I stumbled across an interesting article over at Sound on Sound written by studio design consultant Carl Tatz. I'm not sure if the article is free to read, but the article can be found here in any case: The Elephant In The Control Room | Sound On Sound

To summarize the article, the theory and his experience is that to be able to tame those hills and mountains measured in the lows of a room, the solution isn't about adding even more bass traps. Instead one should look into using a sub to even out the frequency response.
I was just looking about to add (yes even more) bass traps (was looking at the GIK acoustics Soffit Bass Traps http://http://gikacoustics.co.uk/pro...fit-bass-trap/) when I read the article. Now I'm not sure whether to purchase Neumann KH805 to complement my 310's, or more treatment.

I've pasted a few quotes from the article down below so that you can get a grasp of what it's about:
"The common advice given to solve this problem is to add acoustic treatment into the room. Bass traps in particular seem to promise the most ‘magic’. Acoustic treatment is critical in control–room design, and is most valuable for attenuating side wall and ceiling first reflections, and to remove any sort of spurious reverberant ambiance. Any sonic information coming back to the listener at the listening position, other than the direct sound from the nearfields, will cancel with the direct sound to produce comb-filtering, making the playback sound ‘far away’ as well as being inaccurate. Rear–wall absorption and corner bass trapping is generally a good idea and, combined with first–reflection treatments, will render a smoothing effect for the control room’s acoustic character. I would offer a word of caution, however, about bass trapping at the front of the room that directly affects the low frequencies at the listening position: without careful evaluation, these nearfield traps can just as easily be destructive as helpful, causing unwanted frequency dips."

"The observed phenomenon is as follows (cue for the acoustic animal trainer to lead the elephant into the control room). In a personal home–studio–sized control room, nearfield or mid–field monitors are usually mounted on speaker stands behind a desk or atop the meterbridge of a large–format console at tweeter/listener ear height, which is typically 48–50 inches (122–127 cm) above the floor. The overwhelming majority of the time, this configuration leads to a low–frequency acoustic cancellation due to the interaction of the floor, front wall, ceiling, console and listening position, creating a dip in the neighbourhood of 125Hz, give or take 25Hz."

"So how do you tackle this problem? No amount of bass trapping or other acoustic treatment is ever going to satisfactorily attenuate a wide 15dB dip, and if you tried to EQ it away, you wouldn’t be able to turn your monitors up past 45dB without blowing the woofers... So what’s the answer? Subwoofers

Yes, subwoofers. Properly positioned — and two are always better than one for phase–correcting axial modes — they will begin to fill in your Grand Canyon of missing bass information. This is the only way the canyon can be filled, and no room is too small for this to work."

Anyone here care to chime in with their experience regarding this subject? Curious to know
Btw. I'm not having issues with the canyons he's speaking of here, but I'm always looking to tune my room (ok, my speakers) to even more perfection.

He has a good point. Quite often our ears at about half way between two side walls and half way between floor and ceiling. Two nulls often at very close frequencies.
With an eight foot ceiling say 70Hz, 10 foot width say 56Hz. A lot of missing Kick Drum and Bass. The whole backbone of the music is missing.
Harman have published great research showing the benefits of multiple subs. 2 great, 4 brilliant. The LF response of the 310 is IMO amazing. I would be very slow to filter that LF out diverting it to a sub or subs. If you go that direction I would encourage experimentation with all the LF drivers running simultaneously.
Interestingly an opposite approach seems to work well too. http://www.psiaudio.com/en/our-products/avaa-c20/
DD

Adding a second sub solved some significant issues for me in my room in the frequency range described above. I added a second sub after thoroughly treating my room didn't fully resolve it. My room is now +- 5db with 1/3 octave smoothing, and +- 8db without. My room is an absolute pleasure to work in.

As a note, I didn't end up with my sub in the corners as Carl recommends. I already had deep traps there. They are positioned 1/3 and 2/3 of the room width directly under my monitors on heavy custom stands similar to sound anchors.

That looks like a cool idea. I wonder what pricing is like.

Also, as a "bedroom" dweller, I have struggled with getting my cube to be decent enough to make me happy. I have some very basic treatment but nothing ever helped. Finally against most GS recommendations, I went ahead and put a sub in my space and things changed RADICALLY. At least I can hear some bass now. Of course, the sweet spot is about the width of my shoulders, but it works for me.

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That sounds like a mess in practice although theories are always fun and worth a look. The goal is to get the room as good as possible, then play with the type and location of only 2 speakers, IME.

If you get the bass trapped as best you can, and I mean really go for it, you can play with 2 speaker placement to fine tune the response so much easier and more effectively than the interaction of one or even two subs given the crossover options, the rooms issues, the listening position options, the subs location vs the speakers location, etc. Too many players in the game, too many variables.

A studio builder friend of mine and I have been testing the Harmon data and approach as well as the PSI AVAA active cancelling system. We are seeing very dramatic results from mulitple subs in smoothing typical small room modal responses, this stuff is real and it is quite amazing. The PSI AVAA absolutely do work but the results are less dramatic and they are expensive

They are about $2000 a piece. A minimum set would be (2) and ideally you would have 4 in most most smallish rooms

I'll stay with what I have.

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Another exemple by Earl Geddes.

https://mehlau.net/audio/multisub_geddes/

We have tested the Geddes approach too, it definitely works. Compared to the JBL/Harmon is treats each sub woofer as it's own unique "contributor" to the total node cancelling effect and tends to have more complex and extreme EQ curves, but it really works- Geddes is quite an amazing engineer.

When you have spent your life hearing music in rooms with modal issues these multiple sub-woofer solutions are pretty mind blowing. Bass sounds even over a very large area, the clarity of bass notes is striking, the decays are tight and you can hear down lower then you would ever expect in such small rooms. Another strange side effect is that the imaging in the midrange and highs is dramatically improved

This is really quite a big deal, glad it's getting discussed here

in the link below, in French (apologize, my english is not sufficient to translate), a well known french acoustician Jean-Pierre Lafont is against the multi sub to treat the modal issues



http://www.cinetips.com/viewtopic.php?t=21

as too often in audio, each one to judge by himself

https://gearspace.com/board/10016278-post9.html

so you basically need to analyse your current situation and check if your current issues are due to the floor bounce or something else.

But even if not due to the floor bounce; two quality subs and a DSP (to be able to freely choose the x-over, time align, and to carefully add correction filters) can really make wonders assuming the room is properly treated.

I tend to agree (although I haven't read the text you linked to yet).

if you ask me; the best option is to treat the room as much as you can (especially in the modal range, so avoid thin porous stuff if possible) and then add two subs (L&R) on the floor below the mains and integrate them well using a DSP.

Cabasse (French brand speaker) some years ago arrived to the same conclusion : Treatement and two subwoofers

Thanks Jens! I had a very knowledgeable guy measuring my room about a year ago and while I don't remember the specifics we came to the conclusion that it's doing pretty well (in terms of a somewhat flat freq. response but a bit too much ringing/decay in the bass though). After that measurement I added four Vicoustic VariBass to my room to tighten that up a little bit (haven't followed up on new measurements since though). Since earlier I have two Primacoustic FullTraps and four Cumulus (I think they're called), four Vicoustic SuperBass Extreme, nine Golden Age Abzorba 100's, and two DIY corner bass traps in the back of the room going from floor to ceiling.

But the key for this method to work really seems to be adding two subs and not just one? Jeez, this is going to be expensive

Since you probably want to cross over to the mains above about 120-150 Hz due to the floor bounce cancellation (I've done x-overs above 200 Hz and this is fine assuming good subs and appropriate position in relation to mains); you need two subs. Have you considered to build them yourself? It's not rocket science ... and you'll save A LOT of money.

I removed a 15" Rytmik sub, because the problem often are nulls and these are like black holes, they just can't be filled.
It also added too much low end to the room, if I would filter that away, it would only provide the odd fart every once and while below 30Hz. Waste of space and resource in that case.
I loved the sub, extremely capable, used in Sterling as well. But my TAD monitors being full range, it just added complexity. Also intergrating subs properly "in phase" from the listening spot is a real pain
Move your head a bit and stuff is deviated already again. I like my bass TIGHT, with a 100% focus. not a blurry extension.
Might be great for extending smaller monitors or different rooms, or ofcourse when working on discrete .1 channels most of the day.
But for full range stereo you really need to test what it brings to the table.

If the bass sounds like a "blurry extension"; you have modal issues. Don't blame the sub (unless a really crapy one). The only thing that really matters in terms of "fast" or "tight" bass is the room (treatment). Sure; you might be able to hear the difference between different subs in terms of "decay" or "resonance" in a free field listening test (outdoors), but as soon as you add a room (even if very well treated); the room is going to be the weak link.

True, but I'm nitpicking here and for sure not blaming the sub. All I'm saying is that the sub is not always the default solution for every setup.
My TAD's start to roll off at -6dB/oct at 32Hz or so. There was more than enough energy in that area already, with the sub I could extend that to 20Hz or so, but very little content being played in that range. i had a challenge at 50-60hz, but that was a null, near impossible to fill/flatten by the sub without affecting the respons of the rest in that area as well. You don't want your room to sound as a boomcar
I mean, if you have 5,25 or 8" desktop monitors it will work great. Very clean and tight. Like you also mentioned, one needs to understand the issue at hand to see if a sub can bring improvement. I didn't need an 3dB boost extention <80Hz
I needed to flatten a gap between 50-60hz, now flattened further by moving monitor position, desk position and trapping.

What crossover did you try? Did you try asymmetric crossover with different slopes (and if so; did you adjust the time alignment / phase for each attempt)?

In my experience; two subs on the floor (or even better; on the floor but standing if more woofers, acting as a stand for the mains) is always an improvement no matter how expensive or “large” the mains might be (the mains I design usually contains a 12” midrange driver so …). But this is naturally only true if the integration is done right and this means by using a separate DSP that allows for all the stuff you need (signal alignment, freely adjustable X-over filters (up and down), correction filters etc.).

But yes, one cannot expect to fix everything with subs and DSP. If the room is not well treated; that´s what one should focus on first.

Exactly, the correct integration of multiple subwoofers addresses low freq room issues an order of magnitude better than any practical treatments. Here is "before and after" REW screenshot of one of our tests in a 10x14x8 (with block and plaster walls, kind of the worst case scenario). This is using the standard JBL LSR 308 monitors and (4) LSR 310S, total current cost is $2000. Compare this to the results that are possible and the cost of off the shelf "bass traps"....

Keep in mind that, assuming acoustically symmetrical side walls (i.e. geometrically symmetrical AND equal impedance at both sides), if using two subs underneath the monitors you will not activate (or greatly reduce activation) of odd ordered axial modes off the side walls. In other words, you will largely avoid activating the 1st and 3rd axial modes related to the side walls (the 0,1,0 and 0,3,0 modes and so on). This is due to the fact that the side wall low frequency reflections will be out of phase with each other. This reduces the issue of cancellation due to destructive interference when sitting half way between the two side walls, since the second axial mode does not have a cancellation there -- it actually has a peak half way between the two side walls. The second side wall axial mode (0,2,0 mode) will generally be far easier to treat than the first mode since it is an octave higher. Low frequency peaks can also respond well to EQ with DSP if you know what you're doing and aren't able to fully treat that mode.

So overall I'd agree with Jens that two subs position on the floor below the mains is ideal, as you can reduce the 1st and 3rd side wall axial modes, minimize or eliminate the floor bounce, and sacrifice very little in terms of path length distance between the subs and the mains.

I did not have an active crossover to high pass the mains. So it was really about ammending on the low end for me. Rather than chopping off from a higher freq.
My mains are passive and don't have built in crossovers.
The sub did, but cutting these giants to 80hz or so was crippling their strong points. The JBL example above is great, these are smaller monitors where subs can do a much better job.
Anyway sub is sold, response is better, I'm happy.

That makes sense it is hard to configure a single sub in a small room to not do any harm

However, if you got a second matching sub, and with the right placement, you could start to approach the response of the JBL example above. As shown by the Harmon research- 2 is really good 4 is the sweet spot for smoothing, cost, and, practical placement options. Our tests definitely validate those results and since each room really is unique sometimes 2 is great, sometimes 4 is needed, sometimes DSP is absoolutely required and sometimes it is *somewhat* optional

What placement did you use?

The room shown in the REW screenshot is 10x14x8 with (2) JBL LSR 308s at the front of the long dimension and (4) JBL LSR 310s at the midpoint of each wall.

In Toole and Welti's research from Harmon they focused on the most consistent response for a group of listeners, i.e. a home theater situation, we have found this one works particularly well for studio monitoring/mixing and it's practical because of inconveniences such as doors and closets!

You really have to hear this to fully appreciate it, the timing and clarity on the bass is a new experience.

I am assuming no delays or dsp?

Steve, I think you mean the JBL LSR310S,, not a 301s. Yes?

Joe

yes, corrected it to 310s thanks, I am serial typo-ist and trying do this between other tasks!

SteveGTR your measurement with the four LSR310S is impressive. Am impressed with the flatness below 40 Hz down toward 22 Hz. I haven't personally tested modern small inexpensive commercial subwoofers, but because of the published specs on most of them (or in some cases the vague nature of published specs), was skeptical whether they would be useful below about 40 Hz regardless how many one would install in a room.

And being ported, one wouldn't expect much "pressure zone" room bass reinforcement below the modal frequencies.

Am not being critical. It is just interesting they do so well that low. I don't doubt that multiple subs can help flatten a room's bass response. Here are some curiosity questions--

Did you have to go to a lot of trouble tweaking crossover frequencies, level matching or EQ to get such flat results?

How heavily treated was the test room?

If you experimentally compare for instance 1 sub turned on in the room, versus 2, 3 or all 4 subs enabled-- Does room bass decay and group delay stay about the same for all combinations? Is it noticeable that perhaps as the frequency response gets smoother with more subs, that the bass decay time and bass group delay also gets longer?

Just curious, because in my limited experiments with only two subs in a small room, which only had one obnoxious null around 37.5 Hz-- When the subs were carefully balanced and EQ'd to "mostly flatten" that null, it also raised the bass decay time and bass group delay. I guessed that "pumping more energy into the null" was the explanation. Fixing one problem by causing another.

So am curious if this would be an effect one might expect to see fairly often when using multiple subs in small "imperfectly trapped" rooms? Or if the issue is specific to my room but not necessarily a "highly probable side-effect" of multiple subs in a small room?

Also, did you check REW distortion plots on the four LSR310S + LSR 308 sweeps? Really, I'm not being critical. Genuinely curious. So far as I know, one would expect at least one or two percent of low-bass THD on about any speaker which mere mortals can afford. Just curious how much. If the LSR310S also happens to have fairly small low bass distortion it is even more impressive.