Re: [Phys-l] more on sound

["John Clement" <clement@hal-pc.org>]

> These questions were recently asked by students of mine:
>
> 1. For sound intensity level, why is the decibel unit is used instead of
> the bel (1 dB = 0.1 bel)?

It was found that 1 dB is approximately the smallest increase in sound that
can be heard.  In other words if you increase the level by 0.5 dB most
people will not notice the difference.  However, it is well known by
salespeople and audio testers that a slight difference in level below 1 dB
will make a loudspeaker sound better than its lower profit rival.  There is
extensive research carried out on this by Bell Labs in the early 20th
century.  Smaller than 1 dB difference is also perceivable in stereo as a
shift in image location.  Similarly 0 dB was calibrated because it is
approximately the threshold of hearing for most people.

> 2. Do the "f holes" in violin sound boards play any role in the quality of
> the sound produced? Do these openings somehow "compliment" the shape of
> the resonant cavity? Acoustic guitar sound boards are "figure 8" shaped,
> yet the openings are circular.

There have been some good articles on violins, and apparently all of the
features have some effect on the sound.  One of the really significant
factors is apparently the finish.  A chemist spoke at a local AAPT meeting
and presented evidence that he had manufactured violins which sound exactly
like the famous ones that came from Cremona.  They were indistinguishable in
listening tests and in spectral balance.  The finish is a mélange of junk
and all of the famous violins had it.  When the chemist who made it retired,
there were no more outstanding violins produced.  It produced an extremely
hard finish which tended to crack.

But to almost answer the question, yes it does have an effect on the sound.
The acoustics however are not simple.  The good violin makers would
carefully shave the sounding board in strategic points to tune the violin.
I think that the shape is probably more tuning the sounding board than the
resonant chamber.  Incidentally the famous violins generally sounded better
after they had been broken and then repaired.

> 3. In a choir, why are the higher-frequency notes often easiest heard? It
> could be because they sing louder. Or is it because our ears, being closed
> pipes, are most sensitive around 3000 Hz?

There is certainly the effect that singers tend to be louder at high notes.
This may not be the only factor.  There is also the beaming effect if the
choir is facing you.  The lower notes spread more and will often be softer.
And then don't forget that the higher voices get the melody more often and
also get very interesting parts.  Actually older people hear the low notes
better as the high end goes away.  Incidentally choir directors are always
telling the tenors to lighten up on the high notes and not belt them at
maximum volume.  Some people think that tenors sing at you not to you.  As a
tenor, I can see the truth in it sometimes.

John M. Clement
Houston, TX