Digital Audio

This section of the Audacity Tutorial provides an introduction to working with sound and an overview of digital audio.

Recording sound with Audacity means working with a sound system. The range of equipment you may be using could vary from a simple microphone that came packaged with your computer to an expansive studio setup with expensive microphones, mixers, compressors, and other recording gear.

No matter what system you're using, however, the fact remains that all audio systems are based upon the same concept: Taking sound waves and converting them to electric current, manipulate them as you want to, and then convert them back into sound waves.

Analog Recording

In analog recording, the varying sound pressures in sound waves were reproduced with varying voltages produced by a microphone. These varying voltages were reproduced on magnetic tape with a metal coating. For the most part, recordings were made on tape and then distributed on vinyl records. On records, the variations in the width of the groove correspond to the pressure variations in the sound waves. This type of equipment however suffered from noise and distortion. Because of the physical contact with the medium during playback, additional noise and distortion of the original signal was added every time a recording was copied.

Digital Recording

In digital recording the soundwave is converted into an electric signal by a microphone. the physical propterties of the sound wave are converted into digital information which can then be decoded for final reproduction. The representation of an audio signal is not longer related to the sound wave. In digital audio, the value of the audio signal is sampled which produces numbers representing the value of each sample.

Measuring Sound

The intensity of sound is measured in decibels (dB). It represents the relationship of sound pressure level to sound intensity. Twenty decipbels (20dB) is about the sound made by rustling leaves. A jet taking off at 200 feet away can be as loud as 125 dB. The normal "Threshold of Hearing" is 0 dB and represents the hearing of someone with an undamaged ear. All sound pressur levels are positive numbers that illustrate how much louder a sound is than the threshold of hearing.

Sampling

The intensity of the sound signal is measured at regular intervals which is called sampling. The more samples per second the greater the accuracy and quality of the recording. At each sampling point, the audio signal is assigned a specific intensity from a set range of values. CD quality sound is sampled at 44,100 samples per second or kHz. Digital Audio Tape (DAT) is capable of sampling at 32,000, 44,100, and 48,000 kHz. Other commonly used sampling rates are 22.05 kHz and 11.025 kHz.

Clipping

In Audacity, the levels in the digital audio signal are expressed in dB as measured by their relationship to 0 dB, the highest possible level. In digital audio, the signam can never exceed 0 dB. If it does, the peaks in the digital signal will be cut off or "clipped" at the 0 dB level. This clipping caused distortion and should be avoided. It's best to record in Audacity at levels of between -6 to -12 dB to be save. If you see your recording meter peaking and turning red, you should reduce your recording volume.

Bit-rates

Bit-rate refers to how many bits (digital 1s and 0s) are used each second to represent the sound signal. The bit-rate for digital audio is represented in thousands of bits per second (kbps) and is directly proportional to file size and sound quality. The higher bit-rate, the larger the file size and the better the sound quality. Lower bit-rates result in smaller files but poorer sound quality. The recommended bit-rate for recording in Audacity is 32 - bit.

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