[sdiy] Percussion MIDI controller

[Fredrik Carlqvist]

If you use a (plastic) tube, you can put a piezo contact microphone and a
regular microphone inside the tube at the same end, and measure the time
difference between the two. Since the speed of sound in the tube is higher
than in the air inside the tube, you get a linear position dependent
difference. 

At 1.5m distance, the air wave will take about 4ms and the plastic wave
about 1ms.

I guess you have to use a directed microphone so you don't pick up the sound
from the vibrating tube. But the sound wave should be stronger than the
indirect vibrating plastic -> regular microphone signal. 
 

Fredrik C



-----Original Message-----
From: owner-synth-diy at dropmix.xs4all.nl
[mailto:owner-synth-diy at dropmix.xs4all.nl] On Behalf Of Ingo Debus
Sent: den 24 februari 2005 17:32
To: synth-diy
Subject: [sdiy] Percussion MIDI controller

Hi all,

Someone asked me about a pecussion MIDI controller like this: a 1...1.5 
metres (4 or 5 feet) long rod to be played with sticks/mallets. It 
should sense both the impact and the position where the stick hit it.

My first idea was attaching two piezos to the rod, one at each end, and 
sensing the time difference. So I did this and watched the outputs of 
the piezos on a dual trace scope. With wooden rod and sticks, the slope 
is way longer than the time difference itself (the speed of sound in 
solid materials is much higher than in air), and couldn't be seen on 
the scpoe at all. It's probably difficult to measure the time 
difference precisely.
Then I tried a steel rod and hit it with another piece of steel. Now 
the time difference became visible, because of the much steeper slopes. 
But hitting steel with steel makes a lot of noise in itself, so this is 
probably not very well suited for something controlling a synth.

Another idea is attaching a resistive wire to the rod and hitting it 
with an electrically conductive stick, thus sensing the position. This 
would of course require a cable at the stick. Impact would still be 
sensed with a piezo.

Anybody already done things like this?
Any other ideas?

Ingo