Re: [Harp-L] Combs/Vern..you online?

["Vern Smith" <jevern@xxxxxxx>]

.----- Original Message ----- 
From: "Rick Dempster" <rick.dempster@xxxxxxxxxxx>
To: "Vern Smith" <jevern@xxxxxxx>
Sent: Sunday, September 21, 2008 10:35 PM
Subject: Re: [Harp-L] Combs/Vern..you online?


Thanks Vern. That was offlist; deserves to be posted surely? Nevertheless, 
it's going into my own harp-archive!
Cheers,
RD

> > > "Vern Smith" <jevern@xxxxxxx> 22/09/2008 15:31 >>>

----- Original Message ----- 
From: "Rick Dempster" <rick.dempster@xxxxxxxxxxx>
To: <harp-l@xxxxxxxxxx>; "George Brooks" <gbrooksvt@xxxxxxxxxxx>
Sent: Sunday, September 21, 2008 9:00 PM
Subject: Re: [Harp-L] Combs/Vern..you online?


> When I 'twang' (or 'plink') a harmonica reed, is not the sound produced
> the vibration of its mass?

It is not only it's mass at the tip but also the stiffness of the reed
acting as a spring.  That is why when we remove material at the tip,
lowering its mass, the pitch rises and when you remove material near the
rivet, weakening the spring, the pitch falls.

> When the reed is vibrated by my breath, is not SOME of the sound being
> produced in the same way as if I had  plucked, plinked or twanged it?

Yes. There is obviously some sound produced by the reed acting as a bell or
as the tine of a music box.  However, the reedplate and comb of a harmonica
is a very poor soundboard so the sound is very faint.  It is masked by the
much louder sound of the "siren".  Masking prevents you from hearing a faint
sound while you are hearing a strong one. It is why you can't see stars in
the daytime or hear a whisper when a train is passing nearby. See
http://en.wikipedia.org/wiki/Auditory_masking.  I made a harmonica with a
reed protruding from the rear where it could be easily plucked.  When the
harp was being blown, listeners could not hear the plucked reed.  When only
the protruding reed was plucked and the harp was not blown, listeners could
hear it clearly.

> But the fluttering plastic protruding from the car door, and the ruler on
> the table, are being stretched and released, and, while clearly acting for
> the most part, if not all, as a siren, could be said to be influencing the
> sound by vibration of their own mass, just as a rubber band (or guitar
> string?) might.

In a harmonica reed, the mass of the tip and the springiness of the root
determine the pitch.  The flow of air excites the vibration.  The chopping
of the air stream as the reed enters and exits the slot generates the sound
that you hear.

> I still cannot, when it comes right down to it, quite see the difference
> between a struck bell, or plucked guitar string vibrating the air and thus
> vibrating our ear drums, and the breath activated reed vibrating the air.

The bell has a large surface area and so does the soundboard of the guitar.
Because the reed has a tiny surface area and is connected to a tiny, dead
soundboard, it emits only a faint sound.

> I think the answer lies somewhere in the fact that the pitch of the bell
> or guitar string cannot be altered by causing it to vibrate faster or
> slower (ie hitting, plucking harder or softer) because the note is
> inherent in its mass;

The same mass and spring effects that set the pitch of the bell or guitar
string determine the pitch of a harmonica reed.  The resonant pitch of any
mechanical vibration is proportional to the square root of (M/K).  M is the
vibrating mass and K is the stiffness of the spring.

> whereas the reed, plastic, prop, roarer produces a different pitch
> depending on the speed of its oscillation.

"speed of oscillation" and "pitch" are the same thing.

> But then why does the harp-reed produce the same pitch when it is plucked
> as when it is blown?

Because the pitch is determined by the resonant frequency of the reed and is
a function only of mass at the tip and stiffness as a spring near the rivet.

> When we pluck the harp reed, are we hearing a siren, or, effectively, a
> bell (or kalimba tine, guitar string etc.)

When you pluck the reed, you hear it acting as a bell-kalimba-string.  When
you blow air, a much louder siren-like sound is generated.  In your school
siren, the pitch was determined by the speed of the rotor and the frequency
with which the little windows in the rotor passed matching windows in the
housing.   In a harp, the opening and closing of the little slot/window is
determined by the natural or resonant frequency of reed vibration.  i.e. the
reed and the siren generate sound in the same way but determine the pitch
differently.

> I'm not interested in a heated exchange; I'm just interested in the way my
> harmonica works.

So am I.

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