Page 75 - The Master Handbook Of Acoustics

50 CHAPTER THREE

loudness that can be perceived. The threshold sensitivity would be
represented by a single fiber firing. An overall, well-accepted theory of
how the inner ear and the brain really function has not yet been for-
mulated. 3–6
This has been a highly simplified presentation of a very complex
mechanism to which much current research is being devoted. Some of
the numbers used and theories discussed are not universally accepted.
Popularization of a subject such as the ear is an occupation that might
be hazardous to my health, but audio workers must surely be amazed
at the delicate and effective workings of the human ear. It is hoped that
a new awareness of, and respect for, this delicate organism will be
engendered and that damaging high levels of sound be avoided.

Loudness vs. Frequency

The seminal work on loudness was done at Bell Laboratories by
7
Fletcher and Munson and reported in 1933, and refinements have
been added by others since that time. The family of equal-loudness
8
contours of Fig. 3-6, the work of Robinson and Dadson, has been
adopted as an international standard (I.S.O. 226).
Each equal-loudness contour is identified by its value at 1,000 Hz,
and the term loudness level in phons is thus defined. For example, the
equal-loudness contour passing through 40-dB sound-pressure level at
1,000 Hz is called the 40-phon contour. Loudness is a subjective term;
sound-pressure level is strictly a physical term. Loudness level is also
a physical term that is useful in estimating the loudness of a sound (in
units of sones) from sound-level measurements. The shapes of the
equal-loudness contours contain subjective information because they
were obtained by a subjective comparison of the loudness of a tone to
its loudness at 1,000 Hz.
The surprising thing about the curves of Fig. 3-6 is that they reveal
that perceived loudness varies greatly with frequency and sound-pres-
sure level. For example, a sound-pressure level of 30 dB yields a loud-
ness level of 30 phons at 1,000 Hz, but it requires a sound-pressure
level of 58 dB more to sound equally loud at 20 Hz as shown in Fig.
3-7. The curves tend to flatten at the higher sound levels. The 90-phon
curve rises only 32 dB between 1,000 Hz and 20 Hz. Note that invert-
ing the curves of Fig. 3-7 gives the frequency response of the ear in