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Digital Audio Essentials by Bruce Fries, Marty Fries

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Analyzing Sound

Occasionally, when you are troubleshooting audio—say, trying to determine the source of an unwelcome spike or hum—it helps to see a graphic representation of how sounds are distributed over the full range of frequencies.

A waveform view (discussed earlier) shows how the energy in a waveform is distributed over time, while a spectrum analysis shows how the energy is distributed over a range of frequencies. Severe spikes or dips, or any gaps in the response, may indicate problems. For example, a steep spike at 60 Hz might indicate that your recording picked up a 60-cycle hum from the recorder’s poorly grounded AC connection.

A spectrum analysis is strictly a diagnostic tool. Interpretation of the analysis is as much an art as a science. These analyses are often used in combination with test equipment to evaluate hardware, or with test files to evaluate different programs such as MP3 encoders. How you fix the problem depends on properly interpreting the data and may involve applying filters or re-recording the material with different settings.

Sound Forge includes a spectrum (frequency) analysis view, as illustrated in Figure 13-20. A spectrum analysis is useful for troubleshooting problems with digital audio. Because the audio in this example was originally sampled at 22.05 kHz and later converted to 44.1 kHz, there should not be any information above 11.025 kHz (see Chapter 8 for an explanation). The small clump above 10 kHz most likely represents noise introduced by ...

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