[sdiy] Harmonic bandwidth

[Scott Gravenhorst]

Ian Fritz <ijfritz at comcast.net> wrote:
>At 07:01 AM 1/9/2008, Scott Gravenhorst wrote:
>>Ok, now I've done it.  I wrote a C program to generate 3 .wav files using 
>>double float,
>>one with zero starting phase values and 2 more each with different random, 
>>but constant
>>phase values.  The program uses 32 harmonics in the series that produces a 
>>sawtooth.  I
>>was careful to limit clipping and applied the same worst case attenuation 
>>value to all
>>three.  Each sample is several seconds long.
>>
>>I most definately hear a difference in timbre.
>>
>>The question is "why?".
>
>Here are a couple more things to think about.
>
>Room acoustics:  If you play a sawtooth through your speakers you will hear 
>the timbre change as you move your head around.  If you change the phases 
>of the harmonics, then you are introducing different delays which translate 
>to different relative spatial positions of the wave components' loops and 
>nodes. So I think you may hear differences just because of that effect.
>
>Speaker dynamics: If you put a perfect sawtooth into a speaker you would be 
>asking for its displacement to change instantaneously.  If you rephased the 
>harmonics then you would no longer have this step.  Of course you never 
>have exactly this situation, but the transient mechanical response of the 
>speaker to a step should be kept in mind.  For this reason I think the 
>sawtooth does not make a very good test.
>
>An old trick of loudspeaker salesmen is to play at a higher level through 
>the speakers they want to sell.  There was a very large loudness difference 
>in the first pair of examples that were posted.  It is very important when 
>comparing timbre to keep the loudness level the same, probably to within 
>0.5 dB.
>
>I'm not pretending to have the answer here, but there are many possible 
>spurious effects that need to be considered.

Excellent points, so I changed my program to use only 2 sinewaves.  In this case, I hear
no difference.



-- ScottG

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-- Scott Gravenhorst
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