At this point in time, almost any modern USB or Firewire based audio interface will be of sufficient quality to meet the basic needs for FFT audio measurement. I’ve used everything from a $99 Maudio Fast Track Pro to a $4000 Metric Halo ULN8. There is a difference in the quality & sound of these interfaces but fortunately FFT audio measurements are a comparison between a known and unknown signal. As long as you set up your rig correctly, the audio interface for all intent and purpose disappears. With this said, I use my audio interface for FFT audio measurements, audio playback, DSP processing of signals and recording so it’s only logical to use the best audio interface I own at any given time.

My favorite audio interface for portable use is the Metric Halo 2882 & 2882 +DSP units. The only difference between the two is a +DSP license that opens up some pretty spectacular plugins native to the device that can be used for everything from PA processing to processing your live audio as it goes to the console.
Metric Halo – 2882 webpage
For 2 channel FFT audio measurements, I take my Metric Halo ULN2.
Metric Halo – ULN2 webpage
For studio work, I use ULN8s
Metric Halo – ULN8 webpage

All these options are exceptional in their own way but if I was offered one device for putting in my backpack & getting on a plane, it would be the 2882+DSP every time. The swiss army knife of modern audio i/o.

I have many other audio i/o devices. RME, Presonus, Line 6, Maudio. Sometimes,  size is an important factor and having an ultra small I/O like the Presonus Firebox is ideal. When you are setting up, as long as you can measure the device itself and verify there aren’t any issues with it using a virtual loop method, any modern interface is likely to work for audio measurement purposes.

Whichever audio i/o you plan on using, make sure its compatible with the software platform you plan on using.

In order to make measurements quickly, having more than one measurement mic helps a lot. Since we typically use one input/output for a physically loop out and back in, having an audio interface with more than 2 inputs / outputs allows for additional mics to be placed, patched and selected without moving mics or repatching.

An audio interface with 8inputs and 8 outputs is something to shoot for. When I take my 2×2 interface, I always wish I had 2 more inputs. Consequently, I end up taking my 8×8 most of the time.

System engineers who are performing install work will routinely use 8 or more mics. Example positions include: Main L, Main R, Front Fills, Under Balc Delay, Over Balc Delay… In each case, you might have a “PRINCIPLE” mic and a “SECONDARY” mic. It all adds up fast.

For live audio, I rarely see more than (2) mics used but I think this is solely a factor of time limitations.

One of the things to consider when you’re deciding on an audio interface for measurement purposes is whether or not you get one with knobs on the front to adjust input gain and output gain or the type that has only a GUI to control it. Honestly, I don’t much care for knobs. I have tried both types of interfaces when measuring. For knobs examples are the Metric Halo ULN2, RME Fireface 800, Maudio Fasttrack Pro. The problem with knobs is that many times you’re in the dark or at least low light. Seeing which knob is which isn’t as exact as typing in a number on your screen. With a backlit keyboard like the ones on modern MacBooks, you really don’t need any light to work.

I’ve tried using my RME FF800 and MH ULN2 repeatedly and always find myself taking a Metric Halo 2882 with me again. Nothing wrong with the RME FF800 and MH ULN2 but I like a GUI.

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