Sound Wave

In today’s technologically sophisticated world, sound systems of various types are a part of almost everyone’s daily life. We must to be familiar with some general aspects of sound helpful Good sound quality is the goal of any sound system: 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 a physical medium-usually 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 acoustical pressure wave consists of one half-cycle of compression (higher pressure) of the molecules, followed by one half-cycle of rarefaction (lower pressure) of the air molecules. These cyclic pressure changes travel outward from the vibrating object, forming a pattern called a sound wave. A sound wave is a acoustical energy consists of fluctuating waves of pressure changes (cycles) moving through the air.

A simple sound wave can be described by its frequency and by its amplitude. The rate of air pressure fluctuation is called the frequency of the wave. The unit Hertz (Hz) is now used to indicate frequency in cycles per second (cps):

1 Hz = 1 cps

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 musical attribute of pitch. The higher frequencies are called harmonics and are responsible for the timbre or tone of the music. 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.

The amount of time required for one complete cycle of a sound wave is called the period of the wave. A wave’s period is expressed in seconds per cycle, and is found by using the equation:

where: P = Period
f=frequency

The wavelength of a sound wave is the physical distance covered from any point in one cycle to the same point in the next cycle of a given frequency sound as it passes though air. Wavelength is expressed by the equation:

where: λ = wavelength
c = speed of sound
f = frequency
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 ft/sec (344m/sec) in air at sea level on a standard temperature day (which is 59o Farenheit or 15o Celcius).

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.

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

Proportion of Direct Sound vs. Ambient Sound

In contrast, ambient sound, such as reverberation, has a relatively constant level. Therefore, at a given distance from a sound source, a listener (or a microphone) will pick up a certain proportion of direct sound vs. ambient sound. As the distance increases, the direct sound level decreases while the ambient sound level stays the same. A properly designed sound system should increase the amount of direct sound reaching the listener without increasing the ambient sound significantly.

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

Increase and decrease of sound level

A very important property of direct sound is that it becomes weaker as it travels away from the sound source, at a rate governed by the inverse-square law. For example, when the distance increases by a factor of two (doubles), the sound level decreases by a factor of four (the square of two). This results in a drop of 6 dB in sound pressure level (SPL), a substantial decrease. Likewise, when the distance to the direct sound source is divided by two (cut in half), the sound level increases by 6 dB.

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

Standing Wave

One additional form of indirect sound is known as a standing wave. This may occur when the wavelength of a sound is the same distance as some major dimension of a room, such as the distance between two opposite walls. If both surfaces are acoustically reflective, the frequency corresponding to that wavelength will be amplified, by addition of the incoming and outgoing waves, resulting in a strong, stationary wave pattern between the two surfaces. This happens primarily with low frequencies, which have long wavelengths and are not easily absorbed.

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

Echo dan Reverberation

Echo occurs when an indirect sound is delayed long enough (by a distant reflecting surface) to be heard by the listener as a distinct repetition of the direct sound. If indirect sound is reflected many times from different surfaces it becomes “diffuse” or non-directional. This is called reverberation, and it is responsible for our auditory perception of the size of a room. Reverberant sound is a major component of ambient sound, which may include other non-directional sounds, such as wind noise or building vibrations. A certain amount of reverberant sound is desirable to add a sense of “space” to the sound, but an excess tends to make the sound muddy and unintelligible.

Direct Sound and Indirect Sound

Sound can be classified by its acoustic behavior; for example, direct sound vs. indirect sound. Direct sound travels from the sound source to the listener in a straight line (the shortest path). Indirect sound is reflected by one or more surfaces before reaching the listener (a longer path). Since sound travels at a constant speed, it takes a longer time for the indirect sound to arrive, and it is said to be “delayed” relative to the direct sound. There are several kinds of indirect sound, depending on the “acoustic space” (room acoustics).

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

Sound Picked Up

Once a sound has been produced and transmitted, it is received by the ear and, of course, by microphones. In the ear, the arriving pressure changes “push” and “pull” on the eardrum. The resulting motion of the eardrum is converted (by the inner ear) to nerve signals that are ultimately perceived as “sound”. In a microphone, the pressure changes act on a diaphragm. The resulting diaphragm motion is converted (by one of several mechanisms) into electrical signals which are sent to the sound system. For both “receivers”, the sound picked up is a combination of all pressure changes occurring just at the surface of the eardrum or diaphragm.

Absorbed and Reflected

After it is produced, sound is transmitted through a “medium”. Air is the typical medium, but sound can also be transmitted through solid or liquid materials. Generally, a sound wave will move in a straight line unless it is absorbed or reflected by physical surfaces or objects in its path. However, the transmission of the sound wave will be affected only if the size of the surface or object is large compared to the wavelength of the sound. If the surface is small (compared to the wavelength) the sound will proceed as if the object were not there. High frequencies (short wavelengths) can be reflected or absorbed by small surfaces, but low frequencies (long wavelengths) can be reflected or absorbed only by very large surfaces or objects. For this reason it is easier to control high frequencies by acoustic means, while low frequency control requires massive (and expensive) techniques.

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

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)