RT60 (RT = reverberation time) refers to the time it takes latent reflections to drop 60 dB after the signal has been shut off.
It should probably be noted at this point that the accepted professional benchmark for determining audio signal decay in a room is known as RT60. When I refer to "rate of decay" or “decay rate” I mean how fast the sound decays: If we have 85 dB signals in two separate rooms and one fades to the noise floor in 300 ms, and the other in 200 ms, the latter has a faster "rate of decay.” For example, an 80 dB signal will quite naturally fade down to the noise floor in less time than a 95 dB signal.
In the following discourse, when I refer to "decay time" I mean the time it takes a signal to fade away, relative to its gain or loudness. To start, let's clarify a few definitions. Measurements of the decay duration are typically presented in time-domain graphs such as waterfalls and spectrograms. I’ll show some tragic examples of the type in Part 2 of this piece.Ī quick explanation for those who may have no idea of what any of this is: “Ringing” is a succinct term that refers to the time it takes a bass signal to fade down to the room’s noise floor after the signal stops. On the Web you can find manufacturers of equalization equipment, as well as companies that specialize in acoustics, present “before and after” time-domain graphs that claim to show an improvement in ringing, that actually do not. Sadly, these folks are in good company, as such contradiction can be found even among those who should know better. On the flip side, I’ve seen others who claimed to have achieved a 300-400 ms improvement in ringing after subwoofer equalization, which is patently impossible. And others who, after boosting at 20 Hz to compensate for their sub’s lack of extension, were annoyed that equalization actually introduced ringing – completely unmindful of the fact that such an adjustment will make a time-domain graph appear worse than before.
It's common to see people confuse say, mere gain changes (as we often get with equalization) with an improvement (or not) in ringing.įor instance, I’ve seen threads where people were happy with the audible results of equalizing their subwoofer, but were disappointed that it didn’t get them an improvement in ringing.
Equally problematic is the question of how to address ringing, and even more so, how to determine if it has been improved. It’s surprising the amount of confusion and misinformation that still surrounds the subject of low frequency ringing and waterfall graphs after all these years. endured our pathetic musings without throwing up his hands and calling us all a bunch of idiots. And I’d have to include myself in that category as well, at the time. A thread doesn’t go on for 17 pages unless there is a lot of debate over conflicting ideas and interpretations, not to mention people just trying to get a handle on this new (to them) topic.
Over at Home Theater Shack you’ll find an old thread on waterfall graphs, dating back to the early days of the REW program, that runs for 17 pages. Do a search on “rew waterfall graph images” and you’ll find all kinds of crazy-looking stuff. As a result we now have a curious mix of enlightenment and chaos: Someone downloads a sophisticated measurement platform, and in a few days he's an expert dispensing wisdom to the even less-informed on the audio forums. However, the advent of affordable measurement platforms like TrueRTA, Room EQ Wizard and others has made these powerful analytical tools available to the general public, most of whom have no formal training or experience in the relevant sciences.
Once upon a time in a land far, far away, the capability to generate waterfall graphs was limited to expensive software programs available primarily to professional speaker designers and acoustics engineers. Understanding Signal Levels in Time-Domain Graphs