how to use OMNI directional mic to detect your room chracter

Posted: Sat Aug 05, 2006 3:35 pm

hey can anyone tell me how to use an omni directional mic to check your room's characterstics and monitor phase..a little help and guidence will be very appriciated...
thank you

Posted: Sat Aug 05, 2006 6:23 pm

One of the only times that I've used an omnidirectional microphone to check my rooms characteristics, were with a preconfigured audio spectrum analyzer. It contained a pink noise generator. A 31 band LED audio spectrum analyzer along with a specially calibrated omnidirectional microphone.

The customary use for this was to be placed where you would normally sit to record and mix. Assuming you knew the response characteristics of your speakers already, you would get an idea of what the acoustics in your room, coupled with your speakers, were doing by the analyzer display while sitting in the sweet spot. If you want to "voice" a custom built speaker system, you would need this same set up with a properly calibrated room, to evaluate a speaker. You're asking a very scientific oriented question here that really has very little to do with how you make good recordings.

Monitor phase is easily established with a monaural source sent to both the left and right speakers equally. If the speakers are in phase, the bass will sound full and solid. If it seems to disappear, sounds hollow or thin, your left channel speaker is out of phase to your right channel speaker, or vice versa. If you find that to be true, you would simply reverse the 2 wires on only one speaker. If you were to observe this on a spectrum analyzer with an omnidirectional microphones sitting in the mixing position, you would see a huge loss of low frequency response if the speakers were out of phase.

Conversely, if you want to hear what you will room sounds like to a microphone, set up an omnidirectional microphone in the middle of your room. Walk around standing, sitting, playing an instrument in one position and then moving to another position, some conversation would be nice also with a friend and while doing all of the above, record that microphone. Once you're done recording all that stuff, play it back without any modifications. You will quickly hear all of the resonances, reflections, echoes and frequency response aberrations as created by the acoustic space and the contents therein, accentuated and reinforced by playing it back in the same room you created it in. Which of course makes it sound twice as bad.

So I'll imagine your next question would then be "how do I make everything sound good with all this stuff?" Simple! Read a lot of stuff about acoustics and the measurement methods for judging acoustics. You could go to school? Or just drink 8 ounces of Florida orange juice every day. I'm really not trying to sound like a wiseguy here. Some people think you need sophisticated 31 band graphic equalizer's on your speaker system adjusted properly so you have a flat response that your mixing position. That really is not the case. Although I've known plenty of people at plenty of studios who operate that way. I have improved the monitoring in a lot of control rooms by simply eliminating exactly what I just described. You might need to do that if you have a custom soffit mounted Westlake system costing tens of thousands of dollars. In all probability you really won't want to do that with your average, run-of-the-mill commercially available speakers that you purchased at places like Banjo Hut. If you have determined that your speakers don't sound good in your room, it's your room. The only way you can fix that is to make acoustic improvements to the room and not to the speakers. How do you know what kind of acoustic improvements to make? You will need a real acoustic engineer/consultant and they don't usually come cheap. Some people think that putting up a bunch of foam is all you need to do? Nope. Not. Way more than that. Otherwise, you learn how to record and mix in the space that you have. It is a difficult process to learn how what you do sounds on an almost infinite number of different playback systems. Everything is generally a compromise and we are all mostly looking for best translations. It just takes lots of listening and not so much numbers or data.

So your simple question really goes way beyond a simple answer. My overall best suggestion is to gather what you feel to the some of the greatest sounding commercial recordings produced that you love to listen to. These will become your reference for evaluating room's, speakers, microphones, mixing techniques and everything else. None of us track and mix on a single pair of speakers in a single room. We all require a multitude of references so as to be able to produce our highest quality product.

Most of us know if it don't sound right. It ain't.
Ms. Remy Ann David

Posted: Sun Aug 06, 2006 9:47 am

RemyRAD wrote:

Monitor phase is easily established with a monaural source sent to both the left and right speakers equally. If the speakers are in phase, the bass will sound full and solid. If it seems to disappear, sounds hollow or thin, your left channel speaker is out of phase to your right channel speaker, or vice versa. If you find that to be true, you would simply reverse the 2 wires on only one speaker. If you were to observe this on a spectrum analyzer with an omnidirectional microphones sitting in the mixing position, you would see a huge loss of low frequency response if the speakers were out of phase.

Remy,

ALthough this is true - it is also possible that the exact same phenomenon will be caused by room modes........ with the mixing position sitting in a null.

The difference would be that swapping phase would not change the bass response at that location to any discernible degree.

Sincerely,

Rod

Posted: Sun Aug 06, 2006 10:48 am

Remi, Rod,

Remi, as Rod suggested this is too simple as you describe it.
There is a phase relationship of course, but that can be any kind of combination.

E.g. With both speakers in phase you most likely cancel all low order odd y-axis modes, meaning you have a nice and full sound on those frequencies at the mixing position. Reverse the polarity of 1 of the speakers and you super activate those modes creating a deep dip at the mix position.
For the even y-axis modes you have likely the reverse.

I confirm the influence of the phase relationship with the waves, but you can have A LOT of different combinations there.

What you're most likely refer to are the first order x-axis modes.

As Einstein said:
Make things as simple as possible, but not any simpler.

Eric
.
PS: Remi I like reading your posts, showing a lot of insight and experience.

Posted: Sun Aug 06, 2006 11:50 am

Guys (and most distinguished Gals!),

Here's a really easy way to tell if one of your speakers has its polarity (not phase!) reversed:

Play a mono track through any device that has a pan pot, and sweep rapidly from left to right to left etc. If the speakers are correct the sound will go left and right as you'd expect. If one speaker is reversed the sound will seem to go to unusual places around the room. It's not a subtle effect, so it's easy even for inexperienced people to tell if their speakers are okay or not.

--Ethan

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