Sam Lord wrote on Fri, 22 August 2008 05:00 |
The critical point is that those differences are are *fixed* for a given speaker type. I call those differences "absolute offsets," e.g. Otm and Otb. Let's say you're a speaker designer and you've chosen your drivers and crossovers and recorded those absolute offsets. You then draw a box with some slope of front baffle with three points on it to represent the infinite-distance acoustic center of those drivers. Now draw the ear-to-tweeter line and the other two. Is there any ear-to-tweeter distance and speaker angle at which the lines converge at the ear? If yes, you have found your *only* time-coherent listening position for that speaker. Now, if just one driver on the speaker can be adjusted to move forward or back, *and* the other two drivers can be tilted, you will have a large area in which your speakers can be placed wrt your ear yet still achieve time coherence. The model for stereo simply adds the vertical dimension and uses the equation for a sphere, x^2+y^2+z^2=R^2. My spreadsheet models for our adjusting speakers treated each speaker independently, having inputs for the distance between a given listener's ears. Any deviation in floor flatness has to be accounted for.
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I understand the principle you are trying to explain, although I'm not sure I agree with some of the details.
After all there are many good sounding systems that do not have the voice coils of all the components perfectly 'lined-up', and there are some loudspeaker designers that do not consider this an absolute requirement. I also know that there is a margin of 'misalignment' that is beyond human hearing, the misalignment must result in group delay that is beyond the threshold of hearing and my google search turned up this little tidbit:
"Given that the minimum audible group delay is claimed to be 1ms at 2kHz, that amounts to a physical driver displacement of 345mm - assuming the velocity of sound to be 345m/s (22
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« Reply #27 on: August 23, 2008, 08:53:25 PM »
Samc wrote on Thu, 21 August 2008 19:54 |
seriousfun wrote on Thu, 21 August 2008 22:22 |
If the woofer is two feet below the mid driver, sounds common to both of them will be smeared. If the woofer is in a separate box five feet away, common sounds will be smeared worse.
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If this is true......
Quote: | To sum it up - no speaker design is perfect, and placing a subwoofer directly below a main speaker might not be better than placing it elsewhere.
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How can this also be true?...... this situation (you describe) has nothing to do with the inherent problems of loudspeaker design, it is about correcting low frequency issues in the room. This would also seem to be setting up perfect conditions for group-delay issues. ...
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Not contradictory. Moving a woofer far away from a tweeter might always smear common sounds, but the effects of arbitrarily placing a subwoofer below the speakers because the idea seems right to you, resulting in horrid frequency response, *might* be worse. I've placed essentially full-range speakers in rooms where they had to sit in order to present a proper image, but the room made their bass response horrid; adding a properly integrated subwoofer, and placing it where it delivered flat response, would have been a good solution (yes, with potential, predictable compromises everywhere). No speaker design is perfect. A car might have wonderfully precise steering, but if it's not fast enough to drive on a highway, it's still not a very good car (stupid analogy alert).
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« Reply #28 on: August 24, 2008, 06:19:09 AM »
seriousfun wrote on Sun, 24 August 2008 01:53 | Not contradictory. Moving a woofer far away from a tweeter might always smear common sounds, but the effects of arbitrarily placing a subwoofer below the speakers because the idea seems right to you, resulting in horrid frequency response, *might* be worse.
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Doug, I would like to suggest that there is nothing "arbitrary" about placing the subwoofer close to the rest of the system to preserve coherency; science dictates it. Quote: | I've placed essentially full-range speakers in rooms where they had to sit in order to present a proper image, but the room made their bass response horrid;
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It would seem that the acoustics of the room was out of whack and the ROOM needed to be treated. Quote: | adding a properly integrated subwoofer, and placing it where it delivered flat response, would have been a good solution (yes, with potential, predictable compromises everywhere).
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Are you suggesting that splitting up the loudspeaker system, and placing the components around the room as a viable means to treating acoustic anomalies in a room? Why not just treat the room, wouldn't that result in a more predictable situation?
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Sam Clayton
« Reply #29 on: August 24, 2008, 07:18:17 AM »
Sam Lord wrote on Sat, 23 August 2008 04:51 | Hi Sam, thanks for a fine discussion.
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Thank you for the education..... Quote: | That Googled group delay figure isn't close to correct. Think, 1msec at 2kHz is 2 full cycles!! No, if your 2k crossover point between mid and tweeter is off by as little as 1/16", or about 3.5 degrees out of phase, you can positively hear it. Of course you need to be close to begin with, let's say well under 20 degrees of phase or around 0.25", otherwise you run the risk of missing a whole cycle when making adjustments to get coherent wavelaunch. I haven't read AES papers on this, though one recent paper has postulated a minimum audibility threshold for interaural (stereo) time offset of around 2usec, which is about .026 inches distance for sound. That's a very different parameter, but I think it bolsters the argument for tight inter-driver time alignment.
| Lets assume that this is correct... I don't dispute your argument by the way, I'm trying to figure which way is up on this issue, anyway, how do you explain the many good sounding systems that do not abide by this theory? Has anyone ever actually done any scientific tests on this topic? Quote: | I spoke with Billy Woodman of ATC at the 123rd AES, and he thought my 1/16" was a little tight, but certainly believed that 1/8" or 10 degrees mid-to-tweeter phase error was audible. Roger Quested, for all his accomplishments, doesn't get the importance of this.
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I could hardly find two people who agreed on this subject too and hence my question about scientific tests. Quote: | I would only add that tailoring amps for frequency ranges gets you almost nothing. You can use poorer components on sub amps, but internal amp bandwidth really needs to cover the whole audible range to sound good.
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My "tailoring" statement refers only to choosing appropriate amplification type and power for each frequency band. Quote: | These speakers are now about 65 grand, and IMHO not bettered by any.
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What loudspeakers are these pray tell.
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Sam Clayton
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