Page 85 - The Master Handbook Of Acoustics

60 CHAPTER THREE

burst also influences the perceived loudness. Short bursts do not
sound as loud as longer ones. Figure 3-12 shows how much the level
of the shorter pulses has to be increased to have the same loudness as
a long pulse or steady tone. A pulse 3 milliseconds long must have a
level about 15 dB higher to sound as loud as a 0.5-second (500 mil-
lisecond) pulse. Tones and random noise follow roughly the same rela-
tionship in loudness vs. pulse length.
The 100-msec region is significant in Fig. 3-12. Only when the
tones or noise bursts are shorter than this amount must the sound-
pressure level be increased to produce a loudness equal to that of long
pulses or steady tones or noise. This 100 msec appears to be the inte-
grating time or the time constant of the human ear.
In reality, Fig. 3-12 tells us that our ears are less sensitive to short
transients. This has a direct bearing on understanding speech. The con-
sonants of speech determine the meaning of many words. For instance,
the only difference between bat, bad, back, bass, ban, and bath are the
consonants at the end. The words led, red, shed, bed, fed, and wed have
the all-important consonants at the beginning. No matter where they

Sound-pressure level increase to maintain same loudness, dB 20 Tones Random

noise
15

10

5
0
1 10 100 1000
Pulse duration - milliseconds
FIGURE 3-12
Short pulses of tones or noise are less audible than longer pulses as these graphs indi-
cate. The discontinuity of the 100- to 200-msec region is related to the integrating time
of the ear.