Digital Sound & Music: Concepts, Applications, & Science, Chapter 4, last updated 6/25/2013
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Diffraction is the bending of a sound wave as it moves past an obstacle or through a
narrow opening. The phenomenon of diffraction allows us to hear sounds from sources that are
not in direct line-of-sight, such as a person standing around a corner or on the other side of a
partially obstructing object. The amount of diffraction is dependent on the relationship between
the size of the obstacle and the size of the sound’s wavelength. Low frequency sounds (i.e.,
long-wavelength sounds) are diffracted more than high frequencies (i.e., short wavelengths)
around the same obstacle. In other words, low
frequency sounds are better able to travel around
obstacles. In fact, if the wavelength of a sound is
significantly larger than an obstacle that the sound
encounters, the sound wave continues as if the
obstacle isn’t even there. For example, your
stereo speaker drivers are probably protected
behind a plastic or metal grill, yet the sound
passes through it intact and without noticeable
coloration. The obstacle presented by the wire
mesh of the grill (perhaps a millimeter or two in
diameter) is even smaller than the smallest
wavelength we can hear (about 2 centimeters for
20 kHz, 10 to 20 times larger than the wire), so
the sound diffracts easily around it.
Refraction is the bending of a sound wave
as it moves through different media. Typically we
think of refraction with light waves, as when we look at something through glass or that is
underwater. In acoustics, the refraction of sound waves tends to be more gradual, as the
properties of the air change subtly over longer distances. This causes a bending in sound waves
over a long distance, primarily due to temperature, humidity, and in some cases wind gradients
over distance and altitude. This bending can result in noticeable differences in sound levels,
either as a boost or an attenuation, also referred to as a shadow zone.
4.1.7.2 Reverberation, Echo, Diffusion, and Resonance
Reverberation is the result of sound waves reflecting off of many objects or surfaces in the
environment. Imagine an indoor room in which you make a sudden burst of sound. Some of that
sound is transmitted through or absorbed by the walls or objects, and the rest is reflected back,
bouncing off the walls, ceilings, and other surfaces in the room. The sound wave that travels
straight from the sound source to your ears is called the direct signal. The first few instances of
reflected sound are called primary or early reflections. Early reflections arrive at your ears
about 60 ms or sooner after the direct sound, and play a large part in imparting a sense of space
and room size to the human ear. Early reflections may be followed by a handful of secondary
and higher-order reflections. At this point, the sound waves have had plenty of opportunity to
bounce off of multiple surfaces, multiple times. As a result, the reflections that are arriving now
are more numerous, closer together in time, and quieter. Much of the initial energy initial energy
of the reflections has been absorbed by surfaces or expended in the distance traveled through the
air. This dense collection of reflections is reverberation, illustrated in Figure 4.17. Assuming
that the sound source is only momentary, the generated sound eventually decays as the waves
lose energy, the reverberation becoming less and less loud until the sound is no longer
Aside:
Diffraction also has a lot to do with
microphone and loudspeaker directivity.
Consider how microphones often have
different polar patterns at different
frequencies. Even with a directional
mic, you’ll often see lower frequencies
behave more omnidirectionally, and
sometimes an omnidirectional mic may
be more directional at high frequencies.
That’s largely because of the size of the
wavelength compared to size of the
microphone diaphragm. It’s hard for
high frequencies to diffract around a
larger object, so for a mic to have a
truly omnidirectional pattern, the
diaphragm has to be very small.
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