Re: Amplitude and pitch of sound waves

[Leigh Palmer <palmer@SFU.CA>]

> If 'pitch' can't mean frequency, we'd better outlaw the use of
> 'color' to describe visible spectral electro-magnetic radiation!

Herb is quite correct. Both "pitch" and "color" are percepts, not
physical quantities. They are not properly associated with their
physical counterparts in a one-to-one mapping, and it is better
to keep the distinction at all times.

Pitch depends not only on frequency, but it also depends upon
loudness (another percept) or sound pressure level (a physical
quantity). Common misconceptions exist among physicists about
pitch, for example the idea that middle C has a frequency of 256
hertz (called "scientific pitch") is perpetuated by science
supply companies which sell tuning fork sets in this bizarre
scale which no musicians use. There is also the common belief
that an octave (a pitch interval, and therefore a percept) is
always the relation between two sounds the frequencies of which
are a factor of two apart. In fact such an interval is less than
an octave at the upper and lower ends of the range of a piano,
and piano tuners must take this into account unless they are
tuning a piano for a tone deaf physicist.

Color is an even less appropriate term to use in physics, though
here again it is misused quite frequently. The abuses here are
more numerous than they are with respect to pitch. Let's consider
the simple question of colors of spectral light. Most of the pure
spectral light has colors one would describe as "iridescent".
These colors are close to colors we encounter commonly in the
ordinary sunlit terrestrial environment, but are sufficiently
uncommon that we must prepend that adjective. They can't be
matched with a tristimulus colorimeter (see Feynman Vol. I Ch. 35)
except by using negative coefficients, a sure sign of their being
uncommon. Moreover the color of spectral light depends upon its
intensity, dimmer light being perceived as less saturated. This
is because there is appreciable input to the percept from the
scotopic vison, which is insensitive to color, at low light levels.
When one gets away from light colors in isolation and begins to
discuss light in the environment, well the topic becomes extremely
complex, many of the readily demonstrable perceptual phenomena
being (generously) not well understood. Light of the same spectral
composition and same intensity entering the eye in different
circumstances can evoke widely differing color percepts. There
is an extensive litearature on these topics. Unfortunately physics
teachers rarely read outside physics. I got my first introduction
from an Addison Wesley book called "Color Television Engineering"
by Henry Wentworth. It was a complete manual for my first color TV
set, a 1955 RCA model CT-100.

Leigh