The View From 2009: This article was an attempt to enable home recordists to begin to think about and approach acoustical measurements. It’s more about discovery than anything else. Enjoy!

Acoustical Measurements For The Rest of Us

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 assessing with your ears “how those changes sound.” In addition, some recent new products are available which 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 explain how you might try to measure it. But first, a little bit about some measurements that might be of interest to you...

Rooms Vs. Frequency

The idea that rooms have might have some sort of frequency response makes some intuitive sense. How that “room” frequency response sounds to us is a little more complicated. First off, we only hear said room response 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.

This frequency response can be particularly troublesome, variable and expensive to deal with for frequencies below 300 Hz, due to standing waves, which are derived from the room dimensions themselves. Among others, there will be a standing wave whose wavelength is equal to twice each room dimension. So, if you have an 8 foot 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 sort of glom together and 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. Whatever. Ya pays yer money and ya takes yer chances!

Rooms Versus 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 various frequencies. 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. Interesting, eh?

So what we need to do is measure bands of the audio spectrum and see how long each band takes to decay. We can’t really use a sine wave to do this, because of standing waves. So, we use narrow-band noise as the preferred signal.

Decay time is the time it takes sound to die away in a space after the source stops emitting energy. This is usually formally defined 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 6 in order 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.” Also 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. 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 which 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.

	Figure 1. NC Curves from NC-15 to NC-45. A noise floor meets a given NC curve when no part of its spectrum exceeds any portion of a given curve.

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 which 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!

And Here’s The Real Scientific Method!

Yer gonna need lotsa paper. You’ll write lotsa stuff down. Many times. Don’t, repeat don’t, try to carry info in your head. It leaks out. 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 after you’ve written down the data. As any good scientist will tell you, if it wasn’t written down immediately, it generally didn’t happen that way!

Stuff You Can Use

Happily, there are a bunch of CDs you can buy to help with test signals for your measurements. A basic standby that I can recommend is the ProSonus Test 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 (LEDR), 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 (http://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 which 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. Check ‘em out. They are cheap and good.

Above that is the Gold Line stuff (http://www.gold-line.com), which is quite decent. Not only does Gold Line sell Real Time Analyzers, TEF Analyzers and the like, but 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. Buy one!

If you REALLY want to get into this in a more serious way but wanna stay at least sort of low-cost, there are a number of portable measurment devices you can get in the $1-3K range. I’m currently using the Neutrik Test Measuremnt gear quite happily (http://www.nti-audio.com/Home/Products/Minstruments/tabid/56/Default.aspx). Once you have somthing like this, you’ll become a learning fool, wonder how you ever survived without it! Trust me, you’ll never look back!!

Actually Making Measurements

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

• 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 methodology, 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!

It takes time to digest the information. Data needs 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!

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