Wavelength and Speed of Sound

Another characteristic of a sound wave related to frequency is wavelength. The wavelength of a sound wave is the physical distance from the start of one cycle to the start of the next cycle, as the wave moves through the air. Since each cycle is the same, the distance from any point in one cycle to the same point in the next cycle is also one wavelength: for example, the distance from one maximum pressure point to the next maximum pressure point. Wavelength is related to frequency by the speed of sound. The speed of sound is the velocity at which a sound wave travels. The speed of sound is constant and is equal to about 1130 feet-per-second in air.

It does not change with frequency or wavelength, but it is related to them in the following way: the frequency of a sound, multiplied by its wavelength always equals the speed of sound. Thus, the higher the frequency of sound, the shorter the wavelength, and the lower the frequency, the longer the wavelength. The wavelength of sound is responsible for many acoustic effects.

(from Audio System Guide for Houses of Worship, Shure Educational Publication)

Amplitude

The amplitude of a sound wave refers to the magnitude (strength) of the pressure changes and determines the “loudness” of the sound. Amplitude is measured in decibels (dB) of sound pressure level (SPL) and ranges from 0 dB SPL (the threshold of hearing), to above 120 dB SPL (the threshold of pain). The level of conversational speech is about 70dB SPL. A change of 1 dB is about the smallest SPL difference that the human ear can detect, while 3 dB is a generally noticeable step, and an increase of 10 dB is perceived as a “doubling” of loudness

(from Audio System Guide for Houses of Worship, Shure Educational Publication)

Frequency of Sound

A simple sound wave can be described by its frequency and by its amplitude. The frequency of a sound wave is the rate at which the pressure changes occur. It is measured in Hertz (Hz), where 1 Hz is equal to 1 cycle-persecond. The range of frequencies audible to the human ear extends from a low of about 20 Hz to a high of about 20,000 Hz. In practice, a sound source such as a voice usually produces many frequencies simultaneously. In any such complex sound, the lowest frequency is called the fundamental and is responsible for the pitch of the sound. The higher frequencies are called harmonics and are responsible for the timbre or tone of the sound. Harmonics allow us to distinguish one source from another, such as a piano from a guitar, even when they are playing the same fundamental note.

(from Audio Systems Guide for Houses of Worship, Shure Educational Publication)

Sound Wave

Because good sound quality is the goal of any sound system, it is helpful to be familiar with some general aspects of sound: how it is produced, transmitted, and received. In addition, it is also useful to describe or classify sound according to its acoustic behavior. Finally, the characteristics of “good” sound should be understood.
Sound is produced by vibrating objects. These include musical instruments, loudspeakers, and, of course, human vocal cords. The mechanical vibrations of these objects move the air which is immediately adjacent to them, alternately “pushing” and “pulling” the air from its resting state. Each back-and-forth vibration produces a corresponding pressure increase (compression) and pressure decrease (rarefaction) in the air. A complete pressure change, or cycle, occurs when the air pressure goes from rest, to maximum, to minimum, and back to rest again. These cyclic pressure changes travel outward from the vibrating object, forming a pattern called a sound wave. A sound wave is a series of pressure changes (cycles) moving through the air.

(from Audio System Guide for House of Worship, Shure Educational Publication)