MP3 frequency comparison
Most of us at some time or other convert an audio recording to the more compact mp3 format for storage, or for play-back on mp3 players, or for sending audio files through web-space. Mp3 is very convenient for this but is what is known as a “lossy” format – which means the price you pay for a compressed file size is a loss of frequencies throughout the audio spectrum of the recording. You may not be aware of this frequency loss if you’re listening on low-end audio equipment, but it is very definitely there.
For my own interest I’ve done a little research into this effect. For my source I used a few seconds of a CD recording of the violinist Sophie Mutter playing part of Mozart’s A-major violin concerto. I chose this because the harmonic content of her playing on her Guarnerius violin on occasion extends beyond 20kHz (about the top limit for a quality CD), and her playing is accompanied by a full classical orchestra. I copied the extract onto my computer using standard audio properties - bit rate 1411kbps, audio sample rate 44kHz, audio sample size 16 bit, and 2-channel stereo. Using CoolEdit’s spectral frequency analyzer the frequency and harmonic content of the music was very obvious on screen.
I made several mp3 versions of the sample at different bit rates, using eRightSoft’s “SUPER ©” media converter, and compared the spectral frequencies of these mp3 files on screen using CoolEdit.
Here are some of the results (the column tabulation may not show as it should):
bit rate cut-off frequency compression
1411kbps >20kHz 1:1
320kbps 19.5kHz 1:4.4
192kbps 18kHz 1:7.3
160kbps 17kHz 1:8.8
128kbps 16kHz 1:11
96kbps 15kHz 1:14.7
64kbps 11kHz 1:22
32kbps 5kHz 1:44
I think you’d have to have an extremely good ear and absolutely top-of-the-range audio equipment to notice any significant difference between the source (1411kbps) and 320kbps – and possibly as far down as 160kbps (iTunes suggests 160 or 192 for most purposes). When you get down to 128kbps (the “near-CD” quality beloved of mp3 file purveyors on the internet) even my old ears can tell the clear difference between that and 192 and higher. 128 and 96Kbps may be just about bearable if you’re listening on low-end playback, but when you get down to 64-32kbps range you’re definitely in the voice (radio and telephony) recording range – but even that low range can have an application in recording tunes – of which more below.
When you do a mp3 conversion it’s not only the top end frequency that goes (although a top cut-off of 16 or 17kHz is not significant for most adults), but frequencies well down in the audio range are lost or attenuated, the effect of which is some level of distortion. This distortion, as I’ve said above, is apparent when you get down to 128kbps or below, but can be lived with at 192 and above, and certainly with 320kbps. I did a couple of further experiments to explore this distortion effect. I digitally subtracted the 320kbps file from the 1411kbps file (the original) to generate a difference file consisting of the lost or attenuated frequencies. These were at a very low dB level and in practical terms would be inaudible. This was certainly not so when I subtracted the 128 file from the 320 – the lost frequencies, although still at a relatively low dB level, were certainly audible on playback and are the cause of the audible distortion on 128kbps mp3.
I mentioned the 64-32kbps mp3 range. This can have an application for someone who records a tune and needs a little help in transcribing it from the recording. Convert the recording to 32kbps mp3 and you’ll have a top cut-off frequency of 5kHz (just above the top note on a piano). This means you lose a lot of the clutter of hiss and high harmonics; the tune notes therefore sound more distinct and, if you use the spectral frequency option in a sound editor, are very obvious on screen.
Summary: 192kbps and above gives best audio results; 128 is ok for web use and if you’re not too worried about a little bit of distortion, and 32kbps can have an unexpected use in transcribing tunes.