Metering for Dummies
Ever wonder what is the difference between each meter available in a given DAW; you’re not alone- lots of have as well.
Let’s briefly recap reference levels and the audio meters available to us.
VU Meters: simply a volume unit meter, it represents the signal level in a designated channel; originally proposed in the 40s.
The reference reading is 0 VU when it is connected to AC voltage equals to 1.228 Volts RMS with a 600 Ohm resistance which equals +4 dBu @1KHz.
It is important to note the rise time of the VU meter which is how long the needle would take to reach 99% of distance to 0VU.
PPM Meters: They fall into different classifications ; True, Sample, Over-sampling and Quasi which is the most common. It shows the true peak level if it exceeds a certain amount of milliseconds.
For Quasi PPMs, integration time is what determines the extent of the shortfall.
There is more than one way to calibrate a PPM, some of those are :
Nordic,EBU ( Physical level is 0 dBu at a permitted max level of +9 dBu )
SABC ( Physical level is 0 dBu at a permitted max level of +6 dBu )
IEC 60268-18 ( Physical level is 0 dBu at a permitted max level of +9 dBu )
British ( Physical level is 0 dBu at a permitted max level of +8 dBu )
It is important to note that Quasi PPMs are neither loudness or true peak meters, the EBU (R68) defines the alignment level as -18 dBFS, however SMPTE recommends RP-0155 recommends -20 dBFS corresponding to 0 VU
RMS Meters: AKA root mean square; true RMS is proportional to the square root of the average of the square of the curve, and not to the average of the absolute value of the curve.
It is important to note that the precision and the bandwidth of the conversion is entirely dependent on the analog to digital conversion.
In pro audio, RMS metering will give you a more accurate indication of perceived loudness; unlike peak metering it will display an average level rather than an instant peak level.
In order to make use of any type of meter effectively, we need to understand what few level units means, their’ suffixes and reference values.
dBV : usually used for microphone sensitivity
dBu : used to measure voltage regardless of impedance
dB SPL : referenced @1 μPa, usually used to measure sound propgating in air and gas.
dBm : equals a voltage level of 0.775 volts or 775 millivolts, referenced relative to a 600 ohm impedance.
dBFS : is the amplitude of a signal compared with the maximum which a device can handle before clipping occurs.
The ITU.BS.1770 standard and the ATSC A/85 use the LKFS term (Loudness K-weighted Full Scale), while EBU use the LUFS term instead (Loudness Units Full Scale), both terms are identical, both measured in absolute scale. One unit of LKFS/LUFS equals to one db.
It is also very important to note the frequency weighting - seen below-which is applied to the signal.
It accounts for the relative loudness perceived by the human ear rather than just average levels or peaks.
It’s also important to note VU meters use dBVU and PPMs use dBFS; many plugin developers have loudness meters which support the LKFS/LUFS standard and provide actual readoputs with no interpretation readed from the user.
LUFS standard is the de-facto standard used for broadcast TV, game industry and for online delivery.
Several metering timescales have been established and it’s good practice to understand the different between them.
Short Term: For average loudness from 1 to a few seconds.
Long Term/Integrated: For the entire program.
True Peak: Sub-sample accurate and provides intersampling peaks’ measurement.
Lastly; I would recommend using the best that suits your specific application and needs; there is no need to limit yourself to one way of monitoring your levels, and lastly keep checking them frequently as you mix.