moment of truth

[billkilpatrick]

over on the lute list there's a very interesting thread with the same title concerning the key your instrument is made in. #here are the initial questions:

- does a lute maker know how a lute is going to sound as he's building it? or, is he unsure until he has put tensioned strings on the finished product?

- i've heard tales of a lutemaker who could tell by knocking on a tree how it will (most likely) sound ...

- i took my lute for repair to a violin maker and she blew in the sound hole to figure out my instrument's tuning: the bowl was in D, she said.

it's this last entry that really started it off.

what sound does your instrument make without strings (this is not a dharma question...) and how can you find out?

- bill

[toddjoles]

According to Roger Siminoff who wrote the only book's out there on building a mando, for the best bluegrass tone the sound chamber should be tuned to D or D# (preferably D). Although I'm sure you'll find differing opinions on the necessity of tuning the sound chamber.

Todd

[billkilpatrick]

dear todd -

how do you know if your sound chamber is in one key or another?

i tried tapping lightly on the bridge with a kettle-drum mallet near a digital auto-tuner but there was no consistency in the registered sound.

- bill

[toddjoles]

Bill, I've only built one flat top mando and the sound chamber came in at a B once strung up. Siminoff say's D or D# for an archtop. I found that to get the mic to pick up I had to tap the top ,(I got to many overtones tapping on the bridge), and then hum the tone myself to get enough sustain.

I'm by no means an expert so I don't know what a flat top sound chamber shoud be tuned to. I didn't tune it at all, just braced per instructions. I just checked the note of the sound chamber at the end of my build so I would have a point of reference in the future.

[billkilpatrick]

thank you - i'll try it tomorrow....england lost...italy defeated......i need something bright on the horizon to look forward to.

[John Bertotti]

I believe it was once stated here that to find the actual sound chamber tone you needed to immobilize the plates top and back. Then blow air into the sound chamber to get the tone. Dave Cohen I think I read that in one your posts is this correct? John

[Dave Cohen]

The fundamental ("Helmholtz" or A0) air resonance interacts with the body modes whaich are close in frequency to it. So yes, strictly speaking, you need to immobilize the body in order to measure the "true" Helmholtz air resonance frequency. In practice though, the interactions change the air resonance frequency some, but not a huge amount, so you can get a ballpark figure without immobilizing the body.

Helmholtz resonance frequencies have been measured for numerous stringed instruments, so a little homework will save you a lot of trouble. Guitars (both steel string and classical) have the A0 frequency in the range of about 105 - 125 Hz (A "C" is about 130 Hz). I have done the air resonance measurements for most types of mandolins. A Neapolitan ("bowlback") will have an A0 frequency of about 200 Hz. For Gibson ovals, Lyon & Healys, and Vega cylinderbacks, A0 is about 210 Hz. For f-hole mandolins, the A0 resonance frequency is typically in the range 285 - 300 Hz. If you make an instrument with typical dimensions, you will get a typical A0 frequency. If you change the body dimensions and/or soundhole(s) area, the A0 frequency will be changed, but you have to make pretty drastic changes in dimensions to get a relatively small change in the A0 frequency.

[billkilpatrick]

dear david -

thank you for the information. i found this site very helpful as well:

http://www.phys.unsw.edu.au/~jw/Helmholtz.html

i must have been a very boring kid to think physics was boring.

as for trying to establish what key the sound box of my charango is tuned to, i guess what i'll do (with out the aid of a microphone) is simply play notes on my guitar near the sound hole of the charango and see which note sets up the most sympathetic resonance.

thank you again - bill

[Dave Cohen]

The usual way in which we measure the Helmholtz resonance is to embed the intrument backside down in a sandbox (with plastic sheeting covering the sand so as not to scratch the instrument's finish) and place sandbags (plastic sandwich bags filled w/ sand) on the top plate of the instrument. We then feed the output of a speaker driver via a rubber tube into the body cavity of the instrument. The speaker driver is driven by a signal generator (sine wave). A microphone is fed into the body cavity and guided into various positions with a stiff wire (a straightened paper clip). The signal generator output is fed into the x-axis input of an oscilloscope, and the mic output is fed into the y-axis input. The result is a lissajous figure, a tilted ellipse. We then dial the signal generator frequency up and down until the lissajous figure reaches maximum amplitude. That frequency is the Helmholtz resonance frequency. As science goes, the above equipment is very cheap. Careful scrounging could get you all of that gear for under $100. Careful shopping could get you all of that gear new for only a few hundred $.

[billkilpatrick]

fine; sounds very definitve.

is there a low-tech, "stranded on a desert island" approach to discovering the key?

- bill

[billkilpatrick]

in answer to my own question, yes...

quite by chance i found a way to gauge the note in which the bowl of your instrument is naturally tuned.

while on your desert island, stroll into your bathroom (or adjacent duomo - both of which have been conveniently placed there for you on the island) playing the notes of a chromatic scale and listen carefully to the reverberations. the room itself acts as a sound chamber and any notes played in it resonate sympathetically with "the" note of your instrument's sound chamber. you can get better reading of this by playing well away from the bridge.

hope this is of some interest.

- bill