![]() The external ear includes the pinna, which helps capture sound in the environment. Below are some definitions of terms and measures used to describe sound. Measures of time are expressed in various temporal units or can be translated into phase measured in angular degrees. Frequency is measured in units of hertz (Hz), cycles per second. The common measure of sound level is the decibel (dB), in which the decibel is the logarithm of the ratio of two sound intensities or two sound pressures. Sound pressure is proportional to sound intensity (in units of power or energy), so sound magnitude can be measured in units of pressure, power, and energy. There are many aspects to the temporal variation of sound, such as sound duration. Frequency refers to the number of times per second that the vibratory pattern (in the time domain) oscillates. A noise is said to be “white noise” if it contains all frequency components each at the same average sound level.Ī sound waveform has three basic physical attributes: frequency, amplitude, and temporal variation. Noise contains all possible frequency components, and the amplitude of the noise varies randomly over time. A common complex sound used to study hearing is noise. However, everyday sounds are complex sounds, which are made up of many tonal frequency components. Largely because tonal (sinusoidal) sounds are the bases of the frequency domain description of sound, a great deal of the study of hearing has dealt with tonal sounds. ![]() ![]() Thus, one can describe sound as temporal fluctuations in pressure, or one can describe sounds in terms of the frequency components that compose the sound. Thus, the time and frequency domain descriptions of sound are two different ways of measuring the same thing (i.e., the time and frequency domains are functional equivalents). Using the same analytic tools, the frequency domain representation of a sound can also be calculated from the time-domain description. If one knows the tonal components of sound as defined in the frequency domain, one can calculate the time-domain description of the sound. A tonal sound has a time-domain description in which sound pressure changes as a regular (sinusoidal) function of time. In the frequency domain, the spectrum defines sound in terms of the tonal components that make up the sound. In other words, the time-domain description of a sound wave specifies how the sound pressure increases and decreases over time. In the time domain, sound is described as a sequence of pressure changes (oscillations) that occur over time. Sound waves can be mathematically described in two ways, that is, in two domains. Thus, a sound wave propagating outward from a vibrating object can reach the eardrum of a listener causing the eardrum to vibrate and initiate the process of hearing. The sound wave may also be reflected from the object or it may diffract around the object. When the pressure wave encounters another object, the vibration can be imparted to that object and the pressure wave will propagate in the medium of the object. A pressure wave is propagated outward from the vibrating source. ![]() Sound is derived from objects that vibrate producing pressure variations in a sound-transmitting medium, such as air. Hearing allows one to identify and recognize objects in the world based on the sound they produce, and hearing makes communication using sound possible.
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