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



15 inches of water pressure? Vern, have you ever graphed that choking pressure for different gap sizes? That would be fascinating to know. 
How does that go with the pressure and volume? Is that relationship the same as with added sound... say when two reeds play at the same time, the sound is not twice as loud? 
Does the amplitude of the reed travel have a linear relationship with the amplitude of sound? 
 
David Payne
www.elkriverharmonicas.com
www.hetrickharmonica.com


________________________________
 From: Vern <jevern@xxxxxxx>
To: daijoubu@xxxxxxxxx 
Cc: harp-l@xxxxxxxxxx 
Sent: Saturday, December 29, 2012 11:38 PM
Subject: Re: [Harp-L] Embossing and Compression (was Rick Epping, father of embossing)
 
The flow of breath through the slot excites the vibration of the reed and provides the stream of air that the reed chops to generate the sound.  

Up to the point where the reed chokes at about 15 inches of water pressure, more pressure produces louder sound. However, this isn't a linear relationship because a given increase in pressure produces smaller increases in loudness as the pressure gets higher.   I once made some measurements using a water manometer to measure pressure and a sound meter to measure decibels of loudness.

Vern

On Dec 29, 2012, at 8:12 PM, daijoubu@xxxxxxxxx wrote:

> 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
> 
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