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Adjusting Room sound using material damping methods (not DSP)

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  • Re: Bass traps

    Alan,

    I?m afraid there might be a misunderstanding...

    I follow your thoughts about damping out a resonance inside a loudspeaker box by applying felt at the coordinate of maximum particle velocity (minimum pressure), and of course the same principles applies to large "loudspeaker boxes" like rooms. But my idea was to open up holes in the room where the standing waves (resonances) have maximum pressure - that is in the corners of the room - where you would put bass absorbers like the ASC bass traps mentioned above.

    I believe the misunderstanding arises from my incorrect use of the word attic. I now know that the name attic is not used for the space I?m planning to use. Bear me out:

    My listening room has sloping ceiling halves that partially follow the 45 degree slopes of the house roof (with max height in the middle). There is also a horizontal part of the ceiling in the middle with a height of 2,4 metres (I believe the space above this is properly called attic). In order not to allow the room height to decrease to zero along the sides, walls made from13 mm plaster on wood frames have been erected on both sides. It is the space behind these walls I am planning to use. A direct translation from Swedish for this space would be "creep space", for obvious reasons.

    I have made a drawing of the room and the creep spaces that I have scanned to a file, but since my computer skills are very rudimentary, I have not yet been able to attach the file to this reply. I will ask a friend to help me with this, and I would also like to include some SPL measurement that I have made. As I said: bear me out.

    Comment


    • Re: Bass traps, room treatment or room reconstruction

      Hi again

      Tried to convert my scan of listening room into something that can be attached. Hope it works.
      Have not (yet) been able to include the SPL measurements I mentioned. Working on it...
      Attached Files

      Comment


      • Re: Bass traps, room treatment or room reconstruction

        Originally posted by Forberg View Post
        Alan,
        I follow your thoughts about damping out a resonance inside a loudspeaker box by applying felt at the coordinate of maximum particle velocity (minimum pressure), and of course the same principles applies to large "loudspeaker boxes" like rooms. But my idea was to open up holes in the room where the standing waves (resonances) have maximum pressure - that is in the corners of the room - where you would put bass absorbers like the ASC bass traps mentioned above.
        You are right that the pressure has a maximum at walls. This is simply because the walls don't move and the air has no speed there. So keeping the energy the pressure has a max.
        But this changes when you open the walls. There are no walls anymore, so the pressure doesn't have a max where the walls have been. The air will then disappear through the holes and propably come back depending on the type of damping you have inside the holes. I'm not sure - but opening a window is probably better :-)

        But ...! Looking at your picture I would assume that if you open the walls in the corner then you create some kind of tube in the triangle. This volume could propably act as a helmholtz resonator, if you tune it together with the size of the hole in the wall to have a resonance frequency of one of your room modes. If you open all four corners and add some walls inside the tubes you could probably have a tube for each critical room mode. Some damping inside the tube would change the quality of the resonance of the tube. That's my theory - sounds brilliant - but I have no idea if it works ... :-)

        I can recommend the following book: Master Handbook of Acoustics by F. Alton Everest. It will help you to understand the stuff in detail ...

        Good luck,
        T.W.

        Comment


        • Re: Bass traps, room treatment or room reconstruction

          Thank you T.W. for your interesting comments. Turning the tubes into Helmholz resonators is in fact something that I have already considered. Just imagine a Helmholz res with a volume of some 34 cubic metres! (did I hear somebody whisper Guiness?). But, as you pointed out, inside walls would have to be added. However, everybody will realize the problems of doing that; as I have already mentioned, these spaces - tubes - are called creep spaces in Sweden.

          I really liked your comment that it would probably be better opening a window. By opening a window, all energi that flowes out is of course absorbed.That is really what my idea is all about!
          Now, if you consider the fact that these tubes have a triangular cross section of 1,5 times 1,5 m and are 15 m long and that the longest side of the triangle is covered with 200 mm Rock Wool all along its lenght, the total volume of absorbent is 6,4 cubic metres. Each side of the room.
          I think that compares favourably, to say the least, with what can be achieved with absorbers inside the room, whether it be ASC (or other makes) bass traps or the bales of fibre glass in Alan Shaws countryside listening room.

          It would be interesting to calculate the amount of absorption in these tubes. I have a faint recollection of something called Sabines formula that might be helpful I suppose. Perhaps I should get myself a copy of the book you mentioned. Or perhaps I will just go ahead and do it! (the holes, that is).

          Comment


          • Re: Bass traps, room treatment or room reconstruction

            Originally posted by Forberg View Post
            It would be interesting to calculate the amount of absorption in these tubes. I have a faint recollection of something called Sabines formula that might be helpful I suppose. Perhaps I should get myself a copy of the book you mentioned. Or perhaps I will just go ahead and do it! (the holes, that is).
            There's a lot of useful information available to you at this link.
            http://www.realtraps.com/

            Comment


            • Re: Bass traps, room treatment or room reconstruction

              Originally posted by Groovetracer View Post
              There's a lot of useful information available to you at this link.
              http://www.realtraps.com/
              And there are also some nice videos by RealTraps in YouTube:
              1) How to make a Bass Trap Acoustic Panel (Tutorial):
              http://www.youtube.com/watch?v=iyYUpkpL0gw
              2) How to Set Up and Treat a Listening Room:
              http://www.youtube.com/watch?v=lbLVjHfHahg

              I have build some panels with the help of 1). But I didn't want to have fiberglas or
              mineral wool in my living room. So I found a damping material called "homatherm":
              http://www.homatherm.com/int/product...scription.html
              I used two pieces of 60mm thick for each panel. To cover the panel is used IKEA bedspreads:
              http://www.ikea.com/gb/en/catalog/products/00020710
              I like the panels and the overall acoustic in my listening room is much better now. I have one in each corner as a starting point. I will build some more later ...

              Good luck - try it out!

              TW

              Comment


              • A really good listening room - how to make

                This thread concerns those features of our listening room that make for a really great listening experience. Of course, we have to consider the balance between the appearance of the room, our taste in furnishings, our budget, what is permitted by landlords and neighbours and planning regulations. So getting the best sounding room is about the art of compromise.
                Alan A. Shaw
                Designer, owner
                Harbeth Audio UK

                Comment


                • Re: A really good listening room - how to make

                  In some email discussions off-site I have been discussing what makes a really great listening room - what features to look for and what to avoid, if possible. My correspondent convinced me that he had what he considered to be a "really well damped room". When I asked for pictures of the room I was rather shocked; his idea of a 'really well damped room' was a modern minimalist setting in loft conversion in a big city, with glass walls, no curtains, wood block floor, no rugs and just a few pieces of Scandinavian furniture. I asked him to rate, on a scale of 0-10 how 'well damped' he though his room was: answer, 8/10.

                  Now, we must have be talking at cross-purposes, because I'd rate that room 1/10 - just about the worst possible environment (highly reflective) for listening to quality music; only his bathroom could be worse. So it seems that what one listener may consider a good environment for music another may not.

                  I will in turn illustrate some randomly selected rooms I have found on the internet in furnishing catalogues. All of the rooms are beautiful in their own way - and probably very expensive too - but our concern here is solely to identify what features of each room would make for a good or poor listening experience: the style of furnishings, the colour of the room you can ignore. My advice is only general, but if you could at least avoid some of the seriously negative features in your own room, even if you can't do much to bring positives acoustic treatment into the room, it will improve your listening fidelity.

                  The room is a highly critical part of the listening experience. The bigger the speaker, the more extended the bass (M40.1), the more bass energy you pump into the room, the louder you play, the further you sit from the speakers the more attention you must pay to treating the room if yiou want the best possible sound. A little time and money spent on improving the listening room will enhance every speaker system. Let's start with gathering an overall feel for the subject by looking at the extremes of acoustic environments from the highly damped 'dead' acoustic to the 'live' - see next post.
                  Alan A. Shaw
                  Designer, owner
                  Harbeth Audio UK

                  Comment


                  • A reflection-free (dead) acoustic environment

                    At one extreme we have the dead-room, the free-field room also known as an anechoic chamber. This is normally used for completely (or nearly completely) removing reflections from the walls back to the listener. It would be used for measuring microphones or loudspeakers where it's essential to eliminate the contribution of the room to just see what the mic/speaker behaves like.

                    Attached is a picture from the 1960s or a free-field room (Bell Labs I think) where there is no floor: the listener sits suspended on a wire mesh floor in the middle of the chamber surrounded by loudspeakers. Whichever way the sound travels from the speaker it is absorbed by the soft wedges which line all four walls to a depth of many feet. Does such an non-reflective environment exist in nature? Well, almost. A sort of equivalent would be standing at the very peak of a mountain and shouting: there would be reflections (and certainly some from whatever you are standing on, preferably a needle-sharp ridge) but the most obvious echo reflections would be seconds away from nearby mountains (if any) and very quietly. So, we could say that a mountain peak is nature's (virtually) reflection free environment - but how extremely inconvenient to listen there!

                    Another great example of a (virtually) reflection free place found in nature is a corn field. Have you ever walked across one with a companion, and become separated by more than a few feet? No matter how hard you shout, you just can't be heard: all the storks act like mini anechoic chamber wedges and both in the heads and stems soak-up all the sound. If the field is next to a motorway, the muffling of vehicular noise is astonishing.

                    The last picture shows the perfect sweet-spot! Note how the mountain's edges gradually roll away from the peak in all directions, and that there are no nearby peaks to return an echo. Surely a great acoustic there!

                    So, quality rating for the Bell Labs chamber, say, 9.5/10 for reflection-freeness and 9/10 for the second mountain peak (from the plane window) and 8/10 for the corn field.

                    >
                    Attached Files
                    Alan A. Shaw
                    Designer, owner
                    Harbeth Audio UK

                    Comment


                    • A very reflective (live) environment with strong echoes

                      At the opposite extreme to the anechoic chamber is the highly live acoustic. Caves are a good natural example.

                      If you are in South Wales do visit this one at Dan-yr-Ogof. I've been there and the acoustic is lively (lots of reflections from the hard stone walls) but is surprisingly even in overall sound. In fact, when deep inside the cave the lights dim and a small boat rows onto the cave's lake and the audience is serenaded by acoustic instruments. The tone is even, and the reflections seem to work sympathetically with just the one of two guitars, but imagine a full orchestra or even a vocalist: the sound would be come very jumbled and confused. The strong echoes would return to the listener after a second or two, a few noted behind; a very uninviting sonic experience.

                      http://www.showcaves.co.uk/caves.html

                      Score out of 10 for freedom from reflections: 0/10. Useless for listening to full-range reproduced sound over loudspeakers.

                      >
                      Alan A. Shaw
                      Designer, owner
                      Harbeth Audio UK

                      Comment


                      • Real rooms, reflections and what we can do about it ...

                        We've talked of the importance of absorbing as much reflective sound as possible in the living room. There are two ways that sound from your loudspeakers can reach your ears (or the measuring microphone): directly from the speakers in a line-of sight to your ears or indirectly after bouncing off or passing through or around another object. How much of that indirect sound reaches your ears (or measuring mic) entirely depends upon the characteristics of the objects in the way: if they are soft furnishings then significant energy may be absorbed by them; if they are glass or brick walls, then negligible energy will be absorbed.

                        As the BBC's former acoustician Bob Walker said "...Over the normal audio frequency span, wavelengths range from about 15mm to 7m. That nicely encompasses most sizes of objects within rooms, and even the room itself. Plus, the interactions between sound waves and the room and its contents cover the whole gamut of reflections and refraction effects, as well as absorption. In a typical room there is usually at least the floor surface within about 2m of the source. Therefore, from a maximum of about 6ms onwards, the sound field (even outdoors) contains components which have interacted with some surfaces or objects. After 30 ms in a small room with a sound wave front will have travelled in every direction to the boundaries of the room and will have interacted at least once with every object contained therein."

                        James Moir, respected for his audio research work in the 1960s and 70s commented "Achieving a good solid and firm stereo image requires a high ratio of direct to reflected sound, for it is only the direct sound of that carries the basic information about the location of the stereo image in the space between the loudspeakers. The sound reflected from the room boundaries, particularly those in the vicinity of the speaker, can only serve to dilute the basic directional information. Thus to achieve a good stereo image we need to minimise the amount of reflected sound." (Source: Wireless World, Oct. 1979).

                        The essence of absorption is to have the reflected, useless, destructive sound wave that messes up your enjoyment of hi-fi at home pass through a material or structure which it finds hard work, such as Rockwool or fibreglass. Then it loses energy as friction heat so what bounces off it (or passes through it) is diminished in energy. Less energy = less potential to reach your ears. All rooms - even anechoic chambers - have reflections, and real-world living rooms have innumerable reflections. As Bob Walker reminds us, the sound wave from the speakers will touch every single object in the room. Just think about that for a moment: every single object will, to greater or lesser extent, reflect sound off it or let it pass through it. So, logically, if we want to improve the listening room we should identify those objects, how reflective they are and where they are relative to the speakers and our hot spot. If we could exchange all hard objects for soft ones, and move those soft ones into the optimum position for them to best absorb that surplus sound waves splashing around the room, we could definitely improve the listening experience.

                        It goes without saying that there is a practical limit to what we can do at home, and we must work within the 'wife acceptance factor'. And our budget. And our motivation to make changes to our comfortable, known multi-purpose environment. So whatever we do will tend to be at the bottom end of the scale of what we can - or if deadly serious - really should do to improve our room. So now a statement that sets the scene to keep in mind: the more absorptive the listening room (within likely practicalities) the better the stereo image, and overall sound, and the realism. The more lively the room, the less satisfying the experience unless we can train ourselves to hear-through the sub-optimal acoustic; and many people can do that without being aware of it.

                        Now to look at some real world examples of speakers in my listening room, described here. You will notice from one picture that I've set-up the microphone in three positions around the speaker: left, centre and right and measured the in-room RTA response there to high precision one sixth octave*. When over-plotted together you can see that at A, B and C the speaker's response in the middle and lower frequencies is subtly different even just a metre or so off axis! It's surprising how much a small movement in your, the speakers or the microphone's position in room can effect the response.

                        In A, B and C you will notice a slight dip in the response centred around 250Hz. This is not in fact an issue with the speaker, I know from experience that it's actually caused by the so-called Allinson effect of the sound bouncing off the floor and then on to the mic. This effect applies to all speakers, not just Harbeths. This reflected sound off the floor is slightly out of phase with the direct sound and hence partially cancels the direct sound at the mic, hence the dip. If the floor was totally 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.

                        >
                        Attached Files
                        Alan A. Shaw
                        Designer, owner
                        Harbeth Audio UK

                        Comment


                        • Making DIY room-traps

                          Here is an excellent article from my recent archives about damping a room. It involves making DIY wall-hanging absorbers, mainly for damping echoes. However, if the depth was increased and more Rockwool used, the working range could be usefully lowered. I know all three fine upstanding characters in this tale, and as author Hugh was a training manager at the BBC I hope he won't mind me sharing it with you. Isn't it a good think I keep all these historical documents?

                          It's a big file (4MB), so be prepared to wait for the download.

                          (This file has been temporarily removed due to space restrictions. Jan 2010)

                          >
                          Alan A. Shaw
                          Designer, owner
                          Harbeth Audio UK

                          Comment


                          • Room reflections - not in a studio control room

                            After our installation of a Harbeth 5.1 system in the UK's largest TV studio's control room, I wrote a short summary in aswer to a confused customers question about 'big room - small room?'

                            Item is here
                            Alan A. Shaw
                            Designer, owner
                            Harbeth Audio UK

                            Comment


                            • Re: A really good listening room - how to make

                              Dear Alan,
                              May I know which type of Rockwool product I shd use in room with my Harbeth?

                              http://www.rockwool.ro/sw53623.asp
                              "Bath with Music"

                              Comment


                              • Re: A really good listening room - how to make

                                Originally posted by keithwwk View Post
                                Dear Alan,
                                May I know which type of Rockwool product I shd use in room with my Harbeth?

                                http://www.rockwool.ro/sw53623.asp
                                Keithwwk,

                                Fyi here. DIY traps here.

                                Comment

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