[PD] overdriven speaker

[Martin Schied]

Hi!

I'm no speaker modeling expert at all, but I can try to describe what 
produces sounds in an overloaded speaker. There are various sources of 
distortion, symmetrical (mechanic suspension) and asymmetrical (magnetic 
field) and also time variant (temperature) and modulation (doppler 
effect / amplitude modulation) effects. I don't know which effects have 
a stronger or weaker influence, but I describe what I imagine:


High peak amplitude, positive wave:

speaker moves to the front, parts of the coil will not be in the 
magnetic field anymore. The field isn't zero outside the magnet gap, but 
I guess it decreases rapidly and is almost zero (so for example if half 
the coil is inside the magnet, the parts outside will not produce a 
force. So the force is only half as strong as it should. For simplicity 
you could say the field outside the magnet's gap is zero, so you have a 
linear function of excitation / current. Also If the coil moves out of 
the field its impedance will decrease which has influence on frequency 
response for higher frequencies a bit.

Heat

If the impedance is reduced as described above, a higher current will 
flow and heat the coil more than usual. The resistance of the coil will 
increase when it gets warmed and thus the efficiency of the speaker 
decreases (up to -7 dB I read somewhere, but this wasn't meant for 
almost dead speakers but heavy load). The heat needs some time to 
dissipate, so some kind of slow pumping compression effect occurs.

High amplitude, negative wave:

The speaker's coil might crash into the magnet and create different 
mechanical noises. Also the speakers diaphragm will be deformed by this 
crash and create various kinds of noise. Additionally it carries the 
noises the coil created - depending on the material and shape this sound 
different (paper, Kevlar, aluminum, etc sound different). If this crash 
doesn't occur (professional speakers don't have this issue usually) the 
negative wave will not be distorted too much and maybe distortion can be 
ignored.

In both directions the spider (basically a spring) starts to become 
nonlinear. Different manufacturers have different curves, but for small 
amplitudes they all pretend to be linear - so some experiments with sin~ 
or tanh~ might do it here.

Then generally there are happening doppler-effects on all speakers with 
big excursion. You could model them through a variable delay, modulated 
by a differentiated, low pass filtered signal (don't bite if I'm wrong, 
it's already very late... ). Amplitude modulation can be applied the 
same way (lowpass and apply it to higher frequencies).

so to sum it up:

apply symmetric distortion for the spider, split the path into positive 
and negative parts, for positive samples: tanh~, polynomials or other 
wave shapers, for negative parts let the signal untouched or add noises 
of a crashing coil (don't know how to achieve this), then sum both 
signals up, apply doppler effect, amplitude modulation and pumping 
compression. perhaps that sounds like your speakers then :)

I'm not sure if this works at all, but it definitely will sound very 
distorted in the end.

did you already discover http://www.klippel.de/pubs/papers.asp ?



cheers
Martin


On 15.10.2010 21:10, - wrote:
> Thank you for your answer,
> but as I wrote I don't want the sound of simple clipping like clip~,
> tanh~ or overdrive~. I want the sound of a speaker crying for mercy
> because you put just too much through it.
>
> But I don't know where to start. I know there are complex distortion
> effects, which are able to simulate different speaker cabinets after
> variable amps recorded by different microphones. But they all cost big
> money. Also I don't need the physical simulation. I just want the sound.
> If you want I can try to record the sound I'm talking about.
>
> I tried to search for information how to do this but couldn't find
> anything usable. Not even an analysis what happens inside the speaker
> when you torture it like this.
>
> I already know the forum but don't want to doublepost. I really liked
> the post about the oto biscuit. Neat distortion possibility's.
>
>
> Am 15.10.2010 17:12, schrieb George Ker:
>    
>> Hello,
>> I can't really understand, so , you mean something different from
>>      
> [clip~] ?
>    
>> I' m sure you can find really good patches in the puredata.hurleur.com
>>      
> forum searching about distortion , overdrive clip etc
>    
>>
>> -----
>>
>> GeorgeKer~
>>
>> http://tinyurl.com/georgeker
>>
>> -----
>>
>>
>>
>> On 14 October 2010 23:19, -<fallen_devil at gmx.de>  wrote:
>>
>>      Hi,
>>
>>      does anyone know how to simulate the sound of an overdriven
>>      
> speaker? You
>    
>>      know the crunchy sound when you torture it with a strong bass. It's
>>      nowhere near the sound of an normal overdrive with some kind of
>>      
> clipping.
>    
>>      Greetings
>>      -
>>
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