Introduction to Acoustics

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Fig. 13.2 Comparison of a clinical audiogram for a 50 dB
hearing loss at all frequencies (bottom) and the absolute
threshold curve for the same hearing loss plotted in terms
of the MAP (top)
suspected; thresholds are specified relative to the average
threshold at each frequency for young, healthy listeners
13.2 Frequency Selectivity and Masking
Frequency selectivity refers to the ability to resolve or
separate the sinusoidal components in a complex sound.
It is a key aspect of the analysis of sounds by the auditory
system, and it influences many aspects of auditory perception,
including the perception of loudness, pitch and
timbre. It is often demonstrated and measured by studying
masking, which has been defined as ‘The process by
which the threshold of audibility for one sound is raised
Psychoacoustics 13.2 Frequency Selectivity and Masking 461
with normal hearing. In this case, the sound level is usually
specified relative to standardized values produced
by a specific earphone in a specific coupler. A coupler is
a device which contains a cavity or series of cavities and
a microphone for measuring the sound produced by the
earphone. The preferred earphone varies from one country
to another. For example, the Telephonics TDH49 or
TDH50 is often used in the UK and USA, while the
Beyer DT48 is used in Germany. Thresholds specified
in this way have units of dB HL (hearing level) in Europe
or dB HTL (hearing threshold level) in the USA.
For example, a threshold of 40 dB HL at 1 kHz would
mean that the person had a threshold which was 40 dB
higher than normal at that frequency. In psychoacoustic
work, thresholds are normally plotted with threshold
increasing upward, as in Fig. 13.1. However, in audiology,
threshold elevations are shown as hearing losses,
plotted downward. The average normal threshold is represented
as a horizontal line at the top of the plot, and
the degree of hearing loss is indicated by how much the
threshold falls below this line. This type of plot is called
an audiogram. Figure 13.2 compares an audiogram for
a hypothetical hearing-impaired person with a flat hearing
loss, with a plot of the same thresholds expressed
as MAP values. Notice that, although the audiogram is
flat, the corresponding MAP curve is not flat. Note also
that thresholds in dB HL can be negative. For example,
a threshold of −10 dB simply means that the individual
is 10 dB more sensitive than the average.
The absolute thresholds described above were measured
using tone bursts of relatively long duration. For
durations exceeding about 500 ms, the sound level at
threshold is roughly independent of duration. However,
for durations less than about 200 ms, the sound level necessary
for detection increases as duration decreases, by
about 3 dB per halving of the duration [13.9]. Thus, the
sound energy required for threshold is roughly constant.
by the presence of another (masking) sound’ [13.10]. It
has been known for many years that a signal is most
easily masked by a sound having frequency components
close to, or the same as, those of the signal [13.11]. This
led to the idea that our ability to separate the components
of a complex sound depends, at least in part, on
the frequency analysis that takes place on the basilar
membrane (see Chap. 12).
Part D 13.2