Test Microphone positioning

Hi guys! Just a few questions about acoustic testing; hopefully one of you will be able to help, or point me in the right direction to knowledge! Just comming up to testing my LIVE room, and as well as using the room mode calculators etc mentioned on this site, I intend to test my room using a flat response microphone (or near - ive got one of them behringer ECM8000 jobbies) and some test frequencies........ Which brings me on to my first question: is using pink noise or sweeping-frequency tones the best bet? - I guess that both will enable modes in a room to be identified (with correct mic position(?) - question 2 (to follow)), but which does the job best? Am I right to think that single tones will highlight modes at those frequencies which may not be present when a complex sound (pink noise) is played (which I presume will cause additional cancellation/re-inforcement)?? Question 2 - where do I put my microphone??? I understand that different modes will occur in different places in the room, so do I have to run a test with my microphone in every position to find them all?? Obviously in reality such a method is impractical, but are there certain points to record in? Also, I spoke to a semi (self confessed) expert, who suggested ponting my test microphone at a corner is the best bet? Is that right? I luckily have at my disposal an omni directional speaker, which I believe is ideal for this type of testing. Am I right to think that positioning this in the centre of my room when blasting out my test sounds (whatever they will be) will be the right way of playing my sound? Ive tried to look into this everywhere on the net and I still cant fully answer my questions - im sure ive missed something though, and if the answers have been staring my in the face all this time on this site I appologise in advance. Thanks. John

If you are attempting to determine the modal frequencies of your room, place a speaker, a sub if you've got one, in a corner where the floor meets two walls. You don't need a dodec becuase in the freq range we're interested in all speakers are pretty much omni. Put the mic in the exact opposite corner: diagonally across the room right in the corner where the ceiling meets two walls. Get the mic's diaphragm as close as possible to the walls and ceiling without touching them. For this work, what type of excitation signal you use is of little consequence; both pink and sweeps will get the job done. If you need help I'd be happy to send you a test tone and process it for you to show you the modes. I love doing this kind of thing, and every new room is a new chance to learn.

Hi David. Thanks for the prompt reply and generous offer - ill try to have ago and learn this stuff, but if I get stuck Ill defo be in contact! Just a couple of questions: I intend to do tests before and after addding broadband absorbers into the corners of my room, and so I will be unable to put my microphone in the position you recomended when testing the second time (after). Should I therefore place my microphone away from, but pointing at, the corner/ceiling? Am I right to asume that my non-parallel walls will pose no problem to this type of testing - Ill just use the opposite corners with the largest space between them? Cheers John

I would do two tests. To find the modal frequencies, do what I first told you. To study the effects of the traps, put a full range speaker at 38% of the distance between parallel walls in all dimensions and put the mic in the opposite spot in all dimensions. This idea is difficult to elucidate, so an exanple is probably a good idea. If the room is, say, 20 x 14 x 9, put a speaker at 20 x .38 = 7.6 from a lenght dimension wall, 14 x .38 = 5.32 from a width dimension wall, and so on. You really woudn't have to do this; so long as you had the mic and speaker in the same spot for the before and after it would be ok, but the 38% mark should give most of the modes a fair chance at developing.

Briliant - got it! Thanks a lot for your help! John

No prob. 8) Be sure to post back with data.

concentrationface, do you have a camcorder? Could you video tape yourself doing these tests and send me the video? I'm looking to put together instructional videos on studio construction and room tuning/treatment. This would be a great segment to include in that.

Hi sonic clang Unfortunetly I dont have a video camera at this point in time! I agree that a instructional video covering this type of this would be an awsome resource though- i wish I could watch one!! This is going to be the first time I will be doing any testing of this sort, and as such I dont think I would be a good presenter to have in any case - maybe David French (who explained the process to me earlier on in this thread) would be a more suitable candidate? Sorry to be of no use. good luck with your compilation of videos - ill be on to look out for them! John

That's a good idea. David, could you make a video for me? It sounds like you're an expert. :) and willing to do this for strangers.

SonicClang wrote:

concentrationface, do you have a camcorder? Could you video tape yourself doing these tests and send me the video? I'm looking to put together instructional videos on studio construction and room tuning/treatment. This would be a great segment to include in that.

There are VERY GOOD videos @ realtraps.com

Hey SonicClang, I'd do it, but I have no video equipment. I'd be happy to walk you though it, though.

I may take you up on that offer once I'm ready to tune my rooms. At this point I'm not even done with all the major construction. VERY close... but not quite done. I just re-read what you posted about the two tests you'd suggest doing. The one where you're placing the speaker (sub) and mic at opposite corners of the room, that's to find the modes of your room? And a frequency sweep from, say, 40 hz up to 300 hz would be ok for that? Would a pencil style condenser be ok for that? If not, what mic would you suggest? After the room is treated with traps do you suggest doing that same test again? With the mic and speaker as close as possible to the same spot as the before test? Or the second test you posted about? The full range speaker, that would be one with a tweeter, mid, and 15", right? I guess I don't quite understand the second test you suggested. If you'd like to type me up some instructions (since you love this stuff so much) with more detail that would be really cool. Also, how much will it change things if my corners are built at 45 degrees? By that I mean, 6 years ago I was a dumbass, and I thought all the pictures of bass traps people had in their rooms were actually the walls built at 45 degree angles in the corners. Trust me, if I could start over I'd just make them 90 degrees and treat them with traps, but I'm stuck with this now. I'm still going to put bass traps in the corners, I just may have to add two for every corner because I technically have two corners. :) But how much do you think that will change the tests? If nothing else you'll have a good chance to see how that style corner affects a room's response. :)

I just re-read what you posted about the two tests you'd suggest doing. The one where you're placing the speaker (sub) and mic at opposite corners of the room, that's to find the modes of your room? And a frequency sweep from, say, 40 hz up to 300 hz would be ok for that? Would a pencil style condenser be ok for that? If not, what mic would you suggest?

Yes, it's to identify the modes of your room. This kind of thing isn't usually necessary because knowledge of the exact frequencies at which your room resonates doesn't change the treatment strategy. If you're testing, you may as well just do a full 20 Hz - 20 kHz sweep. As far as mics go, you will need an unequalized (flat) omni with a very small diaphragm. The Behringer ECM8000 is a popular cheap choice, but I think the Nady CM-100 may be an even cheaper alternative, though I haven't tried it. It's not a big deal to build a flat omni, so don't worry about quality too much if you just want the mic for testing. If you want a mic that will have value as a recording mic as well, look at the Earthworks mics and maybe the Audix TR-40.

After the room is treated with traps do you suggest doing that same test again? With the mic and speaker as close as possible to the same spot as the before test? Or the second test you posted about? The full range speaker, that would be one with a tweeter, mid, and 15", right? I guess I don't quite understand the second test you suggested.

I wouldn't bother with the modal test since first it's mostly academia and second you will probably need to put sonething in the corners anyway. Try the second test. For the test to be perfectly valid, you shouldn't move the speaker or the mic until both the before and after tests are complete. Also leave the gains and volumes alone. This test is designed so that the major resonances of the room are somwhat evenly excited so that you can see the effect the treatment has on all of them. It's just a good general source/reciever position. By full range speaker, I just mean one that can produce the full frequency spectrum, or as much of it as possible.

Also, how much will it change things if my corners are built at 45 degrees?

What in the...?!? You HAVE to post a picture of this. I will help you when you're 100% ready to roll.

So you're saying this test is somewhat unneccessary? Then I'm not doing it!!! :D Is there a test similar to the one Ethan did on the realtraps site for a live room? Or would the test go down just the same way as in that video? Obviously the mic wouldn't be in a mixing position... Click here http://www.homerecording.com/bbs/showth ... p?t=192327 and check out the pictures on the second page. The best one to see one of the 45 degree corners is in the picture of me holding a beer looking a little bit like Captain Morgan. Hey, at the very least we'll see how a room like this responds. :)

No one said you have to test your live room. The whole idea is just to check up on your treatment and to understand the room from another viewpont besides your ears. Yeah, that space in the corner would've been much better used as absorption, but oh well... all is not lost.

Yeah there's a lot a mic will pick up that your ears just completely miss out on. I found that out when I tested my control room. Man it was pretty bad. but a couple simple bass traps helped tons.