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Earthworks SR71

Phantom Cardioid Microphone By Hugh Robjohns
Published July 1999

Earthworks SR71

Hugh Robjohns tries out a stylish new microphone from US manufacturers Earthworks.

The American company Earthworks was formed by David Blackmer, the inventor of dbx, to produce the best possible audio equipment, and the company has already built a fine reputation with their small range of measurement and recording microphones, along with a couple of extremely high‑quality mic preamps. Many of these products have already graced the pages of Sound On Sound: the QTC1 omnidirectional mic and Z30X cardioid, together with the Lab102, preamp appeared in July 1998, and the OM1 omni mic was reviewed in November 1996.

When I first laid hands on the Earthworks mics for the July '98 review I was extremely impressed with the QTC1, which still remains a favourite omnidirectional microphone, and although I felt the Z30X was overpriced at the time (it was subsequently 'repositioned', I'm pleased to say), it also proved itself to be a very capable microphone indeed with a fantastically smooth frequency response both on‑ and off‑axis. Its very broad polar response would not suit every situation, though, and even the 'enhanced cardioid' version (Z30XL) was not quite as precise in its rearward rejection as might have been liked. However, the particular blend of characteristics and compromises made by Earthworks resulted in a very usable and fine‑sounding microphone.

And so to the latest addition to the fleet, the SR71, which is marketed as a high‑quality, general‑purpose live sound mic. The good news is that this version has been priced at less than half the cost of Earthworks' other cardioid, the Z30X, which puts it in the same territory as AKG's C1000 and the Rode NT1. Earthworks's literature recommends it for a very wide range of applications, including voices (solo and choirs), guitars, pianos and all other stringed instruments, brass, woodwind (particularly saxophone and flute), percussion, snare drum, bass drum and overheads. There is nothing that makes a noise which this mic isn't ideally suited to, apparently!


The model number implies a reference to the infamous USAF SR71 'Blackbird' supersonic spy plane, which seems vaguely appropriate given the mic's sleek black styling. All of the other Earthworks mics I have seen to date have had polished metal bodies, so the anodised black of the SR71 is a company first. Black mics are far better suited to live sound and broadcast applications as they tend to be less visible, and the SR71's finish appears to be able to withstand life on the road pretty well.

The microphone itself is very plain and simple, there being no switches for pads or bass cuts. It is an end‑fire mic, and the business end has a collection of narrow milled slots to allow sound to reach the rear of the capsule. The serial number is printed inside the XLR connector at the base of the mic. According to Earthworks, the mic can withstand being dropped onto a hard floor from a height of eight feet without sustaining any damage whatsoever, so it would appear to be pretty tough and rugged.

The microphone is delivered with a foam windshield and a soft rubber mic stand clip which is designed for the American standard 5/8‑inch thread — no 3/8‑inch thread adaptor was supplied. These accessories come in a six‑inch tube (see the 'In The Tube' box) which is lined with a short user guide and specification sheet, a warranty card, and a MLSSA averaged frequency‑response plot for the specific microphone. The user guide advises of the mic's susceptibility to plosives and wind blasting, recommending the use of the supplied windshield in such situations. However, it goes on to point out that this also reduces the extreme HF response slightly and so should not be used unless absolutely necessary.

Dejà Vû?

When I initially examined the SR71 it seemed very familiar to me, and after checking on the specifications I found that it is exactly the same in every mechanical respect as the Z30X cardioid (and Z30XL enhanced cardioid) mics I reviewed over a year ago. It has the same dimensions: 235mm long and 22mm in diameter (12mm at the capsule end), and the same 225 gram weight. The body shape is identical, and only the milling of the capsule slots and the black finish appear to be different.

Intrigued by this startling similarity, I checked the electrical characteristics of the two microphones and, would you believe it, they are absolutely identical! Sensitivities are given as a healthy 10mV/Pa, both mics draw 10mA from a 48V phantom supply, the maximum acoustic inputs are 145dB SPL with a 5kΩ load on the mic output, and both mics can manage a generous 10V peak output voltage into the same load (falling to 3V with a 1kΩ impedance). The self‑noise of both mics is quoted as 22dB SPL (A‑weighted), and they both drive pin 2 of the XLR positive with positive air pressure at the capsule. The only differences between the two spec sheets at all was that the SR71's frequency response is stated as "50Hz to beyond 20kHz +/‑ 2dB at six inches", whereas the Z30X's is "+/‑ 1.5dB 30Hz to 30kHz at 15cm". In other words, the SR71 seems to have a slightly narrower frequency window with wider response tolerances than the silver Z30X. That, combined with the relatively low price, leads me to believe that this new mic has been designed to employ capsules which haven't quite made the grade for use within the top‑flight Earthworks Z30X cardioid mics.

I would hardly say the SR71 capsules were rejects, though. The frequency response is still commendably flat, making it well suited to live sound applications, since it allows more gain to be applied to the mic and monitors before howlrounds occur, there being no peaks in the output to encourage feedback. The frequency‑response charts show the mic to perform most linearly when placed around six inches (15cm) from the sound source — the distance the designers have chosen as the optimal balance point between the capsule's natural falling frequency response and the bass lift inflicted by the proximity effect. At greater distances the low end falls off gently from about 400Hz, as would be expected, with a ‑3dB point at around 250Hz when placed one metre from the source. Working closer than six inches brings further bass boost, of course.

Close inspection of the polar‑response plots for the two mics show them to be identical too, with the same characteristically broad frontal pickup. The attenuation with angle is unusually gentle, with the 6dB point being at around 90 degrees for most frequencies, although the pattern becomes much narrower above about 20kHz. The close bunching of the polar responses around the sides of the microphone at different frequencies implies a very smooth off‑axis response. In other words, the frequency balance remains consistent as a sound source moves off‑axis, only with diminishing level. This makes the mic very tolerant of positional variations with singers or other mobile musical sources, and also makes it ideal for use as a coincident pair.

At 2kHz the SR71 exhibits an almost perfect textbook cardioid response with a fantastically sharp rear null, but at higher and lower frequencies this is less well defined, although never worse than 12dB of rejection from 500Hz up to 4kHz. Above this point the response starts to move towards hypercardioid, with small side nulls at 135 degrees in place of the 180 degree ideal.

In Use

The SR71 is everything I remembered the Z30X to be — smooth, detailed, extremely transparent, and completely uncoloured. At least, that is how it sounds alongside other mid‑market, and even a few high‑end, mics. For a cardioid mic it sounds unusually open, and an awful lot like an omnidirectional really — in fact it is quite probably better than some omnidirectional mics I have used in the past!

I tried it on a wide variety of sources and found it worked superbly well on all of them, capturing every nuance of the sound with great fidelity. On very close vocals the bass tip‑up gave a slightly warm sound compared to some other small‑capsule mics, but it remained detailed and never showed any hint of harshness or a tendency to sibilance. Moving the mic out to about a foot or so from six and twelve‑string acoustic guitars produced a lovely sound, completely free of boomy resonances but with all the complex harmonic structures perfectly intact. On an upright piano it proved equally competent, and even placing it directly in the firing line of a trumpet did not cause any problems (with the windshield on). Since the mic has been balanced to rely on the proximity effect from close working for a flat frequency response it also proved to be relatively immune to handling noise and stand vibrations.

The broad polar response and modest rear rejection would probably limit the mic's effectiveness in difficult live sound applications, but its very flat frequency response would go some way to making up for it, and the crystal clarity and complete neutrality are definitely welcome. Given the price, it has to be said that this is something of a bargain, not quite living up to the strengths of the Z30X but getting 95 percent of the way there at less than 50 percent of the price. A very high‑quality mic indeed at an attractive price, and definitely one to add to the short list.

In The Tube

Earthworks put almost as much effort into their microphone packaging as into designing the mics! Matched pairs of Z30s or QTC1s are supplied in beautiful wooden boxes arranged like duelling pistols, for example. For the SR71, they have come up with an equally novel packaging solution: the mic is suspended within a tough plastic tube two inches in diameter and about 10 inches long. A simple plastic cap seals the top, but a substantial machined base screws into the bottom. This base resembles a candlestick holder, and the microphone sprouts vertically from it like a candle, being held securely in place during transit by a small winged clamping screw on the side. The whole contraption is innovative and works well, and should protect the mic from roady abuse!


  • Superb sound quality.
  • Smooth off‑axis response.
  • Matt black finish.
  • Nice price.


  • Rear rejection not as good as some other cardioids.


A high‑quality cardioid condenser mic derived from the Z30X, but with a matt black finish, suiting live sound and broadcast applications.