The Fostex Model M88RP printed ribbon bi-directional microphone
Ten black versions of the M88RP were hand-built as a test bed during 2003. This is one of them, with a serial number of 105003. No longer available, they are sought by some collectors.
Fostex RP Microphones Soon after their introduction, the Fostex ribbon mics were being called “Printed Ribbon” Microphones, so fondly did their warmth resemble the beautiful but frail ribbon mics of yesteryear. The reference to “printed” is to one patented aspect of Fostex RP Technology, wherein the voice coil is etched directly onto the diaphragm. But the reference to “ribbon” carried a negative connotation regarding durability. However, Fostex RP Mics have a history which verifies a ruggedness quotient equal to, or better than, the very best dynamics.
In the RP system, a flat, thin diaphragm is driven with absolute phase uniformity, in true piston motion. Eighty per cent lighter than the typical dynamic element, the RP element has the fast transient response of a condenser. Fostex RP (“Regulated Phase”) Microphones use a magnetic design in place of the polarizing voltage design of a condenser. The advantage of this fact can be seen in the ability of the RP design to be applied to any polar pattern, with full integrity.
Extremely good bi-directional pickup pattern is possible due to the symmetrical physical construction of the microphone element.
The basic RP pattern is bi-directional, or “Figure 8.” The acoustic air mass load is the same, in fact, on both sides of the diaphragm. Given this reciprocal pick-up pattern as a basic building block, the RP design can be predictably controlled all the way from a stereo mic to a supercardioid. The high magnet flux on the surface of the RP diaphragm produces outstanding control, reduces diaphragm break-up, and results in lower distortion overall. Fostex RP mics have less than 0.2% THD at 130 dB SPL—a figure unmatched by some of the finest condensers.
The patented Fostex RP design permits capsule construction which has identical front/back response. This design reduces second harmonic distortion to almost unmeasurable levels, lower than some of the best condenser microphones. At 100 dB, RP microphones have less than 0.01% distortion, equal to many high-quality electronics. At 130 dB SPL, RP mics have less than 0.2% distortion. The high magnetic flux, the flat diaphragm and the spiral voice coil all contribute to complete diaphragm control at all sound pressure levels. This patented design results in operating levels so high, they are normally associated only with laboratory microphones. Surprisingly, this kind of performance and reliability is readily affordable. Fostex RP mics are about half or less the cost of world-class condensers, and about the same as the best dynamics available.
Printed Ribbon Microphones The basic construction of the printed ribbon microphone consists of a diaphragm sandwiched between two sets of concentric ring magnets. The magnetic flux from the two ring magnets flows from N to S poles in the direction of the radial axis. The diaphragm is constructed of a plastic film four to six micrometers thick. The aluminum spiral ribbon coil is deposited onto the surface of the diaphragm, producing a single piece diaphragm/coil assembly. The sound waves arrive at the diaphragm through openings between the inner and outer ring magnets, as well as through the center hole of the inner magnet. When excited by sound waves, the diaphragm/coil assembly moves through the magnetic field, and a voltage is induced across the coil winding. The acoustic impedance of this type of transducer is almost identical to the traditional ribbon style, in that the front and rear geometry are symmetrical. Additionally, the diaphragm is symmetrical about the main axis; therefore a Figure 8 polar pattern may be obtained in all planes of the diaphragm axis. As with the traditional ribbon design, if the rear portion of the diaphragm is contained by a sealed labyrinth to prevent sound waves arriving from the back, an omni-directional pattern is obtained. If a small opening is made in this labyrinth to introduce sound waves to the rear of the diaphragm, a uni-directional pattern will exist.
Outstanding Features of a Printed Ribbon Transducer
Non-linear distortion during extremely high sound pressure levels where the diaphragm excursion is very large is kept at a minimum because the coil is in a uniform magnetic field.
Because very strong magnets are located on both sides of the diaphragm, the diaphragm is protected from minute iron particles floating in the air. This microphone is well suited for outdoor use.
Reliability is high and maintenance is simple as the microphone requires neither batteries nor an external power supply.
Due to the extremely small mass of the diaphragm, sensitivity and performance similar to traditional ribbon microphones is achieved while remaining durable.
Extremely good bi-directional pickup pattern is possible due to the symmetrical physical construction of the microphone element.
Specifications
Impedance: 600 ohms
Sensitivity: –52 dB, 2.5 mV/Pa (0 dB = 1V/Pa)
Frequency response: 40 to 18,000 Hz
Switch: Three-step sound selecting
Dimensions: 52 x 46 x 136 mm
Weight: 330 g
Text and illustrations are from A Careful Blending of Design Elements, published by Fostex. Photographs are provided through the courtesy of Mr. John Warshaw.
