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General speaker questions for audio and home theater.

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Here we go again...

Posted by villastrangiato on July 14, 2012 at 11:03:17:
Thorsten babbles:

"Third, the deep notch would be where it is (maybe 130 Hz) if the room in which the measurements where taken had a 2.6 m high ceiling and the measurement hight would be somewhere around 1m to maybe 1.3m...

In fact, given that this speaker has one bass driver very high up at the top of the speaker and another near the floor one would expect to see a shallow double notch, one a very low frequency from the driver high up in the air and another at fairly high frequency from the driver at the floor.

Moreover, disconnecting the bottom bass unit should drastically change the frequency and amplitude of the notch if it resulted from the Allison Effect but it in fact fails to do much at all."

Cancellation or superposition in midbass frequencies has been measured (I've measured it myself many times, thanks) as a result of different path lengths contributing to what the microphone or ear is hearing. Layman is quite correct in that at typical listening heights (40 inches off the floor for example), a woofer mounted at a similar height off the floor will contribute essentially two sounds to what is heard - that which bounces off the floor AND ceiling and that which is direct. Depending on the distance to the listening point, the net difference of path length each portion of the wavefront travels determines whether the frequencies are cancelled or re enforced. Placing a driver near the ceiling or floor - in essence, coupling the driver to these surfaces - minimizes this problem.In such cases, the path length that looked more like the two hypotenuses of two 45 degree triangles begins to look more like the complex interaction of two or three room modes (ceiling to floor vs front wall to back wall vs. side wall to side wall). In that instance, given the wavelengths and inherent energy involved to support further reflection, the amount of cancellation/re enforcement is minimized. This is why experienced designers recommend and have had success in coupling bass drivers close to the floor. The closer such drivers are raised to typical ear height, the more opportunity there will be for cancellation and reenforcement by out of phase reflected wave fronts.

So effectively, there would not be two cancellation notches as Thorsten erroneously suggested - only potentially one. And that would depend on the distance to the listener. Longer distances would reduce the amount of phase difference. As an example, if one calculates the path length difference for a sound reflected from the floor or ceiling from a driver mounted 4 feet high to a person listening at 4 ft height 10 feet away - the path length difference amounts to almost 3 feet! If you double the distance to the listener, it drops in half to about 1.5 feet.