Acoustisoft MP-B-USB Kit ($425): www.acoustisoft.com.(AUDITIONS)(Product/service evaluation)

High quality room measurements are now easier with the Acoustisoft MP-B-USB Kit.

I recommended the Acoustisoft (also know as ETF Acoustic) RPlusD software in Issue 113 but did not discuss options for microphones and soundcards. Doug Plumb has long recommended the Radio Shack Sound Level Meter as the entry point in conjunction with the internal sound card of your computer. The Radio Shack meters have internal microphone preamps and the output, at the back of the unit, is a standard RCA single-ended jack. According to a test in the June 2007 issue of AudioXpress, the quality of the new Radio Shack units have degraded. The current digital version, which gives good results 50-500 Hz, appears to be the best bet. It goes down 3dB at 30Hz, but one could build a calibration file in RplusD to compensate. Unfortunately, the new digital meter has a large peak centered at around 5kHz.

The next step up requires (a) a Creative Labs USB external sound ($50) add-on which provides enhanced performance over the sound card in your laptop and (b) a microphone intended for acoustic measurements such as the balanced Samson MM01 ($90) or the balanced Behringer ECM8000 ($50). A balanced to single-ended microphone preamp ($100) completes the upgrade. Given their lack of calibration, the incremental value of the microphones is bounded, offering improved accuracy over a slightly wider range (20Hz-2kHz).

Acoustisoft, in collaboration with IBF-Akustik in Germany, is offering a combination single-ended microphone preamp and external USB sound card. This custom box includes the RplusD dynamic sound card calibration using the unused left channel of the stereo soundcard. This unit is packaged by Acoustisoft with the IBF-Akustik calibrated single-ended microphone giving accurate measurements over the full frequency range. The package is called the MP-B-USB kit.

This package enables plug-and-play to interface the microphone and computer. The single box of electronics eliminates two analog cables insuring that hook up errors cannot occur. Even after years of use with a separate microphone preamp and sound card, I still make hook up errors given all the required cables.

Before the MP-B-USB, the levels needed to be carefully adjusted in the advanced sound control panel on the computer. But there were three conflicting objectives when optimizing the signal-to-noise ratio of the measurement: (a) the gain of the analog loopback required to measure the frequency response of the sound card (right channel input loop to the right channel output); (b) the gain to amplify the microphone (right channel in); and (c) the output level to the speaker (right channel out). These settings had to be transcribed since different levels were associated with normal computer operation. With the MP-B-USB, you never touch the advanced control panels on your computer since they are set by the MP-B-USB driver.

The MP-B-USB microphone gain is adjustable and has four positions that vary in 10dB increments. The optimal setting is found by monitoring the signal-to-noise ratio reported by the RplusD software, although the position is usually not critical. The MP-B-USB has a front panel LED that indicates when the combination of microphone gain and output level of the speaker is large enough to overload the ...