Page 84 - Master Handbook of Acoustics

noise extends outside the critical band, it will not be masked by the tone. Codecs thus attempt to
contain noise within critical-bandwidth masking curves created by loud tones. One critical band is
defined as having a width of 1 bark (named after German physicist Heinrich Barkhausen).

Loudness of Impulses

The examples discussed so far have been concerned with steady-state tones and noise. How does the
ear respond to transients of short duration? This is important because music and speech are
essentially made up of transients. To focus attention on this aspect of speech and music, play some
audio tracks backward. The initial transients now appear at the ends of syllables and musical notes

and stand out prominently.
As a 1-sec tone burst, a 1-kHz tone sounds like 1 kHz. But an extremely short burst of the same
tone sounds like a click. The duration of such a burst also influences the perceived loudness. Short
bursts do not sound as loud as longer ones. Figure 4-12 shows how much the level of shorter pulses

must be increased to have the same loudness as a long pulse or steady tone. For example, a 3-msec
pulse must have a level about 15 dB higher to sound as loud as a 0.5-sec (500-msec) pulse. Tones
and random noise follow roughly the same relationship in loudness versus pulse length.

FIGURE 4-12 Short pulses of tones or noise are less audible than longer pulses. The discontinuity of
the 100- to 200-msec region is related to the integrating time of the ear.

The region less than 100 msec in Fig. 4-12 is significant. 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