Film sound

Sound Physics and Terminology

Sound is the full partner of image in a movie. A good filmmaker gives a full measure of attention to how sound will be captured during the shoot.

Sound is a rapid change in air pressure caused by something vibrating. Sound travels through air much like the waves in the water of a pond caused by a tossed rock. The relative force of change in the sound waves is perceived as the loudness of the sound. Sound loudness is also referred to as the volume of the sound.

There is a certain maximum and minimum sound loudness that any device can record and this range is called the device's dynamic range. Dynamic range is measured in decibles. A decible is the smallest change in sound volume that can be detected by the human ear.

The best quality analog recorders have a maximum range of about 70 decibles. Digital recorders can record a range on the order of 100 decibles. There is always some inherent noise in any recording device.

The rate at which the sound wave arrive at the listener is the frequency of the sound. The frequency is measured in hertz (Hz). The lowest sound a human can hear is about 20Hz--a very low rumble. The highest frequency is about 20,000Hz although this diminishes as people age and few adults can hear above 16,000Hz. As iPods become more popular this loss in hearing will become more pronounced and at earlier ages.

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When the sound waves are cleanly formed they have a nice tone quality. If the wave become damaged, such as from being recorded too loudly, then we hear the damage as distortion. Sound waves are almost never purely one sound frequency.

Musical instruments introduce a number of quieter frequencies that are usually some multiple of the main sound frequency. These are called harmonics and together with the principle sound frequency they make up the tone quality of whatever is creating the sound.

Any audio recording device has some limits to the range of frequencies it can record accurately. This is know as the frequency response of the device.

With the arrival of digital recorders some additional terminology has entered the field. Digital recorders record the sound as a series of numbers representing the sound pressure of the sound wave at that moment. Each number is referred to as a sound sample. The sound sampling rate of most professional recorders is 48K samples per second. More advanced systems record at 96K samples per second.

The accuracy of the number recorded at each sound sample also determines the accuracy of the sound. All professional recording systems use at least 16 bits for a number range of about -32,000 to +32,000. Better systems use 24 bits for the numbers giving an accuracy of about -16,000,000 to +16,000,000.

A recorder capturing 24 bit sound samples at a rate of 96K is operating with greater accuracy than the human ear.

Recording Devices

The classic sound recording device for movies is the Nagra 4.2 analog tape recorder. These durable, portable machines has been largly replaced by various digital recorders. The latest digital video cameras can capture the location sound in sync with the picture and represent a better solution for the low budget filmmaker than using a separate sound recorder.