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Holy bleepin' Standing Waves!!!

Discussion in 'Recording' started by Southwind, May 23, 2001.

  1. Southwind

    Southwind Guest

    I don't believe it. I just figured out exactly what is wrong with my control room. I am suprised that anything I've ever done in here has been even listenable.

    This afternoon I finally had enough free time to do a simple test: create some pure sine waves at various frequencies and see just where the strengths and weaknesses in my little room are. I had a sneaking feeling that there was a problem around 60 - 100 hz as I always have to really carefull there to have mixes translate.

    I couldn't believe my ears, There is almost NO 80hz at the mix position......Help!!! It was actually rather unsetteling. An 80 hz tone is almost inaudible untill I roll back a foot. Two feet back it is at full volume. It dissapears again near the back wall and then disappears completely behind that.

    O.K.....I guess my plan to tear this place apart and start again from scratch is no longer something I can put off. I am going to re-orient the whole room and build out the corners. SOON

    Any thoughts on the best way to avoid future mode problems would be greatly appreciated. I know that my room dimensions are the problem. Will tuned traps do it? What sort of corners can I build?.......Help

    Thanks in advance,

    Charles Rieser
    Southwind Studio
    Austin, Tx.
     
  2. e-cue

    e-cue Active Member

    Lemme guess. You have parallel walls? If so, find a way to get rid of them that won't be a step backward.
    If your gonna rearrange your control room, I'd start by placing your speakers in the correct height, distance, and direction from the 'sweet spot'. When you tune your room, I'd start with the sine wave tests to check for nodes & anti nodes. Most people stop there. I'd suggest playing several songs that you have heard a zillion times & totally know how they should sound. Headphone checks can also be helpful if your used to the headphones.
     
  3. Southwind

    Southwind Guest

    Thanks e (or is it Mr. Cue?),

    Yea, I got 'dem parallel wall blues. I rent so I can't just knock down the walls, but I can build a sub-wall (?) behind me. I was thinking of several convex sections. Am I on the right track, is this worth the effort? Will treating one wall this way get rid of most of the mode problems or create more?.....

    Thanks again,

    Charles
     
  4. Dan Popp

    Dan Popp Active Member

    Originally posted by Southwind:
    O.K.....I guess my plan to tear this place apart and start again from scratch is no longer something I can put off. I am going to re-orient the whole room and build out the corners. SOON

    Any thoughts on the best way to avoid future mode problems would be greatly appreciated.


    Dear Charles,
    Your problem is not so unusual or so desperate as you might think. Give the folks at Acoustic Sciences Corp (ASC) in Oregon a call. They can probably recommend a solution that doesn't require demolition.

    Yours,
    Dan Popp
    Colors Audio
    USA
     
  5. Southwind

    Southwind Guest

    Dan,
    Thanks much for the tip, I'll be getting in touch with them.

    Perhaps I over stated my intent. While I wish I could take down a couple of walls, I can't. I was think more about adding some corners or angles within the space. Or constructing some tuned traps. What do you do for stuff down that low?

    Thanks again,

    Charles
     
  6. Jon Best

    Jon Best Active Member

    First, go get the F. Alton Everest book _The Master Handbook of Acoustics_.

    I don't think the parallel wall thing is it- the only thing you really need to worry about with parallel walls is flutter echo (you know, that boiiing that happens when you clap between two walls). Modally, splaying walls just makes them harder to predict. You've done the first step- you know where the problems is. Now, find the places (that are out of the way, i.e. you can hang stuff there) where the _most_ of that frequency is in the room. Look up in the Everest book how to put together a diaphragmic absorber that resonates at that frequency. Build one, stick it in the place you found, and see what happens. A polycylindrical diffusor on the back wall and in the corners you can spare will add some high end diffusion, as well as some low end control, too.

    Originally posted by Southwind:


    Perhaps I over stated my intent. While I wish I could take down a couple of walls, I can't. I was think more about adding some corners or angles within the space. Or constructing some tuned traps. What do you do for stuff down that low?

    Thanks again,

    Charles
     
  7. JCG

    JCG Guest

    Non-parallel walls are not likely to help you at 80Hz - the wavelength is so long (over 14 feet if my quick calculation is correct) that you'd need to cant the walls significantly to make them acoustically non-parallel at that frequency (Phillip Newell's books discuss this if you don't trust me on it). As for trapping - this can only affect the amount of "colour" a mode adds to the room - it can't remove a mode or move the nodes and antinodes around the room. It's best to eliminate as many nodal issues as you can by adjusting room dimensions before using trapping to deal with the effects of modes that remain.

    I'd suggest you start by looking at how the modes for your room are arranged - you might find that it is possible to move one wall (or the ceiling or floor) by a small amount to eliminate a coincidence that is contributing to this problem. Knowing which dimension or dimensions are supporting the modes that contribute to your problem is definitely the first step in dealing with it, whether you try to eliminate it by adjusting the dimensions or alter it with trapping or other treatments. There are several mode calcualators around on the Web that will generate the train of modes for your room, given its dimensions (and assuming it is at least roughly rectangular).

    Another thing you might consider is simply moving your mix position to a better spot in the room, although this might not be an option if you have soffit mounted speakers or other features that restrict where the mix position can go.

    Anyway, I hope this helps!

    My $0.02 (Canadian),
    John
     
  8. Ethan Winer

    Ethan Winer Active Member

    Charles,

    > I was think more about adding some corners or angles within the space. Or constructing some tuned traps. What do you do for stuff down that low? <

    What you really need are bass traps, which are meant to solve exactly the problem you described. You can buy tube traps from ASC, and they work, but it is very expensive to buy enough of them to fully solve the problem.

    I wrote an article for Electronic Musician a few years ago that shows how to build some very effective traps for a lot less money than you'd pay for a commercial version. If you'd like to see the article and plans, go to my Magazine Articles page at:

    http://www.ethanwiner.com/articles.html

    And then look for "Build a Better Bass Trap." If you have any questions afterward, I'm happy to answer them here.

    --Ethan
     
  9. lflier

    lflier Guest

    Ethan,

    The bass trap article is great! But I always thought (and mind you I am a relative dumbass about these things) that bass traps were only useful if you were getting a preponderance of low frequencies. I have a similar problem in that the entire bottom end seems to disappear in my studio, and the more you crank the monitors, the more it disappears. This results in mixes that don't translate well of course. The highs also seem to be amplified. The room doesn't have a ton of reverberation, and it has a drop ceiling (acoustic tile), so it's not too "live", but are you saying that the reflected bass signal cancels out the original and that is why one needs a bass trap?

    Also, there is an upright piano directly across from my mixing area (the room is 19.5 feet across). Is this a help or a hindrance as far as treating the room goes? How do you figure out how many traps you need and where to put them?

    Thanks,
    Lee
     
  10. Dan Popp

    Dan Popp Active Member

    Dear Lee et al,
    The "too much bass" and "not enough bass" problems are related - I suggest you read the books mentioned already, or call ASC or another reputable acoustics manufacturer, or even an acoustics consultant. I wouldn't dream of building my next recording space without professional help. It's just really important to get it right.

    Yours,
    Dan Popp
    Colors Audio
    USA
     
  11. lflier

    lflier Guest

    Dan,

    It would be wonderful to hire an acoustics consultant and get all the cool stuff from ASC... but this is a HOME STUDIO, not a commercial facility, and I simply can't afford that. Period.

    Yes I am a bit spoiled from having worked in commercial studios. Therefore, glaring acoustical problems are tough for me to face. :)

    --Lee
     
  12. Ethan Winer

    Ethan Winer Active Member

    Lee,

    > I always thought ... bass traps were only useful if you were getting a preponderance of low frequencies. ... are you saying that the reflected bass signal cancels out the original and that is why one needs a bass trap? <

    Exactly!

    > Also, there is an upright piano directly across from my mixing area (the room is 19.5 feet across). Is this a help or a hindrance as far as treating the room goes? <

    That shouldn't matter much one way or the other.

    > How do you figure out how many traps you need and where to put them? <

    For most smallish rooms you'll probably want to cover the three walls completely (sides and rear). Full coverage is not strictly necessary, and good result can be achieved with just eight bass-only traps - two in each corner. At 19 feet (the longer dimension?) you don't really have to line all of that with traps.

    --Ethan
     
  13. Southwind

    Southwind Guest

    From what I can tell, preponderance and lack of bass are indeed related. Modes and anti-modes are certainly co-habitating here in my control room (though not for long). I have identified a sweet spot where it seams relatively flat and will be working on low end management this weekend.

    Thanks to all who have replied, this has been a great learning experience in the pratical application of stuff I have "known" for years. Soon this place is gonna sound soooo much better.

    Charles Rieser
    Southwind Studio
    Austin, Tx.
     
  14. lflier

    lflier Guest

    Ethan, thanks so much for your response - makes a lot of sense. Just a couple more questions if you don't mind:

    1) The 19.5 foot dimension is actually the shorter one. The side walls (relative to my rig) are almost 24 feet across. There is a futon couch against one of them. I am thinking it probably wouldn't be necessary to treat the side walls. However I am getting cancellations in the low bass AND the high bass, and also I seem to be getting some doubling in the high frequencies, so maybe I could put traps in the corners plus a high frequency absorber or two directly across from my monitors, over the piano?

    2) The basement is a concrete block structure on 3 sides, standard framed interior wall on the 4th side (the one right across from the monitors) and I built an inner wall in from the concrete blocks. Given that, would it be possible to simply mount the fiberglass directly on the walls on those sides instead of building a frame for them, since there's already a second inner wall?

    3) My monitors are very close to the wall that I face while mixing. Would it be an advantage to put any kind of treatment behind the monitors?

    Thanks,
    Lee
     
  15. Ethan Winer

    Ethan Winer Active Member

    Lee,

    > 1) The 19.5 foot dimension is actually the shorter one. The side walls (relative to my rig) are almost 24 feet across. <

    That is a nice size room! I assume it is a single room that you use for both recording and mixing? That's the way my studio is, and at 18x33 our rooms are similar sizes.

    I opted for a live room for recording, knowing that it would be less than ideal for mixing. If you look at the photos in my Bass Traps article you can see that I treated only the area directly surrounding my "mix station." Though to really treat that small region properly I put traps on the ceiling too. I record a lot of classical music and acoustic instruments, so keeping the room live was very important. For most pop music production you can aim for less ambience.

    > However I am getting cancellations in the low bass AND the high bass <

    Standing waves can occur at any frequency. If you play a 1 KHz. sine wave - or even higher - through your speakers, you'll find peaks and nulls caused by acoustic wave "collision." It's just that you hear those by moving your head inches at a time, rather than having to move a few feet to hear them at low frequencies.

    > I seem to be getting some doubling in the high frequencies <

    I'm not sure what that means.

    > so maybe I could put traps in the corners plus a high frequency absorber or two directly across from my monitors, over the piano? <

    I don't want to be vague, but without seeing your room in person, it's hard to know exactly what would be ideal. So let's approach it this way. If I were you I would:

    Start by building 16 bass traps, with one pair (lo / hi) for each corner and four more pairs along the walls, in the general area of your speakers. You should also alternate mid/high absorbers between the traps that are along the walls.

    You can always build more bass traps later if you want even tighter bass. And maybe you'll want to add more mid/high absorbers along the side and/or back walls later too.

    --Ethan
     
  16. lflier

    lflier Guest

    Hi Ethan,

    >>That is a nice size room! I assume it is a single room that you use for both recording and mixing?<<

    Yep. I really like that atmosphere, it works out really well as far as the creativity goes!

    >>I opted for a live room for recording, knowing that it would be less than ideal for mixing. If you look at the photos in my Bass Traps article you can see that I treated only the area directly surrounding my "mix station."<<

    Yeah.... this would be a little tougher for me because my rig is along the center of one of the long walls, whereas you have yours at one end. I am guessing though that most of the problems for me are coming from the opposite wall, and the wall that I face when mixing (and I guess the corners).

    >>Though to really treat that small region properly I put traps on the ceiling too. I record a lot of classical music and acoustic instruments, so keeping the room live was very important. For most pop music production you can aim for less ambience.<<

    I think maybe treating two walls would be a good compromise. I record mostly rock music but I do like a natural sound, I don't use sound replacers and I don't like to use too much artificial reverb.


    >> I seem to be getting some doubling in the high frequencies <

    I'm not sure what that means.<<

    I mean that the high frequencies seem to blow my head off as I turn up the volume, while the bass frequencies start disappearing.

    Your plan sounds good except I won't treat the side walls, other than in the corners. I will start by building 8 traps to go in the corners, and see what that does, then add more along the front and rear walls as I need them. Thanks a TON for your help.

    --Lee
     
  17. Ethan Winer

    Ethan Winer Active Member

    Lee,

    > > I am guessing though that most of the problems for me are coming from the opposite wall, and the wall that I face when mixing (and I guess the corners). <

    Usually parallel walls create flutter echo, and they also color the natural room ambience by emphasising and minimizing some frequencies contained in the natural reverb over others. Standing waves at low frequencies can happen pretty much anywhere, in any shaped room.

    > I think maybe treating two walls would be a good compromise. <

    Yes, at least to start. You can always add more traps later if you determine it's needed. When I built my current room I did not have the four traps in the back wall corners. But it took only a short time to realize I needed them! If I were using my room for mixing only I'd add more treatment. But as it is now, it's a good compromise between being dead enough to make mixing decisions, and live enough to sound good even when the microphones are back a few feet.

    > I added I record mostly rock music but I do like a natural sound <

    All the more reason to start conservatively, to keep as much natural ambience as possible.

    > the high frequencies seem to blow my head off as I turn up the volume, while the bass frequencies start disappearing. <

    Once you get the bass problem solved, you'll be in a better position to determine if the high frequencies are still a problem. If so, you can consider adjusting your tweeters or overall room EQ or whatever. What kind of loudspeakers are you using?

    > Thanks a TON for your help.

    My pleasure!

    --Ethan
     
  18. Ang1970

    Ang1970 Well-Known Member

    That's some great advice, Jon, Dan, and Ethan.

    I would like to expand on one point that Ethan raised... far too often the ceiling and floor are overlooked as culprits for sloppy room response. If your ceiling is on the low side, you might be getting a nasty direct reflection from speaker to mix position. Add to that a direct reflection from the console, and you've got enuf mid/hi's to singe your eyebrows! Sit in the mix position and listen to some white noise, then have someone hold a pillow over your head. Then try the same thing with a pillow on top of the mixing console. This will give you a general idea of how much reflection you've been dealing with, and has the added benefit of giving passers-by the impression that you're a total lunatic! Win-win, see?

    If you notice a problem with the mixer, try raising the speaker height to increase the angle of reflection, or slide them backward so the back of the console blocks some of the sound from hitting the top of the board. Elevating the speakers may also lower the angle of reflection off the ceiling to a point further behind the mix position. Make sure your speakers are in the best possible place -=before=- making any other adjustments to the room.

    The two main options for dealing with a low ceiling are refraction and absorption. Absorption seems to be covered adequately, so here's an idea for refraciton: A simple wedge (2x4 & plywood) mounted on the offending portion of the ceiling will adjust the angle of reflection toward whichever direction the wedge is pointed (degree of effectiveness depending on the area and depth of the wedge, materials used, yada, you know the drill). The back wall is closer than the sides, so if you angle the reflection in that direction the wave may still be reflected back to the mix position to a noticeable degree. If that happens, I would consider a dual wedge to angle the reflection off to the sides, where it will be readily absorbed and diffused by whatever other crap happens to be in the room.

    And of course, YMMV. Without hearing and seeing your room, all this advice may be worth less than you paid for it.

    Good Luck! :)
     
  19. Ethan Winer

    Ethan Winer Active Member

    Angelo,

    > Then try the same thing with a pillow on top of the mixing console. <

    What a great idea!

    --Ethan
     
  20. Dan Popp

    Dan Popp Active Member

    Originally posted by Lee Flier:
    Dan,

    It would be wonderful to hire an acoustics consultant and get all the cool stuff from ASC... but this is a HOME STUDIO, not a commercial facility, and I simply can't afford that. Period.


    Dear Lee,
    Sorry, I don't "get" that philosophy. I have about 4k invested in acoustics in my small room. It's not perfect, but with close-field monitoring, it works. You could easily fill a room with 10-15k$ worth of low-end recording gear and *not know what you were getting* unless you spent some money on acoustics. The time savings alone of *not* wondering "is this really what I think I'm hearing?" is priceless.

    As for Ethan's system, well and good, but I wonder about the tuneability of it. In other words, any room will have problems with specific levels of energy at specific frequencies. I wonder (and I am not an acoustics guru) if the shotgun approach (everybody cover 3 walls and hope for the best) is going to leave some rooms with too much trapping at certain freqs and not enough at others - with the end result that the room response is different, but not necessarily better.

    Yours,
    Dan
     

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