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Critic's Corner Discuss a review. Provide constructive feedback. Talk to the industry. |
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In Reply to: Acoustic polarity, again posted by KlausR. on May 6, 2007 at 02:36:41:
Hi
I read your post again and had a few thoughts.
Loudspeaker drivers are not as simple as a Battery where there is a clear plus and minus.
When you take a battery and touch it to a woofers terminals, it either moves in or out depending on its polarity. Right here one departs from “simple†because some manufacturers make the red terminal such that positive make the cone move out and other companies, it moves in.The next step away from a “battery†and towards how they work is to now keep in mind that while at 0 hz or DC, it is easy to see what the cone does.
With audible sound, now one can’t see that relationship.
A loudspeaker driver like a woofer has an amplitude response curve AND a phase response curve.
Normally a woofer’s acoustic phase is anywhere from near +90 degrees below the box cutoff to –90 degrees mid band, rising to zero at Rmin.
So, what “polarity†or phase the woofer is, is a variable which is dependant on frequency.
For a sealed box at least one can say the acoustic phase will always be within +90 to –90 degrees.
What might not be obvious is that a lagging phase is the same as a time delay which increases with decreasing frequency.
A woofer takes an instantaneous broad band signal and reproduces the highs first, then the mid and then the lows, all spread out in time relative to the original.Now, add a crossover, each section also has an amplitude AND phase response, these are added to the responses of the drivers so what you really have is the sum of both sets of magnitude and phase.
Then connect the second driver to the crossover and unless the sources are less than ¼ wl apart, they do not add uniformly / coherently and the sum is direction dependant.Normally it is assumed that these kinds of problems are not audible, a position taken in marketing products as the solutions have not been achievable until DSP or novel designs came along. Also, if one sets up a computer to manipulate phase alone and compares A vs B, one would conclude that these things (once at a low level) are not big while they are audible. Conversely, if one builds actual speakers with the same phase properties, one would conclude the effects are much larger.
The gap between the two (it seems to me) is a result of the fact that one also hears the self interference at the speaker which in addition to causing the phase issue, ALSO exists in X, Y and Z and so alters the radiation in all directions. I think we can hear that interference as it allows one to localize the source.
The more coherent I have been able to make the sources, the less able you are to close your eyes and identify “where†the speaker is.
This is also accompanied by what sounds like the sound getting simpler, having less “features†but also sounding more real at the same time.
I tried to post about this in more detail on our form, if the spammers don’t have the doors clogged again.Lastly, a humorous situation exists now that people are starting to pay attention to acoustic phase. Many computer based measurement systems (like MLS) have a plot for acoustic phase and yet, when one compares those measurements to ones made with a systems that actually has a phase comparator (TDS), the results are sometimes totally different.
Many of the non-TDS systems produce a phase plot, which is the minimum phase interpretation of phase. This is fine for most things like crossovers and eq but is still not exactly the real thing.
Hope that helps,Tom Danley
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Follow Ups
- Its not a battery, not that simple. - tomservo 07:40:38 05/07/07 (0)
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