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The RCA Type 44-BX
bi-directional Velocity Microphone

RCA Type 44-BX with 91-A umber gray pedestal

Use these audio controllers to play a voice sample and an acoustic guitar.
Both of these recordings were made using the microphone pictured here.

Below is an entire tune entitled Laughing Lucas, composed by Florence McPherson during 1901.
It is 3:41 in length, and was recorded using the 44-BX pictured above on Tuesday, Nov. 30, 2021.

The Type 44-BX Velocity Microphones (MI-4027–B, –D, –H, –J and –K) are high-fidelity micro­phones of the ribbon type that are specially designed for broadcast studio use. They are constructed to withstand mechanical shocks, and to retain sensitivity and frequency response regardless of changes in temperature, humidity, and barometric pressure. Their es­sentially flat frequency response (50 to 15,000 cycles) is suitable for reproducing both voice and music.

An RCA Type 44-BX sans grilles

Courtesy of Jeff Rudisill

The moving element of the micro­phone is a thin corrugated aluminum ribbon sus­pended be­tween the poles of a strong Alnico magnet. The moving air particles that constitute sound waves vibrate the ribbon in the magnetic field. This motion causes an alternating voltage to be generated in the ribbon, the amplitude of which is pro­portional to the velocity of the air particles. The output voltage and the electrical impedance of the ribbon are raised to a value suitable for transmission of the signal to an amplifier by a transformer built into the mic case. The transformer is well shielded against stray magnetic fields by multiple shields of mu-metal.

Directional Pattern about Vertical Axis

Directional Pattern
One of the most useful properties of a velocity micro­phone is its bi-directional or figure-eight directional characteristics. As shown in the directional patterns (figures 2 and 3), the output of the micro­phone is maximum for sounds originating directly in front of or behind the micro­phone, and minimum for sounds originating at the sides, top or bottom. This characteristic is valuable for both vocal and musical pickup. For vocal pickup, its chief value lies in the fact that it enables participants in dialog to face each other across the micro­phone. For musical pickup, it makes it possible to obtain different effects by arranging orchestral instruments about the micro­phone so that the sounds of some instruments are attenuated and others are accentuated.

Directional Pattern about Horizontal Axis

This directional pattern also makes it possible to eliminate acoustic feedback from loud­speakers that often occurs in sound-reinforcement work. In addition, the directional pat­tern re­duces pickup of background noise and reflected sounds. For the same ratio of reverberatory to direct pickup, a sound source can be placed 1.73 times as far from a bi-directional micro­phone as from a non-directional mic.

micro­phone location

Frequency Response
The frequency response of a velocity microphone is essen­tially uniform when the sound source is at least three feet from the micro­phone, but the low frequencies are accentu­ated when the sound source is closer to the micro­phone. This is called the proximity effect. Speakers and singers are often required to stand close to the micro­phone, and the low-frequency boost that occurs is ordinarily undesirable except when special effects are wanted.

Frequency Response


To solve this problem, a jumper connection is provided on the micro­phone terminal board that provides either of two de­grees of compensation for close talking. When the jumper is in the M (music) position no compensation is provided, and the response is essentially uniform from 50 to 15,000 cycles per second, provided that the announcer or musical instru­ment is at least three feet from the micro­phone. However, the announcer may move as close to the micro­phone as 12 inches when the jumper is in the V1 position, or seven inches when it is in the V2 position, without causing objectionable low-frequency boost. A downloadable PDF is available containing appropriate illustrations of these connections.

Quoted from Radio Corporation of America Industrial Electronic Products Broadcast Audio
Equipment Instructions for the Type 44-BX Velocity micro­phone, MI-4027–H, –J, –K
(with insert for –L), IB-24267–5(–5A),
circa 1951.

A 44-BX on a boom

RCA 44-BX on an Atlas Sound AD14B adapter

RCA 44-BX on an Atlas Sound AD14B adapter

“Considered by many as the most natural-sounding micro­phone ever made, ribbon mics were immediately embraced by the broadcast and recording industries. Not requiring any awkward power supply or batteries in their operation, the first commercially produced ribbon micro­phones appeared in the early 1930s.

“The ribbon micro­phone was also known as the velocity micro­phone and was the last of the four basic micro­phone types developed, following the dynamic, condenser, and carbon micro­phones.

“The ribbon’s natural sound can also be made to sound warm, big, and syrupy (Bing Crosby-like) when placed within two or three feet of the talent (generally, you can’t close-talk a rib­bon without having a greatly exaggerated bass characteristic). An adequate breath filter in front of the ribbon is also necessary to protect the delicate ribbon (NEVER blow into a ribbon mic).” —Michael Dorrough, Dorrough Electronics

Music/Voice switchXLR on RCA 44-BX
XLR connector on a 44-BXXLR connector on a 44-BX
CBS modified its 44-BX mics to accommodate a built-in M-V switch and an XLR connector.

“There is virtually no difference between the Type 44-B and the Type 44-BX, except that the Type 44-B cable exits straight down from behind the bottom, whereas the Type 44-BX cable exits at the bottom of the back of the base housing, straight backwards. Later in the Type 44-BX, a significant change was made to the internal damping screens where the cross-shaped perforated metal/cloth-lined screens were eliminated in favor of the fine metal mesh screens on the pole pieces. This cured midrange response anomalies and provided parti­cle protection.” —Stephen Sank, Talking Dog Transducer Co.

Switch modification
Another non-RCA modification to provide external switching between the “M” and “V” settings.

44-BX label
Typical label found inside an RCA mic.

RCA 44-BX ribbonRCA 44-BX ribbonRCA 44-BX ribbon

Elmer Davis
Elmer Davis
Click photo to learn more about Mr. Davis.

Hans von Kaltenborn in 1943
H. V. Kaltenborn in 1943
Click photo to learn more about Mr. Kaltenborn
at the marvelous Radio Days web site.

Raymond Gram Swing
Raymond Gram Swing
Click photo to learn more about Mr. Swing.

Mary Margaret McBride
Mary Margaret McBride
Click photo to learn more about Ms. McBride.

Kate Smith
Kate Smith
Click photo to learn more about Ms. Smith.

Twin 44-BX micro­phones
A fine old pair of 44-BX mics equipped with the now-rare hanging fixtures.

Suspension Hanger
This was known as a Suspension Hanger or Mount­ing, Type UP-4212 (Stock № MI-4071) or Type UP-4212-A (Stock № MI-4071-A); both essentially the same item. Type numbers were supplied by marketing; MI numbers by engineering. Special thanks to Mr. Stephen Sank, to Mr. James U. Steele, and to Mr. Jim Webb for tracking down this information.

Elvis Presley
You accomplished a lot of good work, Elvis.

Hoagy Carmichael
Hoagy Carmichael
Click photo to learn more about Mr. Carmichael.

Download the specifications for this mic.
For historical reference only, original price was $129.
Microphones displayed on this site are not for sale.

Photo credits: Mr. Davis courtesy of the National Archives
Mr. Swing courtesy of James F. Widner, Old Time Radio, Radio Days,
Mr. Kaltenborn, Ms. McBride courtesy of the Columbia Broadcasting System
Ms. Smith courtesy of Guideposts magazine
Mr. Carmichael courtesy of Indiana University

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