Earthworks M30BX Measurement Microphone

By Bruce Bartlett

	Product Points
	Applications: Test and measurement Key Features: Omnidirectional pattern; .25-inch diaphragm; +14 dB switch; battery operated; ships with mounting clip, battery Price: $895 Contact: Earthworks at 603-654-6427, Web Site.

Features

The Earthworks M30BX microphone reviewed here is designed and calibrated for measurement. This device meets ANSI Type 1 specs and is ideal for on location acoustical measurements of loudspeakers, sound systems, and room acoustics.

A free-field, omnidirectional (pressure) microphone, the M30BX was designed not to disturb the sound field it is measuring. Its tapered tip prevents sound reflections back to the mic capsule, and its .25-inch diameter capsule with minimum features results in a polar pattern that is very uniform. Another benefit of the tiny mic capsule is that the diaphragm has very low mass, so it responds quickly to transients and has low handling noise. The M30BX is a great tool for measuring the impulse response of loudspeakers.

The signal-to-noise ratio of small-diaphragm microphones is not the best. In fact, the self-noise of the M30BX is 27 dB A-weighted. You wouldn't use it to measure low-level ambient noise. Its hiss might become audible when recording quiet instruments at a distance.

Inside the microphone is an electret-condenser mic capsule coupled to a wideband impedance converter. According to Earthworks, the "circuitry excludes the transconductance of the input FET from the overall gain structure." As a result, the mic's sensitivity remains stable with the ambient temperature. The output will drive either balanced or unbalanced inputs.

The M30BX works on an internal 6V battery but not on phantom power. The battery is a single type P28 (lithium or alkaline), 4LR44 or equivalent. Battery powering suits the microphone for use with laptop sound cards, handheld analyzers, or recorders that lack phantom power. You access the battery by unscrewing the back half of the microphone. The battery is on only when an XLR connector is plugged into the mic - a convenient touch. According to Earthworks' chief engineer, Alex Khenkin, the sensitivity of the M30BX does not change at all when the battery voltage drops.

If you need an identical mic that is phantom powered, the Earthworks M30 does the job. Also available is the M50 with response up to 50 kHz.

Alongside the battery holder is a switch that adds 14 dB extra gain if needed. The M30BX comes packed with a calibration sheet showing frequency response and sensitivity at both gain settings. Earthworks can supply a computer file of the frequency response data, which can be imported into measurement software.

Also included are a mounting clip, a standard 1/2-inch calibrator adapter, a 6V battery, and an XLR-to-RCA adapter (for use with sound cards and MiniDisc recorders). All this is packaged in an elegant, hinged wooden box. It smelled of fresh varnish and reminded me of the lab instrumentation in physics classes many years ago.

Earthworks recommends the M30BX for recording as well as measurement, especially in the field where electrical power may not be available. The manufacturer also sells a stereo matched pair.

In Use

I found the M30BX handy for measuring my studio monitors, studio acoustics and home stereo loudspeakers. Because no phantom supply is needed, it was easy to plug the mic into a laptop sound card input and make measurements with SMAART software.

I recorded my voice six inches from the M30BX and played back the recording. What a great sounding microphone! Its hiss was not audible at a realistic playback level, and I heard no tonal coloration. The low end was solid, even without any proximity effect. Sibilants were very smooth.

When used on an acoustic guitar, the M30BX had a sound that was warm and smooth, rather than etched and crisp as you might expect from its extremely good time resolution. Similarly, percussion transients were not hyped or edgy. They just sounded like the live instrument.

Summary

The Earthworks M30BX is a well-engineered, precision device with very flat frequency response, tight transient response, and a uniform polar response. Its self-noise is low enough for most loudspeaker and SPL measurements, and for many recording situations. The M30BX sounds extremely smooth, wide-range and uncolored. Because it is battery powered, it is ideally suited for on-location and portable applications. Highly recommended.

Bruce Bartlett is a microphone engineer/technical writer, audio journalist and recording engineer.