Sample Rates
See also Sample Format - Bit Depth for help with choosing the appropriate bit depth.
Contents
Bandwidth
Sample rate is the number of samples of audio carried per second, measured in Hz or kHz (one kHz being 1,000 Hz). For example, 44100 samples per second can be expressed as either 44,100 Hz, or 44.1 kHz.
Bandwidth is the difference between the highest and lowest frequencies carried in an audio stream. The sample rate determines the maximum audio frequency that can be reproduced. Theoretically the maximum frequency that can be represented is half the sample rate (known as the Nyquist frequency). In practice, the limit is a little lower, so the practical upper frequency limit for a sample rate of 44,100 Hz, is a little over 20,000 Hz, but less than 22,050 Hz.
The term bandwidth may be applied to the frequency content of an audio signal stream, or the frequency ability of audio hardware or software. Although an audio interface may support a very high sample rate, for example 192,000 Hz (192 kHz), which could practically support frequencies above 80 kHz, there is no guarantee that the audio interface does support such high frequencies. It is not uncommon for high quality audio interfaces to deliberately filter out extreme high frequencies so as to improve rejection of electromagnetic interference.
Audacity defaults
44,100 Hz is the Audacity default setting. It is highly recommended that you use this setting unless you have good reasons to deviate from it.
Which Sample Rate to use
44,100 Hz
44,100 Hz (44.1 kHz) is the sampling rate of audio CDs giving a 20 kHz maximum frequency. 20 kHz is the highest frequency generally audible by humans, so making 44,100 Hz the logical choice for most audio material. High quality tape decks using metal tape, and medium quality LP equipment can reproduce 20 kHz (higher for top quality LP equipment, though some of this is harmonic distortion inherent in the medium). Note that the upper limit of human hearing falls rapidly with age. While people in their teens can hear 20,000 Hz, many older people cannot hear above 14,500 Hz.
48,000 Hz
48,000 Hz (48 kHz) is the sample rate used for DVDs so if you are creating DVD audio discs from your Audacity projects you may prefer to work with this setting.
Reduced bandwidth recording
Audio may be recorded at below 20kHz bandwidth for a few reasons:
- To reduce file size
- To reduce CPU usage
- Because the source material itself is of less than 20kHz bandwidth.
A lower sampling rate can also be used to remove the highest frequency hiss present in a noisy signal. While in theory there is no loss of quality as long as the bandwidth of the sample rate stays above the audio signal bandwidth, in practice one often does not know exactly what the signal bandwidth is. So for most purposes, a better option is to use noise gating for hiss reduction, which has much more effect and is less likely to compromise the recorded signal. The 'Noise Reduction' filter is a multi-channel noise gate.
32,000 Hz
The Nyquist frequency of 16,000 Hz is above the frequency limit of many medium quality sources, such as ferric cassette tape. On good tape decks, chrome tape can reproduce 18,000 Hz.
FM radio bandwidth is around 15 kHz, so FM radio could be recorded with little quality loss.
32,000 Hz sample rate is thus adequate for:
- cassette recordings from ferric stock
- Speech
- All other audio where smaller files than 44.1 kHz are desired with only slight compromise on sound quality.
22050Hz
22050 kHz (often lazily called "22 kHz") has been a reasonably popular sample rate for low bit rate MP3s such as 64 kbps in years past. Audio quality is significantly affected, with higher frequency content missing. With the general rise in the availability of large file storage space and faster data links, 22 kHz is now of more limited use.
- For speech recording where perceived quality is unimportant, but clarity must be maintained.
- AM radio
- Formerly used to squeeze much mp3 music onto floppy disk & very small mp3 players.
11,025 Hz
11,025 kHz (often lazily called "11 kHz") gives very poor sound quality.
8,000 Hz
8,000 Hz sample rate has less bandwidth than modern telephone systems. 8 kHz sample rate is thus not suitable for any serious recording task, though may be adequate for some sound effects or recording infrasound.
Ultrasonic Recording
Recording and playing with a bandwidth higher than 20kHz gives no audible benefit (assuming replay is not speed altered). It can be used:
- to maintain full bandwidth when the recording will be slowed down afterwards
- for analysis of signals above 20,000 Hz
- for some oscilloscope waveform recording
- to record sig gen waveforms up to 40,000 Hz
- for recording some wildlife
- and other ultrasonic applications.
When recording ultrasonic input it should be borne in mind that not all audio sources can output signals above 20 kHz. To capture ultrasound, all items in the chain of audio must support the full frequency range to be recorded.
Some audio interfaces and sound cards support sampling at up to 96,000 Hz, some do not.
Custom Rates
When recording a signal of known limited bandwidth, you can minimize file size by using a recording sample rate of 2.2x signal bandwidth. This is seldom necessary.
