Acoustic Treatment / Soundproofing

Discussion in 'Speakers' started by sphinxvc, Feb 14, 2016.

  1. Dr J

    Dr J Friend

    Friend
    Joined:
    Sep 28, 2015
    Likes Received:
    94
    Trophy Points:
    18
    Hi,

    first time content post from me, been lurking and learning since Changstar days and introduction post a long time ago.

    I liked these Room acoustics 101s when planning and (still continuing) building a home theater in a basically concrete wall rectangle in the basement. There was a -12dB to -30dB chasm between 100Hz and 200Hz when I started...

    Short, enough information and maths for an amateur like me to get it mostly right, and links to further details.

    http://arqen.com/acoustics-101/room-setup-speaker-placement/
    http://arqen.com/acoustics-101/room-setup-acoustic-treatment/
    http://arqen.com/bass-traps-101/placement-guide/

    I think this is a good list of topics to go through, lifted from the above sources:

    "Step 1: Listening Position
    Step 2: Listening Distance
    Step 3: Speaker Layout and Height
    Step 4: Speaker Distance to Front and Rear Wall

    Step 5: Apply Acoustic Treatment
    Step 6: Optimization Using Acoustic Measurements
    Step 7: Rock ‘n’ Roll!"
     
    • Like / Agree Like / Agree x 3
    • List
  2. sphinxvc

    sphinxvc Gear Master (retired)

    Staff Member Friend BWC
    Joined:
    Sep 28, 2015
    Likes Received:
    3,271
    Trophy Points:
    113
    @Riotvan, unfortunately trapping the window is not an option. There's a heater in front of it.

    @Marvey, LOL. I've had nerd-traps for a few years now, they'll have to do while my teddy bear collection is in storage. And will try the short side.

    @Armaegis, yep, I'm leaning toward that one now. Makes it easy to set up the projector too.

    @Dr J, cool, will check that link out.
     
    • Like / Agree Like / Agree x 1
    • List
    Last edited: Mar 6, 2017
  3. Dr J

    Dr J Friend

    Friend
    Joined:
    Sep 28, 2015
    Likes Received:
    94
    Trophy Points:
    18
    Maybe obvious, but I found the dead zones AKA nulls fascinating when trying to understand room acoustics:

    This could be the null caused by room modes, i.e. when the sound wave starts bouncing back and forth between parallel walls or ceiling and floor, sort of like a instrument string vibrating at its fundamental frequency ("my room is ringing...").

    The room modes occur at fractions (1/2, 1/3, 1/4,...) of the wave length of the sound fitting between the walls, either as nulls or amplifying the frequency, at that fraction of the distance between the walls, and I understand it is advisable to avoid placing the speakers or listening position at these locations. Conversely, at the corners and at the walls these frequencies get amplified a lot.

    The 1/2-wave null in the middle of the room is an example.

    Exception to avoiding the middle of two walls is that you need to be in the middle of the side walls, if you place the speakers place symmetrically with respect to the side walls, as you should to retain symmetry and esp. stereo image. One solution to that is putting enough absorbing material on the side walls to dampen the sound wave bouncing back and forth between them.
     
    • Like / Agree Like / Agree x 1
    • List
  4. Dr J

    Dr J Friend

    Friend
    Joined:
    Sep 28, 2015
    Likes Received:
    94
    Trophy Points:
    18
    From reading, doing practical experiments and consulting a professional room acoustic design guy when I was at my wits end, the answer is "enough, and not a bit more" ;-)

    I have understood that even for a dedicated studio or listening room or room theater, the purpose is not to dampen everything to death and get to sub-0.1s T60 (time to decrease by 60db from an impulse). And it is not practical for normal rooms as the damping depth to get there for the lowest frequencies is huge, like a yard or two of the stuff.

    If everything is damped, you lose a lot of the sense of space that you get in normal listening situations, and sound is too dry. Instead, after a certain amount of treating the basic room issues (first boundary reflections and corner bass traps with absorbing elements), it seems the recommendation is to use diffusors instead to reflect the sound in different directions, to retain the liveness of the room.

    The hazy average of recommendations for T60 seems to be: for normal stereo listening 0.3s - 0.5s, for home theater with multi-channel audio (where the spatial cues come from the speakers) 0.1s - 0.3s. And there are calculation tools freely available that allow you to estimate how much of the surface of a room needs to be treated t get to roughly these T60 ranges, but in the end a mic and REW or similar measurement equipment is needed to check.
     
    • Like / Agree Like / Agree x 2
    • List
  5. Dr J

    Dr J Friend

    Friend
    Joined:
    Sep 28, 2015
    Likes Received:
    94
    Trophy Points:
    18
    I have to say that my understanding and experience is different. Maybe I misunderstand your point or have not come across the same phenomenon?

    As I understood the guidance I read and got, the idea behind having a space behind the panel is to increase the absorption co-efficient for lower frequencies. The mids and highs absorption does not benefit from the gap.

    Lower frequency waves are longer and hence require the absorbing material to be further away from the wall to impact the high velocity/low pressure portion of the wave, or possibly the low velocity/high pressure part, cannot recall. Some rule of thumb for this I read is that for a panel to be effective for a wave length L, it must be at least 1/4*L thick, although some effect is in practice achieved already with 1/10*L thickness.

    This effect is also shown in the published specs of the industrial acoustic absorber panels for building construction that I checked and ended up using. They provide an absorption co-efficient per octave with and without a space behind the panel. They also need to provide measurements data compatible with official standards and by third party neutral bodies to prove the specs (in the local EU bureaucratic way).

    A rule of thumb seems to be that you can increase the efficiency significantly (equivalent to 1-1.6times thickness ) by having a gap behind the panel which is one half to two times the thickness of the panel. More than two times, or possibly already at that, the panel starts to resonate or the reflections/standing waves behind the panel make the absorption co-efficient un-even for different frequencies (not sure about the mechanism anymore) and that is not good as the aim to reflect uniformly across frequencies.

    There is a nice calculator for exploring the impact of different materials, layers and gaps here: http://www.acousticmodelling.com/multi.php. The challenging part of using the calculator was to find out the "Flow resistivity" of the materials, but using any value in the range of 10 000 to 20000 gives an idea.

    For what it is worth, my own admittedly limited measurements seemed to confirm the above theory (more or less, did not do systematic experiments).

    *) as opposed to much more expensive, interior-decoration-friendly panels for audiophiles for which absorption specs are often not available :)
     
  6. Dr J

    Dr J Friend

    Friend
    Joined:
    Sep 28, 2015
    Likes Received:
    94
    Trophy Points:
    18
    I think I am probably writing too much now as this topic has been close to my heart for the past year, will cease for today after this:

    There are some rather nice freeware acoustic related calculation tools to help in designing/understanding the acoustic behavior of a room.

    The above mentioned http://www.acousticmodelling.com/ also has links to other acoustic calculators that are useful for understanding room acoustics.

    As does http://amroc.andymel.eu/amroc_andymel_eu_others.php.

    This helps in calculating the room modes in a nice visual way: http://amroc.andymel.eu/

    This one helps to find the places to put the absorptive/diffusive treatment: http://amray.andymel.eu/.

    Although for the latter the good old "put a mirror on the wall/ceiling/floor and move it around, when you see the loudspeaker from your listening position, that is where the panel goes" works as well and is less effort intensive. But if you need to plan the placement of furniture and panels before you have (access to) the room, it works to get it roughly right.
     
    • Like / Agree Like / Agree x 1
    • List
  7. purr1n

    purr1n Super Friend

    Staff Member Friend BWC
    Joined:
    Sep 24, 2015
    Likes Received:
    75,381
    Trophy Points:
    113
    Location:
    Padre Island CC TX
    ^ This is the happy medium. Really folks, most rooms are OK.
     
    • Like / Agree Like / Agree x 2
    • List
  8. Serious

    Serious Inquisitive Frequency Response Plot

    Friend BWC MZR
    Joined:
    Sep 28, 2015
    Likes Received:
    1,969
    Trophy Points:
    113
    Location:
    near Munich, Germany
    It also comes down to preference. I know my dad likes a livelier room than I do. In terms of measurements I like T60 around 0.2-0.3s, but less is also ok as long as the high end isn't completely overdamped. My dad would probably want closer to 0.5s.
    What's much more important is that you don't get annoying first reflections. More of an issue in a smaller room. Side-walls are my main priority here, but the ceiling also matters more than you might think. The floor I usually don't find as bad. I also find that sometimes the reflection from left speaker to the right wall and right ear (and vice versa) is worse than the one from the left wall that arrives first. This is especially true when you have widebanders with beamy treble that are toed-in so they cross in front of the listening position, which makes the treble ray of death reflect off the opposite wall and hit the other ear. With bare walls you might actually get more reflected energy vs direct energy from the drivers this way.
     
  9. GoodEnoughGear

    GoodEnoughGear Evil Dr. Shultz‎

    Friend
    Joined:
    Oct 25, 2015
    Likes Received:
    2,848
    Trophy Points:
    113
    Location:
    Cape Town, South Africa
    I'm a lucky fucker.

    The listening room I've negotiated in our new house looks like this: 3.6m wide, 4.5 m deep/long and 3.3m high. Beautiful high ceilings in an art deco house. Dimensons are very good for a small room! One window on the short end, 1.1m above the ground and reaching to the ceiling - sash windows.

    I could hardly ask for more. It needs a desk and a single bed in it too, so this is what I'm thinking:

    Listening Room.jpg
     
  10. purr1n

    purr1n Super Friend

    Staff Member Friend BWC
    Joined:
    Sep 24, 2015
    Likes Received:
    75,381
    Trophy Points:
    113
    Location:
    Padre Island CC TX
    Needs torture rack.
     
    • Like / Agree Like / Agree x 1
    • List
  11. Garns

    Garns Friend

    Friend
    Joined:
    Jul 9, 2016
    Likes Received:
    2,067
    Trophy Points:
    93
    Location:
    Sydney, AUS
    So here's a question. I'm looking at getting a new lounge chair, and have the option of one which comes to shoulder level or one with a full head support:

    Screen Shot 2021-12-06 at 10.18.31.png Screen Shot 2021-12-06 at 10.27.20.png
    The full head support is more comfortable, but I read that a chair which goes behind your ears is acoustically compromised for speaker use. Question is, is it really that bad? The chair will be about 2ft from the side wall and 4ft from the rear and there are currently wicked bad reflections off them - putting a pillow behind my head on my (shoulder-height) current chair is a big improvement. I would get it in fabric, so I would expect it to be more absorptive than reflective (steel frame + foam + fabric cover). I'm of the view that you can't get enough absorption so wouldn't this be a great way to absorb the back reflections without making the room look like a padded cell?
     
  12. Armaegis

    Armaegis Friend

    Friend BWC
    Joined:
    Sep 27, 2015
    Likes Received:
    5,146
    Trophy Points:
    113
    Location:
    Winnipeg
    Yes, if your back wall is causing issues, then a soft backed chair is going to be better than listening to all those rear reflections. There will of course be some reflection coming off the chair itself, but it will *probably* be better than those from the wall. More critically important is if the high-backed chair is actually more comfortable. For me I don't actually like a lot of high backed because I feel they slope forward too aggressively and it's uncomfortable on my neck to lean back.
     
    • Like / Agree Like / Agree x 2
    • List
  13. Riotvan

    Riotvan Got lost for three weeks at Delft City Hall

    Friend
    Joined:
    Sep 27, 2015
    Likes Received:
    2,943
    Trophy Points:
    113
    Location:
    The Netherlands
    Ask for a homedemo to hear how the chair sounds. “But sir our chairs don’t squeek.”
     
    • Like / Agree Like / Agree x 4
    • List
  14. Garns

    Garns Friend

    Friend
    Joined:
    Jul 9, 2016
    Likes Received:
    2,067
    Trophy Points:
    93
    Location:
    Sydney, AUS
    I was actually quite surprised that the high back was more comfortable. It definitely slopes forward more but in this case it cancels out the fact that the lower back one slopes back too much...
    Lol, that would be the best approach. But the shop floor only has the high-back in leather!
     
    • Like / Agree Like / Agree x 1
    • List
  15. murphythecat

    murphythecat Self Imposed Exile

    Friend
    Joined:
    Apr 4, 2016
    Likes Received:
    1,147
    Trophy Points:
    93
    Personally, a bed was the best solution for my comfy chair. acts as a early reflection panel for the floor bounce as a bonus.
    I couldnt just sit in a chair sitting upgright for very long.
    Absurd and ridiculous looking but impossible to go back
     
    • Like / Agree Like / Agree x 1
    • List

Share This Page