Decibels - Standard Operating Level

We have spoken about the audio chain and the different stages a sound signal goes through to be constantly transformed into what we need. Every stage receives an electric signal entry, it manipulates it and returns as output the result of the manipulation that has taken place. The result that ensues from this stage will seemingly proceed to the input of the next stage which itself shall operate a further transformation, and so on. To get a better picture let's imagine that the last stage is a power amplifier [Amplification ] connected to a loudspeaker system.

What happens if the output from one stage has a much higher voltage than the amount the next stage is capable of handling? The answer is not at all immediate and includes many factors, but generally speaking we can say that the second stage shall have to manipulate a signal which is too high and for which it hasn't been calibrated. This results in distorsion [Distortion by saturation ] whose intensity shall be greater the more the signal goes beyond the amounts that the second stage is expecting to receive. But what exactly is the second stage expecting to receive? The answer is found in SOL (Standard Operating Level). For example we can say that the SOL of a certain module is 1 Volt, meaning that the audio signal passing through this stage shall have maximum values of around 1 Volt (to be more precise SOL is measured in dB) or slightly more.

In the following table we have the SOL values and their corresponding voltages in various operating contexts:

If we observe this table we can make some interesting considerations. Firstly we can see how every context has its own dB reference value (dBu in a professional field, semi-pro and broadcast, dBV in the semi-professional field). But what is truly interesting is the voltage column on the right. We can see how in the professional field the voltage level relating to the SOL, is 1.2 V , whilst in the Hi-Fi field it is 0.25 V. The first signal has an amplitude which is about 5 times greater than the second and thus allows a far more faithful sound reproduction to take place (to convince you of this fact, try and imagine that you are working with a SOL level of 0.000001V. Would you still be able to appreciate the difference between two voltages such as: 0.0000015 V and 0.0000016V?). So, the higher the SOL (the higher the voltage level at which it operates) the more accurate the reproduction of its wave form. Obviously quality demands cash; circuits that work with higher voltage levels are more expensive, and this truth is applicable in general and shall be reiterated many times within this course.