Sound Levels and Recording (Part.1)

It’s a real pleasure for me to start a series of posts entitled « How to measure sound levels ». The first part will be dedicated to the concept of sound and dynamic and which tools are needed for measurements.

SOUND LEVELS, HOW TO MEASURE THEM ?

In the context of sound tracking, mixing or in the area of computer based music in general, the measure of the sound level imply to pay attention to a very important component : The Dynamic.

No sound has a constant level, there are always variations associated with a sound. That variation is called Dynamic and represents the difference between the highest and weakest levels.

This can easily be understood when ones looks to a sound wave.

The lowest levels of this sound are located inside the red rectangle. In practice, the mean value of the lowest levels is estimated and called RMS level.
The highest levels are indicated by the red arrows and are called Peak level.

PEAK LEVEL

Those peaks last extremely short time and can be measured by extremely reactive tools having a 10 ms reaction time.

They are called Peak Meter and are very useful and even essential to detect and measure digital levels such as found in DAW (Logic Pro, Pro Tools and so on) but also in Radio and Television area.

NB:
A very common language mistake is found in the DAW user’s manuals by calling those Peak Meters as VU Meters.
Of cause, it’s obvious that our DAW also includes VU Meters but the confusion is very frequent and the tools displayed by default are in fact Peak Meters.

RMS LEVEL

The RMS Level represents the statistical mean value of the audio sound included in the red box. It can be measured with audio tools having a longer (about 300 ms) reaction time. Such long detection time doesn’t allow to detect Peaks and such tools called VU Meters are restricted for RMS estimation.

While such analog tools are from the analog era, they are still contributive in the digital world even if the Peak Meters are the post prominent.

Important to keep in mind:
The RMS Level is the indicator which is the closest to our ears. A sound that is heard with a higher level will be characterized by an increased RMS value.

MEASUREMENTS UNITS

VU Meters being used in the Analog area lead to the fact that the results are expressed in dB VU.

In the Digital area the units are different and expressed under the form dB FS.
The formula to detect such value being different, the results will also differ in the scale of measure: An Analog results of 0 db Unit on a VU Meter is NOT EQUAL to a 0 dB observed on a Peak Meter.

0 dB VU IN THE ANALOG DOMAIN

This 0 dB VU represents the ideal level and all the analog hardware has been set to have an optimal functioning at such level.
Of course, a margin of safety is also planned and generally speaking is of 20 dB magnitude (with some little deviation of few dB linked to the devices themselves). This doesn’t include Recording Tape Machines.

Once this upper limit is reached or further, some distortion appears. Analog distortion may not necessarily be unpleasant; it may add some depth in your sound or harmonics if their amount is not too high.

Therefore, analog distortion needs to be under control too. This explains also why in the analog area there is no need for extremely price tools for being detected.

For example, when a distortion appears, on the VU Meters a simple red light flashes to indicate its presence but does not describe its amount.

0 dB FS IN THE DIGITAL DOMAIN

As opposite to the analog area, this 0 dB FS level represents the maximal level to be reached and certainly not an optimal level to work with.
This concept is easily understood by watching at a Peak Meter for which there is no space left after 0 dB FS.

As soon as you exceed this level, your audio signal is damaged and a digital distortion is produced. Nothing to compare with Analog, such Digital distortion is very unpleasant and easily audible.
That explains why the 0 dB FS is estimated by Peak Meters that have greater precision in detection levels.

Ok but…what is the optimal level to work in Digital Audio ?

Another way to express this demand could be : What is equivalent to 0 dB VU in the Digital world ?

That’s what I’m going to deal with this very important point in the second part of the post dedicated to sound levels and recording

Take care and see you soon