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Understanding sound > Frequency (Pitch) - Pg. 349

chapter 15 Sound for production There are four basic properties of sound that are essential to understanding audio and the techniques of microphone placement and recording for film production: Pitch (frequency) Loudness (amplitude) Quality (timbre) Velocity (speed) We plot these sound wave characteristics on the graph shown in Figure 15-3. This common sine wave graph measures the compression of the air molecules that are caused by a particular sound. With this graph we are able to see certain properties of a particular sound. 331 1 2 3 a Frequency (pitch) Sound waves travel in fairly consistent wave cycles. One wavelength is the length of one cycle, from peak to peak, which then repeats itself. A wavelength is plotted from one highest pressure point to the very next highest pressure point. The number of these waves that pass a fixed point over the course of one second is the mea- sure of the frequency of the sound wave. This measure of cycles per second is referred to as Hertz (Hz) and is measured along the graph's x-axis. b c A sound that generates 10,000 wave cycles every second has a frequency of 10,000 Hz, also written 10 kHz. This frequency of cycles per second is actually measuring the pitch of that particular sound. The fewer cycles per second, the lower the pitch of a sound; the more cycles per second, the higher the pitch (Figure 15-4). Figure 15-3 A simple sound (a) can be understood in terms of its wavelength (1), and its amplitude (2), or the degree to which it deviates from normal air pressure (3). The higher the number of cycles per second (b), also called Hertz, the higher the frequency or the sound. Sounds with very low frequencies have fewer cycles per second (c). Neither the human ear nor a microphone can perceive all sound frequencies. The range of detectable pitches for a given apparatus is called the frequency range. An average, healthy human ear can distinguish pitches from 25 Hz to 20 kHz. Dogs can hear frequencies beyond 20 kHz; this is why they can hear high-pitched dog whistles that humans cannot. The fre- quency range that a microphone or a sound recorder can pick up and duplicate is a common measure of equipment quality. The "hearing" range for a particular piece of gear is called its frequency response. Those old cassette decks (good ones!) had a frequency response around 30 Hz to 12 kHz, which is less than the range of human hearing. A typical professional digital audio recorder has a frequency response of 20 Hz to 40 kHz, which is greater than the range of human hearing--just one reason why the cassette tape disappeared. 5200 Hz 150 Hz - 2000 Hz 45 Hz Figure 15-4 A piccolo (the highest pitched instrument in an orchestra, left) can reach a frequency of 5,200 Hz. A tuba (the lowest pitched instrument, right), can create sounds as low as 45 Hz. The human voice is located in the frequency midrange, from 150 Hz to 2,000 Hz.