Sonnie said:
Man that looks neat as grits... just wish I could understand it. :dontknow:
It’s not that hard, Sonnie – let me ‘splain it to you.
The “waterfall” term might be what’s getting you. Actually what waterfall charts show is
signal decay. They call them “waterfall” charts merely because that’s what it resembles. To get a handle on it, let’s take a look at a chart that might be a little easier to read than the ones posted above:
Notice that there are some similarities between this chart and the regular one-dimensional charts we’re more used to looking at. For instance, you see a graph for
dB level (vertical markings), and
frequency (horizontal markings - although this one could use finer resolution). If you focus on just the
top line of the plot, you can see that it reads just like a regular one-dimensional response chart. For instance, we see a couple of peaks between 128 and 164 Hz. And a deep valley between 92 and 128 Hz. Make sense so far?
Now – that top-line plot is the
baseline for the rest of the “waterfall” readings, which are the ones that look like they’re “moving towards you.” The readings that “move towards you” are showing how long it takes that particular frequency to fade away. This is known as “decay.” Decay is caused by reverberation – i.e, the signal bouncing back and forth off the walls and around the room. Decay is also caused by room modes (a.k.a “standing waves”), which are a build up of energy at certain frequencies, caused by the room’s dimensions reacting to the wavelengths of those frequencies.
Now, to be entirely meaningful, the “moving towards you” part of the graph showing signal decay
should have some graduated markings in milliseconds (i.e., all the horizontal lines really close together). This one just says “800 ms,” so I guess that means 800 ms is the line that’s furthest “forward.” (However, that seems at bit ridiculous. Keeping in mind that sound travels roughly a foot per millisecond, 800 milliseconds would be essentially 800 feet. I have a hard time believing that decay times in even the
worse residential rooms is almost the time/distance of a football field! Maybe it’s supposed to be 80 ms, not 800.)
Also, notice that (for the most part) there is a distinct correlation between
signal amplitude and decay time. In other words, the peaks generally take a long time to fade away, while the valleys fade out pretty quickly.
It has much to do with a measurement standard known as “RT60,” where “RT” is “reverberation time” and “60” is “60 dB.” Basically, RT60 is the time it takes for a signal to reduce to 1/1000 of its original level after the signal source stops. A 1/1000 reduction in signal level is 60 dB.
Now, note on the chart I posted, and the ones Oliver and Brian posted earlier, that the lowest SPL reading is 55-60 dB. This is the point where audio is essentially reduced to ambient levels – i.e., the room’s noise floor. This is highly relevant. Obviously we don’t really care about the
actual RT60 of signal that is only 70 dB to begin with. We’re not worried about how long it takes to get down to 10dB! Once it drops below the ambient noise floor, its essentially “outta there,” for audio playback purposes. So, the time it takes a signal to drop to 55-60 dB is what we’re primarily concerned with.
In case you haven’t already figured it out, it’s important to establish that waterfall charts and the delay times they show
are entirely amplitude driven. On other words,
all delay times across the board will rise and fall as you raise or lower the volume level, in relation to the 55-60 dB ambient level. Make sense? In other words - turn the level up high enough, and when that valley between 92-128 Hz hits 95 dB, its decay time will look as bad as the peaks on both sides of it do now (unless of course it's a true null). Make sense?
I’m sure you’ve followed some of those threads on some of the other Forums debating whether or not an equalizer can reduce the long decay times you see with those response peaks. The answer is “yes and no.”
We all know an equalizer will reduce those peaks by reducing the
amplitude (level) of the signal at the frequency where the peak is. And as we’ve established, when amplitude is reduced, delay time reduces, too (i.e., relative to our 55-60 dB ambient level).
However, as we know, once we’ve reduced all the peaks by equalizing, the sub’s level is too low, so we’ll have to turn it up to compensate. What happens then? Well, you’ll see the decay times rise as well, across the board. But the difference now is that since response is relatively smooth, the decay times will also be fairly uniform as well.
So yes, the equalizer reduces the decay times of peaking frequencies, essentially by robbing room modes of energy. However it can’t reduce the decay time the room exhibits as a whole – i.e., its natural reverberation. Fortunately, with the low frequencies this doesn’t seem to be a problem in most cases – at least not a serious one. The overwhelming majority of people who smooth sub response with an equalizer are highly satisfied with the results.
If your waterfall charts show excessive low frequency decay even after equalization, you might want to address it. An audible clue might be if your sub still sounds muddy and undefined (although you have to take care here - some subs are just muddy and undefined!). This can be difficult, however, both expensive and intrusive. The problem is that low frequency reverberation can’t be addressed by the same methods that work for upper frequency reverberation – soft floor coverings, overstuffed furniture, etc. It requires treatments specifically designed for low frequencies, like bass traps and Helmholtz resonators.
Sorry to be long-winded – maybe it’s not so simple after all! But hopefully this helps.
So – to answer Oliver’s original question...
I was wondering if anyone actually tried to calibrate their sub using he waterfall decay instead of FR graph?
...there’s no good reason to, since the goal of equalization is to fix your response, not decay problems.
Regards,
Wayne
