Re: [Harp-L] Opening Cover Plates

["Vern Smith" <jevern@xxxxxxx>]

There is acoustic theory to support your empirical results.

Lets be clear here that we are not discussing "no 
difference"; we are discussing "no perceptible difference."

I try not to let my feelings enter into these discussions. 
It is my purpose to reduce, not create controversy.  The 
acoustics of the harmonica are objective facts.  Although I 
don't claim to understand them completely, that is my goal.

The shape of the covers will not affect the direction of the 
sound because of diffraction.  When a wave emerges from an 
opening that is much smaller than a wavelength (the case 
here), it propagates radially in all directions as from a 
point source..like a pebble falling into a pool of still 
water.

A non-technical explanation  is this.  All of that sound 
came through that tiny .080" x .5" reed slot.  Since the 
opening under the cover at the back of the harp is much 
larger than that, there is plenty of room for the sound to 
escape.

Only for the sake of argument, assume that the amount of 
sound energy reaching the ear is proportional to the area 
under the covers at the rear of the harmonica.  Even if you 
increase the area by 50%, that's an increase of only 1.7 
db...hardly perceptible.

I have a sound meter.  I have covered 75% of the area at the 
back of a diatonic with a finger and have not seen any 
reduction in the loudness on the meter.

Like you, I conclude that opening the covers isn't worth the 
effort.

Vern

----- Original Message ----- 
From: "JohnnieHarp" <johnnieharp@xxxxxxxxx>
To: "harp-l" <Harp-L@xxxxxxxxxx>
Sent: Thursday, November 12, 2009 11:01 PM
Subject: Re: [Harp-L] Opening Cover Plates


> > Opening Cover Plates ,,, Any videos online showing how to 
> > do this to a Golden Melody?
>
> I haven't seen any vids but do have GMs with cover plates 
> "opened up"
> by others (back and side) and by myself (back only).
>
> However, I don't alter GM cover plates any longer as I 
> didn't find a
> difference in terms of loudness, tone, etc, when I compare 
> the altered
> coverplates to unaltered ones, when tested on a specific 
> comb/reed
> plate GM combination. Also confirmed this impression by 
> asking 3rd
> parties and by comparing digitally recorded test samples.
>
> The only thing that changed was that the altered cover 
> plates were
> less resistant to distortion / crushing than when in their 
> pre-altered
> state.
>
> It may be that differing cover plate designs are affected 
> differently.
> The GM cover plates are pretty solid compared to some 
> other models.
>
> Or perhaps we're back to a "comb material"-like 
> controversy. I'm
> interested in what Vern might have to say on this? Perhaps 
> he's
> already made his feelings known...
>
> Here's a link to a vid that Dave Payne made on opening up 
> cover
> plates. If trying on a GM would suggest leaving the middle 
> cover plate
> brace at the back of the cover plates intact:
>
> http://www.youtube.com/watch?v=HAhD_ZGaDcE
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