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absorbing low freq - cuts boom outside room also?

Discussion in 'Pro Audio Equipment' started by stephen_kambeitz, May 28, 2002.

  1. :confused: I have just recently built a project studio in a 2000 sqr/ft lot. The partition wall between my studio and the shop next (I am in a strip mall ) was said to be 14" wide - staggererd studs 5/8 drywall with insulation. To achieve an STC of 55 we built on accordingly. As it turns out the wall between me and the next door is roughly 8" - not on staggered studs but with insulation. My room has 14" high ceilings and I am recording drums in this room. Now problem is since the partition wall is so thin i cannot get away with recording drums during daylight hours - luckily It is only 800hZ and below coming through and not the whole spectrum. My question is - with bass traps or low end absorbers - can I put these in the room and have the low end energy tuned into heat or absorbed to cut down on the boom in the room and keep the spl from building up and going through the partition wall. There for being able to record during the day.
     
  2. RandomGuest

    RandomGuest Guest

    Joined:
    Feb 10, 2001
    Damn good question..!!

    man.... :( a grown man, beating the living sh!t out of a kit... is hard to mute...

    Fingers crossed.. I hope it works..

    What style businesses are either side or above or below you?

    We get into Friday eve setups and tracking all weekend, there is an office above and they CAN hear live drums, but little else from us..

    We can track during the day, but try not to..
     
  3. Astroboy

    Astroboy Guest

    Fourteen inch ceilings?

    Kind of limits mic placement, don't you think?
     
  4. I have never had a roblem with mic placement - what I did to accomodate my '14" ceilings' is purchase my mics and stands from a company that used to make cars called "micro machines". At first I saw it as a problem ...them they made it all easier for me. Then as a christmas gift from santa, he gave me 12 elves as gophers to set up mics and cables in my performance box.....
     
  5. Mike Simmons

    Mike Simmons Active Member

    Joined:
    Nov 5, 2001
    Location:
    Philadelphia
    Home Page:
    You guys are cruel... kinda reminds me of that scene from Spinal Tap with the tiny Stonehenge though. As to the original question, I think F. Alton Everest may cover some of this in his books on studio construction.
     
  6. Astroboy

    Astroboy Guest

    Sorry about the whole fourteen inch stonehenge thing....

    I'm no acoustic doctor, but i play one on t.v.....

    My two cents: Bass traps don't eliminate bass frequencies, they just stop them from repeating endlessly, right?
    So, i guess it makes sense that one set of waves or so would still be pounding the walls.

    Low end transfer is really hard to get rid of.
    All of the floor and wall coupling makes it difficult at best.
     
  7. knightfly

    knightfly Active Member

    Joined:
    Jan 18, 2002
    I've been studying acoustics off and on for the last 15 years, ever since I built the most ^#$%ed up room in existence due to ignorance - granted, most of my comments are from theory, not practice, but here goes - anybody with real experience, feel free to pounce, my skin is tough and my brain loves new input...

    Bass traps work best in corners because that's where the maximum pressure and minimum velocity of the wave is, at least at the prime modal response of that room dimension. The higher the frequency, the closer to the wall you have to get before the velocity is zero and pressure is maximum, which is why bass traps have to be away from the wall to work. The way a bass trap works is to impede the flow of air into/out of the trap with a semi-permeable material, such as rigid fiberglass board. This impedance converts the sound level into heat, caused by air friction thru the semi-permeable material. The thickness of the material and the space behind it, along with whatever diaphragm action of the wall, determines the frequency and amount that is affected. Because you are actually CONVERTING the bass energy into heat, that amount of bass energy would stay IN THE ROOM as heat, and therefore not be heard outside.

    Disclaimer: Books are great, you gotta start somewhere; all I can tell you for sure is that what I built BEFORE I read anything sure didn't work, and I'm still a year or two from building again so no practical experience yet. I keep remembering part of my military training, quote "The Map is NOT the Terrain...) Hey, F. Alton Everest, last I heard you were still alive in California somewhere, wanna join a really cool forum? Pleeeeze... Steve
     
  8. That is exactly how I understood it also. I have ran pink noise through the room and figured out my axial modes - which are the easiest to control but give the most room coloration- the formula is simple...1130 ft/sec is the speed sound travels in normal atmospheric presure. Then take the distance between two paralel walls - in my case the ceiling is 14', the width is 25' and the length is 38'. To plug these into the formula is 1130/2X(distance)...so 1130/2X14 = 40hZ. This is the axial mode that will develope here, plus the 1st, 2nd, 3rd order harmonic. 80, 120, 160 etc - up until 300 hZ - above that modal problems are less noticable or the build up of axial modes are less of a listening problem. So what i have is a sealed room - air tight. SPL builds up in the room as axial modes/waves develope, in essesnce creating a boom since the air inside the room is being excited. Since bass travels through solids faster than through air - it goes through the wall next door...then there is my problem...just as stated above all modes axial, oblique and tangential terminate in the corners of the room ,it makes sense to have absortion in the corners - what a bass trap does in convert sound energy into heat caused by friction as the waves go through the "bass trap" in the corner -therefore cutting down the SPL inside the room - attenuating the low end. bass traps are a loose term - until there are things called treble traps these will still be a play on words.... So If I can attenuate the bass boom inside the room by absorbing what I can but still keeping the room flat and not cutting out to much low end in the room I "think" what will heard out side will be less....
     
  9. Kev

    Kev Well-Known Member

    Joined:
    Nov 13, 2001
    Hey Steve,
    ...even though you've had had some cool s^*t it to say about this stuff here and home at 'tech talk'.

    I think that is the best idea I have heard in weeks! :tu:

    What is the chance of getting a REAL Acoustic person here for a bit of advice to all of us ?

    An Acoustics section would be a perfect addition to Group DIY but how to do it ?
     
  10. knightfly

    knightfly Active Member

    Joined:
    Jan 18, 2002
    "What is the chance of getting a REAL Acoustic person here for a bit of advice to all of us ?" Kev, I'm crushed! (hehehe - not...)
    "An Acoustics section would be a perfect addition to Group DIY but how to do it ?" My sediments perzackly, my "under-world" friend - Unfortunately, the only two I've been very impressed with are kinda spoken for - Eric De Sart, from Netherlands, hangs out on Studiotips.com, and John Sayers, from your side of the planet, hosts an informative site at

    http://www.saecollege.de/reference_material/index.html

    and hangs out on homerecording.com. Of the two, Eric is the deepest, and hardest to understand (needs someone as deep as he is who speaks English as a FIRST language) John is full of practical, hands-on info.

    I figger since I've bought and read 4 or 5 of Everest's books, the least he could do is mod an acoustics forum for us - that doesn't seem unreasonable to the rest of you guise, does it? I mean, hell, he's only about 91; Linus Pauling was older than that when my daughter was helping him organize his reference section at Oregon State U's library... :=)

    Stephen - What is the most "leaky" instrument giving you problems? Kick drum? Bass Gweeter? All the above? Have you tried doing a TEF analysis on whatever's getting thru the most? Maybe a specifically tuned HelmHolz just for kick? Specifically what have you added after their (maybe) STC 35 wall, step-by-step? Controlled flanking over the top of your wall? Forgive me if I'm "carrying coals to Newcastle" here, just trying to help figure out what's next, other than a lawsuit... Steve
     
  11. bonneybear

    bonneybear Guest

    My room is far from perfect, my question is , I have excessive bass on the perimeters of the room and the corners , leaving the middle of the room bass light, will bass traps help the cancelation of bass in the center?
     
  12. knightfly

    knightfly Active Member

    Joined:
    Jan 18, 2002
    "My room is far from perfect, my question is , I have excessive bass on the perimeters of the room and the corners , leaving the middle of the room bass light, will bass traps help the cancelation of bass in the center?"

    At the risk of sounding like I know what I'm talking about (see my previous comments if you are gullible enough to think this) I am pretty sure you won't affect the "bass light" characteristic of the center of your room. Assuming a rectangular room, the fundamental modal frequency of each dimension of your room will have lowest velocity, therefore highest pressure, at physical boundaries (walls/floor/ceiling)and that freq. will have its highest velocity (therefore LOWEST pressure) at the center of the room. This is true also for top to bottom, which people sometimes forget. What this means is that each of the three primary mode frequencies will have the LEAST volume at the dead center of the room. For a room with dimensions of 10 x 16 x 23.3 (a favorite ratio of acousticians, with good reason) this would be a point 5 feet from the floor, centered between each pair of walls.

    If you put a bass trap(s) in the room, they SHOULD be at the point(s) of highest pressure as you said. This will absorb some of the energy of the bass freq. and cause less apparent (and actual) bass at that freq.; however, I don't see any way that cutting the sound pressure at the wall would cause any increase in that pressure/frequency in the center of the room. If anything, it might reduce the level in the center of the room also, since you've just converted some of that energy to heat.

    If you draw out your room to scale, then subdivide it into two, three, four, five, and six sections, and do that in all three dimensions, you will define the points of the room that will have peaks/dips at what frequencies. Let's take the 23.3 foot dimension in the example I gave, which would support 24.25 hZ, the lowest frequency this particular room will support. Taking just that one dimension of the room into consideration, you would also have resonances at (keeping math simpler by rounding off) 48,72,96, 120, etc - If the 24 hZ level is minimum at the center of the room (all will be maximum at the walls) then the 2nd harmonic (48 hZ) will have 3 maximum and two minimum level points in that dimension; max at walls and center, min at 5.82 feet from each end wall. The THIRD harmonic of that dimension at 72.7 hZ will have peaks at 1/3 and 2/3 the length, and dips at 1/6, 3/6, and 5/6 of the length, or 3.88 feet from each wall and at center.

    If you do this exercize for each dimension of ANY ROOM, you will find that the center of the room SUCKS for ANYTHING! Every resonant frequency the room supports will either be peaked or dipped at the exact center of the room. The only solution I am aware of is don't be in the center of the room!

    If you are experiencing nulls in the center of your room, that means they are either the 1st, 3rd, or 5th harmonic of that dimension. To find out which, you could try moving to a point 1/3 the length away from the wall; if you find another peak, then the offending freq. is the 1/3 harmonic. If the peaks come at fifths of the length, then that is the 5th harmonic of that dimension. This gets so complex when you add in 3 dimensions, furniture, people, tangential and oblique modes, that sometimes its easier to just move things until they sound good. F. Alton Everest, in at least one of his books, remarked that for the most part figuring anything beyond axial modes of a room is a waste of time, since the first piece of furniture/equipment you put in the room changes everything anyway. Still, it's good to know what frequencies to EXPECT to have problems with, and which dimensions are responsible for them. The basic formula for finding this out is Frequency = 1130/2L, where L is the length of the room in feet. This will get you the lowest axial mode frequency of the room. Do the same for width and height, then multiply each frequency by 2, 3, 4, 5, 6, throw away any results higher than about 300 hZ, etc, and you have the axial mode response of your room. If you want an easier way to figure this, there is a simple Excel/Lotus spreadsheet I wrote about 10 years ago here -

    http://www.prorec.com/prorec/downloads.nsf/category

    The file is called roomtune, it's under Other Application. It is a self-extracting archive that includes a wordpad info file, and both Excel and Lotus versions of the spreadsheet.

    If you really want to know more about your room, download MODESv2p.xls from this site -



    click on Calculation Tools to get to the file. This sheet does all the things I was going to re-write mine to do, probably better. The cool thing (for me, anyway) is that for once I found it BEFORE I went to all the trouble of writing my own... Some things to watch about this sheet - you have to convert your dimensions to inches before you enter them in the yellow boxes. Then, you HAVE TO follow the directions on the first page (1,2,3) or the sheet won't recalc the graphs, and you'll be looking at someone else's room graphs and YOUR mode frequencies. Bummer...
    This sheet also has some good basic advice on traps/locations, etc - I use it a lot. Be sure to click on the various tabs at the bottom of the first page in order to see all the views of the information generated.

    I hope this answered some of your questions without muddying the water even worse - If not, post back and I'll try again... Steve
     
  13. bonneybear

    bonneybear Guest

    THANKS STEVE, Your approach seems to make a lot of sense for the layperson..........Mike
     

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