Page 148 - Master Handbook of Acoustics

the two types. Figure 6-12 shows the differences in audibility thresholds of continuous and
noncontinuous sounds. Anechoic speech is closer to a noncontinuous sound than either music or pink
noise. At delays of less than 10 msec, the level of impulses for threshold detection must be much
higher than continuous sounds. The threshold curves for classical music (Mozart) and pink noise are

very close together. This confirms the belief that pink noise is a reasonable surrogate for music for
measurements.

FIGURE 6-12 Absolute thresholds of perception of reflections of different types of signals, ranging
from 2 clicks/sec (noncontinuous) to pink noise (continuous). The closeness of pink noise to classical
music (Mozart) gives assurance that pink noise is a reasonable surrogate for music in measurements.

(Olive and Toole.)

Most researchers use sounds having the same spectrum for both the direct and reflected
simulations. In real life, reflections depart from the original spectrum because sound-absorbing
materials invariably absorb high frequencies more than low frequencies. In addition to this, off-axis
loudspeaker response lowers the high-frequency content even more. Threshold audibility experiments
show that radical low-pass filtering of the reflection signal produces only minor differences in
thresholds. The conclusion is that alteration of reflection spectrum does not appreciably change
audibility thresholds.