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Propeller Head Plaza Technical and scientific discussion of amps, cables and other topics. |
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Propeller Head Plaza Technical and scientific discussion of amps, cables and other topics. |
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In Reply to: Re: Hey Tom posted by Soundmind on March 21, 2005 at 09:43:00:
>Any sound coming out of your speakers from hitting a microphone
> or phono cartridge cable is due to vibrations traveling throught the wire
Completely agree. It's just the "phone" kids make with two empty cans and some wire.
That said, microphony in cables _do_ exist, and has two main causes: piezoelectricity and triboelectricity.
It is a pain in the ass when you have to specify connection hardware for accelerometers to airborne
equipment in remote racks.
Microphony in cables creates small charges (in the level of the picocoulomb/Nm2 for mainstream cables).
This small charges creates a current which flow through both the load and source impedances.
This current translates into voltage according to the source and load impedance.
Let be a 100? load (MMphono cartridge oe any gear output) for exemple connected to a 10K? input (any gear)
With a source impedance about 100? (most audio equipement, but intrument mikes, see below)
a 1pC charge (violent shock on the cable) with a 100microsec rising time gives a peak of 2microvolts, which is 115dB
under the standard consumer level. Which means that the phenomenon at nominal level will be completely undetectable.
You just cannot hear it. Well, the shock on the cable itself will be much more audible! (evidence: say the shock
acoustic level is around 70 dB (CLACK!), say your gear is set in order to give a SPL level of 110dB with 1.44V input
(standard, somehow loud...). The noise level (electrical -120dB: good gear) will translate into -10dB acoustical
(inaudible by definition). The shock on the cable will give a 110-115= -5dB electroacoustic (through the speakers) peak.
Strictly inaudible.(and at the same time, you got the 70dB of the direct shock)It can be problematic in one case: instrument (guitar) mikes based on piezo technology. The input impedance of their
preamp is about 10mega?. The piezo mike's output impedance is around 100mega?. The above calculation gives a ...ONE
tenth VOLT peak, which is several dB above the mike's output. Well, in fact, it will be much less because of the cable
capacitance which is in parallel with the load, and slows down the voltage rise, but it will be noticeable without a
doubt!
Same problem with accelerometers, ionic probes and all high source and load impedance connections. In those case, the
solution is to use a cable with low tribo and piezo factors, with its dielectric layered with semiconductor powder. It
exists, ask for accelerometers cables, furthermore, they are very well shielded, given the severity of the airborne
environment they are specified for.
But, again, and apart from the cases listed above, no such trouble with normal audio sources.
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Follow Ups
- Re: Hey Tom - Jacques 14:17:28 03/21/05 (1)
- Ahh ! - hitsware 16:49:52 03/21/05 (0)
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