The sample rate is the number of frames a second. In a raw file it changes nothing in the bytes: the waveform has the same shape whatever rate is chosen. It sets how long the sound lasts and how high it is. The explorer’s Sample rate option takes any whole number from 100 to 100,000,000 Hz, with a list of the usual rates to pick from.
Common rates
| Rate | Found in |
|---|---|
| 8,000 Hz | telephony (G.711 µ-law and A-law, Asterisk .sln), narrowband speech |
| 11,025 Hz | a quarter of CD; early PC and Mac sound and the lowest rate of many WAVE files |
| 12,000 Hz | some speech and low-rate music codecs |
| 16,000 Hz | wideband speech, speech recognition |
| 22,050 Hz | half of CD; early PC audio |
| 24,000 Hz | half of 48 kHz; speech and low-bitrate music codecs |
| 32,000 Hz | some broadcast and digital tape modes |
| 44,100 Hz | compact disc, most music, FFmpeg’s default for raw input |
| 48,000 Hz | film, video and professional audio, DAT; most sound cards |
| 88,200, 96,000 Hz | high-resolution music (2× 44.1 and 48 kHz) |
| 176,400, 192,000 Hz | high-resolution music (4× 44.1 and 48 kHz) |
| 2,822,400 Hz and up | DSD, one bit a sample (DSD and PDM) |
| 250 kHz to millions | software-defined radio I/Q recordings, for example 2,048,000 or 2,400,000 Hz (Where raw audio comes from) |
What a wrong rate does
A rate that is too high plays the sound too fast, too short and too high; too low, the reverse. Rates in whole ratios are whole octaves, so a mistake between related rates is easy to hear as the right tune in the wrong key:
| True rate read as | Speed | Pitch |
|---|---|---|
| 44,100 Hz read as 48,000 Hz | ×1.0884 faster | 1.47 semitones higher |
| 48,000 Hz read as 44,100 Hz | ×0.9188 | 1.47 semitones lower |
| 48,000 Hz read as 32,000 Hz | ×0.667 (slower) | 7.02 semitones lower |
| 22,050 Hz read as 44,100 Hz | twice as fast | one octave (12 semitones) higher |
| 8,000 Hz read as 16,000 Hz | twice as fast | one octave higher |
| 11,025 Hz read as 44,100 Hz | four times as fast | two octaves (24 semitones) higher |
| 8,000 Hz read as 44,100 Hz | ×5.51 | 29.55 semitones higher |
The explorer’s Playback speed option plays at 0.25, 0.5, 0.75, 1.25, 1.5, 2 or 4 times the rate without changing the file or the displayed duration, so you can hear whether a different rate would sound right. Web Audio plays between 8 and 96 kHz; the explorer repeats samples below that range and averages them above it.
Finding the rate
- From the length. If you know how long the recording is: rate = bytes ÷ (seconds × bytes a frame). A three-minute CD-quality stereo file is 44,100 × 4 × 180 = 31,752,000 bytes.
- From the pitch. A known tone or voice that sounds too high or low gives the ratio, and the table above says which usual rate it points to.
- From the name. The explorer reads
44100,16k,44k1,48000hzand radio forms such as2400kor2.4mspsin a file’s name. - From the spectrogram. Speech has most of its energy below about 4 kHz and music with a bandwidth that stops sharply at a frequency often means the rate is twice that (a file resampled from 22,050 Hz to 44,100 Hz shows nothing above 11,025 Hz).
- From the source. The format table for the program or device that made the file; see Where raw audio comes from.