Meters are a second set of ears. They show what is happening inside the computer, the console and the speakers, and they remove doubt from decisions. Knowing how to read and compare them is essential to mastering.
The decibel
The decibel comes from telecommunications. Telephone engineers needed a unit to measure signal loss along cables, and in the late 1920s the bel was named after Alexander Graham Bell. It proved too large for everyday use, so it was divided into tenths - the decibel (dB). By the 1930s it had become the standard unit of audio measurement.
A decibel is a ratio - a comparison - on a logarithmic scale. Useful rules of thumb:
- +3 dB - double the acoustic power.
- +6 dB - double the amplitude (sound pressure or voltage).
- Around +10 dB - roughly twice as loud to our ears. Perceived loudness also depends on frequency.
The letters after "dB" tell you what it is measured against: dBFS, dBu, dB SPL, dBTP, LUFS.
dBFS and peak meters
Digital audio is measured in dBFS - decibels relative to full scale. 0 dBFS is the maximum level; everything below is negative. Above 0 dBFS, digital audio clips.
The standard meters in your DAW are sample peak meters: they read the highest sample values. They are fast and generally accurate, but they can miss peaks that occur between samples - see the intersample peaks chapter.
RMS and VU meters
Peak meters show the instantaneous highest level. RMS (root mean square) meters show an average over a short window, which corresponds much more closely to how loud something feels.
The VU meter
Before VU meters, broadcasters used peak programme meters (PPMs), introduced for radio in the 1930s, with fast rise times and slow fall-back. In 1939, the VU (volume unit) meter was developed by Bell Labs with US broadcasters CBS and NBC, and it became the industry standard soon after.
VU meters are deliberately slow: their needle takes around 300 ms to rise and fall. That slowness makes them a reliable indication of average level and perceived power - excellent for comparing the balance of signals, especially low-end elements, and for comparing your master with a reference.
Ballistics
A meter's ballistics are its rise and fall times:
- VU - about 300 ms rise and fall. Average level.
- PPM - rise times of a few milliseconds, slow fall-back. Peaks.
Calibration
Analogue VU meters are calibrated with a 1 kHz sine tone so that the needle sits at 0 VU at the studio's reference level. Digital VU meters need a calibration point too, because there is no fixed conversion between VU and dBFS. Common choices put 0 VU somewhere between -20 and -12 dBFS; -18 dBFS = 0 VU is very common.
True peak
True peak meters oversample or reconstruct the waveform to estimate the actual peak level of the analogue signal, including peaks between samples. They read in dBTP. True peak is what streaming services and broadcasters specify for maximum levels.
LUFS: loudness
LUFS (loudness units relative to full scale) measure loudness as humans perceive it, weighting frequencies according to hearing. Loudness meters typically show:
- Momentary - the last 400 ms.
- Short-term - the last 3 seconds.
- Integrated - the whole track.
- Loudness range (LRA) - how much loudness varies across the track.
Streaming services use integrated loudness to normalise playback - see the loudness chapter.
Spectrum and stereo meters
- Spectrum analysers show frequency content - useful for checking low end you cannot hear well, and comparing tonal balance with a reference.
- Correlation meters, goniometers and vectorscopes show stereo width and mono compatibility. Correlation near +1 is mono-compatible; readings towards -1 indicate phase problems.
Do this now
- Put a peak meter, a VU meter, a true peak meter and a LUFS meter on your meter bus.
- Play a reference and note its peak, VU, true peak and integrated LUFS readings.
- Calibrate your digital VU meter to -18 dBFS = 0 VU and compare your premaster's needle with the reference's.
Adapted, with thanks, from the lesson notes of J.M. Brooklyn Audio Mastering.