Re: [Harp-L] Re: Combs

["Vern Smith" <jevern@xxxxxxx>]

----- Original Message ----- 
From: "Derwood Blues" <Derwood.Blues@xxxxxxxxx>
To: "harp-l" <harp-l@xxxxxxxxxx>
Sent: Tuesday, September 23, 2008 8:39 AM
Subject: [Harp-L] Re: Combs



> Is not all sound chopped air?

No. see below.

> Either I am missing your point or you are
> missing mine. I was saying that a harmonica is not like a siren in that it
> uses the vibration characteristics of the material to create the frequency
> rather that the velocity of the air. A guitar does the same thing it chops
> air by the characteristics of the moving material, the strings. Other than
> moving air, like all sound producers, I do not see the similarity to a
> siren. But likely I am missing the point and am wallowing in my ignorance.

Let us define "chop".  I may have originally used the term years ago to 
describe the way sound is generated by a reed.   The reed and the siren have 
these things in common:

1. Air pressure different from atmospheric inside the device...higher in the 
case of the siren and higher (blow) or lower (draw) in the harmonica.
2. A tendency for the air molecules to move from the higher to the lower 
pressure.
3. A window that opens and closes at sonic frequency.
4. The opening and closing of the window "chops" the air into separate 
puffs. The resulting pressure variations are transmitted through the air as 
sound.

However, the frequency of the siren is determined by its speed of rotation. 
The old mechanical sirens had a characteristic wail where the pitch rose and 
fell with rotor speed.  The pitch iof the reed is its resonant frequency of 
vibration, a constant.

A more common way of generating sound is used by a loudspeaker  (or guitar.) 
The cone moves toward and crowds air molecules together raising their 
pressure.  AND when it moves away from them, they fail to follow promptly, 
creating a partial vacuum.  These pressure variations travel through the air 
as sound waves.

Once sound waves move away from the generating device, they are very 
similar.  The velocity of sound is a function of the properties of air and 
its temperature but not of the way the sound is generated.

Vern
Visit my harmonica website www.Hands-Free-Chromatic.7p.com