[Harp-L] Full length covers

["geoff atkins" <geoffatkins@xxxxxxxxxxxxxx>]

   3. Re: Full length covers (Vern Smith)

Vern:
The opening along the rear would prevent any possible "organ-pipe" resonance
where the transverse dimension would be related to frequency.  Consider the
effect of cutting away one entire side of an organ pipe or a marimba
resonator.  Even opening the tiny little spit valve spoils the resonance of
a brass instrument.

Geoff: 
You're confusing two things here: the length of an organ pipe determines and
contains the vibrating air column that forms the note. The air oscillation
is initiated by the fipple. In brass instruments, the spit valve isn't
modifying the resonance, but breaking the length of the pipe. Consider a
flute: one vibration source and multiple notes on the same tube.
In a harmonica the basic note is formed by the reed, which vibrates at the 
required frequency. Each note has a different reed.

Geoff: I know for a fact that, say, opening the restricted face on some
cheaper experimental harps has been beneficial,

Vern: How do you know?

Geoff: I've done it and subjectively experienced the improvement.
Geoff: why shouldn't a longer cover have an effect?

Vern: Because there is no resonant cavity under the covers.  If the
fixed-dimensions of the cavity under the covers were resonant, you wouldn't
like it because it would make the loudness different at different pitches.
Musicians call this "uneven response".
I played a diatonic with the rear completely open and also 2/3 closed by
modeling clay.   My sound meter detected imperceptible differences of a few
db in both directions that could be explained by my inability to blow at a 
consistent level.  Such a test would be completely valid if the harp were
machine-blown.

Geoff: I design acoustic attenuators as part of my job, and know some of the
strange things sound does. If your meter's on the dB(A) scale (approximating
human ear response)and you get a "few decibels" either way, you're actually
measuring a very large sound pressure level difference. People can hear a
difference of about 2 dB, and 10dB is already double loudness.

Vern:
I posit that attempts to produce perceptible effects by small changes in
cover design are wasted effort.  The function of the covers is to keep your
lips and hands off of the reeds.  Otherwise, the harmonica sounds little
different with no covers at all.

Geoff: It's the little difference we were discussing. I do agree that the
cover has probably less impact than say correct hand cupping, but I'd like
to hear what the manufactures say about covers.

Best regards, Geoff Atkins