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To: Loopers-Delight@annihilist.com
From: pycraft@elec.gla.ac.uk (Dr M. P. Hughes)
Subject: Re: Klein Electric Guitar
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Resent-Date: Fri, 6 Jun 1997 02:43:09 -0700

>> >>>Strummed acoustically it is
>> >>>quite loud, like a 335. The resonance chamber does that for you.
>
>> >Well, I'm hardly an expert on guitar design, but as I understand, 
>resonant
>> >chambers can add a lot of sustain and harmonic content. I think that's
>
>> I thought:
>> The chamber probably absorbs some energy from the cord and thus 
>diminuishes
>> sustain. The loud acoustic sound confirms that. The sound energy you 
>hear
>> is taken from the string and the more you take the quicker it is 
>without,
>> which means less sustain.
>
>......well, it seems to me "acoustics" differs from this point of view.  
>the string vibrates...the chambers resonate.  It's the amplification 
>properties of the chambers themselves that create the sustain.  Take an 
>acoustic guitar without the huge box and you haven't got much.  Play any 
>hollowbody guitar vs. solid body...play through any good quality speaker 
>cabinet and then through a speaker sitting on the workbench and you start 
>to appreciate what "chambers" can do.

Dammit, this has got me thinking (oh, no, not again)!  The reason an
acoustic shifts air is that the bridge moves the top up'n'down due to
string vibration.  So some inergy is lost, although the guitar top
effectively becomes part of the system (ie you're strumming the strings
_and_ the top), and your initial pick strike will move the top.  There will
be a loss of energy due to the moving bridge, but it shouldn't be any more
significant than the loss due to bridge movement on trem and non-trem
guitars.  OK there's an observable difference but we're still talking
approximately the same sustain, not guitar-vs-banjo.  

The term "resonance" probably isn't very accurate - whilst chambers do have
resonances, these are at specific frequencies - like the famous footage of
Tacoma Bridge finding its resonance frequency!!  Acoustic volume is
entirely down to how much air you're shifting.  If you take a solid gtr,
the acoustic sound comes purely from the string since there's nowhere near
enough energy to vibrate the 2"-thick body to any degree.  In an acoustic,
the string/top system vibrates the top, and the volume is equal to the
total space enclosed by the top in its "extended" and "retracted" positions
(ie the ends of its oscilation). This is the case for, say, an Ovation. 
For regular acoustics, you also couple the _back_ of the guitar into the
system.  The top moves forward, drops the pressure in the body, and "suck"
the back of the body with it.  This causes the back of the guitar to
vibrate introducing another large sound-making area and increasing
loudness.  The shifted air from the exterior of the guitar's top disperses
from all over the top (and is quiet) whilst the change of pressure inside
the chamber has only one place to act through - the soundhole.

 The delay for sound travelling between top and back eill cause resonances
- ie there will be frequencies where top and back will be "in plase" and
summing their volumes, others where ther will be phase cancellation.  Since
the back's contribution will be smaller (due to losses in the body) this
won't result in volume changes, but will "colour" the sound.  I would guess
that the frequency of these in vnversely proportinal to the depth of the
guitar (ie sound gets deepwe ith increasing distance - ie wavelenght
increases) Also, the acoustic coupling is more "spread out" through the
chamber (think reverb) giving rise to more resonances and a richer sound. 
"Resonances" occur everywhere - the top, chamber, back etc occur everywhere
which is why acoustic guitars are perceived to sound more "alive"; however
they aren't why chambered guitars are louder - it's why they sound better. 
Acoustic volume is derived from the same source (the efficiency of the
board) but not through resonances.

To summarise, the volume of an acoustic is lastly louder, and the sustain
only slightly lower, because the string is coupled into a single vibrating
system with the top which is a very efficient way of moving air.  

IMHO...!

Michael

Dr Michael Pycraft Hughes      Bioelectronic Research Centre, Rankine Bldg,
Tel: (+44) 141 330 5979        University of Glasgow, Glasgow G12 8QQ, U.K.
    "Wha's like us?  Damn few, and they're a' deid!" - Scottish proverb