Mailbag: “On Sure Footing”

March 29, 2010

Dear Michael,

I am an Organist and Sound Engineer, and am wondering exactly what a sonic foot is? I know it is different from the English foot and sounds to an 8 Hz frequency fundamental tone, but I am not sure what to make of this. What does it exactly mean in terms of pipe length and increments?

Thomas

 

Thomas,

Sonic foot...hmmm, I was wondering myself what you meant until I read to the bottom of your message.
The 'foot lengths' indicated in organ specifications refer to the relationship between pitch and pipe length. As it turns out, the open diapason pipe sounding bottom "C" on an organ keyboard is 8' in length.
According to the physics of tone generation (this works with strings, too), a pitch an octave higher will be produced by a pipe half as long, thus the 4′ and 2′ indications (and higher). Fractional lengths, such as 2-2/3' play fractional pitches (that are part of the natural harmonic series)...in this case the 'G' one and a half octaves above that low "C".
A stopped pipe (with a cap or stopper in its open end) will sound a pitch double its physical length (a stopped flute at bottom "C" will be only 4'
long).
The organ's 'ensemble' is based on an accumulation of ranks of pipes, building upon the unison, but then strengthening the upper harmonics, creating a brilliant (and louder) collective sound.
I hope this answers your question. This web page may provide some additional clarity.
http://www.danishchurch.vancouver.bc.ca/history/organ.html

 

jmb

 

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