Propeller Head Plaza Technical and scientific discussion of amps, cables and other topics.

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Re: "by the time the reflected hf component arrives, the direct one is gone so there is no combfiltering." = ???????????

Posted by Soundmind (A) on November 16, 2006 at 17:50:16

In Reply to: "by the time the reflected hf component arrives, the direct one is gone so there is no combfiltering." = ??????????????? posted by Richard BassNut Greene on November 16, 2006 at 15:11:21:

"by the time the reflected hf component arrives, the direct one is gone so there is no combfiltering." = ???????????????"

As you know, comb filtering occurs when the reflected sound arrives at the same point as the direct sound. If the source is a steady state tone, then there will constructive and destructive interference depending on whether it arrives in phase or out of phase and by how much. And which you get will depend on where you are measuring and at what frequency. The determinant is the geometry of the room, the location of the drivers, and their spatial radiation as a function of frequency. At any given location, there will be alternating maxima and minima forming a FR graph which resembles the teeth on a comb. BUT...if the input signal is not steady state but a transient, by the time the reflected wave arrives at a given point in front of the speaker, the direct wave from that same transient will no longer be there, hence there can be no constructive or destructive interference.

"With speaker enclosures of any size I've seen used in a home, the front and rear tweeters are going to be no more than a few feet apart, which means their sounds are going to be a few milliseconds apart and WILL interfere with each other (destructive interference due to out-of phase arrivals at our ears)."

If this were a significant factor, then the sound of an insturment at stage center which is reproduced in a two channel stereophonic sound system would have a different spectral balance (other than the fact that if the speakers aren't toed inward you may be off axis, than the instrument would have being reproduced by only one speaker or the other. The interference pattern created by using this "artificial" phantom source instead of a center speaker would cause this alteration. But it doesn't seem to happen. High pitched instruments such as picolos, violins, triangles, sound the same to me whether they are at center stage or off to one side. Is that how they sound to you too?

"You can't improve the treble frequency response accuracy of a speaker by using more than one tweeter."

You can as I've already explained but I'll recap it again. In real rooms we all live in, not anechoic chambers where speakers are tested, not listened to, there will inevitably be reflections off the walls and other room boundaries but because the treble is reproduced by a speaker which confines its dispersion to an increasingly narrow beam as frequency increases, the reflections reaching our ears from the same general direction as the speaker will not have any high frequency components. This materially alters the perceived tone of the instruments and is different from the way these same instruments propagate their sound in the real world. The solution is to compensate by providing indirect HF sound complimentary to the absorption of the room so that the reflections have the same relative spectral balance as the direct sound. This flattening of spectra of total power transfer between the speaker and the listener will result in a sound which is just as clear and much less shrill than is produced from direct radiating only loudspeakers.

"You CAN create a "spacious" sound effect, which some listeners may enjoy, from front + rear tweeter destructive cancellations (comb filtering)"

That is simply untrue. If it were, you could reproduce the same effect by merely installing a comb filter in a direct firing loudspeaker. If you have a 31 or 62 band equalizer try it and let me know what you hear. I think it doesn't work. At least one reason the sound is more spacious with additional indirect firing tweeters is that human sense of direction is most accute at high frequencies. You can move off the axis of the speaker and still have reasonably flat total power transfer meaning high frequencies continue to reach your ears. I also believe that the human ears/brain direction finding system works much like a direction finding dipole antenna. As soon as your turn you head even slightly, your brain can make a subconscous judgement about the relative size of the source depending on whether the sound gets substantially louder in one ear and softer in the other ear or not. With HF sound coming from more directions than just the speaker itself, the apparant source will be more spread out. Even if you turn your head, it becomes somewhat harder for your hearing to get a lock on the direction of the source of the sound and so it appears to be wider. BTW, this lack of sharpness of direction of the source is more like what you hear in a concert hall where the overwhelming preponderence of energy at all frquencies you hear is reflected including early reflections off the stage floor and wall behind the musicians.

"It's very unlikely that the recording or mastering engineer used speakers with rear tweeters to make the recording"

This is true. But we never consider that the best sound reproduction will be obtained by duplicating the studio's monitoring equipment. If it were, most vinyl records made in the US would sound best heard through Altec A7 Voice of the Theater speakers and most vinyl recordings made in the UK would sound best through Tannoy dual concentric monitors. Probably most cds would sound best heard through B&W 801s and similar B&W models.

"therefore that recording will be altered by playback using home speakers with rear tweeters (or Bose 901'swith rear drivers)."

If you mean they will not be reproduced in the same way as they were in the studio, you are right again. But that doesn't necessarily mean worse. It could mean better. As for Bose 901, I have written extensively elsewhere about them, why I think they are flawed, and how I altered mine changing their sound substantially. Interestingly, far from increasing the apparant size of the source, the tweeters for some reason focus them in ways much like a direct speaker (depends to a great degree on how the recording was miked) but when there are many sources playing simultaneously, they are spread out and further back typical of results using the direct/reflecting principle.

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