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Adjusting Room sound using material damping methods (not DSP)

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  • The efficiency is even worse than we thought .....

    Oops. My mistake - thank you.

    OK, so you recalculate that 99.9969% of the energy that is blasted into the room by the speakers is useless and doesn't directly reach our external ear. A shockingly inefficient process.

    But in reality, the efficiency is even worse than that because the actual point of sensing sound is the ear drum and we've made our calculations based on the approx. 8cm2 external ear flap. The ear drum has an area much smaller than the external ear flap. Let's say it's only one tenth. The efficiency calculation is so pitiful that there is no point even recalculating but it's 99.99969% wasteful to fill a room with sound. Almost nothing actually reaches our ear drum and in turn our ear from the sound pressure generated by the speakers.

    Which means that the hifi listener is drowning in sea of echoes comfortably sitting in his untreated listening room! There must be almost total degradation in sound quality due to the acoustics of the room which is why I say, if you attend to no other factor in your hifi system, spend a little time and effort on the room treatment. It's the cheapest, guaranteed way to improve your listening experience!
    Alan A. Shaw
    Designer, owner
    Harbeth Audio UK

    Comment


    • The ear as a sound collector

      Two thoughts:

      1. does the structure of the ear act as a horn, amplifying whatever enters it so that the amplitude of the pressure wave that acts on the eardrum has some kind of mathematical relationship to the amplitude reaching the outer ear?

      2. efficiency of the ear shouldn't affect our relative perception of direct and reflected sound. However there is the question of how much the ear collects of each... are jug-eared people better or worse off?

      Comment


      • Efficiency of ears and 99.99..% not direct sound

        Originally posted by honmanm View Post
        Two thoughts:......
        2. efficiency of the ear shouldn't affect our relative perception of direct and reflected sound. However there is the question of how much the ear collects of each... are jug-eared people better or worse off?
        I think the ears still remain 100% efficient. IMHO, what Alan is trying to convey here is the ears only capture 0.0003% (or whatever percentage in relation to the room) of the sound directly from the speakers. The rest of the sound that our ears capture were the remaining sound that gets bounced of the room surfaces. So in untreated room 99.99..% sound that our ears pick up is not the actual sound from the speakers. In short, you are hearing the room which now acts as a big loudspeaker.

        ST

        Comment


        • Clapping in different rooms

          As I'm at work this morning I have been wandering around clapping in two different spaces:

          My office space which is appr. 5.5m x 4m, the ceiling is low at not much more than 2m and three of the walls are bare block work with the fourth being mdf over timber. Not really 'furnished' in any way but it does contain a large amount of boxes and other soft material.

          The adjoining space is a workshop, also 5.5m wide but with a ceiling that must be close to 7m at it's highest point. The whole space is around 12m in length, at ground level the office space is taken out of that but above the space is continuous.

          The office is not actually as 'ringy' as I would have thought; clapping in the centre of the room the sound is over very quickly and exhibits very little lingering twang - the sound is to me somewhat bright but no more than that. Getting much closer to a wall (30cm or so) does start to bring out a more prolonged tone and moving into a corner the sound is (or appears) generally louder.

          Moving to the workshop the immediate difference is that the sound from the initial clap goes on much longer, it's not a big enough space to hear a separate echo but it definitely reverberates and in that sense sounds to me more 'natural' than the office where the sound appears to disappear very quickly. In addition to the pleasant reverberation though there is a lingering presence of sound ringing around the space. I'm pretty sure that this sound is actually from some large metal components - I also doubt that I can clap with sufficient force to reveal anything further about a space of this size.

          The issue that occurs to me in the context of this thread and the different types of room 'problems' that may be encountered, is that as I understand it the 'signature' sound of a room can be set off from anywhere in that room, the twang that was talked about from post 44 - 60 would (I think) have been the same irrespective of where the (imaginary) starting pistol were fired.

          Other issues, flutter echoes for example, appear to be more directly related to proximity to surfaces. Given that ultimately we are concerned with speakers in rooms, it appears that much energy is spent getting the placement of speakers optimised when in fact the placement is only half the story. I don't want to jump the gun on this and clearly the title of the thread states that it is concerned with the room itself, but is actually it possible to keep separate what the room is doing from what is happening due to the particular placement of the speakers within that room?

          Or put another way, in the context of the issues raised so far which of those are affected by the location of the sound source within the space and which are not?

          Comment


          • Room treatment - needed if I play quietly?

            While thinking about that echoes issue and reverberations, I'm wondering when does the advantage to treat a room start. Let me precise my questioning. Does someone who listen at very low volume will get the same improvement to damp his room versus someone who mainly listen at medium or high volume?

            Sebastien

            Comment


            • Listening loudness and room influence ....

              Originally posted by Sebastien View Post
              While thinking about that echoes issue and reverberations, I'm wondering when does the advantage to treat a room start. Let me precise my questioning. Does someone who listen at very low volume will get the same improvement to damp his room versus someone who mainly listen at medium or high volume?

              Sebastien
              I would guess the greatest improvement will be had by one listening at medium volume. Lower, and the ear will detect imperfections proportionately less than the desired signal; higher, and resolution of discrete sounds becomes less in general.

              Originally posted by A.S.
              For this reason headphones are so efficient and only a few milliwatts of power are needed: 100% of the sound is delivered to your ear = very high efficiency. and zero room interaction.... So, your two ears represent only 0.00031% of the total surface area which means that 99.99969% of the sound energy sprayed into your listening room by the two speakers is not only wasted energy but degrades from your listening experience as it becomes tainted with the room's sonic characteristics. That's why attending to the room's characteristics and damping is the most important upgrade you can make.
              This is an excellent thread and I'm very grateful for the effort that has gone into it. If we prefer speakers over headphones, even in an inadequately treated room like mine (pic attached), doesn't that suggest that the room has a positive role to play despite all the resonances and echoes?

              Thanks again for this thread - enlightening.

              Ben
              Attached Files
              Ben from UK. Harbeth Super HL5 owner.

              Comment


              • Every room has an optimuim listening loudness ....

                Originally posted by BAS-H View Post
                I would guess the greatest improvement will be had by one listening at medium volume. Lower, and the ear will detect imperfections proportionately less than the desired signal; higher, and resolution of discrete sounds becomes less in general. Ben
                Does someone who listen at very low volume will get the same improvement to damp his room versus someone who mainly listen at medium or high volume?
                These two comments have one answer. I've illustrated that virtually 99.999% of the energy blasted from the speakers into the room is wasted. That energy (by implication) bounces around the room until it finally makes its way to our ears having been modified by the absorptive characteristics of the room and all the surfaces the sound waves touch. That is definitely not what we want. We do not want the room at add anything to the sound as recorded and as fed into the speakers. Otherwise our listening room unwittingly becomes part of the performance, and change the room and change the performance. Not good at all. So by implication, the more power we grind into the listening room the more acoustic problems. Which in turn means that if you listen at a moderate 80dB there is far less 'room sound' in what you hear than if you listen at 100dB when there is (perhaps) ten times more sound pressure being forced into the room.

                Key point: every room has an optimum listening level where the absorption of the surfaces can cope with the energy thrown at them. Beyond that point they can do no more than throw the sound back into the room, unabsorbed and that will degrade the high fidelity listening experience.

                As we all here live in the real world, it's of no interest to us real-world people (as opposed to theoretical acousticians) how we could eliminate every little echo. We know that a) we could, if we converted the listening room into an anechoic chamber to damp every indirect sound but b) we would then face divorce, bankruptcy or worse. That is not a real-world solution. What is then? I'd say that whatever we dream-up as a solution should be...
                1. Portable, repositionable, can be hidden away when the wife has her friends around for tea
                2. Low cost
                3. Quick and easy to make and modify with few simple tools
                4. No health issues; stable characteristics
                5. Can be scaled up or down in size
                6. Tolerable cosmetic design
                7. Self-standing if possible

                But before we get stuck into the construction of some sound dampers (and I need to find a garden centre that's open to buy some skin materials first) we should have closer look at those echoes we already identified. Specifically, the curious effect we noticed when echoes trail the direct original sound by only a short time gap and are not actually detectable as discrete echoes. In other words, 'when is a measurable echo not audible as an echo'?
                Alan A. Shaw
                Designer, owner
                Harbeth Audio UK

                Comment


                • Every room has an optimum eplay sound level

                  Originally posted by A.S. View Post
                  ... So by implication, the more power we grind into the listening room the more acoustic problems. Which in turn means that if you listen at a moderate 80dB there is far less 'room sound' in what you hear than if you listen at 100dB when there is (perhaps) ten times more sound pressure being forced into the room...

                  Key point: every room has an optimum listening level where the absorption of the surfaces can cope with the energy thrown at them. Beyond that point they can do no more than throw the sound back into the room, unabsorbed and that will degrade the high fidelity listening experience....
                  1) What can I use, without spending a fortune, to measure the dB that I have at my listening position?

                  2) I already experience this that every room, and recording, has its best level to be listen to.

                  I also found this thread very interesting. Let's move forward!

                  Sebastien

                  Comment


                  • Room damping

                    Originally posted by A.S. View Post
                    Key point: every room has an optimum listening level where the absorption of the surfaces can cope with the energy thrown at them. Beyond that point they can do no more than throw the sound back into the room, unabsorbed and that will degrade the high fidelity listening experience.
                    Aren't damping materials rated by absorption coefficient (meaning a fraction of the sound that is absorbed)? But I guess what you mean is that at lower levels the reflections fall below the threshold of audibility. Long ago with much younger ears and bigger brick rooms it seemed that the system sounded best late at night, but at a lower volume level than daytime listening... perhaps because at the daytime listening level the echoes were no longer buried in background noise?

                    As you have pointed out in the thread on equal loudness contours, each speaker design also has its optimum listening level (one of the things I *really* appreciated about the original-type HL-P3s was their ability to sound "right" at very low volume levels). So "big room" speakers are never really going to be happy in a small room... which is probably why we see very few American speakers in the UK.

                    Not to steal your thunder, but I seem to remember from an earlier topic that when you say
                    'when is a measurable echo not audible as an echo?'
                    the answer is that within a certain time window the echo is interpreted as part of the original sound - leading to spurious observed peaks in the frequency response (I do wonder whether that is one of the root causes of obsessive tweakery...).

                    A question: given two very similar speaker designs (say Monitor 30 and Compact 7) could it be that one will suit a particular room better than the other because the peaks in its response do not coincide with frequencies that are given a lift by the room, speaker position, or listener's position?

                    Comment


                    • Fill the room with water and watch the waves ...

                      Originally posted by honmanm View Post
                      Aren't damping materials rated by absorption coefficient (meaning a fraction of the sound that is absorbed)? But I guess what you mean is that at lower levels the reflections fall below the threshold of audibility...
                      No I'm not exactly saying that. I think you are hugely over-rating the absorption capabilities of a real-world listening room. Do not for a moment think that somehow, magically, a few bits of absorbing material scattered here and there (non scientifically, without test equipment) around the room is going to solve serious acoustic issues and make them 'fall below the level of audibility'! There domestic listening room is simply alive with sound which persists long after the speaker has produced its note - and you can here this room hang over with the hand clap test. As I said just a few posts back, imagine filling the room with water. Assuming that the speakers could be played under water (don't try it!) what you'd see in a fraction of a second was that every molecule of water in the entire filled room was dancing about with energy from the speakers sound waves. Bad enough with just one speaker, but turn on the other one and you see a completely crazed mess of waves. You could simulate that by putting a kitchen blender into the fish tank (don't do it with a fish in there!) and turn it on low ....

                      As Dudley Harwood once said 'if something is inaudible, it's inaudible - end of story' but as I hope to show later, you are seriously kidding yourself if you think the dominant reflections off the side walls, floor and ceiling are somehow buried in the noise in a standard listening room! Sorry to disappoint you!
                      Alan A. Shaw
                      Designer, owner
                      Harbeth Audio UK

                      Comment


                      • Absorption

                        Originally posted by A.S. View Post
                        I think you are hugely over-rating the absorption capabilities of a real-world listening room. ... As Dudley Harwood once said 'if something is inaudible, it's inaudible - end of story' but as I hope to show later ...
                        Sorry, I'm thinking in "engineer" mode, trying to understand the nature of damping materials - their characterisation by a frequency-dependent absorption coefficient implies that absorption (at any given frequency) is linear - although clearly there is going to be some sort of elastic limit.

                        Once at work we did a number-crunching demo that animated wave propagation in a square surface with reflective boundaries so I do have a idea of what could result (although that was a very simple model).

                        But if we assume that the reflections are all audible regardless of listening level, how does this relate to the *perception* of less clutter at low volume levels?

                        P.S. clap test in revised living room layout results in mild reverberation at speaker positions, very little at listening position. And no I don't believe the reflections are inaudible - however my goal is to make them insignificant (sorry more engineer-think there, they say that to an engineer there are only two kinds of material object in the world: things that are broken, and things that need to be improved).

                        P.P.S. and that leads to the question of whether you could produce speakers that are more directional but retain the Harbeth character...

                        Comment


                        • A private black hole?

                          Originally posted by honmanm View Post
                          .... trying to understand the nature of damping materials - their characterisation by a frequency-dependent absorption coefficient implies that absorption (at any given frequency) is linear ...

                          P.P.S. and that leads to the question of whether you could produce speakers that are more directional but retain the Harbeth character...
                          I suspect that the issue is simpler than you indicate. As I laid out in a post a few back, in a real-world listening room (I stress this point) the technical performance of the absorbers (absorption coefficient with frequency, linearity or whatever qualitative parameter you chose as a figure of merit) just isn't the issue I think. The issue is that you need a lot of it as I have shown that 99.999% of the energy pumped into the room by the speakers serves no benefit to the human ear.

                          The core issue is the sheer quantity of sound sloshing around in the room which needs to be continuously soaked up, bar by bar, note by note. And since we can't have our own private black hole sitting quietly somewhere behind the listening seat soaking up all the excess energy like a sponge, we are going to have to treat a lot of the surface area to make any worthwhile or noticeable difference. How much? Well, we estimated the total surface area of a typical room as about 50m2 - so shall we say, 10%? Would that damp down the room's acoustics? Or maybe we should aim for 20%? That's 10m2 of some sort or absorber. Visualise 10 square metres for yourself.

                          And how are we going to discretely smuggle 10m2 of damping into our room so that our partners won't notice? Again: the issue is not the laboratory performance of a piece of absorber: it's the practicality of introducing enough of it into the domestic listening room. I personally couldn't care less what the absorbers specification is under controlled laboratory conditions: all we are concerned about is can we practicably gain benefit from such a solution in a normal domestic listening room or not (at reasonable cost) and without serious domestic strife.

                          Back to the 1951 studio acoustics PDF many posts ago. Just how low were those absorption coefficients?

                          P.S. Change the directionality of a speaker, and you change its sound character, at least in the ordinary room. And we are discussing ordinary rooms not anechoic chambers.
                          Alan A. Shaw
                          Designer, owner
                          Harbeth Audio UK

                          Comment


                          • Domestic absorption ideas - the reality of the listening room ....

                            Though this is a very new topic for me, I get the impression that around 30% treatment is the norm - with the focus being on the first reflection points.

                            For a domestic living room, one can get quite a lot of absorption in the guise of soft furnishings (I envy a friend who looks after his elderly parents and has his system in their distinctly over-stuffed living room - it even has acoustic tiles on the ceiling which was apparently a fashion in the '70s...!).

                            The entire floor can be carpeted and the window wall given heavy drapes (2 of the 6 surfaces). Each sofa is good for about 2m2 in the horizontal plane and another 2m2 vertically - more useful if the back of the sofa is above tweeter height. You've mentioned that open bookcases are good diffusers - e.g. the IKEA Billy that is about 2m x 0.8m - another 2m2. A coat stand in the corner of the room - or coat rack on the back of the door helps a bit.

                            Unfortunately in our present circumstances we are not permitted to change the flooring and blinds in the living room, which is why this has been so much of a head-scratcher. The lady of the house is a decor-minimalist (less to keep clean!) so ideas like ethnic wall hangings probably won't fly.

                            One which might is the idea of making little jackets for the speakers (end-of-winter duvet sales should soon be upon us) as this places the absorption closer to the source of the sound. But you've just pointed out that this may have unintended consequences...

                            Looking forward to your scheme for freestanding absorbers, as there is a convenient cupboard outside the living room.

                            Comment


                            • Correcting myself

                              Originally posted by STHLS5 View Post
                              ... So in untreated room 99.99..% sound that our ears pick up is not the actual sound from the speakers. In short, you are hearing the room which now acts as a big loudspeaker...
                              On further reflection, I do not think that our ears pick up 99.9996% of the sound reflected by the room. Taking Alan's example, our ears will pick up 0.00031% only (not 99.9996%) of the sound reflected by the room. For the sake of mathematical precision, the accumulated sound picked up by our ears are:-

                              1) Direct sound from the speakers =0.00031%

                              2) First reflected sound from the room, also = 0.00031%

                              3) As waves would continue to be bouncing of the walls in undamped room, the ears would continue to pick up 0.00031% of each reflection until the reflections become inaudible.

                              However, the loudness of the reflected sound would be much lower than the direct sound. So if we were to compare the direct and the reflected sound there would be a significant loss in the loudness of the reflected sound. So the ratio is not 1:1 but 1lesser than 1). It gets complex when we consider that low frequencies which could continue without much loss in energy as compared to higher frequencies.


                              ST

                              {Moderator's comment: we can argue all day about this or that percentage. Is it not a fact that the *only* sound we want to hear is the one on the recording, and that means the one that arrives directly from the speaker to the ear not reflected off any other surface (wall, floor etc.)? Since the area of the room is vastly bigger than the area of the ear and *since the speakers cannot focus sound only directly to the listener* whatever the maths the room will have an influence on what we hear unless really well damped.}

                              Comment


                              • What sounds hit the ear?

                                I do think ST is onto something here... actually the surface area of the room should not be a factor in the relationship between direct and reflected sound. Assuming the same listening distance, in a bigger room the sound pressure will be weaker at the boundaries so even though there is a greater boundary area it will not affect the total amount of energy contained in reflections.

                                Think of dropping a stone into a pond - the ripples on the surface of the water are biggest near the centre, and get smaller with increasing distance from the original splash.

                                Again using a simplifying assumption & treating the speaker as a 3D point-source radiator, there is a small conical segment of the radiated sound that directly reaches the ear... the remainder goes into the room and that relationship is independent of room size - it only depends on how far the ear is from the source of the sound. The difference between big and small rooms comes in that reflected sound reaches the ear a lot faster in the smaller room.

                                Where ST has hit the nail on the head is that if the ear is smaller, it receives proportionately less of both direct and reflected sound. To a first approximation, reflected energy that doesn't reach the ear doesn't matter.

                                Imagine putting mirrors all the way around the room... when seated at a particular listening position (A), any place you can see a reflection of the speakers is a reflection point, and the size of the each image tells you whether it is a first, second etc. reflection (apparently someone makes kits to help one do this). Someone seated at a different position (B) will see a different pattern of reflections... most if not all of these reflections cannot be observed at position A.

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