|
Propeller Head Plaza Technical and scientific discussion of amps, cables and other topics. |
|
|
|
Propeller Head Plaza Technical and scientific discussion of amps, cables and other topics. |
|
In Reply to: Question for jneutron re analysis posted by Commuteman on January 13, 2004 at 17:14:12:
Peter: ""If the output signal is due to the piezoelectric effect, then it is due to the deformation of the dielectric at the point of impact, right?""Yes
Peter: ""Let's take the case of an interconnect used to connect a preamp to a poweramp with a 100k input impedance (like my VT100....)
If we simplify the geometry a little, there is an interesting result....
If you think of the dielectric as a sheet of constant thickness that is being compressed at right angles to the plane of the sheet, then presumably the energy generated by this effect is proportional to the area of the sheet that is being compressed (given a constant pressure). Is this correct?""
Yes
Peter: ""Let's say Bruce's finger (given the curvature of his fingernail) has a contact patch (the area that hits the cable) of about 2mm square. This is 4E-6 m2.""
An error here is how the pressure wave props into the cable. I would assume a spherical wavefront.
Peter: ""If the cable is, say, 8mm in diameter and 1m long, it has a surface area of about 2.5E-2 m2. This is approximately 6280 times bigger than Bruce's fingernail contact patch.
This means that the same energy could be produced by a pressure change that is 1/6280 of the pressure exerted by Bruce's finger.
For a load of 100k, Bruce's plots show voltages of the order of 150mV, or -16dBV. For a load of 10k, he shows signals at about 20mV, or -34dBV. It's a pity he doesn't show a 1k load, but it looks like it goes down roughly 20dB for every decade decrease in resistance, so 1k should be around -50dBV.
So, if the preamp output impedance was 1k (not unheard of for tube gear) we would expect a voltage around -50dBV.
Let's assume that the threshold for audibility for this effect might be -80 dBV (I suspect it's lower, but I don't want any complaints...)
That means the pressure that would be needed to reach this threshold would be about 30dB lower, or 188,000 times smaller than the pressure of Bruce's finger flick.""
I don't know about mosquitoes smacking into each other, but that doesn't sound so wildly inaudible to me.
Did I screw up somewhere in the math?""
I didn't see any.
Of course, a lot of your assumptions regarding the actual numbers is guesswork, as was mine. Chief unknowns: linearity of piezo voltage vs delta volume; audio enviro forces on cable.
But, yours is an excellent post. That is exactly the kind of discussion and analysis I had hoped prophead would foster.
By analysis, you certainly do show that based on Bruce's tests, the effect on an IC cannot be discounted..
As you pointed out, new, more specific tests are absolutely warranted.
Thanks.
Cheers, John
This post is made possible by the generous support of people like you and our sponsors:
Follow Ups
- Re: Question for jneutron re analysis - jneutron 06:38:53 01/14/04 (0)
Top of Page ]
[ Contact Us ]
[ Support/Wish List ]
[ Copyright © 2025, Audio Asylum, all rights reserved ]
