Re: [Harp-L] Speaking of amp stands...

[MARK BURNESS <markwjburness@xxxxxxxxxxxxxx>]

Many of the amps commonly used for harp are very directional, especially those 
with 10" speakers, 12" less so, but still it's very apparent. The sound is not 
dispersed uniformly in a hemisphere, the voice coils do tend to beam out hi 
frequency in a very narrow beam...this has been used to effect by studio 
engineers for years, various players, recording engineers & FOH sound engineers 
have sought many and varied ways to lessen the issue. Lower frequencies are much 
less directional, so the sound spreads in a different manner depending on the 
frequncies involved, the more "on axis" you are, the clearer you hear highs & 
fidelity, the more off axis you get the sound gets warmer, less harsh, moving on 
to dull & poorly defined.


 



________________________________
From: Michelle LeFree <mlefree@xxxxxxxxxxxxxxxxxxxxxx>
To: harp-l@xxxxxxxxxx
Sent: Monday, 31 January, 2011 18:27:46
Subject: Re: [Harp-L] Speaking of amp stands...

Vern writes:
> It is easy to over-estimate the directionality of an amp.  The mental image of 
>a beam of sound emanating from the front of the amp like a beam of light is 
>unrealistic.  Because  the speakers are small with respect to the the 
>wavelengths of sound being produced, diffraction* causes the sound waves to 
>disperse almost equally in the hemisphere to the front of the amp.  A better 
>mental image would be to think of throwing the amp into a calm lake and watching 
>the resulting waves.
> 
> Turning the amp at various angles to a sound level meter and plotting the 
>readings would show the directionality of the amp.  A cruder experiment would be 
>to determine the minimum angle that could be detected by a blindfolded listener.
> 
> * diffraction:  
>seehttp://hyperphysics.phy-astr.gsu.edu/hbase/sound/diffrac.html
> 
> Vern

Amazing! Along with his usual flair at describing how physical and acoustic 
science principles apply to the real world of harmonica players, Vern has just 
issued the very first ~footnoted~ Harp-L post I can recall. In fact, perhaps the 
first footnoted email list post I can recall ever! Gotta love 'im! :-)

What is it, 83 years young? And still pushing envelopes. Heck, I'll be delighted 
to even be able to ~say~ "harmonica" at that age.

Thanks again, Vern!

Michelle

PS: Another slightly less crude way to demonstrate an amp's angular sonic 
directivity might be to use a standard stage mic' as a sound level meter placed 
at equidistant angular increments in front of the speaker as Vern suggests. Plot 
the amplitude of the PA's sound level meter as a function of the angle in front 
of the speaker.