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Acoustical Measurements for the Rest of Us
How to make acoustical measurements without a Ph.D.
By Dave Moulton

As Lord Kelvin said, “To measure is to know for sure…maybe.

The acoustical behavior of the rooms we produce music in is often baffling. To make it worse, acoustical treatments are regularly shrouded in hypespeak (as in “Our Soundsucker® Wall Panels will remove all time smear and clarify the reverberation of all acoustic instruments.”).

Often we suspect that something might be wrong with our room, but we don’t know how to find out. When we consider the cost of measurement equipment that acousticians use and the steep learning curve needed to use it, we can easily see why room acoustics remain baffling. They aren’t easy to understand, they’re expensive to measure and hard to learn about, and they’re tangential to what we really want to do—which is to make really cool recordings.

Happily, it’s possible to make some measurements on your own with some fairly basic equipment. A couple of pieces of measurement gear may even be worth investing in.

Then with some basic books on acoustics, you can start fooling around on your own and learn a lot about the acoustics of your studio. You can even begin to make changes and measure their effect physically while also hearing “how those changes sound.” In addition, some recent new products are available that can really help you out if you are interested in penetrating this particular briar patch a little more deeply.

There’s another benefit to all of this. As you struggle to make the measurements described here, you are going to gain some real knowledge about what is going on acoustically around you. This will be of great help as you continue to read about what others have found. Why, you can actually educate yourself!

With all that said, this article is about measurement rather than about acoustics. I’m not going to explain how to deal with reverb time, for instance, just try to explain how you might try to measure it. But first, a little bit about some measurements that might be of interest.

Rooms vs. frequency

The idea that rooms might have some sort of frequency response makes sense; how that “room” frequency response sounds to us is a little more complicated. First off, we only hear it as reflections of the sound source. Second, it changes over time—the reflected paths from the source to our ears have their spectra altered by the walls, floors, and ceilings they bounce off. These will probably have a different spectrum than the spectrum of the sound source.

Due to standing waves—which are derived from the room dimensions themselves—this frequency response can be particularly troublesome, variable, and expensive to deal with for frequencies below 300 Hz.

Among others, there will be a standing wave whose wavelength is equal to twice each room dimension. So if you have an 8’ high ceiling, you will have a standing wave at 70 Hz (divide the speed of sound—1130 ft/sec—by the wavelength—8’—and then by 2). There are also standing waves for your room dimensions in combination, so it gets complicated in a hurry.

In big rooms (like Madison Square Garden and the Mormon Tabernacle) the troublesome standing waves are below the audible range of hearing. When we get to small rooms like yours and mine (any room where the largest dimension is under 50 feet) the lowest dozen standing waves usually have a significant impact on the room’s bass response. Above that, there are so many of them so tightly spaced in frequency that they just average out.

At mid and high frequencies the frequency character of the room is determined by the so-called “coefficients of absorption” of the various wall surfaces and the furnishings. These coefficients vary as a function of frequency for each material, so the resulting room timbre is highly variable. Happily it is also controllable by you, sometimes very cheaply.

So how do you measure the frequency response of the room? You can get a rough feel for the response of the room by crudely measuring the near-field response of your loudspeaker and by then facing the speaker away from your test mic and measuring the response you get with that setup. Then you find the difference between the measured near-field response of the speaker and the reverberant response of the room. That curve will be, in a rudimentary way, the frequency response of the room.

Whew!

Once you know the response, you’ve got stuff to think about. There’s a school of thought that holds that flat room response ain’t good, and that high frequency room reflections should be steeply rolled off. Others think there should be NO reflections (one acoustic equipment manufacturer refers to room reflections as “acoustic distortion”).

My own take is that lateral reflections should have flat response and that floor and ceiling reflections should have the high frequencies rolled off. See ‘Has Dave Moulton Finally Lost It?!’ in the December 2000 issue.

Rooms vs. time

Such a generalized room frequency response only tells us a little of the story. In many respects what we really need to know is how long each frequency takes to die away. This is the concept of reverb time, or for small rooms, decay time.

Once we start measuring reverb/decay time, it becomes painfully obvious that we really are measuring decay time at frequency. If we measure the time it takes for the whole spectrum to die away, for instance, the time that we are really measuring turns out to be the time it takes for the most reverberant frequency to die away.

We could have a really dead room, for instance, with a bell in it that resonates at 500 Hz. The measured reverb time would be the decay time of the bell.

So what we need to do is measure bands of the audio spectrum and see how long each band takes to decay. The preferred signal for that kind of measurement is narrow band noise.

Decay time is the time it takes sound to die away in a space after the source stops emitting energy. This is usually defined formally as the time it takes for the reverberant sound to drop 60 dB in level, called RT60. Common practice now is to observe the time of the first 10 dB of decay and extrapolate RT60 from that.

Aside from the obvious convenience, there is evidence that the “subjective” reverberance of a room will be determined by the rate at which sound decays over the first 10 dB of decay (called the ‘early decay time’). We multiply the early decay time by six to get the RT60 time.

For most control rooms the decay time is quite short, less than 3/10ths of a second and occasionally under 1/10th of a second. Low frequencies tend to decay more slowly than high frequencies.

Meanwhile, the sensation of “deadness” in a room is associated with extremely short high frequency times. Low frequencies can still be comparatively long, making the actual reproduced sound muddy even while the room sounds “dead.” Interesting, eh?

In general, it is believed that low frequencies should take longer to die away than high frequencies. For acoustic recording spaces and concert halls, this definitely seems to be true. For control rooms, on the other hand, it may be desirable for the decay time to be constant across the spectrum.

Critical distance

Critical Distance is the distance from the sound source in a room where the direct sound and the reverberant sound are equal in level. Beyond that Critical Distance, reverberance begins to mask direct sound, affecting intelligibility.

Noise floor

The noise floor of a room is the level of ambient sound in the room when nothing is going on (of a musical nature, that is). Unfortunately, the level varies widely as a function of frequency (bass is usually much higher), so it is important to do it octave by octave. The noise floor will probably be a defining factor for the dynamic range of your playback system.

NC (Noise Criteria) Curves have been developed that establish standards for noise floors across the spectrum. Because of our poor hearing sensitivity at low frequencies, we can tolerate higher noise levels at those frequencies. This is fortunate, because it is relatively difficult and expensive to reduce low frequency noise in rooms. NC-20 is Really Quiet, while NC-40 is Mediocre. See Figure 1.

Vibration

The last interesting thing to measure (it may be the most productive in terms of immediate improvement to your studio) is sympathetic vibration—the inevitable buzzes and rattles that happen in any room. Under normal playback conditions, these are usually masked, at least partially. Nonetheless they can be major, if hidden, contributors to auditory mayhem, blurring and confusing your tracks.

So now we’ve got an array of things to try to measure: frequency response of the room and reverb time in various frequency bands, the Critical Distance and noise floor of your room and the frequencies that excite audible vibrations of the hardware and furnishings in the room.

The trick is to do this without spending a bundle of money. Fortunately, it’s possible to do this, because (a) you don’t have to prove to anybody else that your measurements are perfect, and (b) you can trade the time you spend struggling to make these measurements plus your lack of precision for some significant dollar savings. Finally (c), you’ll learn a bunch by doing this!

The real scientific method

You’re going to need lots of paper to write things down. Many times. Don’t, repeat don’t, try to carry info in your head. It leaks.

Before you measure, write down the test you’re doing. Then measure. Then write down the data. Then go ahead. If you have arithmetic to do, do it only after you’ve written down the data. As any good experimental scientist will tell you, if it wasn’t written down immediately, it generally didn’t happen that way! [Or it actually never happened at all—MM, Ph.D.]

Things you can use

Happily, there are a bunch of CDs you can buy to help with test signals for your measurements. A basic standby I can recommend is the ProSonus Studio Reference Disc CD. This has a batch of test goodies on it that you can use, including swept sine waves, a gated swept sine, noise bands, polarity info, Doug Jones’s ‘Listening Environment Diagnostic Recording,’ and other goodies.

No matter what else you do, buy the Radio Shack Sound Pressure Level meter, about $35. You can use it to establish sound pressure levels at any position in your studio.

If you want something classier, the next step up is stuff from an outfit called Old Colony Sound Laboratory (www.audioxpress.com/bksprods/index.htm). They offer a batch of low-cost test stuff (including the above CDs), most notably a cheap test microphone called Mitey Mic that includes a response correction curve to make your acoustic frequency response measurements flat. They also offer a ton of different little kits and stuff to do things; they’re cheap and good.

Above that is the Gold Line stuff (www.gold-line.com), which is quite decent. Not only does Gold Line sell Real Time Analyzers, TEF Analyzers, and the like, they offer the Gold Line TS-1, which is a combination broadband sine wave generator and a digital meter (frequency and amplitude) for a little under $500. This is a great general-purpose studio tool. I own one and can’t imagine life without it.

If you really want to get into this in a more serious way but wanna stay at least sort of low-cost, check out Sencore (formerly Terrasonde, now www.sencore.com). They make a remarkably comprehensive test set called the Audio Consultant that does everything you could ever want, acoustically and electronically. Once you have one, you’ll become a learning fool and wonder how you ever survived without it. Trust me, you’ll never look back!

[Also well worth checking out: Metric Halo’s SpectraFoo metering software (www.mhlabs.com). And Waves (www.waves.com) PAZ, for a less extensive and expensive program.—Ed.]

Making measurements

When you are cobbling up all these Gonzo tests, there are a couple of things to keep in mind. Warnings:

• There’s no safety net. You need to be extremely careful with levels and patching. Any time you are screwing around with extremes of the spectrum, a wide variety of levels, and simultaneously sending signals to and recording signals from the same acoustic space, the potential for some really exotic feedback loops increases exponentially. You can easily nuke tweeters, woofers, and amps with frequencies outside of the audio spectrum (you may never hear a thing—you’ll just smell the smoke). So proceed with great caution.

• Errors will abound. Aside from the mistakes you’ll make, these tests are lacking in precision. Between your mistakes and the crude methods, expect to see bizarre results. View everything you get with skepticism, and double-check and verify even (especially!) when the results look good. Treat the results as only rough indicators of reality, and use those results to build up your fund of understanding and knowledge. Look for correlations and verifications. If two different tests tend to support the same conclusion, then you can begin to figure that you may be on the right track.

• You will drown in data. At some point, you will mumble, “Oh, man, what am I doing? What does all this stuff mean?” Welcome to the club. You are now a scientist. You have too much data, and it is all confusing and incomprehensible. This is why scientific reports are confusing and incomprehensible: scientists are just as confused as you are, but they have a budget for printing reports! [Sometimes—MM, ex-Ph.D.]

It takes time to digest the information. Data need to be reduced to coherent expressions. It needs to be reasoned through. You may need to repeat many of your tests lots of times, simply to verify that the reverb you thought you measured, for instance, was not simply an inadvertent reverb patch you forgot to pull!

Measuring the frequency response of your room

First, measure the nearfield response of your speaker.

You use pink noise for these measurements. Set your speaker up on a stand or something as far away from reflecting surfaces as you can reasonably get it (outdoors, facing up from the ground, is an excellent alternative). Send pink noise to the speaker at a modest level, say 75 dB SPL. Place your measurement mic about two feet from the speaker, on axis (line it up with the tweeter).

Send the mic signal through its preamp, through an eq (octave band graphic or parametric) and to some sort of level meter. Turn all eq bands all the way down. Turn one up to +12 at 1kHz (if it’s parametric, set the bandwidth at 1/2 octave). Adjust the level to the meter so that it is reading approximately 10 dB below full scale (if you are using a conventional VU meter, this would be -7VU).

Write it down! Don’t touch the levels again for the rest of the measurement. If you have a test CD with octave bands of noise, you can skip the eq and take a heap less time and trouble. See Figure 2.

Next, change the frequency of the eq for each octave of interest. Write down the level you observe on the meter. You’ll have trouble, probably with the bottom two octaves, and maybe with the top octave. You probably have to do a lot of estimation. If the speaker gives any signs of distress at frequency extremes, don’t push it. In any case, the resulting set of levels for each octave will be the so-called “baseline” for the subsequent room measurement.

Next, measure the general response of your room.

Repeat the above measurement, with the speaker in the room facing away from the test mic. The test mic should probably be at your favorite listening position. The speaker should be at least five feet away from the mic, but not right at a wall, and especially not in a corner. In this case, particularly at high and mid frequencies, the levels you observe will be primarily derived from reflected sound.

Once you’ve collected the raw data for this test, you have some arithmetic to do. First, you will “normalize” your room data. To do this, find the difference in level between the room measurement and the baseline at 1 kHz.

In my hypothetical example in Table 1, the room level at 1 kHz is 6 dB below the baseline. So to make them the same, I would increase the room level by 6 dB. Apply that correction to all of the other octave bands (i.e. in my example increase all room levels by 6 dB). The resulting level is “normalized.”

Find the difference between the two levels at each frequency. In my example, for instance, at 63 Hz the normalized level is -14 dB, while the baseline is -12 dB. The net value is -2 dB.

The resulting curve is the net room frequency response. Due to the way the measurement is set up, it should always be 0 dB at 1kHz. If not, you’ve made a mistake somewhere. See Figure 3.

In this case, there is a peak in the 125 Hz range and a dip around 4 kHz. This suggests (a) some sort of room mode buildup due to wavelengths around 9’ long (1130/125 = 9) and (b) a significant amount of absorption in the 4–8 kHz range.

Measuring for Critical Distance

Set up your monitors normally. Generate noise at a moderate level in one monitor. Measure the SPL at two feet from the speaker. Write it down. Measure the level at four feet from the speaker. Write it down.

Is it approximately 6 dB less than at two feet? If it’s significantly less than 6 dB down (say only 3–4 dB), you’ve found the critical distance. If it’s approximately 6 dB down, measure again at eight feet. If you are still within the Critical Distance the level should be another 6 dB down. Keep doubling the distance from the previous measurement (this won’t take long!) until you get to the point where the level is only 3–4 dB below the previous measurement. That point is the Critical Distance.

Often for small, dead rooms the Critical Distance is beyond the walls, which is okay. Mainly, you want to make sure that your listening position is within the Critical Distance. Happily, it usually is. This is a measurement that is very handy in concert halls and other large rooms. See Figure 4.

Measuring for buzzes

Using the sine wave generator at a modest level (ca. 65 dB), slowly sweep across the spectrum. Listen for buzzes in the room. When you hear one, stop sweeping. Note the frequency, find the vibrating offender, and damp it so that it stops buzzing. Continue sweeping.

When you’ve crossed the spectrum, repeat at a slightly higher level (say 70 dB) just to confirm that you’ve got rid of everything. Don’t bother trying to debuzz your room at very high levels because (a) you probably can’t, (b) you will drive yourself buggy trying, and (c) buzzes that occur only at high levels usually either aren’t much of a problem or else they’re such a huge problem you’ve long since fixed them!

Measuring and logging room modes

This is a test that uses a gated swept sine. If you don’t have the ProSonus CD, you need to have a VCA or something with a low frequency oscillator that you can use to turn a signal on and off continuously.

Set the on/off rate at about 1/2 second for a complete on-off cycle. Send the sine wave through this to the speakers. Sweep the sine frequency slowly from say 50 Hz to about 500 Hz. You will hear the sine wave switch on and off.

More importantly, during the “silences” you will hear the sound of the reverberance of the room at each frequency. At frequencies where the room is resonant, the reverberation will swamp the sound of the gate switching; at frequencies where the room is heavily damped, you will hear the sound of the gate clearly, as clicks.

As you sweep, note the frequencies and relative levels of the various major peaks and dips. From this measurement you can get a fairly clear picture of the resonance occurring in your room. Log the placement of the mic and repeat in other locations. Compare the patterns for an even more detailed view. You can even begin to map, in a crude way, the resonant behavior of your room. Whoa, dude!

Measuring reverb time

Using the same gating device, send octave-band noise to a single speaker. The actual reverb time will be too short to measure directly with a stop watch. Instead, speed up the rate of the on-off gate until the reverberant sound definitely merges with the next on-cycle of the gate. Slow it down now until you can hear the reverb begin to die away to about half its loudness, which is the subjective equivalent of approximately -10 dB.

Now use a stop watch to time 16 cycles, for instance. Because each cycle is half reverb, divide that time by 32 and you’ve got the Early Decay Time for that frequency band. Multiply by 6 (remember, you measured only the first 10 dB of a 60 dB decay) for the Reverb Time.

Repeat with each octave band of interest. In general, times are taken for the octave bands from 125 Hz to 4 kHz. I find the 63 Hz octave interesting if I can get a measurement (it depends on the speaker), and both 8 and 16 kHz are interesting for small rooms. However, they may be too short to measure reliably this way. See Figure 5.

Measuring the noise floor

The problem with measuring noise floors is that the microphones and meters we’re using just aren’t quiet or sensitive enough to measure many of the levels we’d like to know about. Our ears, however, are.

So if we can establish a known signal level and then can attenuate the level by known amounts until it just becomes inaudible, we will have determined the threshold of audibility for that sound in our room—which is generally equivalent to the noise floor for that sound. This is best done by octave band.

Establish a known Sound Pressure Level with the source signal (say 1 kHz noise at 50 dB SPL) run through a module of your console at maximum undistorted level and controlling gain with the monitor level pot. Calibrate by experimentation a range of -3 dB increments on that module, hopefully down from your known level as much as 40 dB (which would mean you are measuring to 10 dB SPL if your reference is 50 dB SPL).

Once you’ve got everything set, start reducing the level of the noise in 3 dB increments until it is no longer audible. Write down the level 3 dB above that inaudible level as the level of the noise floor for that band. If you get to your lowest calibrated level and can still hear the signal clearly, note that level with the prefix ‘<’ (that symbol means ‘less than’). Do this for each octave band of interest.

When you are done you will have a response curve for the noise floor of the room. Typically, low frequencies will seem difficult to assess, because you won’t be able to hear them at any kind of low or even moderate level. That’s okay because we don’t hear very well at low frequencies. So those levels will seem high (like 50 dB SPL at 63 Hz, for instance), which is probably the truth.

To create more meaningful and relevant data, you can apply A-weighting, which corrects for the way we hear at low levels (i.e. levels equivalent to 30–40 dB SPL at 1 kHz). Use the corrections shown in Table 2 for each octave band (yup, you subtract 33 dB in the 32 Hz band!).

The resulting curve is the A-weighted noise floor spectrum, which should give you a reasonable idea of how your room is behaving. You can have lots of fun by turning various equipment pieces on and off and seeing how that changes the noise floor. You may be surprised!

The meaning of it all

Hearing is complex. Acoustical behavior of reverberant rooms is complex. These tests should give you an insight into these complex behaviors in a way you can use to go beyond the marketing hype. Using these tests, you can both hear and understand a little better what is happening in your studio and control room and what will happen with your recordings in other rooms.

Happy trials!

Dave Moulton updated this article from a really sleazy one he wrote in 1994.

 




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