SMAART And Reverberation Time
By John Murray

LSI’s technical answer man fields a multipart question from a reader

If I want to measure an acoustic parameter such as RT60 or frequency response in a small room like a home theatre or a recording studio, can I use a (SIA) SMAART Live acoustic-based (computer) measurement system? Danny Murniadi

A small room really doesn’t have an RT60 (reverberation time). Reverberation time requires an even slope of level decay, which is a smoothly sloping, dense field of reflections that has an even and measurable drop in level with respect to time. This smooth decay slope is what determines how many seconds it takes the sound to drop by 60 dB (RT60).

Without a dense field of decaying reflections, RT60 cannot be determined. A small room only has relatively few early reflections before the sound dies altogether. You may be able to get some measurement systems to produce a number for RT60, but this really is meaningless until you get to larger rooms with a mathematically statistical (sufficiently dense) decay field.

See figures Small Room 1, 2, & 3 (below). Note that a variation of only 0.24 seconds in RT60 indicates more than a 50 percent difference in room absorption, comparing the shortest calculation to the longest ( 0.47 and 0.71 seconds). All that was done differently in the three small room RT60 calculations was to change the data points that determine the calculations.

Now, see figures Large Room 1, 2 & 3 (below). The data point selections still produced a 0.26 seconds variation, but this is only a four percent difference in room absorption rates. This is a much more reliable variation and will be much more useful as a predictive tool.

I would advise against using RT60 calculations as a valuable measurement for rooms as small as home theaters and recording studio control rooms. It’s better to pay attention to room modes and flat or concave reflective surfaces within the room than to rely upon RT60 numbers.

In addition, no acoustic space has a single, overall frequency response. A room does impart an acoustic signature on a signal source, and it is called the impulse response.


This is the character that a room, a loudspeaker or even an electronic effects box applies to a neutral impulse source. If you have a perfectly neutral impulse, test loudspeaker and measurement microphone, and capture the reproduced impulse on an oscilloscope, this will “trace” the impulse response of the room relative to the position of the test loudspeaker and measurement microphone.

Unlike an effects box (or, to a lesser extent, a loudspeaker within its coverage pattern), a room imparts an infinite number of unique impulse responses for every point in the room. Every position has its own unique pattern of reflections in time and level, and therefore also has a unique impulse response. Some software packages can actually extrapolate a room’s impulse response for a given point in a room, and this can be applied to an anechoic recording. It can be heard binaurally (in stereo) on headphones as if located in the stereo recording microphone’s position with the source at the test loudspeaker’s position. This can sound very realistic.

SMAART Live will measure the RT60 of a sufficiently large room. However, the SMAART AcousticTools package is more intended for this function, offering a better feature set for RT60 than the SMAART Live package. SMAART Live 5 does allow the user to draw an RT60 slope line, but it is very basic. Be sure you have the right tool for the intended measurement.

MEANS OF COMPARISON

Can SMAART act like stand alone RTA such as Audio Control SA-3050 or Linear-X pcRTA? Also, what kind of benefits will I get using SMAART compared with Linear-X pcRTA for room acoustics and electro-acoustic measurements? - Danny

SMAART is a stand-alone RTA of sorts, with inclusion of a measurement mic, a good preamp, and the computer/software as the measurement system. However, even though SMAART does have an RTA measurement, it’s not really a conventional Real-Time Analyzer we’ve become familiar with over the years.

RTA’s are a time-blind, two-dimensional (energy vs. frequency) measurement system. SMAART is not timeblind and is a three-dimensional (time vs. energy vs. frequency) measurement system. It provides not only level and frequency information, but has time-oriented measurements as well.

Standard RTA’s are time-blind because they accept all acoustic energy for a relatively long time period, measurement-wise, before calculating levels (generally 1/4 to one full second). For a system measuring factors lasting 1/20,000th of a second, like a 20 kHz sine wave, 1/4 of a second is an eternity.

SMAART is a semi-anechoic measurement system. In this way, it can measure only the signal from the loudspeaker. Meanwhile, it ignores sound reflecting around the room except at the very longest wavelengths of the lowest bass where room reflections are a shorter path length than the wavelength of interest.

At shorter wavelengths, the measurement is identical to that of an anechoic chamber.

Only at the extreme low-end does the room interfere with the loudspeaker’s signal and renders the measurement non-anechoic. This makes SMAART a semi-anechoic measurement system, and it is far superior to a common RTA.

Why is a semi-anechoic measurement system better than an RTA? Because where the measurement is anechoic (at the shorter wavelengths), a room cannot be equalized, only a loudspeaker. However, low-end buildup can be equalized where mutual coupling occurs between multiple transducers and nearby surfaces. Not coincidently, this is where SMAART is not anechoic.

GETTING AROUND LACK OF LINE INPUT

My laptop (IBM ThinkPad R32 P4-256 RAM) doesn’t have a line input. Which method do you suggest for solving this problem? Buy a PCMCIA card and Audio Control MP400 preamp, or use a USB preamp from Sound Devices? Danny

A good PCMCIA sound card, such as Turtle Beach (for example), will solve the lack of a line input and provide MUCH better audio input circuitry for lower distortion and greater dynamic range in measurement. The Audio Control MP400 will provide microphone preamplification, 48-volt phantom power for a good measurement microphone, and patching so that input channels can be swapped easily and an equalizer can be optimized in the signal chain without installing it in the system rack.

The Sound Devices USBPre combines measurement quality electronics that are better than any sound card with the preamp and phantom power of the MP400, but without the input switching or EQ patching capabilities. My suggestion? For ultimate accuracy, pick the USBPre, but for flexibility, add the MP400. You can have it either and/or both ways!

Live Sound Technical Editor John Murray is a 26-year industry veteran, working for EV, Midas, MediaMatrix and TOA. He has presented two AES papers, chaired three Syn-Aud-Con workshops and is a member of the TEF Advisory Committee and ICIA adjunct faculty. We encourage you to send technical questions to John at jmurray@livesoundint.com.