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Your First Measurement Rig…

Meaningful Metrics – By Pat Brown

Many first-time buyers of measurement rig are confused by the multitude of choices in the marketplace. This article will help you sort through the offerings and spend your money wisely.

If you are tooling up to do sound system work, you are doing it at a good time. There are many excellent acoustical measurement platforms to pick from. It is not so important which one you use, as to how you use it. Plan on lots of study, practice, experimentation and head scratching.

In addition to software, you will need some associated goodies to round out your measurement rig. Since this question comes up often in the seminars, I decided to cover it in a short article that can be added to our course manuals. Here is a rundown on what you will need for a basic measurement rig.

I never like to recommend specific makes and models, but the measurement landscape is getting so confusing that I have provided some links to push you in the right direction.

Computer

The vast majority of practitioners use notebook PCs for measurement work. The processing power, storage capacity and display resolutions of anything made in the last few years should suffice. The main decision is PC or Mac. There are more Windows-based software choices, but the Mac is gaining ground. If you are buying a unit specifically for measurement work, I would consider a netbook running WinXP, as these combine small size, low cost, and a mature operating system. Make sure that the display resolution is supported by your measurement application.

Photo of a laptop

Figure 1 – Netbook PCs are cheap enough to justify dedicating a PC to your measurement rig.

Software

The unofficial standard for measurement software is the dual-channel FFT, usually in the form of a PC-based application with supporting hardware. These analyzers usually feed a test stimulus into the system, and then collect the system response from an appropriately placed microphone. A comparison of these two signals yields the response of the system, which essentially says “this is what the system-under-test did to the signal.” Examples from the market place include SmaartLive, EASERA, EASERA Systune, RoomCapture and ARTA. I would suggest ARTA for getting your feet wet, as it is low cost and extremely capable. You will likely add another platform later to complement it’s capabilities, and by that time you will know what additional features you are looking for.

Hardware

All of these applications require external audio hardware. You will need a minimum of two-channels (one for the reference signal and one for the microphone). You will probably want to use a notebook PC, which narrows the field to USB and Firewire units. Of those two, only USB is typically supported on all PCs, so I would lean in that direction. A minimum feature set includes XLR inputs, 48V phantom power and balanced outputs. There are many devices in the marketplace that meet these criteria, so let me add some additional considerations that will narrow the field.

Chassis

Avoid plastic boxes. A metal chassis with all connectors bolted to it will survive the inevitable drops and jerks from connected cables.

A professional 2-channel USB audio interface

Figure 2 – A professional 2-channel USB audio interface

Outputs

Balanced is a must, but balanced via an XLR connector is a big plus. This narrows the field dramatically, but it allows you to use any mic cable to connect to the system. If you must use 1/4-in TRS, then at least make sure that the jack is bolted to the chassis with a nut. Many units have plastic connectors that simple protrude through holes in the chassis – bad news for a lot of reasons.

Knobs

There are two schools of thought on this. Physical knobs make it easy to tweak levels, but make it harder to maintain an absolute calibration. Since the vast majority of audio measurements are relative, you might go for the convenience of a knob. Some units use digital encoders to set levels so that a fixed absolute level can always be dialed in. Other units are controlled completely by software so any level changes have to happen there. Ironically, most units without knobs actually cost more (like fat-free food) as they are targeted at more sophisticated users.

The Audix TR-40A

Figure 3 – The Audix TR-40A

Prices range from $150 – $1000. Audio boxes have evolved into a gray marketplace of consumer vs. semi-pro vs. professional units. Check out Edirol, MAudio, Presonus, EASERA Gateway and the Duran D-Audio, but skip the low end offerings from any of these brands. Don’t make the common mistake of buying two or three different models en route to a professional model.

Microphone

Your first microphone should have two main characteristics: omni-directivity and a physically small capsule. At one time it was common to use general purpose omni’s for measurement work, but these days microphones designed specifically for measurement abound. Most practitioners will eventually end up with two mics – one for general field work and another for that occasional precision application. I’d look at the Audix TR-40A (~$200) at the low end and the DPA 4007 ($1700) at the high end. This will give you a feel for the range of choices. There are many units to pick from between these two extremes in the marketplace, including Earthworks, Josephson, TestMic and Gold-line

Some Frills

The previous items form the bedrock of any measurement rig. There are some other items to consider that will add speed, convenience and accuracy to the basic kit.

Calibrator

Many acoustic measurements do not require absolute calibration. It is enough to know the deviation from some reference response. SPL, STI and noise measurements require absolute calibration. The easiest way to accomplish this is to use a calibrator for your microphone. Calibrators are fitted to the mic and produce a reference level (typically 94dB or 114dB ref. 20uPa) for the measurement system. A low cost calibrator may not be traceable regarding certification. If this is an issue, the price goes way up. Don’t sweat it. If you need one of these you will know it.

Wireless

I have never met anyone who has invested in a wireless system for measurement that has regretted it. This may cost as much as the rest of your rig, but few items will justify themselves as quickly. I recently measured a system with two large main arrays, three delay rings and a host of fill and special purpose loudspeakers. A suitable mic cable would be measured in hundreds of feet, and the wireless rig definitely fast-tracked the work. Maybe that’s bad if you are billing by the hour. In this arena the Lectrosonics digital hybrids are the most widely used.

Lectrosonics UH400A

A wireless system suitable for measurement work

Figure 4 – A wireless system suitable for measurement work

Tall Stand

One of my main measurement peripherals is a very tall mic stand, used to get the mic away from the audience plane reflections. Of course, a long piece of half-inch conduit works in a pinch, but a telescoping stand travels better. Some units are too tall for transport when completely retracted, so be sure to check that out. I recently replaced my worn out Bogen light stand with a Pic branded unit. The local machine shop threaded the end with the appropriate 3/8″ thread. A leveling leg is recommended if you work on a sloped floor, but this increases the size, weight and price. Hymnals or gaff tape can be substituted in a pinch.

Conclusion

I’ve been investing and re-investing in measurement gear for three decades. Many of my mainstream tools are not the most expensive tools that I own. As with all other endeavors, it is all about balance. If you buy quality gear and distribute your investment appropriately between the various components you will spend far less money than trying to do it on the cheap. The most expensive “anything” is the one you buy twice! pb