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Measuring NuVerb for Fun, Profit and Confusion.

In the interest of consumer information, it seemed reasonable to me to measure NuVerb just like I'd measure an acoustic room. So, I patched the TEF analyzer straight into the console and picked up the test signal from the Stereo Bus for analysis. Thought I'd just confirm that delay times were as advertised and maybe do a couple of illustrations to demonstrate the effect of predelay.

Well, I sure opened a can of worms and managed to severely warp my mind in the process - it should be fine as soon as it dries out. Not for you to worry. As far as I can tell, the various reverbs and predelays have times approximately as advertised, but boy do I have a lot of research questions now, and a lot more studying to do.

When you make such a measurement in an acoustical space, you use the test signal to excite the space. That signal becomes the "direct" sound and it is an inherent part of the signal. In an electronic reverb, you can dispense with the dry signal altogether, so you are observing only the reverberance. Cool! But if you do that, then how can you determine the reverb time, which is the time it takes to decay from the direct signal level to a level 60 dB down? Logically speaking, you can't. So, naturally you include the dry signal in your analysis. Problem is, early decay time is dependent on the relative amplitude of dry vs. wet signals. Which is the right level? Damned if I know. In reality, we mix by ear, balancing the desirable qualities of reverberant wash against the desire for clarity, transparency and intelligibility. So, there is no single to way to do this, and the reverb times vary wildly depending on how you proceed.

The next thing I noticed was that the behavior of the reverberance over time was quite a bit different than I have gotten used to seeing in the acoustic rooms I have become accustomed to measuring. With a patch like Default Medium Hall (18 ms predelay and 1.3 seconds Decay Time), f'rinstance, the onset of reverb takes place over about 250 ms, rising approximately 10 dB as it does so, and then decaying at a reasonably constant rate from there. If you reduce the parameters Spread and Shape to 0, the reverb decay begins immediately. The decay rate seems to be about the same, but the beginning of decay is changed in time. All of this suggests that the idea of decay time as a single number is a pretty shaky idea.

When I started testing extreme settings (Large Hall with .2 sec decay time, f'rinstance), measured decay rates began to diverge significantly (get longer) from the indicated rate. Nonetheless, the audible quality of decay correlates well with the shown settings. I know this is an issue that has been engaging David Griesinger, who does most of the reverb programming for Lexicon. His query goes something like, "What are the physical qualities that affect our perception of reverberance? What values do we use to decide on the loudness of reverb?" It is commonly held that the decay rate over the first 10 dB of decay is subjectively the most important, and this analysis seems to support such a premise.

More important, Griesinger's idea of running reverb loudness, which can be thought of as the ongoing subjective loudness of the reverberance while the music is happening, may be more important to our musical work than the rate of decay. The reason I mention this here is that the measurements make it clear that by fooling with the onset of reverb through controls like Spread and Shape, it is possible to generate a quite stable and long-lived running reverb. Because in the electronic realm we have direct control over the relative loudness of dry vs. wet (in acoustic spaces we have to take what the room gives us), we can then easily and intuitively set that loudness of reverberance at the subjectively "proper" level.

All this is to say that reverberance is not simple in either the physical or perceptual realms, and simple physical measurements (if an Energy Time Curve over a range of frequencies can be though of as simple!) don't correlate all that well with what we hear. What this means to you is that you should use given values as subjective guideposts and reference points, but not as fixed points of truth. This is one place where your ears are really the best guide, probably the only meaningful guide.

Special thanks to Bob Riordan of Lexicon and Sean Penn of Parsons Audio for their technical assistance, guidance and advice regarding this project.

Dave Moulton's dogs like the Wild Spaces Program while the cats seem to prefer Rolled Nutz. Dave himself sort of goes for the 747 Hanger!