Adjusting Room sound using material damping methods (not DSP)

[T.W.]

The winter is long and I'm going to start a new DIY project for a panel absorber. The reason for that is to get rid of room resonances / bass problem below 100 Hz. My other DIY panels that I have placed in all four corners improve the acoustics very well, but seem to have almost no effect at very deep frequencies. (and that's no wonder).

I also have some electronics to beat the bass, but I want to try to solve as much as possible "mechanical" before the computer does the rest. Maybe it's even possible to go without any electronic filtering later...

Before buying something commercial like this:
http://www.mbakustik.de/main.php?tar...sorber&lang=en
I want to try how far I can move myself.

The theory is pretty clear and a good description how we find the final formular is here:
http://dogbreath.de/misc/PlaneAbsorberResonance.pdf
Everything ends up with the mass of the panel and the depth of the box as parameters. Quite surprising!

The question is: Has anybody of you done this already? What is your experience? Is it worth to give it a try?

Thanks. Any thoughts welcome!
T.W.

[hifi_dave]

I've constructed many panels for dealing with mid and high frequencies but not attempted low frequency absorbers. From what you've shown and what I've read elsewhere, the construction is relatively easy and I reckon could even be incorporated into a cupboard or bookcase etc. for a neater effect.

Fortunately my current demo room has walls which are plasterboard over a wooden frame, filled with 6-9 inches of fibreglass. The floor is ceramic tiles over concrete and the resultant sound is clean and tight.

Good luck with your endeavours and keep us up to date with your progress.

[A.S.]

As noted, putting a thin absorptive skin (foam or similar) in contact with and on the surfece of, say, a wall has no effect whatsoever at low frequencies. None. Complete waste of effort. A surface treatment will only effect the upper frequencies.

We covered relatively thin surface treatment boxes here and they do work in the higher frequencies effectively. As you say, for LF absorption you need to suck-out those frequencies by absorbing their very considerable energy by causing a (relatively heavy) membrane to flex at those frequencies. The flexing converts the acoustic energy into heat, and so it disappears as a sonic problem.

Back in 1951 (and I'm sure before that) the design and tuning of these membrane absorbers was well understood and the de facto standard for BBC control rooms and studios. Look here. I think it shows exactly the same concept of a mass of air in a box covered with a flexible skin. As we well know in quality audio, all the fundamentals were thoroughly understood fifty plus years ago.

[T.W.]

hifi dave's room is probably a huge panel absorber. But that's nothing that I can (and want) to build at home. The funny thing about the bass absorption is that you find tons of products state are meant to deal even with deep frequencies, but I still wonder if this works.

So the panel absorber is probably better - at least in theory. Ok, everything is related to the mass and the volume that gives a resonant frequency. Damping inside without touching the membrane changes the characteristic and makes the curve flatter and expands it.

But what about the size? Let's say that we have two M40s in the room that create the bass. Since the room doesn't add any energy we have to "eat" parts of the energy that the speakers produce. Giving the size of the membrane and the excursion of the membrane there is a volume of air that we can calculate with. Since the panel doesn't really move if would have to be quite large to eat all of this energy. (But we don't really want to do this :-)

- Are two panels of 1 square meter a good starting point or too small?
- How do I measure the resonant frequency / characteristic if the panel is done? Microphone inside and some pink noise?
- Does the size ratio of a panel matter. The paper I mentioned says: 1:1 to 1:1.5, 1:1.7

T.W.

[A.S.]

If I were you, I'd print out, carefully study and run a highlighter over key points in that 1951 studio design paper I mentioned here. In my experience, there is more wisdom, more pragmatism and more elegance in the way our grandfather's generation solved (acoustic) problems than any of the modern papers. I selected that paper with care - it really does give you lots of answers ... and just quickly looking at it your questions 1 and 3 are really answered by Fig. 12 and the Worked Example.

In short, you are wasting your time with a couple of 1m2 absorbers.

[T.W.]

If I were you, I'd print out, carefully study and run a highlighter over key points in that 1951 studio design paper I mentioned here. In my experience, there is more wisdom, more pragmatism and more elegance in the way our grandfather's generation solved (acoustic) problems than any of the modern papers. I selected that paper with care - it really does give you lots of answers ... and just quickly looking at it your questions 1 and 3 are really answered by Fig. 12 and the Worked Example.

In short, you are wasting your time with a couple of 1m2 absorbers.

Yes, you are right. I looked at the paper again and everything makes sense to me. They first analyse the room, come up with a target of what to achieve and then build the stuff. That's an engineer's approach. The figures you mentioned are very detailed and it should be possible even for me to adopt some of the ideas for my own "project". I'm probably not able to and don't want to re-design and the entire room, but after doing some homework I hope to come up with an idea what to build. One constraint will be that everything has to moveable and doesn't look too ugly.

I will start doing some measurement to find out what I have in my room. From calculations I know my trouble frequencies but have never really verifed this with a measurement. I also have no idea about reverberation time yet. My DIY panels that I mentioned helped to make this better and speech sounds quite good. They also helped to remove the echo from clapping but I have no idea what actual values I have.

I recently started with SynRTA to look what response I have in my room and what the electronics are doing. But I have just been playing and one challenge was to tell my Vista notebook to accept the M-Audio Transit soundcard and also to use it. With the newest drivers from M-Audio SynRTA also seems to run under Vista (although they recommend XP) and the results look quite reasonable.

Many people publish these nice looking waterfall diagrams. What software do you use for those and RT measurement? I downloaded the RoomEQWizard from Home theatre shack and I'm about to have this also running.

I'll keep posting my findings here...

T.W.

[george_k]

Alan,

That 1951 paper you mention has been taken down temporarily, can you repost it?

[A.S.]

Ok, this must have been one of those huge PDFs that couldn't be taken from the old version of the HUG to the new. I'll put it on my list of jobs to do. (I think I'll have to split the PDF).

[derekhughes]

It is worth reading what is probably one of the most wide ranging (and free!) documents on the whole subject of acoustic room treatment, as seen from a BBC perspective.
I think this probable answers 90% of questions on this subject (except where to get the money to do acoustics properly :-) )

http://downloads.bbc.co.uk/rd/pubs/a...icpractice.pdf

Derek

[unleash_me]

Building a Reference Grade Listening Room - This is an interesting article of how one of the best listening room is created!

[keithwwk]

Thanks for the article. I wonder how much the owner had spent in that room in order to let his speaker to sound good.

I feel very lucky my Harbeths do not need such room to sound half good. :P

[Sebastien]

From my archives, I attach an easy to follow approach to the design of a (BBC) studio. Even though this was published in 1951 (nearly sixty years ago) nothing has changed in the world of acoustics...

Where can I find the link for that article?

Sebastien

[honmanm]

Since our move earlier this year I've battled with a thin midrange, the first time this has been an issue in any previous iteration of our system as the components are usually selected for midrange quality above all else.

For many years I've been happy with small Magneplanars but all good things come to an end and about the time the Maggies finally died I first encoutered Harbeths in the the form of the original HL-P3 - my first brush with natural, unfatiguing sound in a "box" speaker.

So shortly before the move my beloved Quad 33/303 was sold to help fund a pair of P3ESRs, and the combination of new speakers, stop-gap amplifier, and new room has really had me going.

The first step was to eliminate the P3ESRs as a source of the problem by trying them in other' systems (Audio Research and Luxman electronics - both wonderful matches but way way out of my price league).

Although I have since moved onto a more competent amplifier (albeit still somewhat lean sounding in comparison with the above-mentioned Audio Research setup), the room has turned out to be the main culprit.

AS recently posted a PDF of the seminal Shorter paper. The perceived frequency response in our living room is something like the top curve in Fig 1. (p3).

Shorter writes "if a progressive decline in response with increasing frequency is followed by an increase, the upper frequency range will be heard to stand out in unnatural relief, even though the response may nowhere rise above the mid-band level. It should be noted that this type of frequency characteristic modifies the spectrum of the reproduced sound in a way not experienced when listening to natural sounds. Progressive attenuation with increasing frequency is an everyday occurrence - it is experienced, for example, when listening to sound that has travelled round a corner - but, apart from a few isolated cases of specular reflection, selective reinforcement of the upper frequency range does not appear nature."

Now to the room - it probably represents typical modern construction in the UK, with the interior surfaces being plasterboard over cinderblock. It is a hard flat surface that makes a distinct "tack" when rapped with the knuckles. It's not solidly attached to the cinder blocks, and can even be provoked into booming (hard to believe with mini-monitor bass).

The smallish size of the room (about 3.5 x 4.5m) and hard surface make it prone to flutter echo, so the room ends up giving reinforcement to the top and bottom ends of the frequency range - and hence the "hole in the middle".

I suspect earlier iterations of the Harbeth mini-monitor may be more tolerant of this setting on account of their LS3/5a style "presence bump" which has a "double whammy" effect of filling the hole and doing so at a lower SPL so that more of the HF reflections fall below the threshold of audibility (but that's a guess!). I also wonder whether the improved bass of the P3ESRs is actually counter productive in this setting.

Sorting out the room is a work in progress, the owner of the property wouldn't permit some quick fixes like heavy curtains. (The P3ESR measurements published by Stereophile have been very useful, by the way).

Oddly enough the best speaker positioning (so far) follows rules that worked with the Maggies,

• a clear wall behind the speakers (more midrange energy than HF escapes to the rear of the speakers)
• tweeters point at the shoulders of the listening position, at ear level (avoid the suckout that occurs when ears are above tweeter level)
• slight backward tilt to the speakers (HF reflections don't go back and forth forever)

In this room it is quite important to point the speakers almost at the listener's head as this also seems to result in fewer reflections bouncing back and forth between the front and back walls. This is counter to what I'd normally do, listening a bit off-axis to reduce the amount of direct HF energy.

Reluctantly I've come to the conclusion that proper room treatment is going to be needed - but as it's a normal living room the challenge will be to find something that is discreet, inexpensive, and effective.

This has been quite an essay but hopefully it will be of use to other P3ESR owners (and hopefully there will also be some useful suggestions that come in response to it).

[Sebastien]

... I've come to the conclusion that proper room treatment is going to be needed - but as it's a normal living room the challenge will be to find something that is discreet, inexpensive, and effective.

I face the same challenge so any suggestions are welcome.

Sebastien

[HUG-1]

Thwe new User Guide that will be supplied with our speakers adds some more information about room treatment. We will make a version available here on line shortly.

[BAS-H]

That would be much appreciated, HUG-1, many thanks.

@honmanm, I have a similar experience to you, though in a slightly different way. I have P3ES2s in a small room, constructed from plaster board fixed to block walls. The room gives a bloom in response in the mid-bass, somewhere around the low cello range. I've taken to simply ignoring the effect; I don't let it bother me. I went to a live concert recently. The live sound is so massively different from reproduced sound at home anyway, that I've come to accept the shortcomings of the latter. Once I got my mind round to this, I found it very liberating and enjoy music at home all the more for it.

Seek perfection in your room by all means, but don't look too hard!

Best wishes, Ben

[cornelius]

honmann - it's interesting to see someone going from Maggies to Harbeths. For a while I was only interested in "non-traditional" speakers - Maggies, Ohm, Vandersteen... Harbeths are the first "box" speaker that I've heard (that's not a $Million) that disappears and doesn't sound like a box.

I'm starting phase II of a renovation in my apartment, and had to move a bunch of things into my living room/kitchen area while the work is being done in another room. The room is now significantly more damped, and although not set up optimally, my P3ESRs are sounding better! My living space reminds of a control room now :0

Good luck with your room treatments!

[tein3]

Sorting out the room is a work in progress, the owner of the property wouldn't permit some quick fixes like heavy curtains. (The P3ESR measurements published by Stereophile have been very useful, by the way).

Reluctantly I've come to the conclusion that proper room treatment is going to be needed - but as it's a normal living room the challenge will be to find something that is discreet, inexpensive, and effective.

This has been quite an essay but hopefully it will be of use to other P3ESR owners (and hopefully there will also be some useful suggestions that come in response to it).

Hi,

I would like to share with you on my recent journey to set my listening room... and how it had been a gruesome but enriching one... :-) What I have gone through might be of help to you, I hope.... or at least you could try out with... if you look at the pictures section, you may find pictures to my previous room and current room.

I do agree with you that proper room treatment is needed if we wish to enjoy our music to the best possible without the room affecting us... and something discreet, so as the living room still look like a living room... but that is truly tough challenge. I am fortunate to have my "serious" listening in a room and "casual" listening in the living room.

Experiences in my listening room set up:
- basically... all the materials I used initially in this room, are from my previous room... same curtain, same pictures from my old room... The photographed pictures printed on coarse paper helped my fluttering echo in my old room, but it did not help in my new room... :-(
- So I went around to look for economically price canvas painting... these assisted my in fluttering echo treatment
- I also learnt from a friend recently when he demoed to me, how his room curtain assisted him in fluttering echo, where he has them on both rear and sides of speakers... and also, for rear sides, how he uses nice carpets hung on walls to assist that...
Both the above materials, you could place around to treat it as a decor and at the same time... room treat the place

But in my journey to this new room of mine... the bass bloom was the trickiest! It went to the extend that I gave up all "powered" power cords, to tame the amps and source down... that cuts the bloom to some extend... BUT Cello or Double bass pieces still gets the problem....
- Then I replace away my tube amps, as they have a less control bass or ... what we call they do not have tight bass.... Transistor amps works better in the room... there goes... the amps and new amps came on board... :-( sad but a happy upgrade too :-)
- How was the bloom eventually solved? This is classic... as I would marked in my learning journey... I had Sonex placed everywhere I could... front of speakers, rear of speakers, side of speakers... and even took mattresses and pillows to fill the rear of the speaker's space.... BUT none assisted, I even home trial a China made bass trap (works in the owners's shop but not my room...).
And where did it assist...? The sonex was placed behind the curtains you see in my picture? Stand upright with the absorbing side facing the room, somewhere off middle of the room near to the right speakers. That killed the bloom!

In my living room... the C7s, each have diffusing at their rear and partial sides... where my customized TV and equipment rack is, I have asked the carpenter to have strips of wood strips... in high and low height.... to help diffuse the sound sound wave around the speaker. It works, and no major bloom. Now that my room problems are resolved... I am working on my living room now... :-)

From the above... in summary...
Try using curtains, canvas oil paintings, paper drawings... to help your room fluttering issue and still decor the room :-), but please note (as you would already know)... different cloth thickness gives a different effect.
Try absorptive materials for your bloom issues, try all areas in your room and see where helps... it can be at the most un-thought of location... if you need to use sponge/foams... see how you can hide it behind strategic areas, using the curtain to hide them? OR some other forms... or look for ... Chinese "curtain"? You know... those... emm... wood panel pieces that can be folded? And once you stretch them open.. it becomes a "wall", this can break sound waves... and some comes with porous matetrials, you can hide your foams behind them... so you do not see the foams but a nicely decorated panel :-)
Try carpets... hung on walls... it helps, right in the middle, it helps lots in imaging... sides or "funny" corners, it helps bloom issues too...
Try your cables first... if you are using "hi end" cables, now is time to dig out all your old basic cables... try reverting all power cables and RCA cables... all to basic first. From here... see if cables are your issues... (btw: all my cables are back to basic, normal belden power cables, normal belden RCA cables) and system is sounding great in this match in this room. You will be surprised cables that works for your past room but not in the new room... that happens to me! :-)
Try speaker placement.... fluttering issues and bloom can be cured with this too... use the basic position... if it does not work... then try using they most unknown or untried angles... and tune from those angles to the best... for you and your room (only try unorthodox speaker placement method... if all traditional methods, does not work...)

I hope you can understand my writing... and the information will be of some help to you :-)

[honmanm]

Thanks, that's a lot of useful pointers.

One thing that has helped - at least while Mrs H is on holiday - is to put pillows on top/behind the speakers. I hadn't expected it to help, but it has reduced both flutter echo and bass bloom.

[A.S.]

The basic truth of high fidelity sound reproduction in the home is that the acoustic signature of the listening room is the final link the the reproduction chain.

In fact, starting from the microphone, we have the acoustic signature of the recording studio picked-up by that microphone, delivered to the loudspeaker and then played over the speakers in the listening room. But if we think about it, the loudspeaker cabinets themselves are miniature rooms. So in truth we have the acoustic signature of the studio overlaid by the acoustic signature of the speakers and finally overlaid with the acoustic signature of the listening room.

Put crudely: we have the acoustics of the studio (a box) reproduced over the loudspeaker (a box) in the listening room (another box). We can take this one step further: consider that we are reproducing a wooden instrument such as a cello or even piano: now we have the instrument (a box) in the studio (a box) reproduced over the loudspeaker (a box) in the listening room (a box).

A box in a box in a box in a box. No surprise then that each box will have a modifying effect on the preceding signal.

The characteristics of the instrument are as intended by the maker, resonances and all. The loudspeaker designer (presumably) controls the contribution of the speaker box, so that leaves the recording studio and the listening room as the biggest variables, and the ones which benefit from some acoustic treatment with the sole objective of minimising their contribution to the reproduction chain. We at home have no influence over the studio (or hall) that the recording was made in, so that leaves only the listening room as within our capability to modify. Some rooms can be inexpensively treated; others can't. Let's look at them.

To be continued .....