Re: [Harp-L] Full length covers

["Vern Smith" <jevern@xxxxxxx>]

----- Original Message ----- 
From: "geoff atkins" <geoffatkins@xxxxxxxxxxxxxx>
To: <harp-l@xxxxxxxxxx>
Sent: Thursday, April 09, 2009 2:21 AM
Subject: [Harp-L] Full length covers


> But could an extra property of longer covers be the 
> changed resonance?
>
> I'm measuring Hohners here, and using effective lengths, 
> as some edges are
> splayed:
> We have Short covers of 84mm effective transverse length 
> and a full cover
> would be about 104 mm. The longitudinal direction is the 
> same, 27mm, and
> open-faced. The profile varies from Pro Harp to Deep 
> River, and affects
> embouchure as well as the free air gap over the reed 
> length.

> The 84mm has a primary resonance of 4083 Hz,  between D8 
> and D#8
> and the 104mm would resonate around 3230 Hz,  that's G7 / 
> G#7 on an equal
> tempered scale.

> Eighth wave resonance (the last that has much of an effect 
> on perceived
> tone?) takes us down to roughly 500 - 400 Hz, G4 to B4, 
> where we play, and
> higher too into areas where quarter wave harmonics are 
> present.

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

How do you know?

>  why shouldn't a longer cover have an effect?

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.

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.


Vern