Are you dazed by decibels? Struggling with spectrograms? Modern audio gear and software comes loaded with meters to try and give visual indications of signal levels and a host of other information. The problem is that a lot of us don’t really understand exactly what our meters mean and what they’re trying to tell us. There are dozens of ways that understanding how to interpret meter readings can help you, so in this month’s Knowledge Base we’re going to take a closer look at decibels, then examine some of the various meters, dials, gauges and indicators in more detail, to help you understand how they all work and how they might be able to help you in your productions.
What is a decibel?
The hardest part about metering is getting to grips with exactly what a decibel is. In fact, many highly experienced engineers and producers struggle with the scientific definitions. A decibel is most accurately described as a logarithmic method for measuring the magnitude of a signal relative to a given reference level. Only when we know what the reference point is does the decibel measurement take on any real meaning. As such, there are a number of different signals that can be measured using decibels, although for audio purposes we’re mainly interested in acoustic sound signals and (analogue) electronic sound signals.
Part of the reason why so many people find this confusing is that most of the time, we foolishly insist on referring to each of these different measurements simply as ‘a decibel’ and abbreviating them all to ‘dB’ rather than specifying what the reference point is. The result is that we often have to infer the reference point from the context. There are three main areas where you’ll find decibels used for audio measurements, and the context should give you a clue about which type is being used.
Probably the most common everyday use of decibels is as a measurement of the level of intensity of an acoustic sound. This is more accurately termed dB(SPL), and is referenced to sound pressure level in air, where 0dB(SPL) equates to 20 micropascals (μPa), the lowest pressure (and therefore quietest sound) which can be perceived by the human ear.
For electrical audio signals, the most common scales used to measure level are dBu (the professional standard) and dBV (more common in consumer equipment). dBu is referenced to a signal with a root mean square magnitude of 0.775 Volts (RMS is essentially a method of calculating the average magnitude of a wave with positive and negative elements), while dBV is referenced to 1 V RMS. If you’re working exclusively with professional equipment then you can just about forget that dBV exist and concentrate on dBu. The only caveat is that you should pay attention to capital letters – dBu was originally known as dBv and is occasionally still referred to in this way by Americans. dBv is exactly the same as dBu. We warned that decibels can be confusing!
Decibel meters on hardware and in software are usually used to display dBFS or dBVU, measurements of the level of a signal relative to a defined maximum (in digital equipment this is the point just before clipping occurs, whereas in analogue equipment it refers to the highest level before the signal starts to degrade). In digital equipment, 0dBFS is the highest possible signal level. Attempting to push a signal beyond this level will instantly result in the waveform becoming squared off at the peaks. On professional audio equipment, 0 dBFS is usually set at +4 dBu.
Commercial equipment tends to use -10 dBV as the 0dBFS reference point. Analogue equipment tends to use dBVU (referenced in Volume Units) in a similar way to dBFS, where 0dBVU is the nominal maximum signal level. The Volume Unit is a hangover from the days of analogue radio broadcasting and tape recorders, where 0 VU was the highest allowable signal level. Whether you’re working with digital or analogue, any signal above 0 dBFS or 0 dBVU is usually very bad news in terms of sound quality.
One of the most important things to remember about all of these measurements is that the decibel scale is logarithmic. As such, a 100dB signal doesn’t have twice the power or amplitude of a 50dB signal and isn't perceived to be twice as loud (power, amplitude and perceived volume are related but not directly proportional). The base-10 logarithmic scale used by decibel measurements means that a 3dB increase in signal level equates to doubling the signal’s power, while a 6dB increase equates to doubling the signal’s amplitude.
However, the human ear’s response is also logarithmic, so it takes a level increase of approximately 10dB for the signal’s volume to be perceived as twice as high. Bear in mind also that this is true no matter whether you start at 10dB or 100dB, so 20dB(SPL) is perceived to be twice as loud as 10dB, while 110dB is twice as loud as 100dB. Small numbers on the decibel scale can relate to significant perceived level changes, so always be suspicious of anyone who uses phrases like, “it’s only a few dBs”!
VU and PPM
A very specific type of level meter known as a VU meter was commonly found on hardware in the analogue era. VU is sometimes erroneously used to refer to any mechanical signal level meter, but the VU metering system is just as clearly defined as the various decibel scales. VU meters display the average level of the signal, with the meter taking 300 milliseconds to rise from zero to its full level when a signal of 0VU amplitude is applied. Since the VU meter is so slow to respond, it isn’t useful for monitoring peak levels but should be used to monitor overall averages or average perceived loudness. Many VU meters feature an integrated peak level LED indicator to aid peak detection. Take note that some level meters in hardware and software exhibit pseudo-VU style damped readings without truly following the VU specification.
Care should be taken whenever you’re unsure whether a true 300ms integration time is being used. Peak program meters (PPMs) are similar mechanical meters but with a much shorter 5ms integration time in order to display audible peaks in signal level. Unlike VUs, which fall as quickly as they rise, PPMs typically take between 1.5 and 3 seconds to fall back 20 to 26dB after they hit a peak (the fallback time varies by country). This allows peaks to be held by the meter, making it easier to read their level.
Let’s take a closer look at peak indicators, those tiny red LEDs that tell us a signal is too hot for our gear. On analogue equipment, it’s not the end of the world to see the peak LED flicker momentarily every now and again (pushing into the red might even result in pleasant analogue distortion) but on digital equipment it should be avoided at all costs, especially when recording. In the days of analogue tape, it was desirable to use up every last bit of headroom in order to maximize the signal-to-noise ratio and dynamic range of the recording. However, this practice is no longer necessary with digital equipment, due to higher dynamic range and improved SNR. Furthermore, exceeding 0dB results in the painful and highly undesirable sound of digital clipping. At best, that means another take to fix the problem. At worst, you've ruined the recording and won’t get another chance.
Although many recording engineers will have their own slightly higher or lower preferences, we’d advise you to try and hit a peak level of around -15 to -12 dBFS when recording to a 24-bit digital format. Leaving this much headroom should minimize the chances of clipping while giving you dynamic range and SNR far in excess of analogue tape. Even when mixing in your DAW, remember to keep your levels down. It’s a common mistake to push all the faders up and then use a peak limiter plug-in on the master buss to bring the output level back down to 0 dBFS without realizing you’re harming your signal. Aim to leave headroom in every single step of the signal chain (including between effects plug-ins) and you’ll ensure that no damage is being done. Advanced metering Every DAW will include basic metering functions on channel strips, in the mixer section or as a utility plug-in, but if you want to take your metering to the next level, there are a number of more advanced ways to visualize your sound signals, most of which involve measuring multiple variables.
Metering starts to get a little more interesting when we measure another variable alongside the amplitude of the signal and the most common way of doing this is to measure frequency. For example, a spectrum analyzer displays the individual levels of a number of frequency bands in order to show their relative amplitudes. Oscilloscopes measure amplitude and time in order to give a visual representation of the waveform you route into them. Spectrograms add a time element to the spectrum analyzer principle, plotting three variables simultaneously in order to show how the spectral composition of a signal develops. These meters are incredibly powerful tools and a thorough understanding of how to use them will pay dividends in the long run.