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Adjusting Room sound using material damping methods (not DSP)

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  • Time and frequency room echo characteristics

    Originally posted by A.S. View Post
    ...Aside from the drone or honk or twang we've heard in the room's reverberation there are other listening room (or studio) acoustic issues which are at least as serious, and may be even more irritating. One of these is sonic bounce of flutter. Sound waves behave just like a tennis ball, and given just the right circumstances they will efficiently bounce backwards and forwards between (generally opposing) walls. With each bounce they'll lose a little energy (just as the ball would)...
    Another thread has started recently which is covering part of the same general subject of room acoustics that we are discussing here, specifically the Left-Right balance as perceived in the listening seat at the sweet spot. The appreciation of room acoustics as I'm explaining in this thread (will) completely explain why the balance of sound could indeed seem to shift left or right and once that's understood yield the inescapable remedial conclusion. It may be more fun to get hands-on but personally, I'm at an age now where if I can think through a solution without moving from my seat, I'd much rather do so.

    So, assuming that we are all travelling together on this room reflection issue, and assuming that you have listened to the previous sound clips, we should be in agreement that there are two (obviously) interlinked echo characteristics in the listening room:
    • Frequency related room characteristics such as a note or notes (a drone) which adds energy in one or more frequency bands but probably not across the wide audio band (example Clip 12) and/or
    • Time related room characteristics where an echo results in a mirror image of the wideband audio sound being splattered around the room (example Clip 17) and can effect many frequencies or all

    Time and frequency are two sides of the same coin and are intimately connected, so these two are indivisible, and classifying an echo as only a time or frequency issue doesn't make sense. So we are going to have to sidestep that academically and just look at how in the real world listening rooms these two characteristic echoes manifest themselves.

    To save me the trouble of dreaming up a room, can anyone upload a line drawing of their listening room viewed from above showing the position of their furniture and speakers with approx. dimensions?
    Alan A. Shaw
    Designer, owner
    Harbeth Audio UK

    Comment


    • Room plan

      Originally posted by A.S. View Post
      ...To save me the trouble of dreaming up a room, can anyone upload a line drawing of their listening room viewed from above showing the position of their furniture and speakers with approx. dimensions?
      Sorry, I am still experimenting with WCL, meanwhile I have attached my room plan which is too basic. Anyway...

      Room dimension is about 370cm x 440cm.

      Speakers (tweeter) to front wall = 116cm, Speaker to side wall = 102cm. Sitting position from rear wall from head to wall = 106cm. The sketch did not include other room treatment materials and two small CD rack next to the speakers. There are no windows, except for two wooden door one left and right.

      ST

      Comment


      • Excellent. Many thanks. It's better to work from a real-world room rather than me draw an entirely imaginary one. I need to 'scan' your drawing to a CAD program - can you confirm how it is scaled? As a cross check, can you tell me what size you drew the speaker boxes at and what the scale factor is to real life? Ditto the room itself. With this plot and your confirmation of scales, I'm limbering up to move on to some simulations of the situation in your room over Christmas.
        Alan A. Shaw
        Designer, owner
        Harbeth Audio UK

        Comment


        • Drawn to scale

          Originally posted by A.S. View Post
          .......I need to 'scan' your drawing to a CAD program - can you confirm how it is scaled? As a cross check, can you tell me what size you drew the speaker boxes at and what the scale factor is to real life? ......As a matter of interest, have you tried the hand clap test throughout the room? Are there any zones where you can hear anything 'in' the echo as it dies away?
          I am reattaching the plan drawn to scale. The scale is 1:27.54. Speaker height is 107cm. Room height is 286cm. The table is flat wooden board with the PC LCD monitor. And a small bare wooden chair (not stool). One side table that I forgot to add.

          OK, a Merry Christmas to all.


          ST

          Comment


          • Improving the sound in my room

            Originally posted by A.S. View Post
            ... The appreciation of room acoustics as I'm explaining in this thread (will) completely explain why the balance of sound could indeed seem to shift left or right and once that's understood yield the inescapable remedial conclusion.
            I'll look forward on this subject since I like to improve the sound quality in my room. This is my project for 2011.

            Sebastien

            Comment


            • Originally posted by STHLS5 View Post
              I am reattaching the plan drawn to scale. The scale is 1:27.54.
              Thanks. I've imported your PDF via another program into AutoCAD and confirm the scaling as 1:2754. That means, from the speaker baffle to your ear, seated, is about 2.0m.

              Now a thought to leave you with about echoes. Suppose you stand with your nose literally touching a wide, tall brick wall, perhaps the side wall of an apartment block, sports centre or town hall. You clap your hands. What do you hear? You step back 5m, still facing the wall. You clap again. What do you hear now? And another 5m back. And another until you are at least 15m or more mtrs. away from the wall. Let us know what you discover; listen very carefully to the echo.
              Alan A. Shaw
              Designer, owner
              Harbeth Audio UK

              Comment


              • Echo chamber

                Originally posted by A.S. View Post
                Thanks. I've imported your PDF via another program into AutoCAD and confirm the scaling as 1:2754. That means, from the speaker baffle to your ear, seated, is about 2.0m.
                It is 2.36m. I have re-measured and the PDF scales is accurate with a margin of error plus minus 3cm though now I realised that I missed out the carpet and the equipment racks.


                Now a thought to leave you with about echoes. Suppose you stand with your nose literally touching a wide, tall brick wall, perhaps the side wall of an apartment block, sports centre or town hall. You clap your hands. What do you hear? You step back 5m, still facing the wall. You clap again. What do you hear now? And another 5m back. And another until you are at least 15m or more mtrs. away from the wall. Let us know what you discover; listen very carefully to the echo.

                What I hear in the emergency stair case (a perfect echo chamber) of my building is there will be a lingering "twang" or "mmmmmm" or slap when you shout or clap near the wall. The further you are the 'mmmmm" becomes distinct, i.e you hear the second and third sound of the original. I mean echo.

                I am glad I took this topic seriously even though I was thinking I know enough of room acoustics to set up my system. Your slow and methodically teaching now taught me look at echoes and reverberation more critically and it teaches me distinguish pure sound and coloured sound and to know what causes the colouring. It would be nice if more could join in to provide feedbacks.

                ST

                {Moderator's comment: It does feel very lonely here sometimes with the paucity of feedback. I'll leave Alan to comment to your echoes.}

                Comment


                • Distinct echoes

                  Originally posted by STHLS5 View Post
                  ...What I hear ... The further you are the 'mmmmm" becomes distinct, i.e you hear the second and third sound of the original. I mean echo. ...
                  Excellent observation! Couldn't have asked for a better response. Perhaps you don't appreciate yet just how significant your discovery is. Let me repeat what you said in slightly different language ...

                  'When you are really close to the wall, you cannot hear the echo even though you know for certain that there is an echo being created. As you move back from the wall, you can begin to hear the echo (or echoes) as a separate, distinct sound source in addition to the direct sound from your hand clap to your ear.'

                  Did you know that there is a name for this phenomena, named after the researcher who first discovered it? I attended a presentation concerning the King's Cross Station fire at which there was tremendous loss of life. One factor was the unfortunate communication difficulties with the public due to the hard tiled-wall acoustics underground (easy to clean) and the PA system. So this issue of fidelity in the presence of echoes is not just a scientific curiosity - it has great importance in the design of safe public spaces.

                  I'll slightly re-scale your existing room plot. No greater precision is required as any conclusions we draw cannot be better than educated guesses. We don't know the precise nature of your absorptive surfaces (walls, floor etc.) so we can never do better than a crude approximation to the real world. Even so, we may be able to tease out something useful, and applicable to all listening rooms with a little more work.

                  Next we need to have a look at the nature of human sound perception relating to echoes.
                  Alan A. Shaw
                  Designer, owner
                  Harbeth Audio UK

                  Comment


                  • The "Haas effect"

                    This phenomenon is called the Haas effect which is well known by public address engineers everywhere, or should be! What this quantifies, in principle, is the inability of the human ear to separate two similar sounds if they arrive within a short time of each other (if I remember correctly, up to about 40mS).

                    When digital delay systems started becoming affordable, this principle had a significant impact on the intelligibility of the sound reinforcement in some, suitable, venues. At some audience seats it became possible to time-align the arrival of the reinforced sound (electronic transmission) with the residual stage sound (acoustic transmission) which could create a massive improvement in intelligibility, and in turn meant that the system could be run rather quieter than was otherwise possible, with obvious advantages. However, setting up such delay systems is not for the feint-hearted or unskilled novice, for it can truly open a huge can of worms. Think about what might happen should a residual of the delayed, reproduced sound be audible on the stage by the performers!

                    Comment


                    • The "Precedence effect"

                      Actually, having read the foregoing postings a bit more carefully, I wonder if you are actually describing the Precedence effect, of which the Haas effect is a special case.

                      Comment


                      • Haas effect?

                        Originally posted by Pluto View Post
                        Actually, having read the foregoing postings a bit more carefully, I wonder if you are actually describing the Precedence effect, of which the Haas effect is a special case.
                        Originally posted by A.S. View Post
                        I'm not surprised; that's due to the Haas effect which is another slightly off-topic discussion for later.

                        Originally posted by EricW View Post
                        .. Clip 10 is that it's possible to hear the reverb almost as a separate sonic event from original impulse. It has a character of its own.

                        Reading A.S.' response to EricW's, I would guess it to be Haas effect which describes reverb arriving to ears after 3ms. (Moreover, Alan gave another clue that the phenomenon was named after a person.)

                        ST

                        Comment


                        • Sound waves, ray guns and bombs in fields ....

                          OK, regardless of what the effect is called (and I admit that I combine two effects under one umbrella) we should now have a close look at how humans detect echoes.

                          First we must remember that we do not live in a vacuum, we live in air and the air particles are so extremely small that it's easy to ignore that fact. But they must either be extremely small and/or far apart because on a clear day you can see for many km, and certainly as far as the horizon so image has passed through countless air particles from the source to your eyes. The combined weight of those countless trillions upon trillions of air particles presses down on the ground and gives us our barometric pressure. And that pressure can be measured with a old fashioned ink pen barometer. If we substitute water for air, we can imagine that we are walking around in a planet-sized swimming pool several miles deep. If we did that we may be able to see the ripples and waves of passing fish as they agitated the water and pressing against it, propelled themselves along. A fish would find it much easier to flip along in water than air because the more dense water gives it something to push against: and to "do work" you need resistance to work against, otherwise the power goes nowhere.

                          So how do we develop a wave? In the swimming pool we make a flipping motion with our legs. In air, we could make a wave by taking a sheet of card or wood and wafting up up and down. Would we hear either wave? No, because the frequency of the wave would be too low. If we wafted the card once a second that would produce a 1Hz wave and we'd need to waft the sheet at least 20 times a second (20Hz) to sense it as a vibration, and an impossibly fast 50 or 100 times a second to hear a distinct tone.

                          As I boy I used to read those space comics where the characters had laser guns or death rays or the like. I wish I hadn't because for a long time it confused me about how sound waves propagate; it's down to the interpretation of the words wave and beam. Let's just say that unlike a laser beam which from the source (can) radiate as a pencil thin ray in the direction it is pointed, a sound wave behaves in a very different way: it tends to radiate in all directions from the source unless there is something that blocks its path. So that implies that if you detonate a bomb in a flat field, regardless of whether you stand to the north, south east or west of the detonation at any given distance from the explosion you will sense an equal loudness. That is, the sound waves are not focused in one direction, and have no preferred vector and do not seek you out to do you harm. You could say that a bomb is an extremely wasteful, inefficient device because 99.9999% of the energy it creates radiates in directions other than the target, up down, left right, front, back. The schoolboys laser gun conversely applies 100% of the energy to the target.

                          How does this relate to loudspeakers in the listening room? The point I want to make is that your hi-fi speakers do not know that it is you and you alone sitting at the sweet spot for whom they were brought into existence. They do not know that you have neighbours. They do not know that you are listening in a real-world undamped room as opposed to an anechoic chamber or on the peak of a mountain. So they cannot direct their sound directly and exclusively to you and you alone and they spray every square cm of the room with sound without exception. They are as wasteful as the bomb in the field - every fraction of a second your speakers are 'exploding' their sound bombs and blasting your entire room with sound, but only a really tiny, tiny percentage of that energy hits your ears*. And the rest? It goes bouncing around your room as the echo of the bomb blast would from far away hills.

                          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.

                          * We can make a calculation about the surface area of our ears compared to the total surface area of the room:
                          • Let's say our external ear is 4cms x 2cms. That's 0.04m x 0.02m = 0.0008m2 per ear x 2 = 0.0016m2 for both
                          • Let's assume our listening room is 4m x 3m x 2m tall.
                          • So, side walls are 4 x 2m = 8m2 x 2 = 16m2
                          • Back wall 3 x 2m = 6m2 x 2 = 12m2
                          • Floor/ceiling 4 x 3m = 12m2 x 2 = 24m2

                          • Total surface area of this (typical) small room is 16 + 12 + 24m2 = 52m2 (fifty two square metres)
                          • Surface area of two ears = 0.0016m2
                          • Ratio of two ears to total room surface = 0.0016m2/52 = 0.00000307:1 [oops! error of one decimal point noted by STHLS5!]
                          • Two ear surface area compared to total room surface area = 0.00000307 * 100 = 0.00031% [Corrected by STHL5 see next post]

                          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.

                          Please check my maths!
                          Attached Files
                          Alan A. Shaw
                          Designer, owner
                          Harbeth Audio UK

                          Comment


                          • Directionality - good or bad?

                            Originally posted by A.S. View Post
                            unlike a laser beam which from the source (can) radiate as a pencil thin ray in the direction it is pointed, a sound wave behaves in a very different way: it tends to radiate in all directions from the source unless there is something that blocks its path. ... How does this relate to loudspeakers in the listening room? The point I want to make is that your hi-fi speakers do not know that it is you and you alone sitting at the sweet spot for whom they were brought into existence. They do not know that you have neighbours. They do not know that you are listening in a real-world undamped room as opposed to an anechoic chamber or on the peak of a mountain. So they cannot direct their sound directly and exclusively to you and you alone and they spray every square cm of the room with sound without exception.
                            Planar speakers [diaphragms are sheets not cones e.g. electrostatics, ribbons] are a lot more directional - to the extent that the most common gripe about them is that they "beam" and the sweet spot is anywhere between narrow and very narrow! Floor and ceiling interactions are almost irrelevant & side wall reflections are also less of an issue [because the beam directs sound at 90 degrees to the panel and away from the die walls]. However planar speakers need a lot of space and their bass is usually a let-down.

                            The following article has some graphs showing, across the audio frequency band, the ratio of direct to reflected sound [i.e. sound aimed at the sweet spot compared with total output] for "a large planar electrostatic", "a mini-monitor" and "a BBC monitor" measured in an unspecified room.

                            Audio in Modern Times

                            See the section "What actually makes speakers work in rooms" starting on page 16.

                            Comment


                            • An extra zero

                              Originally posted by A.S. View Post
                              ...Please check my maths!
                              It is 0.0031% and 99.9969%.

                              ST

                              Comment


                              • Originally posted by A.S. View Post
                                ..As I boy I used to read those space comics where the characters had laser guns or death rays or the like. I wish I hadn't because for a long time it confused me about how sound waves propagate; it's down to the interpretation of the words wave and beam. Let's just say that unlike a laser beam which from the source (can) radiate as a pencil thin ray in the direction it is pointed, a sound wave behaves in a very different way: it tends to radiate in all directions from the source...
                                I first learned about sound waves when I was 13. Until very recently I thought sound waves mean some sort of wavy energized particles or atoms, sort of sine waves, snake through the air to reach us. It only very recently I learned that there's no atoms travel from the source to our ears. But many still have the misconception.

                                ST

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