Page 97 - The Master Handbook Of Acoustics

72 CHAPTER THREE

adjusted until the tone is just hidden or masked by the noise. Only
the noise in the critical band centered on 1,000 Hz is effective in
masking the tone. If the noise is expressed in sound-pressure level
of a band 1 Hz wide, the voltage of the tone then corresponds to the
1-Hz sound-pressure level of the noise at the masked point. Adjust-
ing the voltage until the tone is just masked should yield one of the
points on our noise spectrum graph. For convenience, let us assume
that this voltmeter is calibrated in dB referred to some arbitrary base
such as 1 volt (dBv). Referring to Fig. 3-11, note that the critical
band centered on 1,000 Hz is 160 Hz wide. This can also be
expressed in decibels by taking log 10 160 = 22 dB; this 22 dB, repre-
senting the width of the critical band as it does, must be subtracted
from the voltmeter reading in dB. This gives one point on the noise
spectrum graph. By repeating the process for other frequencies, a
series of points is obtained that reveal the shape of the noise spec-
trum. If the recording and the entire measuring system (including
the observer’s ears) were calibrated, the absolute levels for the noise
spectrum could be obtained.
The important point here is that there is such a set of filters in our
head that could be put to such a task, not that this method will ever
replace a good sound level meter equipped with octave or one-third
octave filters. Surely human variables would far exceed sound-level
meter fluctuations from day to day, and what the observer eats for
breakfast has no effect on the sound-level meter, although it might
affect the dependability of the readings made with physiological
equipment.

Meters vs. the Ear

There still remains a great chasm between subjective judgments of
sound quality, room acoustics, etc., and objective measurements. Con-
siderable attention is being focused on the problem. Consider the fol-
lowing descriptive words, which are often applied to concert-hall
acoustics 19,20 :
warmth clarity
bassiness brilliance
definition resonance
reverberance balance