Dynamic Range Compression Pt 1 – What Is Dynamic Range? 9


Introduction

Dynamic-Range-Compression-VolumeTo master dynamic range compression we must first define what dynamic range in audio is, and secondly how we measure it.

We will see how better technology has allowed us greater dynamic range in recordings, but also question if there is any point. We will start at the fundamentals, later we will see how it relates to topics like the ‘loudness war’ and making great mixes.

What Is The Dynamic Range Of Human Hearing?

We have evolved to be able to hear a certain range of volume from very quiet to very loud. The dynamic range of our hearing is the difference between the quietest sounds we can hear (the threshold of hearing) and the sounds that are so loud that they cause pain (the threshold of pain).

How Do We Measure The Dynamic Range Of Human Hearing?

We measure it in decibels (dB). The threshold of human hearing is measured as 0dB SPL (sound pressure level) and the threshold of pain 120dB SPL. Therefore the dynamic range is 120dB.

The decibel (dB) is a logarithmic unit. I will skip the maths here, but basically in terms of power, 120dB is about 1,000,000,000,000 times more powerful than 0dB. That is a very wide range of volume for you to enjoy.

What Is The Possible Dynamic Range Of Recorded Music?

The dynamic range of a recording medium (such as a sound card or tape) is the difference between the loudest sound that can be recorded without distortion, and the quietest sound that can be recorded without disappearing into background noise. In the days of tape this background noise was the hiss of the tape, nowadays in digital it is the amount of bits we have available to record.

Here are some approximate dynamic ranges of different recording mediums:

Dynamic-Range-Compression-Phonograph

78 RPM Phonograph Discs

Dynamic-Range-Compression-Cassette

Compact Cassette

Dynamic-Range-Compression-Computer

Digital

= 40 dB quickly reduced to 30 dB
and worse due to wear
= 50 dB 16 Bit Audio            = 96 dB
24 Bit Audio            = 144 dB

What Is the Useful Dynamic Range Of Recorded Music?

Here are some approximate dynamic ranges of different styles of music performed live:

Symphony orchestra – 50dB
Chamber music – 30dB
Rock band – 20dB

If we are to make our recorded music the most enjoyable for the most listeners we have to take into account with what equipment and where they are likely to be playing back the music. Let us assume it will be in an average domestic environment with 40dBA background noise [1] (the A in dBA is a special type of dB measurement that takes into account the ear is more sensitive at mid frequencies).

We will also assume the max volume they are going to play the music is at 100dBA, that’s loud.

100dBa (max volume) – 40dBA (background noise) = Dynamic Range of 60dB

We are now down to the average Hi-Fi listener only being able to hear 60dB dynamic range, but its lower than that! Having the most quiet parts of the music at the threshold of their hearing is not going to be fun, its going to be a struggle to listen.

We can make the following generalisation for the scope of the article:

A useful dynamic range for recorded music for the average listener is 30dB

So we are right back at the usable dynamic range of worn 78 rpm vinyl 🙂

Why Is Dynamic Range A Good Thing In Recorded Music?

If a piece of music is the same loud volume all the way through, is it really loud at-all? If there are no quiet parts, then how can we define the loud parts? Often a chorus naturally wants to be louder than a verse to give it impact. Going from a whisper to a scream requires a large dynamic range. When we create and produce music we can use the dynamic range to aid musical expression. We can have gently plucked guitars and booming techno bass lines all in one song.

So What Exactly Is Dynamic Range Compression?

Now we understand exactly what dynamic range is, we can very easily answer the question what is dynamic range compression. It is simply making the difference between the quietest and the loudest parts less. This is done for many different reasons, including:

  • To make a recording sound louder by reducing its loudest sections, thus increasing the volume of the quiet sections (in mastering).
  • To make individual elements in a recording stay at a more constant volume, for instance compressing a dynamic vocal so the quiet parts are not lost under the rest of the mix and the loud parts don’t leap out (in mixing).

Compressing dynamic range  is different from Normalizing, to learn exactly what that is check out my article How To Normalize Audio – Why Do It? Everything You Need To Know.

Conclusion

The human ear is an amazingly delicate device, it can hear a huge range of volumes from the wind gently rustling in the trees to an exploding bomb.

With today’s recording and playback technology we have more dynamic range available than we can ever use. We have had 96dB of dynamic range available since the 80’s with CD. Earlier we demonstrated that the maximum range the average listener could hear was about 60dB, and would actually ever want was 30dB.

Very little recorded music today has 30dB of dynamic range. Many modern styles of music don’t require that much. Often dynamic range is reduced too much in mastering, this is the dark side of dynamic range compression and resulted in ‘the loudness war’.

We will soon talk about the art of dynamic range compression in mixing and mastering, compression, limiting, compressor settings like attack/release/threshold and more. For that, we must wait until part 2.

* [1] http://www.easa.europa.eu/rulemaking/docs/research/Background_noise_report.pdf

 

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9 thoughts on “Dynamic Range Compression Pt 1 – What Is Dynamic Range?

  • Keith Sanborn

    This is an amazingly clear discussion of dynamic range. One point: you might want to fill in why the effective dynamic range drops from 60db to 30db. I guess the missing point is: whether we like it or not: the quietest parts are just going to be inaudible. If we use that logic: then a 78rpm disk would be completely inaudible! 10 db below our effective threshhold of hearing. Or is there something like how far away from the speakers we might be? were homes quieter during the 78rpm era? And finally, another point you might want to make in Part II: a loud of people listen on in-ear headphones effectively increasing the dynamic range as background sounds are blocked—sometimes making them a hazard for walking city streets— at least by certain type of headphones.

  • g.r.i.p.

    This actually gave me some needed insight on understanding dynamic range. I recently started attending audio engineering school and have a desire to master this art. Thanks for the simple yet informative explanation.