Re: tuba audio physics - KNOW KNOWS ?


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Posted by Gerald J. on March 10, 2000 at 21:28:27:

In Reply to: tuba audio physics - KNOW KNOWS ? posted by Joe S. on March 10, 2000 at 10:13:17:

Its all the air, practically nothing in the brass...

According to my handy Chem Rubber Handbook of Chemistry and Physics:

Warming yellow brass from 68 to 98 F expands it 300 parts per million.

Warming dry air from 20 C to 37 C changes the velocity of sound propagation from 343 to 353 m/sec, a change of 28,000 parts per million (in round numbers).

Raising the humidity of 20 C air from 50% to 90% increases the velocity of sound propagation from 1128.8 feet/sec to 1304.44 ft/sec a change of 1456 parts per million.

A half step is about 59,000 parts per million.

20 C is 68 F, 37 C is 98 F.

Essentially all the brass does is make it hard to warm up the air, then hold the air at some elevated temperature. If a tuba was to be made to hold pitch independent of air temperature it would need to self adjust a quarter step from 68 to 98F internal air temperature.

So yes a lot of hugging will help get the tuba up to temperature but the air has to circulate to be warmed by the brass. That furnace vent sounds to be the most effective. Maybe a furnace for all the brass would get them warmed up in minutes instead of half an hour...

A tuba that has a range of temperatures should have different pitches on notes of different octaves depending on the locations of the nodes of the note vibrations and the changes in taper and diameter.

Gerald J.


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