[Harp-L] Re: Reed Stress and Temperature

[patpowers <patpowers@xxxxxxx>]

Vern,

Just for the record, I agree with most of what you said.  One could argue that hitting a resonant frequency amplifies the vibration, perhaps even multiplies it, and therefore drastically increases the amplitude of the vibration.  Same can be said for frequency increasing the root mean square(rms), as the result is more amplitude per given time period.  So, in a purist's sense you are absolutely correct, and we're back to just debating temperature's effect, if any.

I still say a cold reed will fail faster.  ;)


Keep Harpin'!
Pat Powers


----- Original Message -----
From: "patpowers" <patpowers@xxxxxxx>
To: "Vern" <jevern@xxxxxxx>
Cc: "Larry Sandy" <slyou65@xxxxxxxxx>, harp-l@xxxxxxxxxx
Sent: Tuesday, September 23, 2014 6:52:40 PM
Subject: Re: Reed Stress and Temperature

Thanks Vern - I love a good debate! ;)

OK, I will concede the material properties of plastic and brass are vastly different.  I was using the "plastic spoon" example to exaggerate the effect of temperature on material properties.  Perhaps I exaggerated too much, but my point is all materials get stiffer, and loose elasticity, as temperature decreases. And, I wouldn't necessary dismiss a 50ÂF to 80ÂF change in temperature as negligible.  On the flip side, I wouldn't consider brass to be brittle at 50ÂF either, but it is less elastic than brass at 80ÂF, especially when stressed and already fatigued.

Regarding your statement "Reed behavior that concerns us takes place within the elastic zone below the yield point."  If this were entirely true, then reeds would never fail.  Obviously the elasticity eventually diminishes to the point where the behavior exceeds the yield point.  At some point the metal fatigues and fails, and I still say colder temperature will accelerate this process.

Your argument is based upon the postulate that amplitude is the only stress factor effecting the reed.  You're not acknowledging temperature effect, or the effect frequency (omega) has on a vibrating mechanical system.  It is well known that harmonic frequencies can prove catastrophic to any mechanical system that vibrates.  I will exaggerate again to make my point, and use the example of Ella Fitzgerald shattering a wine glass with her voice.  It's not just the amplitude of her voice, but also the frequency (actually a harmonic frequency) matching the resonant frequency of the object.  I understand that glass is much more brittle than brass by orders of magnitude, but my point is all materials have resonant frequencies, and breaking points, and as frequency increases, so does the root mean power (amplitude/time) of the system.  You can't just ignore the effect frequency and temperature have on a mechanically vibrating system.  I agree that amplitude may be the primary contributing factor, but it is not the only contributing factor. 

High frequencies and harmonic distortion can be extremely damaging. Everyone knows that harmonicas give off harmonic frequencies, hence the instrument being called a "harmonica", and many of those frequencies are well above the hearing range of the human ear.  It;s not the amplitude of a harmonica that hurts a dogs, it;s the high frequncies.  Those high frequencies can also accelerate metal fatigue, and bending notes creates a huge amount of harmonic distortion, especially during the the transitional phase, which can be catastrophic. When sweeping a frequency that give off high harmonics, you'll eventually hit a frequency that matches the harmonic frequency of the material.  I theorize harmonic distortion is what makes bending so damaging to the reeds. I mean, if not just the amplitude, or the temperature, what else could it be?

Thanks for diving in!  :)


Keep Harpin'!
Pat Powers

----- Original Message -----
From: "Vern" <jevern@xxxxxxx>
To: "patpowers" <patpowers@xxxxxxx>
Cc: "Larry Sandy" <slyou65@xxxxxxxxx>, harp-l@xxxxxxxxxx
Sent: Tuesday, September 23, 2014 4:58:35 PM
Subject: Re: Reed Stress and Temperature


While it is true that the properties of brass and plastic change with temperature, the magnitude of these changes are different by orders of magnitude,  I donât question the result of breaking refrigerated plastic in your experiment. I disagree that you can draw conclusions that apply to brass.  Reed behavior that concerns us takes place within the elastic zone below the yield point.  Your spoon breaking takes place above the yield point . (You exceed the yield point when you change the gap of a reed and its rest position changes.)  Brass and plastic are apples and bananas.

The change in elasticity of brass is finite but negligible over the range of temperatures encountered in harmonica playing. If you plot reed stress against tip displacement from the rest condition, you get a not perfectly but very straight line.  If you lower the temperature of the reed you get a parallel line that lies almost on top of the original but displaced a âteeny-tinyâ bit toward higher stress.  This has no perceptible effect on the response of the harp.

I doubt but cannot refute your claim that bending stresses reeds more than normal playing.  If it does however, it is because the amplitude of one of the reeds is greater than normal. I posit that reed stress is a function of vibrational amplitude only. Other conditions affect reed stress only to the extent that they affect amplitude.  It seems reasonable to me that a reed will vibrate at a higher amplitude at its natural frequency than when bent to another frequency.

Vern