Wayne A. Pflughaupt
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Adventures in Waterfalling Part Deux
Multiple Subs, Free Bass Traps, and More
Multiple Subs, Free Bass Traps, and More
NOTE: Readers are encouraged to wade through my previous article on waterfalls and signal levels before delving into this one, to fully understand the terms and concepts presented.
Do Positive-Gain EQ Filters Cause an Increase in Ringing?
It’s known that EQ filters cause ringing, especially positive-gain (boosted) filters. That’s because equalizers use phase shift (which is time-domain) to accomplish frequency response changes. It’s part and parcel to the beast. In addition to whatever electronic ringing the filter may contribute, increasing the signal level at a specific frequency via a boosted filter means the signal is going to take longer to fade away.
This isn’t necessarily a bad thing: If your frequency response curve has a depressed area, its decay time is less than what you have at adjacent frequencies. Raising the signal level of the depression (with equalization) will naturally increase* its decay time – the two can’t be separated. There’s no downside to that, unless you happen to think your sub sounds better with certain frequencies attenuated below the rest (hat tip: it doesn’t).
But does a boosted filter increase the depressed area’s rate of decay along with its decay time? In other words, after EQ, will a time-domain measurement show that area now takes longer to fade away compared to adjacent areas, like you would see with a room mode?
A participant on that epic Home Theater Shack waterfalls thread I mentioned in the Waterfalls / Signals piece offered the following graph that shows ringing from a 1/6-octave, +15 dB filter. Further supporting the ringing contention, you can plainly see that spacing between the slices where the boosted filter is employed is much tighter than what is seen at frequencies above and below the filter. Even worse, we can see that the decay rate has burgeoned from less than 150 ms to well over – well, who knows. This indicates a dramatic and detrimentally longer decay rate.
Graph 1: Electronic Waterfall
The problem with the theory is this: The filter, and waterfall graph, was generated in the REW program via a loopback connection. In other words, the graph is an electronic representation, not an acoustic representation. Although it’s no doubt accurate from an electrical standpoint, what really matters is what happens with an in-room measurement.
Naturally, it would require like-for-like filtering to definitively explore this idea in the “real world.” Unfortunately, or perhaps thankfully, 1/6-octave +15 dB filters should never be necessary when equalizing anything. But perhaps we can still make a point with equalization that’s more “real world” than that.
Here’s a waterfall graph of the subwoofers I had in a former residence after equalization, where I applied a combination of boosted and cut filters. The graph shows where the filters were set in the frequency spectrum, along with the amounts of boost or cut. The 22 Hz filter was a fairly wide one employed to enhance extension. (NOTE: I used a linear graph for this example as it makes the upper frequencies easier to study.)
Graph 2: Waterfall with Positive- and Negative-Gain Parametric Filters
Window = 350 ms, Floor = 45 dB
You can see by studying the spacing between the slices that the boosted areas do not show an appreciable difference in decay rate compared to other areas of the graph. Indeed, the boosted filter at 85 Hz shows the same spacing as the negative-gain filter at 42 Hz.
How can this be, if positive-gain filters increase ringing, or more precisely, the rate of decay? Obviously, a waterfall graph generated in a sterile electronic environment bears no resemblance to one generated in a messy acoustical world. Any electronic inflation in decay rate from boosted filters is obviously swamped by the characteristics the room naturally exhibits. It would require a room heavily dampened with bass traps to show that boosted EQ filters get a real-world increase in the low frequency decay rate. For the rest of us, it’s a non-issue. There may be good reasons not to use boosted EQ filters (most of which are arguable as well), but an increase in ringing isn’t one of them.
* Increase (transitive or intransitive verb): To make greater in quantity, extent, value, or amount, etc.; to add to; to extend; to lengthen; to enhance; – opposed to diminish. Collaborative International Dictionary of English
Do Multiple Subwoofers Reduce Ringing?
Most proponents of using multiple subwoofers in a room claim they offer benefits such as reduced ringing and elimination of modal peaks.
It should be obvious that adding more subwoofers has the potential to raise signal levels, and as such make a time-domain graph look worse. However, if adding subs can prevent the formation of room modes, then it could legitimately be claimed that multiple subs reduce ringing.
I haven’t seen anyone present baseline “before and after” time-domain graphs showing the effects of additional subwoofers. By that I mean, a measurement of a single sub that shows a room mode (i.e. no EQ), followed by another measurement after additional subwoofers were added (again, with no EQ). And of course, the second graph would have to be level-matched to the first’s room mode.
The closest thing I’ve seen are the graphs below from a popular on-line magazine and forum, but unfortunately the “Multiple Subs” graph also includes the effects of equalization. The graphs aren’t precisely level-matched, but at least are closer than most.
Graph 3: Primary Seat, One Sub, no EQ
Window = 576 ms, Floor = 50 dB
Graph 4: Primary Seat, Multiple Subs, with EQ
Window = 576 ms, Floor = 50 dB
The results are mixed, as we shall see. Part of the problem with this example is that the display window is long – 600 ms instead of a more standard 3-400 ms. That makes the room mode at 58 Hz look rather insignificant, even though it is actually taking over 100 ms longer to decay to the graph floor than the areas around it (~345 ms vs. ~230 ms).
You can see by comparing Graphs 3 and 4 is that the slice spacing at 58 Hz is the same in both graphs. In addition, even though its signal level in Graph 4 is reduced by a few dB, 58 Hz is still extending all the way out to 345 ms. So, adding more subs did not make an improvement in the 58 Hz mode’s decay rate.
Turning at the 38 Hz peak, its decay rate did indeed improve between Graphs 3 and 4, even though its signal level actually increased a few dB. In fact, the spacing of the slices between the two is significant, showing very wide spacing in Graph 4 compared to Graph 3. This means the room mode at 38 Hz has been effectively eliminated. So that’s a positive outcome. However, since the graph includes equalization, we can’t tell if it was the additional subs or the equalizing that eliminated the mode – we’d need a before-EQ graph to determine that.
As I said, the results of this example are mixed. Of course, every room is different. You may get an improvement with room modes with additional subs, or you might not. But now you know how to tell.
Either way, I doubt many will dispute the notion that there are definite benefits to adding more subwoofers. For instance, other graphs in the same article show relatively consistent bass at multiple seats, which is virtually impossible to achieve with a single subwoofer.
But even if extra subs do prevent the formation of room modes (which is certainly a good thing), they aren’t going to reduce the overall decay rate beyond what the room would naturally exhibit. That requires absorption. Anyone who believes that additional subwoofers will take your room from this...
Graph 5: Empty Room
... to this...
Graph 6: Room with Bass Traps
Courtesy of Real Traps
...also believes you can reduce flutter echo in a cinderblock room by adding more speakers.
How Much Improvement in Decay Rate Can EQ Give?
It’s a legitimate question: Just how much improvement in the decay rate of a room mode can be accomplished with parametric equalization?
Below is a graph showing a room mode at 42 Hz. We can see that it takes 450 ms to drop to the 60 dB mark (indicated by the horizontal line across the graph face).
Graph 7: Baseline Waterfall Graph at 450 ms
Window = 450 ms, Floor = 50 dB
Now let’s look at the same room and subwoofer after applying an EQ filter to the room mode. This graph is level-matched to the previous at 42 Hz.
Graph 8: Equalized Waterfall Graph at 350 ms
Window = 350 ms, Floor = 50 dB
As we can see, the former room mode has now dropped to the 60 dB mark in 350 ms, and at that point is exhibiting modal behavior similar to surrounding frequencies. In other words at 350 ms, 42 Hz has dropped as fast as it’s going to. Thus we can see that EQ brought an improvement of 100 ms in the room mode’s decay rate.
I don’t consider this to be hard proof as to the amount of decay-rate improvement you can get from equalizing a room mode. It’s a big world, and I’m confident someone, somewhere has had a worse room mode. However, even accepting that, it seems to me that a level-matched improvement of 3-400 ms after equalization, as I’ve seen some claim, is highly improbable. That would mean their mode was originally taking a full 800-900 ms to fade down to the noise floor, assuming a measurement peaking in the 90-95 dB range. If that’s the case, the room has more issues than an equalizer can address. No, more than likely any “improvement” on the order of 300-400 ms is merely the result of a huge loss of signal level due to severe equalization (see “Manufactured Waterfall Malfeasance” in Waterfalls / Signal Levels article).
Want a Better Waterfall? Get a Smaller Room
Something I was recently made aware of is that larger rooms exhibit more ringing than smaller rooms (although on further reflection it should have been a no-brainer).
Here is a waterfall from our current master bedroom, which measures 12’ x 18’ for 770 cu. ft.
Graph 9: 770 Cu. Ft. Room
Window = 300 ms, Floor = 45 dB
Here is a graph from the living room of our former home, which had a soaring vaulted ceiling and was open to the entry, dining room, kitchen, breakfast room, and upstairs landing. The total volume was a fairly cavernous 9000 cu. ft. or more.
Graph 10: 9000 Cu. Ft. Room
Window = 300 ms, Floor = 45 dB
The relevant data here is between 20-60 Hz. You can easily see that the large room, even with most of that range at a lower SPL than the small room, nevertheless shows a much longer decay rate than the small room.
If that weren’t sufficient to drive home the theory, let’s “go extreme.” The following graph was generated with my full-range bass guitar rig, as measured in a smallish church sanctuary. The system consists of a 15” EAW subwoofer with a Genzler top. The sanctuary is 3800 sq. ft. with total cu. ft. coming in at 46,000 – much larger than any home theater room. With this, you don’t even have to level-match the SPL or graph floor to make the point.
Graph 11: Small Church Sanctuary
Window = 300 ms, Floor = 50 dB
Despite the sad looking waterfall, this is actually a great-sounding room. The floors are a combination of hardwood and carpet, and the side-wall construction offers tremendous dispersion. Well-padded chairs also help with absorption. Music from a three- or four-piece band and singers is completely coherent, even with house speakers that are merely average.
Free Bass Traps?
I’ve heard it said that openings in a room can function similar to bass traps and reduce ringing. I suppose the idea is that providing the bass an “escape” will make a difference.
So I dragged my new Hsu ULS-15 MK2 sub to our master bedroom and fired up REW to see what I could find. The bedroom is 215 sq. ft / 770 cu. ft. and has three doors – the main one that opens to the living room, and two others opening to a closet and a small bathroom.
The following graphs have no equalization as we want to see any potential room modes in all their umm, glory. Here is a baseline with all doors closed:
Graph 12: Master Bedroom All Doors Closed
Window = 300 ms, Floor = 50 dB
Here is the graph with only the main door open. As you can see, ringing below 30 Hz does appear to be improved:
Graph 13: Master Bedroom Main Door Open
Window = 300 ms, Floor = 50 dB
Opening the other two doors got this:
Graph 13: Master Bedroom All Doors Open
Window = 300 ms, Floor = 50 dB
Either the closet or bathroom, or both, appear to be acting as a kind of “resonator.” The peak at ~23 Hz has increased by 2 dB, and the decay rate below 30 Hz is definitely longer.
All these results were repeatable, by the way. So it appears that the theory holds, at least to some extent, even though I honestly doubt doors opened or closed is going to make a difference that anyone can actually hear. But experiment for yourself to find out if the “open door policy” makes a difference in your room.
Conclusion
As we can see from the above examples, there is a lot that can be determined from waterfall graphs, once you know how to properly read them.
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