Room acoustics

Discussion in 'Audio Science' started by Ardacer, Jun 16, 2020.

  1. Ardacer

    Ardacer Almost "Made"

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    I've been stuying the topic for some time now, as much as work permitted, and came to a few conclusions. Mostly read work from Linkwitz and JohnK.
    Above the schroder frequency, the best sound is achievable by either making the room very dead, absorptive, or by employing speakers designed for controlled directivity (whichever way, horns, dipoles, large baffles, should work). Below the Schroder frequency, best approach seems to be dual subs, with apparently dipole or cardioid dispersion.

    For smaller normal (not overly or at all acoustically treated) rooms, cardioid bass might be better as it would behave like omni below the first fundamental, benefiting from room gain. For larger rooms, dipole stereo bass when set up properly should in theory be best.

    For smaller rooms, it might also be befeficial to use tightly packed speakers like coaxials or similar due to better integration.

    Did I miss something?

    After building and living with lx521, I'm planning to design my own version of full range dipoles, but ones that are even slimmer. Actually, I'll try to make them simmilar to gainphile's s20Z, footprint - wise.
     
  2. Hrodulf

    Hrodulf Prohibited from acting as an MOT until year 2050

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    By killing all reflected sound you'll get a very headphone-like soundstage. Apply absorption only to primary reflection points, the wall behind you should be diffusive.

    Controlled directivity largely contributes to a correct tonal response of reflected sound. Controlled directivity in bass frequencies makes room integration easier, but I don't think it's a must, if you can experiment with positioning and bass EQ.

    Coax radiation principle is always better everywhere. At least in theory. In practice coax speakers are notoriously hard to get right.
     
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  3. Ardacer

    Ardacer Almost "Made"

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    There is still crosstalk from speakers which doesn't exist in headphones, so it should be bit different. I wouldn't advocate trying to dead the room that much anyway either, or at all. Just mentioned it as a possible alternative to controlled directivity, which I personally believe to be the way (Mandalorian reference). Only if you have a really audible and precise problem, then surgically apply room correction via absorbers/difussors (or even dsp if it makes sense).

    100% agree on the rest. Also, it might not be practical to play around with sub positioning, that's the main reason for ease of integration. If you have space, then you have no problems, usually, anyway.
     
  4. Xecuter

    Xecuter Brush and floss your amp twice a day

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    What about internal to the wall (rock wool)? Can you have too much absorption in the wall cavity? I don't think so, but not sure.
    Because you can liven stuff up internal to the room with diffusion again, but of course utilizing absorption at the first reflection point on the wall and ceiling?
    I'm just planning my build again and see some people saying not such thing as too much absorption and others saying it can be a massive problem?
     
  5. Ardacer

    Ardacer Almost "Made"

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    To me, it seems logical to have absorption (or even better, empty space) behind you, and diffusion in front of you, with speakers of good dispersion properties. (at least for midrange and above)
     
  6. Hrodulf

    Hrodulf Prohibited from acting as an MOT until year 2050

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    For a standard painted drywall/rockwool sandwich the filling doesn't absorb much. There is some low frequency absorption due to drywall sheet flex, but generally most of what could've been absorbed by the wool gets reflected by the drywall surface.

    I've done speaker testing in a huge anechoic chamber which works down to 50Hz. In soviet times materials were cheap, so they made them huge af with 1m long glass wool spikes. Let me tell you, it's not a very comfortable place to be in. The main problem with absorption is that lower frequency absorbers eat up mids and highs like mad. Sometimes this will be alleviated by wrapping low frequency chunky bass traps with some sheet that partially reflects higher frequencies.

    Sure. The question is, what is the correct mix of direct vs reflected sound? Too much reflected sound makes everything mushy, yet too little loses ambiance. It's generally up to you to make the right compromise.
     
  7. Dr J

    Dr J Friend

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    Too much absorption and you end with dead sound and reduced sense of soundstage that is not at all natural, as Hrodulf pointed out. We do not live in acoustically dampened environment, hence our brains expect to hear reflections e.g. when listening to a live acoustic orchestra.

    T60 of 0,3s-0,5s or more for two-channel, for home theaters might go down to 0,1-0,2s (because the surround speakers create the space). In smaller rooms hard to avoid short T60. (EDIT to fix the T60 values).

    Also, if you put a huge amount of absorbing materials in the walls or as traps in the corners, it may eat up your speaker's bass response.

    For soundstage, important to have at least 10f space and/or diffusion behind you.

    Treat the first reflection points on the side, front walls, ceiling, even the floor. Put basstraps in the corners. All of these in moderation to fix problems, as you said.

    Standard trick to have panels or structures with diffusing surface and behind that absorbing material.

    Bass under Schroeder is considered to propagate in all directions equally and fill the room, regardless of the direction where the element is facing. Hence you cannot avoid the room modes (infinte dips and high peaks) by changing the direction of the speaker EDIT:, only by changing the speaker and listening location. Might fix other bass stuff that I do not know enough of.

    The more subs you have, the better you can place asymmetrically to fill the space equally with the bass and avoid dips and peaks from the room modes. However, integration to the main speakers is considered a challenge.

    The above is based on what I read from pretty solid, consistent sources, and got as advice from an acoustics consultant when building my listenting room/home theater. Might not be the full truth or contain misunderstandings, as I am not a professional.

    There are already several threads around this topic, I provided some starting points to posts and threads here for instance:

    https://www.superbestaudiofriends.o...me-throw-rocks-at-the-idiot.9299/#post-300489
     
    Last edited: Jun 17, 2020
  8. Ardacer

    Ardacer Almost "Made"

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    Thanks. I was more interested in the sense that I'm going to be designing my own dipoles and their interaction with different rooms, not really about how to fix my own problems. I know, I didn't really do a good job with thread title, but it is actually room acoustics, couldn't think of a better one.
     
  9. Serious

    Serious Inquisitive Frequency Response Plot

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    @Ardacer What you mentioned regarding the lack of pressurization below the lowest room mode with dipole speakers certainly seems to be largely true. In my room the lowest mode is at 24Hz. At the listening position I'm getting a response linear to 24Hz and then it drops down 20 to 25dB at 20Hz! The anechoic response doesn't have near the same rolloff. The more "leaky" I make the room (opening doors and windows), the lesser this effect becomes.
    So if your room is small enough to make this a big issue I'd consider monopole subs crossed over where the OBs can't pressurize the room anymore.

    More or less just repeating what you and others have mentioned, but as far as overall room acoustics go I'd just go for large first reflections absorbers, ca 7-10 cm thick on the side walls (for both speakers). With OBs the floor and ceiling likely don't matter as much. You might want to add diffusion between the speakers on the back wall. Measure speaker and listening position for smooth bass response. As @Dr J mentioned I'd go for either a lot of distance behind you or close to no distance to the wall behind you. I've heard rooms that were set up with a couch directly against the wall with only slight absorption and it sounded pretty good with surprisingly great imaging. The trick is to sit uncomfortably close to the wall, with the back or your head practically pressed against it.

    @Dr J You must've forgotten a zero for the T60 figures, no? T60 of 10-50ms all seem very low to me. FWIW I think a good compromise is usually a T60 or 300ms that's as flat as possible above the Schroeder frequency.
     
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  10. ergopower

    ergopower Friend

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    This is anecdotal, but I do intend to do some REW measurements to see before and after and to help with speaker toe-in, etc.

    I have a room with built-in bookcases that define where I can put speakers. Before the bookcase install, I had the same speakers (Emotiva ERM-1 from a Yamaha RX-A1060 from a Sony UBP-X800, lower end of mid-fi, I know) about 2 feet from the rear wall and about the same lateral/vertical position. I'm fortunate that the back of this room is largely open so I don't have rear reflections to deal with. I was happy enough with the 2 channel sound that I didn't mess around much with positioning and no room treatment. One way I judge is being able to hear differences in how music was mastered, from the diffuse garbage of 70's rock to fairly precise presentations (Lyle Lovett 'No Big Deal', Guy Davis 'Come Back Baby' are 2 tracks I test with plus some opera).

    Once the bookcases were in, I was appalled at how bad the system sounded in 2 channel. It took some listening to figure out what was going on, but to me it sounded like the sound was very spread out and 'fuzzy' for lack of a better word, regardless of how good or bad a recording. Couldn't localize anything, nothing had a clear sound, it was very incoherent. I figured reflections were the problem, so I bought NuFoam batting at a fabric store and put 1" thick batting on the back and undersides of the shelves around the speakers. I also taped some 24" x 36" x 2" batting to the side walls at the 1st reflection point. This batting is pretty dense, it's designed for furniture cushions where you don't want to use foam. I'm sure partly absorbs, partly diffuses, but couldn't tell you specifics.
    Amazing difference. Back to being able to hear production choices, probably better than before the bookcases.

    Despite the plebeian components, the rig can get me engaged in the music, or infuriated at how effed up some of my absolute favorite old rock is. I tend to listen to that mostly in the car, I think that's what a lot of it was mastered for.
    I was playing Guy Davis last evening, sounding pretty good. I went down to the basement for some reason, and as I was coming back up, I could swear it sounded exactly like coming up the steps to a 2nd-floor club while the band is playing - maybe also a sign that the basics are right?

    A few pics of the room and the absorber/diffusers I built for esthetics.
    If anyone is interested, I have more pictures on the build process, and I have since figured out a way that would be much cheaper to build, you could maybe get down to < $30 each. These are taller than 36" because to my eye that looked better on the wall, it is 48" tall to get the absorber/diffuser just slightly above mid-height, I used some leftover 1" batting at the top to make up the difference. The 24" x 36" x 2" piece is exactly centered on the 1st reflection point as judged from a mirror test. The window on the right is also a limiter for width, so in a more open room it might be worthwhile to go a bit wider. The side chairs are new to the room after the bookcase build, and I'm sure they are helping as well.

    IMG_1553.jpg

    IMG_1514.jpg IMG_1516.jpg
     
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  11. wbass

    wbass Facebook Friend

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    I'm contemplating moving into a quite large open loft apt. About 1500sq ft, no walls (except for the bathroom). Concrete ceiling, wood floors, drywall walls. Should be interesting to see how my Klipsch Forte's and KEF LS50's do in such a space. I'm a little worried about them sounding small and lost. I have, however, conferred with the folks at GIK Acoustics, and they think the space could sound amazing--no problems with bass pressurization and no need for diffusion--though they say the clarity of the system will be improved with a liberal use of absorption panels, which I think I agree with.

    We'll see. Should be a *very* different listening environment from my current and somewhat too small room.
     
  12. Ardacer

    Ardacer Almost "Made"

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    Uh. I'm afraid of fiberglass, get the shivers. I know it's supposed to be safe, when properly isolated, but still...

    I'd bet they will be just fine. If they manage in small rooms, I see no reason why they wouldn't perform in large ones. Not optimally as a bigger speaker could perhaps, but still they should be fine. I'd keep them for a smaller room as a secondary setup or something once you get some "better" stuff (something that can utilize your space better)
     
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  13. murphythecat

    murphythecat Friend

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    look up early reflections. do not apply treatment at the secondary reflection points, only treat early reflection (ceiling, side walls, and behind your listening position). treating the early reflections is ime the most obvious "upgrade". make them as thick as possible so that they also act as bass traps

    then, big bass traps in as many corner of your room as you can.
     
  14. Psalmanazar

    Psalmanazar Most improved member; A+

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    1. Buy a measurement mic
    2. Don’t make a rape dungeon like a 70s-80s dead studio. If you want to whip women, fun fun fun fun fun. If you want something that doesn’t sound like it’s coming out of a black hole into outer space, bad bad bad.
    3. Most of the better mastering rooms are closer to really good domestic rooms with a vaulted ceiling and no square room. There also aren’t weird reflections from consoles and massive racks of gear, and screaming deaf musicians. They aren’t totally dead and use free standing full range, 3 way speakers for translation as opposed to shit like ATCs set up specially to fill in the bass with subs and room coupling to play back everything clear as day like the cleanest, word of god, PA system from the highest heavens.
     
  15. ergopower

    ergopower Friend

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    It's not fiberglass, it's very fine polyester filaments
     
  16. murphythecat

    murphythecat Friend

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    roxul safe n sound is perfectly fine. cover it up, and dont throw yourself at them and you will be fine
     
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  17. Armaegis

    Armaegis Friend

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    That's a huge space. How high are the ceilings? If you're not blasting music, you probably don't need anything on the sides but I might be tempted to hang panels from the ceiling.
     
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  18. murphythecat

    murphythecat Friend

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    agreed, depending on how far your speakers and your listening position are in relation to the distance from the side walls, treatment might not be required.
    iirc, the distance needs to be at least 7 meter away from the side walls.

    unless he has 20 feet high ceiling, a cloud is needed, as well as floor treatment (if optimal SQ is wanted)
     
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  19. Dr J

    Dr J Friend

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    Thanks for catching that, should not write while tired. Will fix in the original post.
     
    Last edited: Jun 17, 2020
  20. Dr J

    Dr J Friend

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    Yes, this a new topic that I think I have not seen discussed (unless in the speakres or DIY corner that I do not frequent). I saw it starting to drawi in people who are interested in the acoustic properties and treatmet of rooms, so though to put some of my standard trail markers for those topics. As the discussion is a bit scattered.
     

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