- The speed of sound is 343 m/s. For practical purpose the speed of sound can be considered as 1200 ft/s. It will take about 1ms (millisecond) for sound to travel a distance of 1 foot. For 50 feet it takes 50ms.
- Air is an elastic medium. Sound is a vibration which modifies the atmospheric pressure levels. These vibrations propagate as waves. The waves are measured in cycles per second and the unit is “Hertz” (Hz).
- The range of frequencies a human can hear is called the Audio Spectrum. Human hearing range is commonly given as 20Hz to 20,000Hz.
- For a normal person the listening sensitivity varies with frequency, as shown by equal-loudness contours.This means, to perceive equal loudness as in higher frequencies, the sound level of the lower frequencies needs to be increased, especially at lower listening levels. The perceived loudness level approaches a flat response at higher listening levels. Our hearing is most sensitive in the 2Khz – 5Khz frequency range.
Above figure shows the “Equal loudness” contours. - Sound Pressure Level is measured in “Pascal” (Pa). The threshold of human hearing is 20 MicroPascal. Sound Pressure Level (SPL) is measured in “Decibel”. It is a logarithmic value of the ratio between threshold and measured values.
- The frequencies from 30Hz to 500Hz are bass and rhythm, 500Hz to 2Khz defines speech intelligibility, 2Khz to 8Khz gives presence to speech (where consonant sounds lie), 8Khz to 16Khz gives brilliance (bells and ringing of cymbals).
- Sound gets reflected, absorbed or diffracted depending upon the characteristics of the material in its path. Acoustics is the properties of a room or indoor venue that determines how sound is transmitted in it.
- The wavelength of a 50Hz sine wave is 22.5feet (6.86meters) for 5000Hz it is 2.7inches (0.23ft) and for 10,000Hz 1.35inches (0.11ft).
- High frequencies have a smaller wavelength than low frequencies hence they are very directional. Objects placed in the live path of HF block them, whereas LF tend to bend (diffract) around them. A subject positioned behind an obstruction will hear an attenuated HF, hence a dull sound.
- Reverberation is the persistence of sound after a sound is produced. Resonance is a physical phenomenon in which an external force or a vibrating system forces another system around it to vibrate with greater amplitude at a specified frequency of operation. Echo is a sound or sounds caused by the reflection of sound waves from a surface back to the listener.
- Inverse square law states that, in a free field the intensity of sound drops by 6 dB for each doubling of distance from the source
- The audio spectrum is divided into sets of frequency called “Bands”. When the upper frequency of a band is twice that of its lower frequency it is called an Octave Band. The frequencies of an Octave Band are 32Hz, 63Hz, 125Hz, 250Hz, 500Hz, 1Khz, 2Khz, 4Khz, 8Khz, and 16Khz
