ABX Testing: 128k AAC vs Lossless

Most people can not hear the difference between 128K AAC and lossless audio:

• The human ear can hear sounds, vibrations in the air, between 20 and 20,000 Hz with older people gradually losing the ability to hear higher frequencies in that range, such as 16kHz and above. Sounds are typically conceived of as waves in the air, propagating through it with the air vibrating as the wave travels though it.
• Sound at different frequencies is heard as different pitches, with higher frequencies heard as higher notes and lower frequencies heard in the bass part of the audio spectrum.
• Digitisation of music involves a bit depth and sampling rate. How many bits are used to encode the music when its sampled, and how many times a second it is sampled. Thus 16/44.1 is 16-bits sampled at 44.1kHz (cycles per second) or 44,100 times a second, and this is CD-level sound. Everything between sample points is lost.
• The bit rate says how many kilobits of data there are per second of audio. The higher the bit rate the larger the output file. If the bit rate is too low we can hear sounds in the music that are not in the original music. These are called artefacts and may affect cymbals and guitar sounds.
• The stream of audio data is sampled and compressed. Typical musical encoding starts at 128kbit/s with 320kbit/s being the highest rate. There are different MP3 encoders with different quality output and you can drop the bit rate down to 8kbit/s for rather muddy low bandwidth speech purposes.
• A constant bit rate (CBR) can be used throughout a piece of music. Alternatively a variable bit rate (VBR) can be used, with fewer bits used during silences and simple music passages and more bits being used for complex, multi-instrument parts of the music.
• A CD-level recording has a 1,411.2 kbit/s bit rate. A 128kbit/s encoding of this represents an approximate 11:1 compression ratio.
• FLAC - Free Lossless Audio Codec - with codec being coder/decoder, is a lossless form of compression. No incoming audio signals are discarded. It is one of several lossless audio codecs, with WavPack and WMA being others. It provides bit-perfect copies of CD-quality music (16/44.1) or even higher (24/192k - known as studio quality). FLAC can reduce audio sources to 50-60 per cent of their original size. FLAC is quite widely used now with websites offering FLAC files.

The general consensus is that, while a low-quality MP3 (128Kb/s) might be discernible from a lossless file (~1,411Kb/s) file, higher quality MP3s (320Kb/s) rarely -- if ever -- are. Of course, this can differ depending on the type of music (classical music is often easier to discern), how familiar you are with the music and how nice your audio equipment is. You'll need some high end audio equipment if you even have a hope of hearing the difference between the two.

Try it yourself:

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