Re: [Harp-L] triple vs quadruple reed harps

[Vern <jevern@xxxxxxx>]

On Sep 15, 2012, at 3:38 PM, Rick Epping wrote:

> Hi Vern,
> 
> I agree that it's deflection that stresses a reed.

OK, we agree on this important point.  

>  I believe that the greatest degree of deflection occurs at the moment when a reed is brought into play from a rest position, by the initial burst of air that the player gives it in order to make it respond.  The more suitable the reed's environment, the less breath pressure will be required to make it respond and the less deflection will occur during the initial attack.

You seem to be saying that reed deflects more during the startup transient than during the steady-state note. 

This raises some questions:

Q. If over-deflection/stress occurs as a result of a "burst of air", why would not a smaller burst achieving normal deflection be sufficient?

Q. Can the diaphragm and lungs produce a burst whose rise time is less than a few periods of normal reed vibration? I posit that they cannot, especially at higher pitches. At (middle) C4, a period is about 4 milliseconds, 8ms an octave down,  and 2 ms an octave up.  The burst could not just slam the reed to its max deflection on the first cycle but would give it many cycles to more gradually get going as the pressure ramps up..  A rough idea of the rise time of a "burst of air" can be obtained by how frequently a player can reverse blows and draws......lets be generous and say 10 times a second for a period of  100 ms.  That is many times longer than the vibration period of even a low-pitched reed.    

Q. Assuming for the sake of argument that the reed is stressed a bit more during the start-up transient, will this affect reed life?  The stress remains below the elastic limit if the gap remains unaffected. Fatigue is a function of stress even below the elastic limit but it leads to failure over millions of cycles.  I posit that a bit higher stress over a few cycles at the beginning of each bent note would be a tiny proportion of the total number of cycles and have a minuscule effect on reed life.  
 
>  An example of this can be found in a comparison of the high notes of the 270 Super Chromonica with the old comb, with those with new comb, in which the space in the reed cells has been reduced.  It's curious that, while a reed responds better in the presence of an opposing reed with a lower frequency, it also responds better in a reed cell with a higher resonant frequency than its own natural frequency.

 Reducing the depth of the highest cells in the 270 comb has raised their resonant frequencies to above the natural frequencies of their reeds, with the effect that these reeds require less breath pressure to respond than with the old comb.  Similarly, the improvement in response given to a reed by the presence of a lower-pitched opposing reed results in less breath pressure being required to make it respond, and therefore less deflection and less stress at the moment of attack.

Q. Can a reed cell connected to the player's embouchure be said to have its own resonant frequency?   If it did have its own resonant frequency, cells of the same volume would have vastly different effects on reeds of different pitches.  We would hate that!  However it is possible for the player to tune the combined volume of the small cell and his larger embouchure to meet the needs of each reed.

If we could measure instantaneous reed deflections, these questions could be answered but our interesting discussion would end.

I deeply respect your harmonica knowledge and am grateful for your willingness to bat these ideas around.  I would like to reconcile my analysis of what happens at the reed level with yours.

Thanks for your response,

Vern




 
> 
> 
> Best regards,
> Rick
> 
> On Sat, Sep 15, 2012 at 9:15 PM, Vern <jevern@xxxxxxx> wrote:
> 
> On Sep 15, 2012, at 9:20 AM, Rick Epping wrote:
> 
>> "..................................  I tend to believe that playing a
>> 
>> reed in an environment which allows it to oscillate closest to its natural,
>> or plucked frequency, is less likely to stress it than in other, less than
>> optimal environments......................."
> 
> Based on the mechanics of materials, here's a counter-argument:
> 
> Amplitude/deflection (shorter radius of curvature) is the only thing that stresses a reed.  
> The greatest amplitude/deflection will likely occur at the natural (plucked) frequency.
> It follows that the amplitude and stress will be less in "less than optimal environments."
>  
> You may be incorrectly projecting the player's stress onto the reed. The reed doesn't care if the player's eyes are bulging, his embouchure is convoluted, and he is red in the face with bending effort.
> 
> Vern
> 
> 
>