Re: [Harp-L] Embossing and Compression (was Rick Epping, father of embossing)

[daijoubu@xxxxxxxxx]

Is it the interaction with the sound wave or the interaction with the reed that changes the sound?

Rob
Sent from my BlackBerry device on the Rogers Wireless Network

-----Original Message-----
From: Vern <jevern@xxxxxxx>
Sender: harp-l-bounces@xxxxxxxxxx
Date: Sat, 29 Dec 2012 16:01:48 
To: David Payne<dave@xxxxxxxxxxxxxxxxxxxxxx>
Cc: Harp L Harp L<harp-l@xxxxxxxxxx>
Subject: Re: [Harp-L] Embossing and Compression (was Rick Epping,
	father of embossing)

On Dec 28, 2012, at 8:54 PM, David Payne wrote:
>  "....... it seemed really obvious that ANY change in air flow changes the tone....." 

It is not at all obvious to me. Your explanation doesn't fit with my understanding of harmonica aerodynamics and acoustics. You will find the following statement to be controversial.  It is followed by a justification. I claim that when you discount your intuition and look at the problem quantitatively, a different picture emerges.

I posit that: "The flow of breath through the reed chamber and under the cover of a harmonica has no perceptible effect on the sound."

These are the reasons:

The velocity of sound is about 1125 feet/second.
The cross sectional area of a 0.18 inch high x .18 inch wide diatonic reed chamber is about 0.032 square inches or .00023 square feet.
The cross sectional area under the  0.16 inch high x 1 inch deep covers is 0.16 square inches or .0011 square feet. 
A player expels about 1 liter (0.035 cubic feet) of air in 8 seconds, a volume flow of about .0044 cubic feet per second.
The velocity of breath through a reed chamber is about .0044 / .00023 = about 19 feet /second.
The velocity of air under the cover is about .0044 / .0011 or 4 feet per second. IF the player does not hand-cup to completely block flow out the rear, then the velocity is even lower.
4 ft/sec x 3600 sec/hr / 5280 ft/mile = 2.7 mph

Because the breath under the covers is moving at less than 1% ( 0.36 % ) of the speed of sound, there is little interaction.  The behavior of sound waves in slowly moving  air isn't very different from their behavior in still air.

Think of holding a conversation in a gentle 2.7 mph breeze.  

Vern