Adjusting Room sound using material damping methods (not DSP)

[A.S.]

The DIY trap looks like an excellent idea - maybe Harbeth should make something like that - but I seriously doubt the claim that it is effective at 80Hz.

To be effective at 80Hz or so, the absorber material would have to be about 1/3 wavelength deep; the wavelength of 80Hz is 4.2m and one third of that is 1.41m. Clearly this DIY absorber is about 0.3m deep which means that its efficiency at absorbing 80Hz will be negligible, but it will be effective at absorbing about 300Hz at which it is about 1/3 of the wavelength deep.

However, it could be that the combination of the three pieces of (what look to me like fibreglass) on top of the hidden inner cloth and then the small air gap under the cloth to the wall could somewhat increase the absorption, but I'd say, from appearance, that this DIY trap whilst effective in the midrange - and therefore most worthwhile - is ineffective or useless at low frequencies. It's just doesn't protrude from the wall far enough to prevent any obstacle to the low frequencies in the room. To absorb LF you need a very sophisticated absorber made from sprung sheets of heavy, roofing felt or similar held over a deep frame and tuned so that the sheets flap (very slightly) in the presence of the LF and hence energy is transferred from sound to heat at those frequencies. In other words, you need a clever combination of mass, springiness, depth and natural absorption: a few floppy pieces of fibreglass will present no resistance to a low frequency sound wave at all.

Also - I strongly recommend not to use fibreglass for health reasons. I believe Rockwool to be far less irritating when inhaled.

[A.S.]

I don't think it matters much. There are no identifying codes on the Rockwool. Remember: to absorb low frequencies you need very thick absorers protruding perhaps a metre or two out into your listening room! Clearly this is not viable domestically. So whatever (wall hanging?) absorbers you can live with are only going to be effective in the middle and upper frequencies. They will have no useful absorption in the low frequencies. That's the physics of sound waves.

[A.S.]

After the server upgrade, I've been able to add this PDF. See previous post (click the PREV button above this posting).

[STHLS5]

totally [/I]reflective, there would be a deep crevasse at that frequency, but in the real world, some energy is lost in the floor carpet so less than full cancellation results.

Can we prove this? Yes, we can do that easily and for almost zero cost. If we put a slab of Rockwool on the floor between the mic and speaker, we can greatly reduce the energy of the floor reflection, so the speaker's true flatness is clear. If we put that slab on the walls we'll have the same energy-loss benefit, ditto the ceiling. We can tune the room's absorption to soak up just the right frequencies by adjusting it's thickness and position: to absorb bass frequencies we need soft and thick material. To absorb middle and high frequencies we need soft and thinner material. We decide what frequencies need treatment and then we adjust the absorbers to do their work. As you can clearly see from the image damping-benefit2-sc.jpg, that the Rockwood does a fantastic job in smoothing out the floor reflections - this speaker is now measures extremely flat, in-room and sounds so.

Getting the best room sound is ultimately, about damping the room a little or a lot. Reflections always degrade the quality of hi-fi listening because they force the brain to work harder to 'hear through' them.

>

Hi,

Just something to share.

I was having the same dip around 250 to 315hz (varies with even a slight movement of SPL meter or the listening chair). It turned out to be from the rear wall despite having a 4 inch 80kg/m Rockwool with 1 inch of gap between the wall. The node/null calculator could not explain the dip. I took a wild guess that the non covered area of my rear wall was contributing to the null in a complex manner.

I finally put another 2 x 4ft 4 inch 80kg/m Rockwool few inches behind my listening chair and the problem was solved.

Regards,
Chelvam

[davidlovel]

Getting the best room sound is ultimately, about damping the room a little or a lot. Reflections always degrade the quality of hi-fi listening because they force the brain to work harder to 'hear through' them.

>

There is evidence that suggests otherwise.

i am in the middle of reading Floyd Toole's book "Sound Reproduction - Loudspeakers and Rooms". He catalogues evidence gathered over his 25 years research in this field which shows unequivocally that there are some room reflections that improve the listening experience. He documents subjective tests in typical domestic-size listening rooms which show that the presence of the first side-wall reflection adds noticeably to the stereo effect stage width and recording ambience perceived by the listener. Listeners consistently preferred the sound with undamped side walls. The damped room you describe may be appropriate if the listener wishes to experience a close-miked recording as heard in the control room. For acoustic recordings in a 'natural' venue, which are probably recorded with a mike(s) much nearer to the musician(s) than most of the audience, the 'mix' may have some ambience added from a mike further away in an attempt to get a sound closer to what a typical audience member would have heard.
Toole remarks on the characteristics which differentiate the concert halls (Concertgebouw, Amsterdam, Musicverein, Vienna) that are generally regarded as the best sounding from others. The former have a shoe-box shape with reflective walls and the performers sited along the short wall, hence the majority of the audience will experience the first reflection as well as the direct sound. Toole points out that while there is a large difference in scale between a concert hall and a home listening room, the same basic principle applies. Another important finding mentioned by Toole is that the reflection angle for the first reflection must not be too great otherwise there will be no benefits. Again he draws a comparison with the relatively poor perceived sound in modern concert halls that tend to be much wider (i.e. nearer square) so that the angular difference between the direct and side-wall reflected sound is much greater.

In my listening room the HL5s are positioned across the shorter dimension of a rectangular room (all walls are brick or blocks) that has a window along most of one long wall. The other long wall has minimal damping. I find that there is a noticeable loss of stereo spread and ambience when moderately heavy curtains are drawn across the window at night.

Toole does note that all other reflections tend not to be liked, causing confusion to the listener.

I strongly recommend that anyone interested in this subject read Toole's book (it was reviewed in the June 2009 issue of Stereophile).

David

[Champion]

I think both ideas - maximum damping vs allow some reflection in the room - have their merits. It depends on what you are trying to achieve. If your aim is to hear what is in the recording (normally true for recording engineers), then you must have as little reflecfive sound as possible, otherwise, you are hearing the room. However, this also means that you will have small soundstage width (and depth?) - in theory, the width cannot expand outside the speakers.

On the other hand, if you want to have more a better soundstage, and don't care if you hear a bit of the room, then a reasonable amount of reflective sound is good. However, by definition, you are hearing a mixture of the room and the speakers now.

[T.W.]

Here is a very interesting article about speakers in real rooms.
Linkwitz:Room reflections misunderstood?

[A.S.]

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. The techniques that apply to studio design are equally applicable to home listening, although at home, we should not aim for such a well damped acoustic - nor will we tolerate deep absorber panels on the walls. However, this is an insight into the subject with a worked example. In the various graphs if you see the axis marked as Absorption Coefficient, this simply means the efficiency of absorption; 0 means no absorption and 1.0 (not achievable) total, perfect absorption. You can see in Fig. 5 that unpainted brick walls have no absorption (close to zero) and heavy curtains much better at about 0.5. So, listening in an untreated brick room will give you a mess of reflections.

If you can, take a look at Fig 1 and 2 as they highlight the real problem of poor studio or home listening room design. Fig 1 shows how the ear expects the sound in a room to decay evenly with the passing of time right across the frequency band from low to high frequencies. This is an ideal situation as it sound soft and natural, and no frequencies linger which would give emphasis to particular notes. Fig. 2 shows a more typical situation: note how the decay curve is of different gradient at different frequencies and the 'needle' pulses of sound (flutter echo) which sound like a 'twang'. You can create your own flutter echo if you clap your hands or shout under a railway arch or in a tunnel; what you hear is like a burst of machine gun fire as the sound bounced around. Flutter echo must be avoided in the listening room at all costs!

Note: curtains provide a generally useful, wide band absorption. If they are spaced away from the wall, the air gap can be tuned to selectively absorb (some) lower frequencies. Curtains remain the simplest and neatest way of soaking up room reflections - and can be opened or closed to suit.

>

[STHLS5]

Fig. 2 shows a more typical situation: note how the decay curve is of different gradient at different frequencies and the 'needle' pulses of sound (flutter echo) which sound like a 'twang'. You can create your own flutter echo if you clap your hands or shout under a railway arch or in a tunnel; what you hear is like a burst of machine gun fire as the sound bounced around. Flutter echo must be avoided in the listening room at all costs!

>

Hi Alan,

I have a unique situation in my room. I do hear flutter echo which can only be heard when I clap while sitting in my listening chair. No other spots in the room causes the problem. I knew exactly which spot to treat to address the flutter echo ( about 0.5 sq/m to my immediate right and left side wall). However, it doesn't matter if I treat the spot or not as the sound remains the same. In fact, I think it sounds better without treating the particular spot.

What's your opinion, please?

Thanks.
ST

[A.S.]

Are you absolutely sure that the echo is from the side to side walls? I know a modern-style BBC control room where the management were fed up replacing the carpet around the mixing desk, worn away by the castors on the sound engineers chair as he moves up and down the desk. Management's solution was to replace the carpet around the desk with a nice, modern, hard wearing wood-block floor. The flutter echo at the sound engineers position is now inescapable; it must be a floor to ceiling problem; the producer, sitting on a raised couch behind the engineer hears none of it. The producer is higher up the hierarchy, so I don't suppose it will ever be sorted.

Did you notice the line in the 1951 article that said that sound absorption at right angles to the echo direction was completely ineffective? As it said, that means that in a cube with six surfaces, to damp all reflections, at least three dissimilar surfaces must be treated i.e. floor, short wall, long wall. So, in your situation, treating one (pair) of walls, the other two dimensions are consequently untreated.You may be better removing the treatment from one of the wall-pairs and positioning it at 90 degrees i.e. on the ceiling. That would make an interesting cross-check on the real source of the echo.

[STHLS5]

Yes. I did treat those area with diffusers and the echo disappeared. I am actually wondering if you could provide a correct way to determine the echo spots and whether they should be treated at all. My thinking is since the source of the sound are the loudspeakers, I started to address room treatment by first clapping/playback vocals with one loudspeakers at the loudspeakers position and to see if I can hear any echo at my listening spot. and I gradually added absorption and diffuser until I get the best possible sound without any colouration.

I did add extra absorption to the side wall but that made the room sounded dead. Since sound coming out from the loudspeakers should reach the listeners' ears without echoes/room colouring I thought it is not necessary to treat echoes originates from source of sound other than from the loudspeakers and near it placement area. What do I know, I just listen to music

ST

[A.S.]

I've just returned from a conference in New York, during which we (briefly) discussed room acoustics. I know that I've said here that 'all echoes are bad in the listening room' but it seems that this may not be completely true. Current thinking is that some echoes, those which closely follow the music (in time) and decay evenly may fuse together with the music in the brain and suggest a bigger sound stage. The key to making this work is to keep the reflective surfaces near to the speakers (hence the reflections follow the notes very quickly) in a relatively small room.

By implication, large untreated rooms will have a longer decay and unless this decay is really even across the spectrum and fast (unlikely) then there will be some confusion in the brain as to the direct and reflected sound. Under those conditions I hold my original statement that 'all echoes are bad'.

[davidlovel]

Not sure if you have read my recent post on this thread, Alan:
http://www.harbeth.co.uk/usergroup/s...=6182#poststop
I am sure it is very subjective but evidence of the potential value of the first side-wall reflection has been around for a few years.

David

[A.S.]

I did, hence the subject discussion in New York, and my subsequent comment.

But - and this is a really big but - the reflections must be evenly balanced in character and die away smoothly. This cannot be assumed. It is most unlikely to occur in the real room outside the lab. What is not clearly stated is that if there are any irregularities in the enchos - like the flutter echo mentioned previously, then the ear will be greatly troubles by the timbre of the echo. So, as with all things audio, we have to balance the theory with how things will work in the real listener's room.

[davidlovel]

Thanks for the response Alan.

Yes I am sure that the typical 'real' listening room deviates considerably from the idealised version used in the experiments that I referred to.

I have a lot of admiration for a CEO who not only sets up a company Forum for users, but also manages to read and respond to so many threads - one reason why your products are highly regarded I guess (Paul McGowan does a similarly excellent job for PS Audio).

I assume that your "discussion in New York" was at the AES meeting: the programme looked interesting but as an almost-nearly-retired engineer I have other priorities now (I was watching whales in Nova Scotia!)

David

[A.S.]

...I assume that your "discussion in New York" was at the AES meeting...

No, most definitely not the Audio Engineering Society which was (apparently) running at about the same time in New York.

As the maths alone tells you nothing about how a speaker sounds I'm afraid that as a humble speaker designer I do not have the intellect, imagination or patience to wade through impenetrable maths of the type that is the AES these past years to be no further forward in subjective evaluation. Harwood was the last really great writer - but the greatest of all was, of course, Michael Faraday - my personal hero. He brought science to the ordinary man through his public lectures.

No, my training session was outside AES and is here.

I have more to comment on this later. It was remarkable not for what we covered, but what we didn't.

[yeecn]

Not sure if you have read my recent post on this thread, Alan:
http://www.harbeth.co.uk/usergroup/s...=6182#poststop
I am sure it is very subjective but evidence of the potential value of the first side-wall reflection has been around for a few years.
David

Talking about side-wall reflections - Sam, the Harbeth agent in Malaysia, set up a pair of SHL5 in a small room, about 4.5m X 7m. The speakers are placed on the short wall. One of the side wall has a big glass window looking out to the outer room. A rather think foam is placed over it. The concrete wall on the other side is not treated. Sam did it that way on purpose to make the room 'alive'.

I was impressed by the demo. In fact Harbeth was the first speaker I auditioned after I decided to look for an alternative to my 12 weeks old speakers. I was so impressed that I placed the order for a pair of C7 and a P3ES2 right away. There is a magical quality in Harbeth that lead to an instant recognition that I don't have to look any further.

Anyway back to the side-wall reflection. The 'live' factor was impressive indeed - but I was troubled by a discordance band around the high overtones of the human voice. It was a very narrow band of frequencies, but I find it jarring and disconcerting, almost like two slightly out of tune instruments playing together. It lead me to think that the reflected voice is causing a destructive interference around that particular frequency band.

I pointed it out the Sam, but he was not the least troubled by it - so I am wondering whether my perception is correct.

[STHLS5]

Hi,

Please read my PM to you.

Thanks

[ryder]

I've just returned from a conference in New York, during which we (briefly) discussed room acoustics. I know that I've said here that 'all echoes are bad in the listening room' but it seems that this may not be completely true. Current thinking is that some echoes, those which closely follow the music (in time) and decay evenly may fuse together with the music in the brain and suggest a bigger sound stage. The key to making this work is to keep the reflective surfaces near to the speakers (hence the reflections follow the notes very quickly) in a relatively small room.

By implication, large untreated rooms will have a longer decay and unless this decay is really even across the spectrum and fast (unlikely) then there will be some confusion in the brain as to the direct and reflected sound. Under those conditions I hold my original statement that 'all echoes are bad'.

Although I am not an expert in room acoustics, I can offer some thoughts based on my experience. Echoes are caused by reflection of sound off the walls of the room and will be present in any listening environment. To eliminate echoes completely, all wall surfaces including the floor and ceiling can be covered with absorptive material which will render the room to be acoustically dead. There will be no reverberation at all. An acoustically dead room is not conducive for proper listening since all life and dynamics will be sucked out from the system. Personally I have not experienced listening in an acoustically dead room before but I can imagine how a system would sound like in one.

Speakers will always sound different in different rooms irrespective of the electronics that are used. I have listened to the SHL5 in 4 different rooms and all of them produced a different kind of presentation. Generally a smaller room will give a more focused sound, more precise imaging and better bass definition while the bigger room yield a "bigger" and life sound but reduced bass impact and tautness. It is difficult to say which is good or which is less preferred as each has their own merits. One thing I discovered is the Harbeth does not require a lot of treatments to sound good. In fact I am not using any absorption in my pretty big listening room now and love the organic and life-like sound in the room. I have 8 pieces of 2'x2' diffusors on one side of the wall though and they brought an appreciable difference, although not as huge as when compared to when they were in the smaller dedicated room.

[yeecn]

By implication, large untreated rooms will have a longer decay and unless this decay is really even across the spectrum and fast (unlikely) then there will be some confusion in the brain as to the direct and reflected sound. Under those conditions I hold my original statement that 'all echoes are bad'.

I believe that artificial echo may be added during the mixing stage. I have listened to many voice books. Some recordings seems more 'roomy' than others, and I believe that these are artificially added 'special effects'. I also vaguely remembered playing with voice recording software that allow me to add various effects to the recordings. I have to say, I find these special effects quite pleasing.

My Denon AVR and my car CD player both contain setting for various simulation modes, like Jazz Club, Rock Arena. I think it is a set of equalizer profile and echoing effect. They make marked differences to the reproduced sound, but I have never been bothered to play around with them though.