The sound masking effect shows up in places you might not expect. One of these is near and dear to many people – digital music files. Yes, those MP3s (and other formats) you download are partially the result of masking!
Analog music contains an astonishing amount of information in a continuous stream. Because it isn’t broken down into discrete bits of data, you can’t really give an approximate file size. The act of digitizing converts analog music into a quantity of bits of data. However, some of the analog information is immediately ‘lost.’ The digital file size depends substantially on the method used to convert the music. As a baseline reference, a CD stores music at just under 9 MB per minute in an uncompressed format, meaning a five minute song takes up 45 MB of storage, which is much more than most people would like.
As a result, a number of methods have been developed that allow music files to be compressed into much smaller formats. Some of these are ‘lossless,’ in that they don’t result in a further loss of data from the ‘original’ CD or digital recording. However, these lossless files are very large, with a full CD still taking up 200 to 300 MB of storage space, even with compressed file formats.
So, methods have been developed to further reduce file sizes. And one of the key methods of reduction is through psychoacoustic models that eliminate music information according to what the models say we can actually hear – in other words these file formats exclude music information that is masked.
There are several ways in which one sound in a piece of music may mask another. First, there’s simultaneous masking, where two sounds occur at roughly the same time and one ‘drowns out’ the other. For this to take place, you generally have to have two sounds of similar frequency where one is louder, but, interestingly, lower frequency sounds can also mask higher frequencies. Second, there’s temporal masking, where a sound that occurs very shortly after another is eliminated as being inaudible.
By eliminating the information in music that we cannot hear via one form of masking or another, file sizes are substantially reduced. It’s just one more way that you experience masking every day and perhaps weren’t aware of it – until now!