# Sound

SOUND FUNDAMENTALS

Sound is the variations of air pressure over the audible range of human hearing. This frequency range is within 20 to 20k Hz

• A factor of 2 time a frequency is defined as an octave; where 1000 Hz is an octave above 500 Hz.
• A factor of 10 times a frequency is defined as a decade; where 10000 Hz is a decade above 1000 Hz.

The speed of sound propagation in air at normal temperature is approximately 1,130 feet per second (344 meters per second). At higher temperatures, the speed of sound increases slightly, while at lower temperatures, the speed is less. The precise values for the speed of sound in air are given by the following equations:

Speed at 0° Fahrenheit = 1,052 feet/second (+ 1.106 feet per degree above 0°F)

Speed at 0° Centigrade = 331.3 meters/second (+ 0.607m per degree above °0C)

The period of the wave is the length of time (in seconds) required for a single cycle, and it is equal to 1/frequency.

Considering a frequency of 1 kHz, the corresponding period will be 1/1000, or 0.001 seconds (1 millisecond).

The distance from the start of one cycle to the start of the next cycle will be 1,130 divided by 1,000, or 1. 13 feet (0.344 meters).

This quantity is known as the wavelength. Wavelength is often expressed as the Greek letter “” (lamba). The relationships among speed of frequency (f), and wavelength () are: c = f , f =c/ and = c/f.

Another quantity is the amplitude, of the alternating pressure of the propagating sine wave.

The static pressure in acoustics uses the International System (SI) of units and is measured in Pascals (Newtons per square meter).

A sine wave with a maximum amplitude of unity has an RMS (root-mean-square) value of 0.707. This RMS value gives us the effective steady-state value of the wave form.

The average value of 0.63 is simply the value of the signal averaged over one-half cycle. This value is used in making power calculations, and it is directly proportional to the measured value with a sound level meter.

Two sine waves of the same frequency may bed is placed from each other in time, creating a phase relationship between them. You may see one signal leads or lags the other by the phase angle (the Greek letter Phi), which we is stated in degrees.

With the sum displaced of the sine waves in the same period may generate a new sine wave with a different amplitude and phase angle.

Two sine waves of unity amplitude with a phase relationship of 90° will combine to produce a new sine wave with an amplitude of 1.4 and a relative phase angle of 45°.

Two sine waves of the same amplitude with a180° phase relationship will cancel completely if they are summed.