Handbook for Sound Engineers

28 Chapter 2

At the higher radio frequencies (VHF and UHF), the
wavelengths become very short—1 meter or less.
Antennas to receive such waves must be of comparable
physical size, usually one-quarter to one-half wave-
length. When waves become too short for practical
antennae, concave dishes can be used to collect the
waves. It should be pointed out that the highest
frequency that humans can hear (about 20 kHz) is a
very low frequency when considering the entire electro-
magnetic spectrum.
An acoustic wave is one that is propagating by
means of vibrating a medium such as steel, water, or air.
The propagation speeds through these media are rela-
tively slow, resulting in waves that are long in length
compared to an electromagnetic wave of the same
frequency. The wavelengths of audio frequencies in air
range from about 17 m (20 Hz) to 17 mm (20 kHz). The

wavelength of 1 kHz in air is about 0.334 m (about

1.13 ft).

When physically short acoustic waves are radiated

into large rooms, there can be adverse effects from

reflections. Acoustic reflections occur when a wave

encounters a change in acoustic impedance, usually

from a rigid surface, the edge of a surface or some other

obstruction. The reflection angle equals the incidence

angle in the ideal case. Architectural acoustics is the

study of the behavior of sound waves in enclosed

spaces. Acousticians specialize in creating spaces with

reflected sound fields that enhance rather than detract

from the listening experience.

When sound encounters a room surface, a complex

interaction takes place. If the surface is much larger

than the wavelength, a reflection occurs and an acoustic

shadow is formed behind the boundary.

Figure 2-8. The audible spectrum divided into octaves (a 2 to 1 ratio) and one-third octaves. Courtesy Syn-Aud-Con.

Octave Band

Center Frequency

Band Limits One-third

Octave Centers Band Limits

Voice

range

Articulation

center