Bit Rates
There’s an important piece of terminology related to audio encoding that we need to mention next: bit rates.
The bit rate of an audio file is the number of bits that are processed per unit of time, usually expressed as bits per second (bit/s) or kilobits per second (kbit/s). Higher bit rates produce larger files. Some encodings such as AAC or MP3 let you specify the bit rate to use when compressing the audio file. When you lower the bit rate, you lose quality as well. Unlike other computer-related units, 1 kbit/s is actually 1000 bit/s, not 1024 bit/s.
You should choose a bit rate based on your particular sound file – try it out at different bit rates and see where the best match between file size and quality is. If your file is mostly speech, you can probably get away with a lower bit rate.
Here’s a table that gives an overview of the most common bit rates:
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32kbit/s: AM Radio quality
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48kbit/s: Common rate for long speech podcasts
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64kbit/s: Common rate for normal-length speech podcasts
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96kbit/s: FM Radio quality
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128kbit/s: Most common bit rate for MP3 music
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160kbit/s: Musicians or sensitive listeners prefer this to 128kbit/s
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192kbit/s: Digital radio broadcasting quality
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320kbit/s: Virtually indistinguishable from CDs
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500kbit/s-1,411kbit/s: Lossless audio encoding such as linear PCM
Sample Rates
There’s one final piece of terminology to cover before we move on: sample rates.
When converting an analog signal to digital format, the sample rate is how often the sound wave is sampled to make a digital signal.
Almost always, 44,100Hz is used because that is the same rate for CD audio.
What’s Next?
Next up in the Audio Tutorial series I talk about converting audio files and recording audio files on the Mac.
