Review of the Earthworks M30 microphone (for linguists)

September 2010

Acknowledgments

I would like to thank Earthworks, Inc., and in particular Dennis Hurd, Director of Manufacturing, for his invaluable help with the review.

Introduction: About Earthworks

Earthworks Inc., founded by David Blackmer, is a New Hampshire (US) manufacturer, known primarily for its high definition microphones. Each Earthworks product is made with great care, meticulous attention to detail, and a strong emphasis on quality. As stated on the company's website: "Earthworks prides itself in making only the very best in professional audio equipment intended for mission critical sound applications."

About Earthworks microphones

Earthworks is a very unique company. It has been able to achieve, the improbable, though significant success by remaining true to the idea of the purity of sound. Let me explain. The vast majority of the manufacturers of professional audio equipment are driven by the notion of subjective sound quality. Every microphone, pre-amplifier, and loudspeaker is designed to simply sound great. The advertising literature, as well as equipment reviews, are filled with highly imaginative descriptive terms such as clarity, presence, punch, vibrance, timbre, etc. All of these terms represent subjective impressions of the quality and nature of sound reproduced by the devices. Manufacturers pride themselves on tailoring the sonic qualities of their equipment to make it sound as pleasing as possible.

By doing so, the original spectral properties of the sound source, be it speech, vocals, guitar, piano, or drums, are effectively, and often significantly, altered to satisfy the engineers' (and listeners') ideas of what constitutes excellent sound. Most changes are non-linear, affecting the frequency, time, and amplitude domains of the original source. For example, if you look through the catalogs of Sennheiser, Shure, or Audio-Technica, you will find a vast number of microphones with uniquely designed polar patterns, highly stylized frequency response, and uniquely engineered transient response. This has been the dominant status quo in the professional audio industry for decades. Earthworks has decided to provide an alternative approach. Instead of coloring the sound captured by microphones, it strives to reproduce sound that is physically as close to the original as possible, thus flaunting the well-established principles of the majority of the professional audio industry. As Dennis Hurd points out "We never say that our microphones are better, we simply say that they are different." And, different, indeed, they are.

Improving impulse response

While working on new loudspeaker designs, David Blackmer used the biomechanics of human hearing as the conceptual basis for the new model of a high definition microphone. Instead of trying to manipulate frequency response (as was commonly done at the time), he focused on improving time-domain response. He developed a microphone cirtuitry that allowed a remarkably fast diaphragm settling time, which translated into better reproduction accuracy, improved sonic realism, and an extended frequency response.

Extended frequency response

As is well-known, the healthy human auditory system is capable of perceiving frequencies from about 20 Hz to 20,000 Hz. However, while this may be true in a typical laboratory psychophisical task, there is some evidence that the human auditory system should be able to resolve frequencies beyond that range, as well as very small differences in the temporal domain. According to David Blackmer: "TO FULLY MEET the requirements of human auditory perception I believe that a sound system must cover the frequency range of about 15Hz to at least 40kHz (some say 80kHz or more) with over 120dB dynamic range to properly handle transient peaks and with a transient time accuracy of a few microseconds at high frequencies and 1°-2° phase accuracy down to 30Hz. This standard is beyond the capabilities of present day systems but it is most important that we understand the degradation of perceived sound quality that results from the compromises being made in the sound delivery systems now in use. The transducers are the most obvious problem areas, but the storage systems and all the electronics and interconnections are important as well." You can download an interesting paper by Blackmer, which further considers the issue here.

The clean slate

The high definition microphone has proven to have broad applications throughout the professional audio industry. The microphone provides a sort of "clean slate." It records sound that is as close to the original as possible. It gives the sound engineer a unique opportunity to shape and color the sound as much as they please. This approach has been very successful for Earthworks, which has enjoyed a growing following among professional musicians, particularly in classical music and jazz. Earthworks microphones are now endorsed by the likes of George Duke, Anton Fig, and Steve Gadd. Musicians often remark that the recordings captured with Earthworks microphones sound "real" and closely resemble the actual sound of their instruments.

The clean slate approach is, of course, absolutely the right approach for the speech and hearing research. If you look through the equipment reviews on this website, you will see that I am constantly trying to find research microphones and pre-amplifiers that can reproduce the cleanest, most neutral, and spectrally rich signal possible. The Earthworks microphones and pre-amplifiers are, therefore, very intriguing to researchers, promising excellent performance for a great deal of laboratory applications.

The M30 microphone design and specifications

The M30 (Figure 1) is a so-called "measurement microphone." It is based on the Earthworks signature omnidirectional capsule with a stellar transient response and a flat, extended frequency response. The capsule is built into a beautifully finished stainless steel housing. The microphone has a very solid, well-made feel. Backed by a two-year warranty, it does inspire confidence.

Figure 1. The Earthworks M30 microphone

Measurement microphones are designed to record ambient sound for the purposes of acoustic measurement and calibration. For example, they are commonly used for setting up listening environments for both industrial, live concert, and audiophile applications. The idea is to provide the diaphragm with unobstructed vibration, without any significant reflections or comb filtering effects. As a result, the microphone requires special care to minimize handling noise and popping. There is no parylene coating on the stainless steel housing, no wire mesh grille around the capsule, and no windscreen.

The M30 has impressive specifications (Table 1). Of most interest to speech and hearing scientists are: an omnidirectional polar pattern, a flat and extended frequency response (Figure 2), high sensitivity, and a high resistance to overloading. The M30 should make an excellent laboratory tool, but can it be used as successfully in the field? Since a lot of modern linguistic research involves field recording, I typically review equipment that is most useful in the field. The M30, due to its handling characteristics, is probably not the best choice for fieldwork. Still, the microphone is so uniquely suited to quality-critical speech research that it deserves a closer look, hence my decision to review it and introduce its merits to the science community.

Figure 2. The Earthworks M30 microphone frequency response (according to the manufacturer)

It must also be noted that Earthworks offers a battery-powered microphone of similar spectral characteristics, the MB30X, which can be easily interfaced with small digital recorders and laptop computers.

Warranty and service

The M30 microphone comes with a two-year parts and labor warranty. Should you have a problem with the microphone, you can fill out a web form to obtain an RMA. You then ship the microphone to Earthworks where it is repaired and calibrated, in house. Earthworks then ships the microphone back to the consumer free-of-charge. Also, you get a one-time free check-up, after which you can send the microphone back for additional $50 calibration.

The microphone ships with its own frequency response calibration chart (Figure 3). In addition, you can download the microphone's electronic calibration files for use with your own measurement software. This is particularly important for high-precision acoustic measurement and calibration applications.

Figure 3. Earthworks M30 microphone documentation, including a calibration chart, registration card, and owner's manual

Recording technique

Simply put, the M30 microphone requires perfect recording technique. It must always be mounted on a stand, preferably, with a shock mount (e.g., the Earthworks MC2). The stand should be isolated from the surface with acoustic foam to further avoid low-frequency vibrations reaching the microphone housing. The microphone should be positioned about 2 inches from the mouth, off to the side (Figure 4). One should try to advise the talker to not move their head during the recording session so as not to unpredictably change the acoustics of the recording.

Figure 4. Earthworks M30 microphone recommended recording technique

I sometimes receive questions of whether one can mount a measurement microphone on the ceiling of a sound-treated booth. Of course, you can do that, but if the subject is farther from the microphone than, say 5-10 inches, you are going to lose a great deal of spectral detail and allow much of the room's acoustics to shape the spectral characteristics of the recording.

Audio tests

I put the M30 through my usual audio tests. While the microphone performed admirably, it was significantly more difficult to handle than well-designed head-mounted microphones. No, it is not a flaw of the microphone! It is just a reminder that stand-mounted omnidirectional microphones have a steeper learning curve. However, with a bit of practice, the Earthworks M30 will reward you with excellent recordings of speech. I used a Tucker-Davis microphone pre-amplifier along with the Sound Devices USBPre A/D converter. I must also point out that Earthworks makes a line of excellent microphone pre-amplifiers, built on the principle of delivering an unbiased, uncolored sound path for the microphone. While I haven't yet tested Earthworks pre-amplifiers, I am inclined to believe that they perform every bit as well as the M30.

Self-noise

The Earthworks M30 microphone is considered to be a small capsule microphone. By design, small capsule microphones have higher self-noise than large-capsule microphones. However, small diaphragm microphones are most common in fieldwork (see, for example, most microphones reviewed on this website) due to their smaller size and portability. Also, large-diaphragm omnidirectional microphones are generally much more expensive. The most effective technique for minimizing self-noise is using close microphone placement. Along with a low-noise pre-amplifier, close placement ensures that the inherent noise of the entire recording chain is kept to a minimum. You can read more about microphone placement in this post.

Since we record speech at relatively low sound pressure levels (50-60 dB SPL), we require the entire recording chain to be as quiet as possible (to have a low noise floor). Microphones of medium-to-high sensitivity (such as the Nady HM-10 or the Sanken COS-11D) may require your recorder's pre-amplifier gain to be turned up to perhaps 50-75% of its range, which, depending on the recorder, may or may not generate appreciable levels of self-noise. The M30 has rather moderate pre-amplifier gain requirements, as it's sensitivity is rated at 30mV/Pa, so I only needed to set my Tucker-Davis pre-amplifier gain to about 25% of its range.

You can read more about testing equipment for self-noise in this post. The idea is to measure real-world inherent noise in the recording chain. I use the same methodology in all of my microphone reviews, so you can compare the present results with those obtained in the other tests.

The Earthworks M30 does not disappoint and delivers truly superb self-noise performance. Figure 5 shows a spectrum of self-noise generated by a recording chain consisting of the Earthworks M30, a Tucker-Davis microphone pre-amplifier, and the Sound Devices USBPre (reviewed here), calibrated to the peak level of -12 dB SPL, and normalized to the RMS of 70 dB SPL.

Figure 5. The Earthworks M30 microphone self-noise

One word of caution: watch out for low-frequency hum. As you can see in Figure 6, there is a 60 Hz peak of around 32 dB. Low-frequency rumble is virtually unavoidable in any industrialized urban environment. In addition, your own equipment may generate 60 Hz (or 50 Hz outside of the US) hum due to ground loops or induction from power lines. This is especially true when using any of the popular USB recording interfaces with a laptop computer. The hum may come both from the interface and the laptop itself. Check your laptop's A/C adapter, as many of them are "dirty," generating high levels of noise that leak into your precious recordings. You may want to try the Ebtech Hum X hum eliminating device (reviewed here). It does work wonders for hum originating from ground loops.

Low frequency response

Low frequency response is one of the critical features of a speech recording microphone. I must admit that I am somewhat biased in this regard because the bulk of my own acoustic analysis and synthesis lives below 500 Hz. However, if you are interested in the analysis of pitch, phonation, spectral tilt, F1, nasalization, breathiness, laryngalization, lateralization, and other articulatory features with low-frequency correlates, you will need a neutral low-end. I tested the Earthworks M30 microphone with my usual setup. I generated a waveform with peaks of equal amplitude at the frequencies of 50, 100, 200, 300, 400, and 500 Hz. I then played the signal out of a flat-response loudspeaker and recorded it with the M30 microphone about 3 inches from the sound source. The spectrum in Figure 6 shows the low frequency response of the Earthworks M30 microphone. The microphone reproduces each center frequency really well, with no appreciable attenuation or amplification above 50 Hz. There is, however, a slight dip around 50 Hz, but it is very slight, indeed, and should not negatively influence your recordings. In fact, I am quite impressed with the Earthworks M30's performance below 500 Hz. From my point of view, the Earthworks M30 passes the low-frequency test with flying colors. It is one of the flattest low-frequency microphones I have ever tested. If used with a flat-response microphone pre-amplifier, the Earthworks M30 will deliver rather respectable low-frequency response.

Figure 6. The Earthworks M30 microphone low frequency response

LPC analysis / re-synthesis

One of my favorite practical (and rather demanding) tests of spectral detail is LPC analysis/re-synthesis. For the synthesis to be successful, the recording must have exceptionally high degree of spectral detail and a very favorable SNR. Of course, a lot of that is accomplished due to close placement, but the microphone capsule must be able to capture the detail in an unbiased, distortion-free manner. Figure 7 shows a rather successful LPC analysis/re-synthesis of the Polish word "bordo" with a 100 Hz decrease in F1 and a 200 Hz increase in F2 (both marked in red) throughout the ten-frame range. The processing was done with the synthesis tools available in Akustyk 1.8.1. All frames, including the voiceless /r/ in the middle, appear to be properly resolved, with no frame dropouts (at a 0.01 s time step). Once again, I must stress the importance of recording technique, particularly that of close (and constant) placement.

Figure 7. The Earthworks M30 microphone LPC analysis / re-synthesis formant tracks

Below is an LPC analysis/re-synthesis of the American English vowel /ae/ (Figure 8). The synthesis is desgined to emulate the Norther Cities Vowel Shift pronunciation of the so-called /ae/-breaking. The synthesis is successful, with no frame drop-outs.

Listen to MP3 at 128 kbps (the first token original; the second re-synthesized):

Figure 8. The Earthworks M30 microphone LPC analysis / re-synthesis formant tracks

Spectral detail

The M30 microphone delivers excellent spectral detail and a very natural spectral envelope (Figure 9). Transient response shines, as evident by the initial consonant bursts at the beginnings of each word in the phrase. The microphone is impressive at capturing even the slightest detail in the release burst spectra of /cz/ and /k/.

Figure 9. The Earthworks M30 microphone; spectrogram of the Polish phrase "czarna krowa"

Figure 10 shows a spectrogram of the American English phrase "why would you grow a red violet" showing superb spectral detail.

Figure 9. The Earthworks M30 microphone; spectrogram of the English phrase "why would you grow a red violet"

Figure 10 shows an FFT and LPC spectra of the vowel /o/ in "bordo." The formant peaks are nicely defined and, more importantly, the spectrum shows a beautifully natural envelope. It is rare to see such a realistic, "textbook" spectrum of a vowel. The waveform, shown in the analysis window below, is very symmetrical about the axis and showing no evidence of phase distortion. The flat response of the M30 contributes to a very accurate result.

Figure 10. The Earthworks M30 microphone; FFT and LPC plots of the vowel /o/ in "bordo"

Conclusions

The Earthworks M30 microphone was designed with acoustic measurement in mind. It has an exemplary transient response and a flat and broad frequency response. In addition, the microphone proved to have impressively low self-noise and a very neutral low-frequency response. These qualities, alone, make it a perfect candidate for a speech recording microphone. It would make a welcome addition to any speech and hearing laboratory. The price of about $700 is high, but perfectly justifiable. The professionalism of the tech support lends yet another argument in favor of Earthworks products.

However, I must point out that the microphone requires nearly flawless technique. If you decide to purchase the M30, I encourage you to spend some time in the lab learning how best to utilize its superb acoustic qualities. If you do, you will be rewarded with wonderfully rich recordings of speech.

Still, I would not recommend the microphone for fieldwork. Head-mounted microphones are by far a better, easier-to-use option.

Download audio files

	Description	Quality	Comments
	With Tucker-Davis pre-amplifier and Audio-Technica CP8506 48VDC 4-Channel Phantom Power Supply	48,000 Hz; 24-bit; mono	Captured with USBPre in a sound booth
	With the USBPre; test recording in Polish	48,000 Hz; 24-bit; mono	Captured with USBPre in a sound booth