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Name: Christina K.
Status: student
Age: 14
Location: N/A
Country: N/A
Date: 1/22/2004


Question:
I am just wondering why temperature effects how a musical instrument sound. My band teacher told me that depending on the temperature, the sound can go sharp or flat.


Replies:
It's because the speed of sound in air depends on the temperature of the air. According to

http://hypertextbook.com/facts/2000/CheukWong.shtml

The speed of sound in air at 0 C is 331.5 m/s, and it increases by 0.6 m/s for each increase of 1 C. This means a 331.5 Hz tone has a wavelength of 1 meter in 0 C air. At 1 C, a 332.1 Hz tone has a 1-meter wavelength. Most people would not notice this difference, but at 10 C, a 337.5 Hz tone has a 1-meter wavelength, and this is roughly a third of a semitone away from 331.5 Hz -- easily noticeable.

Tim Mooney


Christina,

The speed of sound through the air depends on temperature. At higher temperatures, air molecules bounce around faster. This causes sound waves to travel faster. At high temperatures, the sound gets to the end of the instrument too fast. At low temperatures, the sound takes too long to get through the instrument. In both cases, the sound waves will sound wrong.

Dr. Ken Mellendorf
Physics Professor
Illinois Central College


Hi Christina-

Temperature affects lots of physical things a little bit. And musical instruments are tuned pretty precisely.

One octave is a factor of 2 in frequency. There are 12 notes in an octave, so the notes are the (1/12th) root of 2, abut 1.059 apart. That is only 6%. So if an instrument's pitch goes 2% higher or lower, your music teacher will definitely notice.

All the instruments are changed differently by temperature. On a steel-stringed wood guitar, warming makes the wood get longer, but the steel does not change nearly as much.

So the string is stretched tighter, and its vibrating frequency goes up. Similar with a piano, I think, but I think the change is not reversible. The wood or the tuning pegs creep to a different position as the wood expands, and they never bother to creep back when the wood contracts. On a nylon-stringed wood guitar, maybe the nylon expands and sags even more than wood, so the pitch goes down with heat. Only a violinist would know what the violin does.

A metal flute will go sharper when hot, because the speed of sound in air goes as the square-root of temperature. I can calculate this one: changing from 60F to 80F, our temperature above absolute cold increases about 4%, so sound goes 2% faster, so the pitch at 80F will be 1/3 of a note higher than at 60F.

If a flute is 2% sharp, and on the piano some strings are 2% flat and some are not, that band will not sound so good.

And then maybe harmonics in any one note can be affected by different amounts, changing the "taste" of the sound on that instrument.

But that might be pretty subtle.

Jim Swenson



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