[Harp-L] Re: Reed Stress and Temperature

[patpowers <patpowers@xxxxxxx>]

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

 

On Sep 23, 2014, at 12:21 PM, patpowers <patpowers@xxxxxxx> wrote:

> Agreed, the amplitude is not affected by temperature.  However, the pliability of the reed material is, and a cold reed will break easier than a warm one.  If you'd like to test this, take 2 identical strips of metal, or any normally pliable material really (2 plastic spoons will do), and put one in the freezer overnight. Then clamp them both in a vise and bend them back and forth until one breaks. I think you will find the warm material will bend easier, spring back better, and last longer than the refrigerated material. Chances are the refrigerated strip/spoon will break easily.    
> 
> And, maybe it's just my technique (or lack thereof), but I tend to blow and draw harder when bending notes.  The amplitude of the audio signal may not change, but the pressure differential definitely does. I'm not sure of the exact physics involved; the interaction with the other reed in the slot, etc... but I can say with certainty that I've never damaged a reed while play straight notes, only when bending them, and the notes that I bend the most are the reeds that fail first, so the evidence supports that bending notes (at least the way I bend them) stresses the reeds more.  I would be interested in some Finite Element Analysis on the subject, though, as I think multiple factors would play into the stress equation; signal amplitude, differential pressure, temperature, humidity, frequency, harmonic distortion, etc...  We could really deep dive into this, or just postulate that bending notes stresses reeds more, and argue the temperature effect on reed longevity.  I say cold reeds break easier.
> 
> Thanks for the debate. ;)
> 
> 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 1:01:14 PM
> Subject: Re: [Harp-L] Re: Harp-L Digest, Vol 133, Issue 22
> 
> The stress in a reed is a function of the amplitude of its vibration. This amplitude is affected by the pressure difference across the reed.   Do you know of any reason that a cold reed would vibrate at a greater amplitude than a warmer one? Do you blow/draw harder when the harp is cold?
> 
> Do you know that a âdeep bendâ causes either reed to vibrate at a greater amplitude?  You may be making the unwarranted assumption that the greater effort that the player must use to bend is felt by the reed.
> 
> Vern
> 
> On Sep 23, 2014, at 6:22 AM, patpowers <patpowers@xxxxxxx> wrote:
> 
>> Thanks guys - that is some really cool information!  But, please note (yes, pun intended again) that my recommendation had nothing to do with pitch, and was simply to reduce the risk of over-stressing the reeds.  I would not recommend grabbing a cold harp out of the case and immediately start doing deep bends on it.
>> 
>> Keep Harpin'!
>> Pat Powers
>> 
>> ----- Original Message -----
>> From: "Vern" <jevern@xxxxxxx>
>> To: "Larry Sandy" <slyou65@xxxxxxxxx>
>> Cc: patpowers@xxxxxxx, harp-l@xxxxxxxxxx
>> Sent: Tuesday, September 23, 2014 5:18:33 AM
>> Subject: Re: [Harp-L] Re: Harp-L Digest, Vol 133, Issue 22
>> 
>> If you cooled a brass reed by 20 degF, its pitch would INCREASE by about 1 centâ.imperceptible among the other variables affecting pitch.  (derived from the chart at http://www.engineeringtoolbox.com/young-modulus-d_773.html ) Note that E (modulus of elasticity) decreases with increasing temperature and that pitch is proportional to the square root of E.  
>> 
>> If the pitch is decreased by moisture on the reed, the effects would tend to cancel.
>> 
>> Vern
>> 
>> On Sep 22, 2014, at 5:36 PM, Larry Sandy <slyou65@xxxxxxxxx> wrote:
>> 
>>> Pat, that's interesting.  I have never heard that cold reeds are less flexible. That must translate into them playing a LITTLE FLAT until they warm up. I know cold reeds collect breath moisture quickly though, which would also lower the pitch. Interesting parallel.  I wonder if cold, moist reeds are double flat?  ð<br/><br/>Lockjaw Larry
>> 
>> 
>