Musical dynamics are specified in relative terms. One passage of composition can be louder or quieter than another but that doesn’t really say how much energy is going to be delivered to the audience. However, when a microphone is placed in the auditorium the acoustic energy…the sound in the air…gets transduced into electrical energy and that can be measure.
I teach my students that microphones are little electricity generators…kind of like little “Hoover Dams”. The air molecules impact and the movement is converted into small electrical signals…alternating (AC) current. Its voltage is very low and an amplifier immediately brings the signals up to a safe level.
The most common studio alignment is -20 dBFS at +4 dBu (0 VU), which is 1.23 VRMS. Please note that 0 dBFS is typically 20 dB above +4 dBu, (or 20 dB above 0 VU).
The microphone level is brought up in the console or external preamplifier to line level, which is identified as -20 dBFS at +4 dBu (0 VU), which is 1.23 Vrms. Measurements rise of fall from this arbitrary point. A typical analog VU meter can reach plus 3 dB (sometime 6 db) in the positive direction and descends to minus infinity as it progresses to the left. Segment VU meters expand the upward portion of the scale to 20 dB. This is done to accommodate modern digital recording systems.
Some physical or software based VU meters also include a “peak” LED. It’s usually red and illuminates whenever the input level eclipses the maximum capability of the system. In an analog system that “headroom” might be as high as 20 dB. Analog meters also have a “peak” light but it’s hard to know exactly when it lights up. In the distant past, levels as low as plus 3 or 6 would show as an overage. As tape formulations and electronics have improved, that level has increased to plus 12 or so.
The “peak” light is similar to the fuel gauge in your car. When it shows “empty”, your car isn’t actually out of gas. But you know that you only have 20 or so miles left until you’re going to be calling AAA. The metering on recorders, consoles, and digital audio workstations provide a buffer as well. But recording engineers know that plowing into the red is not a safe thing to do. It happens…it’s happened to me. So you check the playback and determine whether you’ve got a real problem. A clip or an overage isn’t going to ruin a track.
In digital recordings, there is a cliff. Once you modulate every bit and max out the digital words, there’s no place to go. The sound heads over a cliff and very bad things happen to the sound.
In the next installment, we’ll investigate RMS, Peak Level, Crest Factor, Loudness, Momentary, Short Term, Integrated and Loudness Range.