Decibels - Dynamic Range

By dynamic range we mean the interval, measured in dB (the dB varies depending upon the dynamics of the contexts we are dealing with), between the lowest value that an audio signal can have and its highest. In nature, sounds have a certain dynamic. A gust of wind has a small dynamic because its highest dB level isn't much greater than the level you'd have in absence of sound. The sound dynamic generated by a hurricane on the other-hand, is much wider. Moreover, in nature there is always some background noise which in a relatively noisy city environment is measured at around 30 dB_spl. So, sounds that have a dB_spl-level which is lower than 30 can be ignored, seeing that they would be covered by the background noise and therefore wouldn't really be perceived. In our general example we can say that the majority of sounds don't go beyond 100 dB_spl and therefore we'll use this value as our SOL reference value. However, it sometimes happens that for brief periods of time more intense sounds can be produced, let's say of no more than 120dB_spl (a value that corresponds approximately to the pain threshold of the human ear). On the left hand-side of the figure beneath, we can see the scale with the values we have fixed:

Dynamic Range

The dB difference between SOL and the background noise is called the signal to noise ratio (SNR- Signal to Noise Ratio) and gives a value for how much a sound is 'louder' than the background noise. The dB difference between the maximum value of the dynamic and the SOL is called Headroom. The dB sum of the Headroom and the SNR is the Dynamic Range (To get a clearer picture of these quantities take a look at the left-side of the previous figure). Once these values have been defined on a physical plane we can see their electronic equivalent (right side of the above figure). Firstly let's focus our attention on the noise. Any electronic equipment makes some noise (for example the thermal noise of electric components, or the natural whirring of a magnetic tape). This time, however, we have an electronic noise and therefore it is measured in dB_u and no longer in dB_spl, and we have a background noise level of -66 dB_u. Our SOL, seeing that we want to work with professional gear, shall be +4dB_u (equivalent of 100 dB_spl) whilst our Headroom shall be 20 dB_u, in order to keep things realistic.

If we make a few calculations we obtain an SNR level of 70dBu and therefore a dynamic of 90 dBu. With these values, we can be sure of correctly reproducing any sound between 30 dB_spl and 120 dB_spl, in other words, with a dynamic of 90 dB_spl. If we just think that tunes played at nightclubs get compressed to the point that they reach a maximum dynamic of 30 dB, we can easily imagine how with 100dB in our hands we could do some pretty awesome things.

A good example is recording an orchestra. In this case we'd be going from very low dB_spl values in the parts where only one instrument is playing, to extremely high levels when, for example, all the instruments are playing together in a triumphant crescendo. With 90dB_u at your disposal, you can record all these different sounds at all these varying intensities with the same high fidelity.

Another example is recording a voice in a song where the singer is required to go from whispering to shouting. Generally one would set up various microphones and the preamplifiers would be set on different SOL levels, each optimized for certain sound intensities. In the mixing phase one would combine the various recorded sections so as to make sure that the sound reproduction of the track is faithful in all its parts.

Now we can perhaps better understand the values in the preceding paragraph's table. Greater SOL values, and therefore higher voltage levels, are more distant from the background noise and thus allow greater dynamic.