Compression - The sound engineer’s view

Tuesday, February 26th, 2008

In our daily work as professional sound engineers at Hoff mastering, the number of audio files we receive from our clients that have already been ‘compressed’ ( ahem… inappropriately …if we may say so) is substantial.We realize that it is often difficult for non-professionals to realize that good pieces can sound real flat if it is not applied properly. This article presents a view of compression starting from a few basic principles, which reflects the way we view the answer to the question : how much compression you can apply yourself, and what you should leave for the boys in our lab ?

Compression, or reduction of the audio dynamic range, is essentially the art of making louder parts of a composition appear softer, and conversely, the softer parts appear louder. On one hand it maintains maximum loudness (would anyone buy an uncompressed piece by Wagner ?) and maintains higher average loudness, but what if we over-compress an average voice recoding with a dynamic range of only 10dB ? The voice sounds abnormally dull, and flat. Our experience over the years show that highly compressed styles (like the disco) soon become monotonous, and fade out of style. Human ears have their own inbuilt compressor systems inside our inner ears (cochleas), and anything that sounds ‘unphysiologically’ compressed appears unappealing. Good compressors have designer circuits to emulate the human ear – by varying the amount of compression according to the average signal level. Excellent compressors have a second layer of circuits : to clamp those peaks in loudness which go beyond a defined level of the average signal level. With human beings living longer, and the higher incidence of hearing impairment seen in all populations, compressing is becoming a fine art – too much and your CD is exceedingly dull, too little and it can get uncomfortable very fast. In the coming years, a progressively bigger market in music for the senior generations would demand very careful compression skills.

Compelling reasons to avoid ‘overall’ compression without professional help

Overall compression can be easy to apply to your mixes by the flick of a mouse button, but the results can often be unsatisfactory ! Consider this :

Professional mastering services have much more sophisticated compressors with defined parameters about attack, ratio, and release times exactly right for your type of music. Experience teaches a lot about the compromises, advantages and disadvantages of overall compression.

Each tune needs a slightly different approach to compression within the same album, unless for some reason you want all of them to sound uncannily ’similar’ !
Listeners playing CDs intuitively expect all commercial CD albums to play at roughly the same sound level on their music system. Professional mastering services strictly adhere to this ‘industry level’ of compression.

Professional mastering engineers apply mastering on a non-destructive basis – after preserving a ‘master copy’ as a reference that can be returned back to if you are not happy with EQ, compression or anything really. Digital software-based EQ or compression at home is however destructive - resulting in an irreversible loss of digital information with each successive step, called dithering, rounding or truncation. The end product might be quite different from what you expected.

Damage to your sound file by non-professional compression is most often irreversible. Loss of life, loss of transience are familiar to most mastering engineers as examples of "enthusiastic compression". Having said that, folks at Hoff Mastering have few ‘tricks of the trade’ to undo some of the damage.

Compression can be a tool to reach your intended market

The subtleties of compression that we advice our customers on, is based on years of experience on the music industry. For instance, in the case of many thrillers, as the movie progresses, so does the dynamic range of the background music ! Towards the climax, as you might have noticed in Hitchcock’s movies, the sound ranges from a barely audible whisper in one frame to a massive polyphonic crescendo in the next.

Coming to music CDs, good music that people buy to listen to over and over again is often skillfully composed- fast paced and loud, bold and dynamic. ‘Over compression’ can kill the flavor that wide dynamic range imparts – Mozart is still the champ when it comes to playing with dynamic ranges. As we have mentioned before, we believe that disco was killed primarily because of its ‘over compressed’ monotony. We would never let that happen to you !

On the other hand, for furniture music, music for relaxation and Gregorian chants the pattern of compression to achieve a subtle, flowing, rising and falling melody can be a different prospect all together. As experienced mastering engineers, we at aim to bring you the wealth of our experience on compression : by producing music intended to reach its desired audience.