Why are reflections off the walls so bad when it comes to room acoustics?

Discussion in 'Audio Hardware' started by Dean Shias, May 25, 2019.

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  1. Dean Shias

    Dean Shias Member Thread Starter

    Location:
    07054
    Why are reflections off the walls/ceilings so bad when it comes to room acoustics? What benefits will room diffusers/aborbers have?
     
  2. Davey

    Davey NP: Portishead ~ Portishead (1997)

    Location:
    SF Bay Area, USA
    Not all reflections are bad, but the ones that occur too closely with the direct wave from the speakers give your brain the clues it needs to locate the source and the room boundaries, and much of the image collapses. If you can avoid cabinet diffractions, and the early reflections from the walls and floor, the soundstage will get much wider and deeper and taller, and much more fun too :)
     
  3. George P

    George P Notable Member

    Location:
    NYC
    How does one avoid cabinet diffractions?
     
  4. Emergency Whiskey

    Emergency Whiskey Forum Resident

    Location:
    Kansas City, MO
    This is a good overview of sound and room effects.

     
  5. Davey

    Davey NP: Portishead ~ Portishead (1997)

    Location:
    SF Bay Area, USA
    Buy speakers that are designed to minimize cabinet diffractions.
     
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  6. George P

    George P Notable Member

    Location:
    NYC
    I did some Googling and I think my B&W 704 S2s are designed to do that.
     
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  7. Hipper

    Hipper Forum Resident

    Location:
    Herts., England
    There's something called the Precedence Effect:

    Precedence effect - Wikipedia

    'The precedence effect or law of the first wavefront is a binaural psychoacoustical effect. When a sound is followed by another sound separated by a sufficiently short time delay (below the listener's echo threshold), listeners perceive a single auditory event; its perceived spatial location is dominated by the location of the first-arriving sound (the first wave front). The lagging sound also affects the perceived location. However, its effect is suppressed by the first-arriving sound.'

    'For time delays above 50 ms (for speech) or some 100 ms (for music) the delayed sound is perceived as an echo of the first-arriving sound. Both sound directions are localized correctly. The time delay for perceiving echoes depends on the signal characteristics. For signals with impulse characteristics, echoes are perceived for delays above 50 ms. For signals with a nearly constant amplitude, the echo threshold can be enhanced up to time differences of 1 to 2 seconds.'

    This plus reflections that you do hear that come within the Precedence Effect time will keep the direction but alter frequency response. Sound that comes later then the Precedence Effect time will all sound like echoes, both adding to the frequency response and causing confusion as to the source - muddying the sound.

    Of course our brain is used to dealing with all sorts of reflections but in recorded music we hope that the stereo effect will reproduce the original placement of sounds in their recording environment including all its reflections. If we add other sounds (from the reflections that both apply the Precedence Effect and add echoes) it changes the recording.

    Floyd Toole in his book 'Sound Reproduction' writes that studies show that most of us prefer side wall reflections when listening to music. I thought I did too but after I tried to eliminate them I found I got a crisper sound which I prefer. Others might think it a bit 'dry'.

    All I can suggest is that you try for yourself. Be aware that a lot of diffusors need a certain distance to work properly (six feet perhaps). If they're too close you can hear them. Also different speakers have different sound distribution patterns. Some are narrow and lead to more direct sound. Others are wider. For example my ribbon drivers are said to have a wide horizontal dispersion but about thirty degrees vertically. For this reason I don't get ceiling reflections (floor and ceiling reflections also need to be considered).

    In all this discussion we have only talked of the higher frequencies - above about 200Hz. Below this figure (to be exact, the Schroder Frequency for your room) sound behaves differently and requires other methods to deal with any problems (principally bass traps, mostly in corners). In fact dealing with bass issues is far more important then the higher frequencies.
     
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  8. chervokas

    chervokas Senior Member

    A couple of things happen with reflected sound when you're trying to play back recorded music (vs. trying to design an acoustic space for the performance of music) --

    First, it arrives at your ears in some kind of time relationship to the direct sound, and presuming that you're trying to hear recorded music and what is encoded in the recording, not the sound of your room as an acoustic performance space, the arrival of the reflected sound at some time after, but not very long after the direct sound from your speakers, screws with your ability to hear the information on the recording and adds a layer of timing and reverb information from the room on top of your experience of what's on the recording.

    Second, reflected sound waves cross the direct soundwaves in some kind of phase relationship to the direct waves and these cause cancellations and reinforcements relating to the distances between the listener and the room boundaries and the speakers and the room boundaries. The results are often big nulls -- like -30 dB frequency response holes -- at certain frequencies, especially in the bass frequencies; or boomy reinforced peaks; or weird comb filtering effects at the listening position; or widely different imaging and frequency response if you move a foot this way or a foot that way or move your head in the listening room.

    Third, room reflections don't just involve frequency by time. You can have sound bouncing back and forth between parallel untreated walls creating a flutter echo (that "ping" kind of sound when you clap your hands in an empty room) -- this will totally screw up your stereo imaging and might make a room sound annoyingly bright, so you wind up buying dark gear, then you move or you put up a bookshelf in your room and you wonder why the gear sounds so dark; or, you have rooms with rugs and overstuffed furniture and heavy curtains in them where the decay time of high frequencies might be quite short but the decay times of low frequencies might be quite long an boomy and music sounds muddy and indistinct and lifeless.

    The trick is to use room treatments and design to control reflections both in terms of frequency and time in relationship to the speakers and the listener such that the frequency response and decay times are as even as possible across the frequency range at the listening position, and, if you have enough room to create some diffusion too, so that the imaging and wall-to-wall sound fill of the room is pretty even and consistent across a wide listening area. When you have the room right, and with the right stereo recordings, the speakers will disappear as an apparent sound source and the image will fill the room from side to side and have layered depth front to back and with be even in frequency response and tight and clean in the bass and it will be like being transported into the original acoustic space of the recording.
     
    Last edited: May 26, 2019
  9. Richard Austen

    Richard Austen Forum Resident

    Location:
    Hong Kong
    Also note that in the video example above - he was comparing a dead empty room. Most all of us have various furniture - paintings on the wall - if they are on canvass they work as absorption - a leather couch? Sofa. Book cases with books etc all serve as absorption etc. So the comparison of a fully furnished room for example and an empty room with the pricey aftermarket room treatments may not be much different if at all to your ear.

    It's one of the advantages of corner loaded speakers like mine or the ole Klipsch K-Horns and various other corner horns in that you can design in the reflection to the overall sound - The idea of corner loading beyond massive gains to bass and speaker sensitivity is that the reflected wave off the immediate side wall will follow the direct wave so quickly that both waves arrive at your ear at virtually the same time so as to be inaudible (on my speakers for example it is just 4 milliseconds). Thus, there is no side wall out of step reflection that is perceptible by the human ear. When done correctly this eliminates the need for room treatments that unfortunately have a habit of over deadening the sound. Similar to various noise reduction techniques - they all take out sound you don't want but they also take out the sound you do want. Especially true in over-damped rooms you get a clear but often overly stayed presentation. In other words most all free standing loudspeakers sound boring and because they don't have the aid of the corner they almost never pressurize instruments or load a room properly.

    What the fellow did in the video with the clap echo test is what everyone should do initially. Simply walk to all corners of your room and simply clap your hands loudly - if you are hearing reverb (the echo) with a long linger then you need to do something - in an empty room it is usually awful - but once all your furniture is in you SHOULD hear significantly less echo.

    As this video notes - you probably want some echo in the .2 to .5 seconds range otherwise the sound will be too dead. Contrary to popular belief you do NOT want your room to be emulating an anechoic chamber.

    This video may also help. And note what he says about treatments - they do NOT work miracles. They make minor fixes as the video above noted - the comparison only seems BIG because he was comparing an empty room! This guy notes more natural treatments.
    At 10 minutes you hear the actual examples of echo/reverb so at least watch that part on.

     
    Last edited: May 26, 2019
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  10. Mondie

    Mondie Forum Resident

    If you want to invest some time into really understanding the science, this superb site written by John Lenard Burnett is a great reference resource:

    An explanation of critical distance

    Acoustics: Critical Distance

    Which leads into the science of rooms and treatments

    Acoustics: Sound Absorption
     
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  11. SandAndGlass

    SandAndGlass Twilight Forum Resident

    What I was gonna say was...

    If you live in an enclosed house and have a stereo, you are going to have reflections.

    As Richard said...

    I have walls and I have furniture and I have reflections. I'm fine with that, I don't get all bothered by them.
     
  12. Richard Austen

    Richard Austen Forum Resident

    Location:
    Hong Kong
    I tend to view it as if you don't get clap echo in your room you are 95% there. Perhaps use a set of good headphones like the Sennheiser HD600 - if your stereo is as crisp and clean at the listening chair as you get from that kind of a headphone - then your room/stereo set-up is fine. If you can't make out the words or the treble is blaring or the bass is booming then you probably have a speaker/room issue.

    Once I got my AN J/Spe and OTO set-up properly and my room treatments on the walls in the correct areas - the HD600s headphones basically sat for the next decade.
     
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  13. Twodawgzz

    Twodawgzz But why do you ask such questions...

    My personal opinion is that room reflections are fine when listening but not fine when recording (unless a live performance is being captured or simulated).
     
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  14. SandAndGlass

    SandAndGlass Twilight Forum Resident

    A lack of room reflections can made for a dead sounding room.

    Reflections can give some life to your listening environment.
     
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  15. MusicNBeer

    MusicNBeer Forum Resident

    Location:
    Florida
    Great post! All reflection are definitely not bad. You don't want your room sounding like an anechoic chamber. The music will be completely lifeless.

    All reputable manufacturers will design to minimize diffraction. The biggest offender is usually the grille which can affect the upper frequency response starting at 2-3 KHz. B&W is definitely designed correctly.
     
  16. layman

    layman Forum Resident

    Location:
    New York, NY
    They are not bad.

    Home speakers are designed to disperse sound in a room. They sound much better in a room (with all its reflections), than they would outdoors (with a lot less reflections). You can test this yourself.
     
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  17. harby

    harby Forum Resident

    Location:
    Portland, OR, USA
    The biggest challenge is when you have a non-symmetrical room. Open to the left and a wall on the right, and you will have significant challenges that require both narrow dispersion speakers and acoustic treatment.
     
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  18. Doctor Fine

    Doctor Fine "So Hip It Would Blister Your Brain"

    Out of control reflections that muddy up the sound are a no-no.
    Like putting your speakers next to a wall and hearing them "honk" from too many ADDED frequencies that now boost and cut notes inappropriately.
    Mud is "anything that DEGRADES the original signal."
    Your job is to REMOVE mud when you set up your system---not ADD it.
    But there are two reflections that should ADD improvements to your playback soundstage (Proscenium) without adding mud to the recording.
    These are:
    1) Some slight wall reverb reflection to "add a little air."
    These reflections must not be introducing frequency aberrations by being too near your speakers.
    But in fact without any "air" your room will sound unnatural.
    So let the speaker sound develop clearly FIRST and then in the far back, ceiling and far sides add a little splash of "air" sounds.
    2) It can also be extremely useful to also put a flat reflection in the dead center in between your speakers to create the "center channel."
    You may hear the center without adding a reflector---however without a solid reflector to gather and focus these center notes the recording will appear to have no "depth."
    If you do it correctly your reflection "flat space" will be at the same distance from your chair as the two speakers are from your chair.
    In other words left-right and center flat will describe an "arc" if you drew it on paper.
    I suggest you do NOT leave your gear sitting in the center right there.
    A rack with irregular stuff on it will blur the focus of your phantom "flat reflection" channel.
    If you do not have a center channel effect in your setup you may wind up not creating a "Proscenium" or stage effect to your playback.
    A "proscenium stage effect" is extremely desirable as it anchors the playback spatially and lets you know front to back depth.
    A proscenium effect will recreate the SOLIDITY that STEREO (which means SOLID in Greek language) is SUPPOSED to have.
    It amazes me how rarely I find a proper setup out there in hobby land.
    Most guys do not build a stage.
    Which considering that is how STEREO is supposed to work---is madness.
    Some guys are content just to have a pretty tone and think that is key.
    I contend that being able to hear all three effects (left-right-center) and a little "air" is CRITICAL to playback.
    But it's OK to do it wrong as almost nobody does it right.
    And then you can join those folks who inform you how "imperfect" stereo playback is and how silly to think it can be "perfect."
    Which is too bad because perfection simply means the darn thing works properly.
    Oh well.
    My two cents.
     
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  19. George P

    George P Notable Member

    Location:
    NYC
    Where would this reflection go? On the front wall, between the speakers? And about how big should this flat reflection be?

    Thanks
     
  20. Ontheone

    Ontheone Poorly Understood Member

    Location:
    Indianapolis
    Because what we want is for the sound from the speaker driver to reach our ears and not have a delayed sound reach our ears later from a reflection.
     
  21. Doctor Fine

    Doctor Fine "So Hip It Would Blister Your Brain"

    Look at any recording studio control room---
    It goes on the front wall in between your two speakers.
    You may already have some sound deadening material (sonex?) (draperies) to help tame the out of control wall reflection.
    But ADD a "flat" area or simply clear away a space in the middle of the curtains to expose the front wall.
    Experiment with bigger and smaller "reflectors' until you feel it is just the right size.
    If there is too much reflection from the front wall it will sound shouty and boomy.
    If too little the center notes will be "too dry and dead"
    Just right?---the center now sounds EXACTLY like the left/right speakers!
    It is like there is another speaker in there!
    Thanks for asking and now---GO GET EM.
     
  22. George P

    George P Notable Member

    Location:
    NYC
    Thanks. Cool idea. So if I have a wide TV about a foot behind, but between the speakers, centered, maybe it would be a good idea to use two towels to cover the left and right sides of the TV, leaving a spot open in the middle?
     
  23. Doctor Fine

    Doctor Fine "So Hip It Would Blister Your Brain"


    Look around at some of our other members room photos who have done EXACTLY that.
    On the other hand "close enough" does the job.
    Once you actually hear a Proscenium effect where you have a focused stage area in between your speakers---you may decide it is really a HUGE deal and find ways to improve it.
    In my case after I realized how much a Proscenium adds to the sound---I started DESIGNING systems with a Proscenium in mind.
    I have a room where I tried NO gear in between the speakers, AND a huge flat front wall space in between.
    That probably approximates YOUR setup with a flat screen TV in the middle.
    It was great but a little "shouty" and the images were a little blurred.
    Then I moved an enclosed rack in between the speakers with a flat front face with two doors on it for reflection and that was OK too.
    It moved the stage dimensions slightly forward (obviously) but still reflected a useful amount.
    Then I OPENED the doors to the rack so the inside rear wall of the rack was doing the reflecting when the doors were open.
    Doing so completely enclosed the Proscenium and made a little "concert hall" cutout.
    In other words the INSIDE of my cabinet was holding the sound and the open flat rear wall was still working correctly so my sound was now "boxed in and the center was "on stage."
    So now if I want to hear a PERFECT stage I have to leave all four doors OPEN!
    Oh well.
    That's what happens when you know how stuff works.
    But BOY does it sound REAL when those doors are open!!!!
    And it costs NOTHING to experiment with this free acoustic phenom.
    Go ahead and EXPERIMENT.
    If a terrific "you are THERE" stage is to your liking---THAT'S how you get there.
    With most recordings it really adds the missing spatial information lost during recording!

    CLOSED
    [​IMG]


    OPEN
    [​IMG]
     
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  24. Doctor Fine

    Doctor Fine "So Hip It Would Blister Your Brain"

    PS an open FIREPLACE can also make a terrific Proscenium!
    I had a house set up that way and a guy heard it and exclaimed "there are PEOPLE walking around inside your fireplace---that's AMAZING!!!"
     
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  25. George P

    George P Notable Member

    Location:
    NYC
    Very interesting. I haven't seen any of the photos that do that.

    My TV is 45" wide. Could you suggest a starting amount (width) to leave open in the middle? Should I try splitting the TV into thirds, 15" each, with the middle third open?
     
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