Audio Principles 13frequencies does sound become directional enough for the head to shade the distant ear,
causing what is called interaural intensity difference.
Phase differences are only useful at low frequencies and shading only works at high
frequencies. Fortunately, real-world noises and sounds are broadband and often contain
transients. Timbral, broadband, and transient sounds differ from tones in that they contain
many different frequencies. Pure tones are rare in nature.
A transient has a unique aperiodic waveform, which, as Figure 1.7(c) shows, suffers no
ambiguity in the assessment of interaural delay (IAD) between two versions. Note that
a one-degree change in sound location causes an IAD of around 10 μ s. The smallest
detectable IAD is a remarkable 6 μ s. This should be the criterion for spatial reproduction
accuracy.
Transient noises produce a one-off pressure step whose source is accurately and
instinctively located. Figure 1.8 shows an idealized transient pressure waveform
following an acoustic event. Only the initial transient pressure change is required for
location. The time of arrival of the transient at the two ears will be different and will
locate the source laterally within a processing delay of around a millisecond.
Following the event that generated the transient, the air pressure equalizes. The time
taken for this equalization varies and allows the listener to establish the likely size of
Figure 1.8 : A real acoustic event produces a pressure step. The initial step is used for
spatial location; equalization time signifi es the size of the source. (Courtesy of
Manger Schallwandlerbau.)