Room treatment - how effective?

OK, similar but less prominent than the problems I had.

Firstly open mineral wool absorbers are flow mode and don't work significantly for low frequencies when place at the room boundaries (or corners).  For bass traps you need pressure mode devices (limp membrane, damped membrane or Helmholtz resonators).

Secondly at the reflection points you don't want absorbers you want diffusers (you already have enough mid and HF absorption from your soft furnishings).

Third, you should have an even more prominent resonance at 24.3Hz.

My solution...

To damp the primary bass resonances (39.5Hz, 44Hz and 78Hz) of my room I used mineral wool sealed in a polyethylene membrane and placed in the corners of the room.  The LF pressure wave at resonance presses on the membrane and the energy is then lost by moving the mineral wool fibers.  I have achieved a -12dB reduction in the resonance at 39.5Hz, 44Hz and -10dB at 78Hz (from +22dB down to +10dB)  However this required 18% of the wall area to be covered by these absorbers.

The last +10dB I handled by placing my main speakers in bass null positions (-8dB at 42Hz) and using a sub to fill in the lower bass.  The sub is connected using a DSP that gives -10dB cut at 42Hz, which in combination with the -8dB from the position of the main speakers gives an overall residual of +3db at 42Hz.

The 1/3rd octave room is essentially flat, being +/-4dB from 15Hz to 18kHz.

Be careful. Room treatment is great for home cinema where the goal is to delete the signature of the room in order to transport the listener to the location of the film.

But, unless you have a very problematic room, treating a room to extremes for stereo can result in a dead lifeless musical experience. Especially since the goal of stereo is to put the band in the room (so the exact opposite of surround).

I say this from experience, having treated several rooms right down to having windows filled in sound absorbing styrene, walls drapped in heavy specialist sound deadening fabric, and floors covered wall-to-wall absorbing underlay and very heavy deadening tiles. This was back when I was purely into home cinema and the result was fantastic. But music in that room was soooo boring. 

Thanks, Huge.

The large bass trap I mentioned is a damped membrane type. What overall dimensions are the ones you built yourself? (And how many did it take?)

It took lot of playing with positioning to find somewhere tolerably level in my room, but that levelness is far from smooth being up to about 10-12dB between peaks and troughs: and just a foot or so further back or forward, or speakers slightly differently positiond, makes quite a difference. The Indicated corner is additional +15dB at ~46Hz and at ~78Hz. Curiously I don't have a prominent peak at around 24.3 in that corner, though elsewhere in the room there is a peak, but not as pronounced.

And thanks, Feeling Zen. That echoes (sic!) the effect when I tried Dirac, killing the music. But maybe its a matter of sufficient to even out significant peaks and troughs, rather than trying to dampen out the whole room. Received wisdom is that bass traps can work wonders with less than perfect bass evenness - but possibly requiring a large number, as in Huge's 18% of wall area - and reducing early reflections can improve clarity. But any significant physical treatment has negative WAF...

I also agree with Huge - he always has wise words.

Ultimately it is about doing just enough to remove clearly audible problems but not so much that you lose the "in the room" effect and start feeling like you are an ant inside a giant pair of closed headphones. Nowadays I just practice careful size and placement of throw rugs and the material of the curtains coupled with a speaker that has a light touch to not aggravate the sprung floor.

After dabbling extensively with room treatments in my dedicated room and the living hall, "destroying" my dedicated room along the way when I did trial and error placing of the treatment panels, I have come to a conclusion which is based on my own opinion.

My opinion. Try to avoid treatments of any kind if your system sounds acceptably fine. No system is perfect and there will always be some colouration from the room. A bit of colouration can be considered as a live sound, and it can be pleasant or enjoyable. The main reason of doing away with room treatments, especially if the system is in the living room is aesthetics. Most of these room treatment products look rather unsightly. Some absorption panels in the form of painting can blend nicely in the room but diffusion will not blend in to the decor of the room. Ditto chunky bass traps or bass panels.

One may eliminate the need of treatments with proper placement of speakers and seating arrangement. Try to work around with the acoustics of the room with carpeting, curtains (wooden blind or fabric etc.). Things which do not affect the aesthetics of the room so much.

It will only be worthwhile to explore room treatments if the system sounds terribly bad due to poor acoustics. If the system is in a dedicated room, it may also be worthwhile to explore room treatments if the room is dedicated just for music listening or home theater and nothing else.

In summary, room treatments are effective, but it is not necessary depending on the circumstance.

feeling_zen posted:

Be careful. Room treatment is great for home cinema where the goal is to delete the signature of the room in order to transport the listener to the location of the film.

But, unless you have a very problematic room, treating a room to extremes for stereo can result in a dead lifeless musical experience. Especially since the goal of stereo is to put the band in the room (so the exact opposite of surround).

I say this from experience, having treated several rooms right down to having windows filled in sound absorbing styrene, walls drapped in heavy specialist sound deadening fabric, and floors covered wall-to-wall absorbing underlay and very heavy deadening tiles. This was back when I was purely into home cinema and the result was fantastic. But music in that room was soooo boring. 

Music can be exciting if diffusion is used. Excessive diffusion will give a very live sound. Excessive absorption, on the other hand, will cause the system to sound dull and boring, or dead.

I will admit to taking a more ad hoc approach to room treatment. This largely due to my asymmetric room dimensions, a vaulted ceiling, and lack of practical locations for panels/traps in my room (a large window, two open entrances, WAF). Also due to the fact that I have some hearing issues and I'm satisfied to let my ears be the judge rather than take an approach that includes assessing results by measurement. I hope to live without a sub as well.

Foam is cheap and as such my treatment has been ongoing, experimental, fairly inexpensive, but not without some success. Foam corner bass traps (stacked vertically from the floor in the lone corner I have available) have been effective in greatly reducing but not eliminating bass modes. This cleans up the overall sound and less overriding bass is definitely more in terms of quality. I've also been able to hang by string or stack on furniture lightweight foam panels in various locations throughout my room to reduce side wall reflections. This has led me to the conclusion that ceiling treatment is warranted and I've dabbled with that to some success.

I've found through recent speaker demos that widely varying the speaker position and toe-in in the room has a great effect on side wall reflections (obviously) but also bass modes. The listening position should be varied accordingly with speaker movements as well.

All that said, I'm of the opinion that you can pay to have a consultant come into your room and make a big initial improvement in sound quality, but to get at the finer nuances that will satisfy your ears in the long-term, be willing to do some research and be involved in the process.

Innocent Bystander posted:

Thanks, Huge.

The large bass trap I mentioned is a damped membrane type. What overall dimensions are the ones you built yourself? (And how many did it take?)

It took lot of playing with positioning to find somewhere tolerably level in my room, but that levelness is far from smooth being up to about 10-12dB between peaks and troughs: and just a foot or so further back or forward, or speakers slightly differently positiond, makes quite a difference. The Indicated corner is additional +15dB at ~46Hz and at ~78Hz. Curiously I don't have a prominent peak at around 24.3 in that corner, though elsewhere in the room there is a peak, but not as pronounced.

I B,

Sorry for the delay, I'm rather chaotic!

The membrane traps I have aren't the more conventional limp membrane, but a lightweight damped compression membrane.  They are 1.2m x 0.42m x 0.45m and can potentially absorb on all four sides (but they are less efficient on the sides due to the type of stabilised fiber block used and the surface facing the wall is significantly masked).  There are 12 of them (but they only cost £20 each).

If you don't have any treatments then u would suggest starting with corner bass traps. That being said I do have treatments on my front wall, that really need with my current speakers, that helped tremendously when I had Mirage speakers and was into HT more than stereo. I just purchased 2 Vicoustic Wavewood panels to go on my back wall also. I'm not sure I need these but I like the looks and something needs to go on that wall. I've read diffusion is best for stereo setups and the front and back walls seem to be best along with corner bass traps, from my reading. 

Huge - Could you give any more details on the "lightweight damped compression membrane" you use. £20 each seems very good value for acoustic treatments which tend to be much more expensive. Thx. 

HardBop posted:

Huge - Could you give any more details on the "lightweight damped compression membrane" you use. £20 each seems very good value for acoustic treatments which tend to be much more expensive. Thx. 

Hardbop beat me to it! Any info, or pointers to info sources would be greatefully received. I've seen some designs, but picking what is best is not always easy.

OK, Initial form...

Knauf Insulation DriTherm cavity slabs - still sealed (i.e. tape up any larger holes!) in their polyethylene packaging.

Frequency / Absorption coefficient
30Hz - 40Hz / 0.2
40Hz - 60Hz / 0.5
60Hz - 250Hz / 0.6
250Hz - 350Hz / 0.5
350Hz - 500Hz / 0.3

N. B. This is VERY approximate: may be +/-50% in absolute value -  I don't have the tools to measure the absolute value accurately, but the ratio between different frequencies is within +/-20% as determined by re-emission measurement.

At some stage I intent to re-test using a thicker (130micron) polyethylene with a less tight packing.  I believe this will move the absorption to lower frequencies and increase the coefficients slightly.  However the current value is actually quite a good match for my room.

Many thanks, appreciate the further details.

...but probably not quite what you expected!

Thanks, Huge.

Your previous post gave the thickness as 0.45m, so presumably 3 x 150mm mineral wool slabs. Those are huge! My interest in a tuned membrane was hopefully to get away with something much thinner...  Out of interest, have you covered them, or do you have a decorative scheme where mineral wool adds to the decorative effect? Fabric covering would alter the damping characteristics of the polyethylene membrane if in contact.

Absorbing bass energy below 125 Hz requires thick material as well as air space behind to absorb the rebound. So yeah, typical room modes in the 45-80 Hz range means quite big and obtrusive traps. Tuned resonators are a more space efficient alternative but they are relatively expensive and you'll likely have two or three modes to treat at several locations. This is where some sort of digital room correction becomes attractive if you are a streamer.

IB, they are 6x75mm thickness.  When I repackage them they'll be reduced to 4x75mm (i.e. 300mm thickness).  This is comparable to the thickness required for a mass loaded limp membrane to work at 40Hz.  You can't see the mineral wool, it's covered by the membrane.  Using fabric in front of them is my next step.  Even if it contacts the membrane, it won't have much effect on the operation of the membrane as that's already damped by full contact with the mineral wool (that's how the membrane looses the energy it takes from the air).  This isn't like a resonant system where damping the membrane would reduce efficiency.

Joe, I'm not using the fibre to absorb energy from the air, the membrane absorbs the energy from the air, the fibre absorbs the energy from the membrane.  It's true these have to be larger then a tuned resonator, but nowhere near as large as a porous absorber.  Semi-tuned limp membrane absorbers tuned for 50Hz will work about 40Hz to 70Hz (and less efficiently to about 125Hz) but have twice the efficiency (i.e. you need half the area).  However, I'd also need separate absorbers to the 70Hz to 120Hz region, to cover the for the 79Hz resonance, negating that advantage.  True tuned absorbers (such as Helmholtz bottles) are even more specific, tuned for 41.75Hz (the midpoint between the 39.5Hz and 44Hz resonances) they'd still only cover 40.5Hz - 42.75Hz, even if I detuned them somewhat.  To cover both resonances I'd have to detune them so much they'd be very inefficient, and I'd still need another set for the 79Hz resonance.  Alternatively I'd have to make three sets to cove all the three main room resonances.

My experiments with applying DRC to the data feed to the streamer weren't that satisfactory, and I certainly wouldn't recommend this approach.  This is why I only apply DRC to the feed to the sub.

Thanks again Huge.

I have an understanding of both limp membrane absorbers and tuned membranes, as well as simple absorbers, just not sure of the best dimensions for practical DIY - thare are a range of approaches people have applied available on the internet, just not easy to pick through, with precious little objective assessment. I am drawn to the tuned type because available space is very limited. Initially I thought of DIY, but available time is a limitation, so I investigated commercial ones - they may still be appropriate for rear wall wideband absorption, as I could then have them printed with photos and visually take the place of pictures on the wall, but bass ones would be better made to measure (and the tuned one I was considering was £500 for one, and only making 1-2dB difference).

Joe, I did have a play with Dirac Live, but found it killed the music - presumably over-correcting. And while DSP can reasonably be expected to reduce peaks in response, anything more than minor troughs put a serious extra load on amp and speakers: a trough of 12 dB requires 8 times the power output from the amp and load on the speakers, which is fine at low listening levels, but if you ever want to play at what I would call realistic listening levels you would need very high power amp and speakers capable of taking it, so in practice it only really lends itself to correction of minor troughs. Judicious treatment of the room can reduce the size of troughs as well as peaks. 

Hi IB,

Yes I found that trying to fill troughs was a waste of time and stressed the amp and speakers; it also had a particularly deleterious effect of the musical presentation (presumable because of the excess stress on the engineering).  Even when using DRC just to reduce peaks, I found that it affected the presentation (mainly time smearing causing a confusion to the textures and internal detail in the music).  Limiting the DRC to just the sub cured these problems.

When I looked at LF absorber systems I realised that the only designs for which I could find well documented sound engineering data were those described in the BBC's papers.  Most commercial suppliers quoted a figure or two determined at one optimal frequency for the design, and then gave a frequency range without specifying the limits in the amplitude domain - this omission renders those figures totally useless!

The only truly tuned designs I came across where Helmholtz resonators and then they still needed detuning to make them useful (see the BBC paper referenced below).  Some suppliers of membrane absorbers claimed their products to be tuned, but then claimed wideband performance as well!

In the end for me it came down to building both scaled up and scaled down versions of the BBC A10 designs
(http://downloads.bbc.co.uk/rd/.../reports/1992-10.pdf)
or trying the damped lightweight pressure membrane system I use.  I measured the 'in-room' performance for the damped lightweight membrane system and although I needed about 1.5 times the area than I'd have needed with the modified BBC designs, the cost was a lot less, and only a tiny fraction of the time was needed.

IB, Joe,

To understand the limitations of DRC you need the concept of whether a system is minimum phase.

https://en.wikipedia.org/wiki/Minimum_phase
http://www.acousticfrontiers.c...lls-room-modes-html/
http://www.roomeqwizard.com/he...ml/minimumphase.html

Normally, some parts of the 'in-room' response of speakers in a normal domestic room fail to meet the criteria of 'minimum phase' and are therefore not correctable (in my room I have this problem at 195Hz to 210Hz and to a lesser degree at about 500Hz).

Hi Huge,

thanks for further info. I have decided I will DIY, and put the money saved towards somethibg else (DIY is one thing I do, whether this or rewiring/replumbing a house, designing and fitting kitchen or bathroom, designing & fitting multizone whole house ventilation...)

Oh, and I eventually twigged: the plastic packaging you mentioned is the outer wrapper of a pack of min wool slabs: I was a bit surprised to interpret that they were individually wrapped, unlike other such products I've seen.