[sdiy] Thoughts on drum synth

[Magnus Danielson]

From: Andre Majorel <amajorel at teaser.fr>
Subject: Re: [sdiy] Thoughts on drum synth
Date: Mon, 1 Apr 2002 14:06:04 +0200

Hi Andre,

<think your message missed the list... >

> On 2002-04-01 03:46 +0200, Magnus Danielson wrote:
> 
> (too much to quote)
> 
> To add to what Magnus said, and to further complicate things, a
> few more considerations :
> 
> - The material of the shell matters (thickness, type of wood).

Eg. how much it "flexes" or rather, which acoustical impedance it
represents. High impedance - hard material, low impedance - fluffy material.

> - You rarely hear a bass drum in total isolation. There's some
>   acoustic coupling with the other pieces of the drum set. The
>   timbre of the snare drum certainly contributes to the sound.

Certainly, this is allways the "problem". That is valid for any drum
in the set, they will be in there.

I was once involved in the preparation of a nigthclub show, where they
had a troubleing ringing in the bas drum. They tried different mikes,
tried laying the mikes on filts etc. I on the other hand brought out
my calculator and got out the halfwave distance... looked under the
stage and found that distance... the wooden floor on which the drum
sat on and down to the concrete floor below was totally unobstructed
space. It formed a large resonance right there... ;O)

BTW, that show went well ;O)

So yes, other resonances can bring in their colouring to the sound.

> - In percussive and plucked instruments, higher partials decay
>   faster (in general).

Right, but you can change that by damping out the circular resonances.

> - At the beginning, there is a short burst of noise. Partials that
>   are not among the natural resonance of the drum decay very fast.
> 
>   The initial spectra depends among other things on the material
>   of the bat. With felt (the normal material), it does no reach
>   very high because the initial pulse is soft. Some people have
>   used solid nylon, which I imagine sounds much richer in higher
>   partials.

Also, you usually have a larger area which you hit with the soft
material, this means you will hit both phases of nodes, so that they
cancel out each other. This means that you will induced energy into
lower modes since they have larger nodal areas where as higher modes
will not get much energy. Hitting with a hard stick on the other side
brings more energy into the higher nodes, and thus have a less "dull"
sound.

I think you can view the induced energy by the pressure impulse that
the stick (or whatever) makes on the skin. While this is a gross
oversimplification, the impulse form is indeed the basis for analysis,
just that it is not easy to exactly define which impulse we really
get, since that will be distributed over the skin.

> - Unless the drum is very well tuned, the tension is not exactly
>   the same around the rim. This causes certain mode to have
>   several slightly different frequencies, which beat against each
>   other. This is most noticeable on toms.

Indeed. I've learned that trade from a drummer who also been a car
mechanics for a moment of his life, however, his car mechanics
analogies did not work with me... since I'm not into that kind of
stuff.

Anyway, the variation of tension makes the acoustical impedance of the
skin different and all of a sudden the whole nodal pattern and
frequencies where they occur change. Also the decay-time (loss
component for that node) changes.

> One csounder wrote two articles on the subject :
> 
>   http://csounds.com/ezine/Autumn1999/synthesis/
>   http://csounds.com/ezine/Winter2001/synthesis/

Thanks for the pointers.

Cheers,
Magnus