur.
Maybe I'm confused but Magnepan rates their larger models into 20's for bass response. I have heard various maggies, and measured with an RTA and there is definately bass below 150.
Are you saying bass is not possible below 150 with planars or are you saying it is just more susceptable to being cancelled by the back wave?
All drivers are dipolar (planar or moving-coil). Bass drivers are no exception, and the only way to achieve (preserve) bass, is by preventing the out of phase back-wave cancelling the front wave.Low bass can be achieved using planar/electrostatic technology through several means. Firstly, you could enclose the bass driver, preventing the back-wave from interacting with the front-wave. Secondly, you could augment the bass with a conventional enclosed moving coil drive unit (hybrid designs like the Martin Logans). Moreover, big size helps. Big size prevents the back-wave working its way to the front (at least down to the upper bass), but at really low frequencies, you would need an infinite baffle (an enclosure) to prevent these destructive interactions.
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