Have you ever noticed that a lot of reverbs — in both plug-in as well as hardware versions — have a parameter labeled “RT60?” What does that mean? The “RT” part is easy. It’s an abbreviation for Reverb Time. The “60” portion is a bit more complicated, so let’s start with that.
Reverb time (also referred to as Reverb Decay Time or Decay Time) is usually expressed in seconds or fractions of a second, and can be very loosely defined as the time it takes for a sound to die away after the source has stopped. Suppose you walk into a quiet church, clap your hands once, and it takes four seconds for the sound to die away. You could say that the church has a reverb time of four seconds. But what if you’re with someone who has poor hearing? They might say that the sound died after two seconds, and argue that the reverb time of the room is two seconds. Who’s right?
That’s where the “60” comes into play, referring to a 60 dB change in level of the sound’s decay. Legend has it that the definition of RT60 comes from the idea that 100 dB is a good representation of a crescendo for orchestral music, and that a well-built performance hall has an ambient noise level of 40 dB. So, if the orchestra plays a hit at 100 dB, the average person should be able to hear the decay of the reverb until it disappears into the room’s noise floor at around 40 dB (100 – 40 = 60).
Let’s return to that church with an SPL meter and suppose that when you clap your hands, the meter shows an SPL of 105 dB. You watch the meter as the sound dies out, and four seconds later the sound level in the room measures 45 dB. That’s a difference of 60 dB, so the reverb time for the room is four seconds. Reverb time (RT60) is the amount of time it takes for the reverberation to decay by 60 dB below the SPL of the original source.
Decay time is a fundamental acoustic characteristic of any space. It varies with room dimensions and the absorption characteristics of the materials inside the room.
Can You Hear My Reflection?
Rooms with large areas of reflective surfaces allow sound to bounce around more efficiently, increasing reverb time. Old gothic churches often have a lot of stone or marble in the interior construction. Marble is hard and non-porous, which means it has a very low absorption coefficient (it does not absorb sound very well). In English, that means it’s very reflective — great for reverb, terrible for intelligibility. Smaller rooms with carpet on the floor and acoustic treatment on the walls absorb a lot of reflections, decreasing the reverb time and improving intelligibility. That’s an important point because intelligibility is our business.
Here’s where the concept of RT60 gets tricky and this affects your mix: reverb time in a room varies with frequency. Remember that room with a lot of marble in it? Marble has an absorption coefficient of under 0.02 at frequencies ranging from 125 Hz to 4,000 Hz. In other words, it’s very reflective across a significant portion of the audible band. Carpet laid on top of foam padding absorbs differently at varying frequencies. Carpet on foam has an absorption coefficient of about 0.70 at 4 kHz (very absorptive) but an absorption coefficient of about 0.08 at 125 Hz (very reflective). That’s why laying carpet helps reduce high frequency reflections (and therefore HF reverb time) but doesn’t do much to reduce low frequency reflections (and therefore LF reverb time). The point here is not to quiz you on the absorption coefficient values of several thousand different building materials; the point is to acknowledge that different parts of the audio spectrum decay at different rates depending upon surface materials, and that means that the tone of your reverb actually changes over time.
It may not be obvious, but the RT60 parameter in a software or hardware ‘verb typically sets the midrange decay time. There may be additional controls for low- and high-frequency decay times, and possibly controls for the crossover points between the LF and mid, and the mid and HF decay times.
If you look at Fig. 1, you can see the screen output of a reverb for a snare drum hit seen through Rational Acoustics Smaart Di v2. The top half shows frequency (horizontal) versus amplitude (vertical). The bottom spectrograph shows intensity (blue = low amplitude, green and yellow = increasing amplitude, red = high amplitude) versus frequency (horizontal) versus time (vertical). The Reverb Time has been set to 1.5 seconds. This particular reverb has a separate parameter for LF Reverb Time, and Screen 1 shows how it looks when it’s set to 0.15 seconds. Fig. 2 shows the same snare hit through the same reverb, but the LF Reverb Time was changed to 6.6 seconds. Notice the longer green and blue tails in the spectrograph between 150 and about 400 Hz showing this increase. The reason you care is because long decay times in the low frequencies will make your mix muddy.
Some reverbs don’t provide an LF Decay Time parameter but may give you a parameter called “Ratio” or “Multiplier.” Fig. 3 shows Waves TrueVerb. The Frequency Response Graph (bottom window) has Reverb Damping controls for Low and High. The multiplier numbers tell you how the Low and High reverb decays relate to the “Decay Time” parameter. Sorry folks, but you’ll have to do some math: If the Decay Time is set to 1.2 seconds, and the Low Multiplier is set to 0.60x, then the Low Frequency reverb time will be 0.72 seconds (1.2 x 0.60). If the Low Multiplier is set to 1x, then the LF reverb time is the same as the Decay Time.
This is useful info because when you are adjusting reverb parameters you want to pay attention to the RT60 across the spectrum. I suggest you start with an RT60 somewhere in the vicinity of 1.5 seconds, with both multipliers less than 1. Setting the LF Decay Time shorter than the midrange RT60 keeps the bottom-end tight, which is particularly useful for those rare occasions when you need to add reverb to a kick drum. As a matter of habit, I’ll set the LF and HF multipliers to around 0.5x the decay time, which prevents the bottom end from rolling around like a pig in a poke, and tames the otherwise splashy “s” and “t” sounds in the high-end. If you want to make the reverb “warmer” sounding, increase the LF Decay slightly; if you want to make it brighter, increase the HF Decay.
A common rookie mistake is making the RT60 too long (which reduces clarity in a mix), but you need to put the reverb in context with the performance space. Use long reverb times for ballads, when mixing in very dry rooms, and for outdoor shows. Keep RT60 short for up-tempo songs. I rarely use much reverb in a live mix indoors because the room is probably adding more than enough natural reverb. If your show is in Madison Square Garden, you probably won’t need to add artificial reverb!
Next month, we’ll investigate some of the other important fine points regarding reverb.
Steve “Woody” La Cerra is the tour manager and FOH engineer for Blue Öyster Cult. In the 1980s, he used up a lot of the available reverb, so there wasn’t much left for recordings in the 1990s. In the early 2000s, the IRC (International Reverb Coalition) grew more reverb to avert the shortage.
