From: mbk@lyapunov.ucsd.edu (Matt Kennel)
Newsgroups: rec.audio
Subject: Re: Some bunk, was Re: polarity inversion
Date: 3 Jan 1993 22:59:07 GMT
Organization: Institute For Nonlinear Science, UCSD
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DPierce@world.std.com (Richard D Pierce) writes:
: I don't intend to necessarily challenge or support the notion of absolute
: phase, but there are some misconceptions bordering on bunk here...
: 
: In article <1993Jan3.064402.19553@guinness.idbsu.edu> Dave Jensen <djensen@claven.idbsu.edu> writes:
: 
: > The audibility of absolute polarity (which, by the way, is not a function
: > of time, as is phase) will be most noticeable with a high-transient,
: > low-frequency signal (below 60 Hz). The impact of a prominent, well
: > recorded kick drum will be weaker if the speaker polarity is opposite that
: > of the assumed correct recording polarity where a positive pressure
: > produced a positive voltage at the microphone. 
: 
: There is no such thing, by your description, as a "high-transient, low
: frequency signal (below 60 Hz)." This is a common misconception. First,
: the term "high transient" is pretty meaningless. If a signal exhibits
: transient behaviour, showing abrupt and short term chnages, it contains,
: by definition, a lot of high frequency information and relatively little
: low frequency information. If it is predominately low frequency, it
: cannot, by definition, have "transient" characteristics. If it has both,
: it has broadband characteristics. 

Dick Pierce is naturally correct.  This somewhat unsatisfying answer points
out some of the disadvantages of classical Fourier analysis, in that
spectral analysis does not easily distinguish transients with broadband
frequency content from long-term broadband noise signals.
{it also doesn't distinguish deterministic chaotic signals from broadband
noise but that's totally irrelvant here}

It may be that the
physical realization of some systems may be described more faithfully
with a mode of analysis that can exploit this difference.  The mathematical
foundations of this field is called wavelet analysis, but it remains to
be seen how much success it has in solving physically relevant problems.

Quoting from Numerical Recipes,

"What makes the wavelet basis interesting is that, unlike sines and
cosines, individual wavelet functions are quite localized in space;
simultaneously, like sines and cosines, individual wavelet functions are
quite localized in frequency or characteristic scale.  As we
will see below, the particular kind of dual localization achieved by
wavelets renders large classes of functions and operators sparse, or sparse
to some high accuracy, when transformed into the wavelet domain."

: > A very simple test to satisfy your curiosity would be to listen to the
: > difference in woofer sound quality as you apply a battery across the
: > terminals of your speaker cabinet. I believe you'll find the compression
: > wave produced by a positive voltage on the positive terminal (pushing the
: > speaker outward) will yield the more satisfying thump. 
: 
: There are a wide variety of reasons why such an experiment might generate
: positive results. First, the whole issue of which constitutes a "more
: satisfying thumb" is purely subjective. Secondly, such an experiment test
: the speaker's step response, something speakers are universally and by
: definition very poor at, so what does the experiment prove? Third,
: depending upon the voltage of the battery, you may very easily succeed at
: uncovering the common mechanical and electro-magnetic unsymmetries in most
: loudspeakers, so you may be doing nothing more than deciding which kinds
: of distortion you prefer.

I agree 110%.

It'd like to mention an orchestral bass drum, in which the first
initial strike moves the membrane inwards, thereby causing a rarefaction
wave to be the first propagated. 

It's also at least 6' in diameter, which may be why no ordinary hi-fi
setup I've heard really comes close to feeling the same---in addition
of course to the fundamentally different acoustics.  (i play
in the university wind ensemble)

: -- 
: |                Dick Pierce                |
: |    Loudspeaker and Software Consulting    |
: | 17 Sartelle Street   Pepperell, MA  01463 |
: |       (508) 433-9183 (Voice and FAX)      |

--
-Matt Kennel  		mbk@inls1.ucsd.edu
-Institute for Nonlinear Science, University of California, San Diego
-*** AD: Archive for nonlinear dynamics papers & programs: FTP to
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Newsgroups: rec.audio