[PD] overdriven speaker

Martin Schied crinimal at gmx.net
Fri Oct 22 03:50:21 CEST 2010


I agree with Martin Peach in most points and want to add some sentences.

On 22.10.2010 01:17, Martin Peach wrote:
> A perfect speaker will reproduce the sound exactly by transforming the 
> instantaneous voltage to a displacement in or out. That's called 
> 'compliance' in the speaker biz. The ideal speaker has zero mass and 
> is totally rigid.
> So nonlinearities will show up:
> 1> when the speaker is massive and can't reach the ideal position 
> quickly enough.
this mainly filters the signal in a way of an analog lowpass filter and 
influences the intensity and phase spectra. It doesn't add new 
frequencies to it, so it's still called 'linear'. However throug the 
mass the system also has to deal with the stiffness and this causes some 
nonlinear effects.
> 2> when the speaker deforms as it is accelerated from the centre but 
> lags at the edges.
I'm also sure these material nonlinearities will produce a main part of 
distortion frequencies in the spectrum. Especially when the speaker 
surround and spider are at their limits their imperfection will cause 
the whole cone to move 'sideways' and build up different modes. So this 
is definitely of interest for a (dying) speaker simulation. Probably 
this is one of the most challenging too...
> Also the stationary magnet is driving a coil in the speaker cone. When 
> the speaker is overdriven the coil will be pushed away from the field 
> of the magnet on the outward stroke and so the coil will be less able 
> to move the speaker, so a kind of soft clipping will occur.
> But on the inward stroke the coil will bottom out and slam into the 
> support structure, giving a hard clipping and possibly some bouncing.
dependant on the construction of the speaker this slam will not occur, 
because the spider prevents this. (In most new speakers the spider is 
linear for small amplitudes and starts to become very hard and thus 
nonlinear at both ends. In a simulation one might want to chose if that 
slam (or i called it crash) happens or not. In a real speaker this often 
is really the death of the speaker because the coil is damaged 
permanently and then scratches on the magnet surface on every movement - 
I don't feel like I want to simulate these sounds :-\
> I don't think the nonlinearities of the air are relevant here, or the 
> doppler effect of the moving speaker.
I also agree that doppler effect is not the dominant parts of the 
distortions in a conventional speaker enclosure for music reproduction. 
Countermeasures are taken to reduce doppler effect as good as possible 
in these systems. In real speaker enclosements doppler effect is largely 
reduced by separating the audio spectrum on several speakers, so the 
bass speaker which does most of the movements only has to produce 
frequencies up to several hundreds of hertz. For example bass speakers 
in 3-ways systems are driven below 150 Hz in most cases - doppler effect 
becomes very low in this spectrum and mid range speaker have almost no 
visible travel anymore. However the doppler effect occurs in every 
speaker with moving parts and especially for full range speakers this 
becomes important. Guitar amps often use bass drivers in full range, so 
this has some practical relevance for musicians too.

I'd be interested what kind of speakers fallen_devil was interested in 
when originally posting his question to the list. Also a record of the 
noise would be nice for comparison...

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