Acoustisoft MP-B-USB Kit ($425): www.acoustisoft.com.

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 analog-to-digital converter. The level of the speaker under test should be adjusted using the electronics volume control.

Another advantage of integrating the sound card with the mic's preamp is a lower noise level in the signal path from the microphone to analog-to-digital converter (ADC). The integrated unit also eliminates crosstalk between the loopback channel and the microphone cable and its associated measurement errors.

The added expense of a balanced microphone can then be mitigated because the MP-B-USB can be placed close to the microphone. Instead, a long USB cable can be used to connect the MP-B-USB interface to the computer. The only other cable is the analog line output from the MP-B-USB to the amplifier that drives the speaker. Even if this cable is long run, it should not be a problem although some care should be taken to insure the capacitance of the cable does not result in a high frequency roll-off. The output impedance of the MP-B-USB line level output is not specified, but I expect 10 foot cable runs should not be a problem.

The $425 cost of the MP-B-USB Kit is a function of the Euro/Dollar exchange rate. Shipping is free but custom fees from Canada to the US are not. The microphone is calibrated with a Bruel & Kjaer model B& K 4133 half-inch free field mike and the microphone calibration file is on a floppy disk. The calibration files can also be sent by email from Acoustisoft for those who cannot access a floppy drive. The electronics are not sold separately. Acoustisoft will only quote a separate price for those who have purchased a calibrated microphone from the company in the past.

The RplusD software is $179. While the price is not cheap, the worth of the data collected by the system is indispensable when using a user adjustable active room equalizer (analog or digital) of expensive passive acoustic room treatments. Without a room measurement system you have no idea whether the passive and active room correction is optimal, is making things worse, or is somewhere in between.

If you are concerned with system performance below 500Hz, the Radio Shack sound level meter may be all you need. The setup is not the simple plug and play of the MP-B-USB but the instructions that come with the RplusD software explain how to set the advanced sound control panel on your computer. Even if you are not using equalization I recommend the RplusD software and Radio Shack meter for anyone purchasing a $3,000+ speaker to assist with room placement. Poor room or seating placement can readily dilute the speaker's value-added performance. If you are using a subwoofer you need RplusD regardless of system price given the mess an improperly placed subwoofer can make of your system's midbass response

For monitoring the placement of passive acoustic panels designed to reduce first bounce reflections, microphone response over 500Hz is required although it does not have to be flat. The MP-B-USB kit is indispensable when evaluating the performance of automatic speakers and/or room correction systems if they try to EQ above 500Hz. I have found these systems can make errors if the measurement microphone is not optimally placed. You need an external measurement system with flat response to verify that the auto EQ has done its job correctly.

--DAR