mic pre for acoustic measurements?

[Syncamorea]

If you are on a Mac, you could just use the mic in and get a phantom power source for the mic at a local store. I've seen them at Guitar Center for ~25 bucks. I have never used that Motu and don't know how colored it is.

[boggy]

Originally Posted by darrella

Hi all. I'm nearing a point in my studio build where I will start taking acoustic measurements prior to installing room treatments. I dug out this old mic I've never really used for the job...turns out it's an Earthworks M30!

The situation is it needs phantom power to run. The 'cleanest' piece of equipment I have for this is my MOTU 896 HD, which I think will color the sound significantly....

Any recommendations for pre amps that would fit the bill? As far as price goes, the Earthworks 1022, at Sweetwater price of $1,999, is too rich for me....

For Earthworks M30 you need only a good phantom power supply.Output from PPS put directly to the balanced input of your audio interface. No need for any microphone preamplifier.

[darrella]

Thanks, guys! Of course, the answer was too simple for me to figure out!

[jwl]

I'm sure just about any mic preamp with phantom power will be fine for your testing. Most modern preamps are relatively clean and distortion-free if the gain staging is set correctly.

We did a measurement mic shootout a while ago, and while the high-dollar test mics were a bit better at the high frequencies, for measuring your room the inexpensive mics are fine. I'd expect the same from any modern preamp.

RealTraps - Measuring Microphones

[OpusOfTrolls]

Solid state preamp, advertised as clean, with a low noise floor and THD rating, and phantom power. All it takes really.

[ejbragg]

Originally Posted by darrella

Any recommendations for pre amps that would fit the bill? As far as price goes, the Earthworks 1022, at Sweetwater price of $1,999, is too rich for me....

The thing about preamps is that is kinda does matter. But so does your monitor choice. In fact, the entire chain (microphone, preamp, converter, and monitor) plays a heavy role in finding the characteristics of your room.

The Earthworks preamp you mentioned is an excellent choice. But because monitors are guranteed to produce coloration when filling the room with test signals, the only way to remove the error is something that will work with everything in your chain: take everything you have outside to a quiet area, away from buildings and do your test, first. Then you'll see how your set-up acts away from any reflection or standing waves. Take the spectral view snapshot (probably most useful is the pink noise), and flip it 180 degrees (or rather, upside-down). Then while doing your indoor analysis, add the flipped signal, removing your system's internal flaws, and all you're left with is the room's characteristics.

It's not perfect, but it allows you you use "decent" preamps, microphones, converters, and [especially] monitors, and still come out with a fairly accurate average. And "average" is an important word. Take several readings outside and average the results. Inside the room, take averages within different spots, as your standing waves will definitely change as you move the microphone - and I assume your source will be placed in one of the two primary positions where they will be. I also assume you'll only be using one monitor; two will create some more flutter that will just muddy things.

[boggy]

Originally Posted by ejbragg

The thing about preamps is that is kinda does matter. But so does your monitor choice. In fact, the entire chain (microphone, preamp, converter, and monitor) plays a heavy role in finding the characteristics of your room.
.......

[bolded by me]
Earthworks M30 has response flat within +/-1dB. Most (transformerless, solid state) mic preamplifiers has even better response (even if they is not needed in this particular case). So measurement mic and microphone preamplifier cannot play even a noticeable role in the room response, in regard to actual measurement results.
Also, M30 may have its own compensation file, so its influence may be fully negligible, for any purpose, even for loudspeaker design.

Also, loudspeakers cannot play a "heavy" role, because room response is almost always a much worse than (possibly) any commercially available monitor loudspeaker which is not broken.

[Jens Eklund]

Unless it´s a really crappy preamp, the only interesting values for a preamp intended for measurements is the noise floor (convenient to have a low noise floor then doing decay measurements thus voiding multiple takes but the noise floor of the room is of course often the bottle neck) and the phase response. I use the Grace design Lunatec V3:

Response = +0.1/-0.4dB for 20Hz-82kHz
Max phase deviation = < 8% for 50Hz-20kHz
THD+N @ full gain (60 dB) and +20dBu output = 0.0046%
Noise @ 60dB gain 50 Ohm source < -130dB

Good enough. The preamp is the least important (together with the AD/DA) pieces of the chain.

[ejbragg]

The Grace preamp you mentioned has a great frequency response, as well as a great transient response. A "dull" preamp may provide a decent frequency response, but will likely provide sluggish transient responses. The transient response measured in a room is a good way to measure spectral decay in a room - one of the last things to correct. Of course anything resembling a transient signal can be pushed out through the monitor and accurately measured, as far as overall decay is concerned, even if it sort of "ramps up". But the more intricate information of the room will be more difficult to find with a slow (not-so-sensitive) preamp.

Also, if opposite corners of the room at its greatest span is less than 28 feet, you'll be fine with a preamp that cuts off at about 20Hz. In other words, the distance from the corner at the ceiling to the corner at the floor in the opposite end of the room. This dimension will allow a full and half-length waveform to build up. If not from your monitors, then possibly from someone's bass cabinet. 28 feet will be the half wavelength of the primary standing wave, so 56ft is the resultant lowest possible standing wave for the room. 1130ft/sec divided by 56 = 19.5 Hz. If that dimension is any longer, you might do well to borrow a preamp and mic pair that will pick up that signal, as well as a monitor that will produce it (flatly). Even if you are not interested in using that extreme low end, it might be handy to know if it exists. It's better to treat the room to be flat than have to use more E.Q. than necessary. Often, if you find such a low primary standing wave successfully, and you successfully treat it, all the other secondary waves up the spectrum that were also problems will clear up along with it. Then if you wish to place a high pass filter on all your mixes just to be safe, that's ideal.

Extra low frequencies, even though you cannot hear them, will affect the higher frequencies in weird ways. And those who build non-parallel walls to fight the standing wave problem may have succeeded for upper and mid freqs. But low end standing waves cannot be fought this way. Odd shaped rooms will not prevent them. All that happens is that they become more difficult to predict. You can certainly calculate an estimate of where your low freq problems will be. And if your equipment limits you from measuring them, then estimation may be your only viable answer. But if you can measure them, that is, by far, your safest approach.

[boggy]

Originally Posted by ejbragg

The Grace preamp you mentioned has a great frequency response, as well as a great transient response.........

True, but microphone preamplifier for measurement microphone, if used only for room modes measurements (and NOT for loudspeaker design), has nothing to do with its "fast" transient response, because all that we need to measure is usually below 300Hz (approx.).

Gain control repeatability may be useful, if we measure absolute SPL value.

[ejbragg]

Originally Posted by darrella

Hi all. I'm nearing a point in my studio build where I will start taking acoustic measurements prior to installing room treatments. I dug out this old mic I've never really used for the job...turns out it's an Earthworks M30!

The situation is it needs phantom power to run. The 'cleanest' piece of equipment I have for this is my MOTU 896 HD, which I think will color the sound significantly....

Any recommendations for pre amps that would fit the bill? As far as price goes, the Earthworks 1022, at Sweetwater price of $1,999, is too rich for me....

To more directly answer the question, that's an excellent mic for the job. I would recommend a transformerless class A preamp. The Earthworks pre you mentioned is about as clean and "perfect" as they come. But True Systems and Grace make very nice preamps, as well. The other thing about the Earthworks is that it will actually capture (theoretically) 0.5 Hz, which is rather insane, and pretty much useless, since you'd be hard pressed to find a microphone that will pick that up, or monitors that will reproduce it. But like I said, I'd find that lowest primary standing wave and focus on that as your lowest freq of concern and go from there.

If you're converting a bedroom (another example) that's say, 10' x 8' x 8' then you have a longest length of sqrt(10^2 + 8^2 + 8^2) = 15' = half the primary wave length. Primary is therefore 30' long [a wave within a room can and will fold back on itself at (or actually opposite) its nodes, where pressure is highest. This happens twice in one cycle, once at the peak of the up-swing, once at the peak of the down-swing].

So 1130 ft/sec (speed of sound at sea level) divided by 30 ft = 37.8 cycles / sec (Hz), or approx 38 Hz. In this room, you probably won't need to measure anything lower - all fundamental wave problems will be cenetered at 38 (so maybe start just below- like 32 Hz) and go up from there.

Be sure to treat the low frequencies first, re-measuring as you go. Everything on the low end should be treated with bass traps in the corners. After that, mid freqs, which should probably be treated with diffusion first, but absorption as well, if the room decay lasts longer than you require. The high frequencies seldom need to be treated once this is all done.

"low freqs" -> 500 Hz & lower
"mid freqs" -> 500 Hz - 4KHz
"high freqs" -> 4KHz & higher