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.
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
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
