Announcement

Collapse

HUG - here for all audio enthusiasts

Since its inception ten years ago, the Harbeth User Group's ambition has been to create a lasting knowledge archive. Knowledge is based on facts and observations. Knowledge is timeless. Knowledge is human independent and replicatable. However, we live in new world where thanks to social media, 'facts' have become flexible and personal. HUG operates in that real world.

HUG has two approaches to contributor's Posts. If you have, like us, a scientific mind and are curious about how the ear works, how it can lead us to make the right - and wrong - decisions, and about the technical ins and outs of audio equipment, how it's designed and what choices the designer makes, then the factual area of HUG is for you. The objective methods of comparing audio equipment under controlled conditions has been thoroughly examined here on HUG and elsewhere and can be easily understood and tried with negligible technical knowledge.

Alternatively, if you just like chatting about audio and subjectivity rules for you, then the Subjective Soundings sub-forum is you. If upon examination we think that Posts are better suited to one sub-forum than than the other, they will be redirected during Moderation, which is applied throughout the site.

Questions and Posts about, for example, 'does amplifier A sounds better than amplifier B' or 'which speaker stands or cables are best' are suitable for the Subjective Soundings area.

The Moderators' decision is final in all matters regarding what appears here. That said, very few Posts are rejected. HUG Moderation individually spell and layout checks Posts for clarity but due to the workload, Posts in the Subjective Soundings area, from Oct. 2016 will not be. We regret that but we are unable to accept Posts that present what we consider to be free advertising for products that Harbeth does not make.

That's it! Enjoy!

{Updated Nov. 2016A}
See more
See less

Adjusting Room sound using material damping methods (not DSP)

Collapse
This is a sticky topic.
X
X
  • Filter
  • Time
  • Show
Clear All
new posts

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

    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.
    Alan A. Shaw
    Designer, owner
    Harbeth Audio UK

    Comment


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

      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.
      Alan A. Shaw
      Designer, owner
      Harbeth Audio UK

      Comment


      • Re: Making DIY room-traps

        After the server upgrade, I've been able to add this PDF. See previous post (click the PREV button above this posting).
        Alan A. Shaw
        Designer, owner
        Harbeth Audio UK

        Comment


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

          Originally posted by A.S.;(...deleted for brevity...)

          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 [I
          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

          Comment


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

            Originally posted by A.S. View Post
            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

            Comment


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

              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.

              Comment


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

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

                Comment


                • The acoustical design of a studio

                  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.

                  >
                  Alan A. Shaw
                  Designer, owner
                  Harbeth Audio UK

                  Comment


                  • Re: The acoustical design of a studio

                    Originally posted by A.S. View Post
                    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

                    Comment


                    • Re: The acoustical design of a studio

                      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.
                      Alan A. Shaw
                      Designer, owner
                      Harbeth Audio UK

                      Comment


                      • Re: The acoustical design of a studio

                        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

                        Comment


                        • Re: The acoustical design of a studio

                          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'.
                          Alan A. Shaw
                          Designer, owner
                          Harbeth Audio UK

                          Comment


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

                            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

                            Comment


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

                              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.
                              Alan A. Shaw
                              Designer, owner
                              Harbeth Audio UK

                              Comment


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

                                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

                                Comment

                                Working...
                                X