[sdiy] Harmonic bandwidth

[Needham, Alan]

-----Original Message-----
From: synth-diy-bounces at dropmix.xs4all.nl
[mailto:synth-diy-bounces at dropmix.xs4all.nl] On Behalf Of Jim Palmer
Sent: 09 January 2008 01:06
To: synth-diy at dropmix.xs4all.nl
Subject: Re: [sdiy] Harmonic bandwidth

REF 1:-
first i mixed 4 harmonics (1 second sine waves at 60,120,180,240)
at -6dB each to avoid clipping.  then clipped a random portion of
each sine wave file from the end and pasted it to the front, playing it
looped to make sure no artifacts were introduced.  then i made a
composite wave of the random-phase-start clips the same way
as the i made the in-phase composite clip.
they do sound quite different.

REF 2:-
the most interesting thing is i reversed the in phase pseudo sawtooth
clip 
and can
definitely hear a difference between the normal and reversed sound.  i
think 
this is
a clue as to what is going on here.  inverting the clip sounds identical
to
reversing it (at least this makes sense)...
_______________________________________________
...TOTAL SPECULATION...
REF 1:-
Is it possible that mixing sines like this IS actually creating
"mathematically" Identical sounds BUT the loudspeaker reacts differently
when presented with a nice, tidy pseudo-saw (plenty of time to respond)
and random-phase (it is expected to track the much more convoluted shape
to give the same sound but, as the wave now effectively 'changes
direction' more often the 'speaker cone fails to comply ?

REF 2:-
If I read this correctly, it disproves the above -unless-
Any loudspeaker cone will respond differently when given a positive
transient (voicecoil moves out of magnet field and loses sensitivity)
compared to a negative transient (coil into field and increases
sensitivity). This would amount to a non-linear transfer function in the
audio path.
IF the speaker is in a small cabinet (versus infinite baffle) then it
will be expected to create a vacuum inside the cab for a positive cone
transient and a positive pressure for a negative transient. Given that a
pressure is easier to create than a vacuum, this could be reinforcing
the above effect.

OK I will shut up now !
	Alan


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