The RCA Type 44-BX bi-directional Velocity Microphone
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.
General 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.
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 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.
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.
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.
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.
“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
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.
Another non-RCA modiﬁcation to provide external switching between the “M” and “V” settings.
Typical label found inside an RCA mic.
Elmer Davis Click photo to learn more about Mr. Davis.
H. V. Kaltenborn in 1943 Click photo to learn more about Mr. Kaltenborn at the marvelous Radio Days web site.
Raymond Gram Swing Click photo to learn more about Mr. Swing.
Mary Margaret McBride Click photo to learn more about Ms. McBride.
Kate Smith Click photo to learn more about Ms. Smith.
A ﬁne old pair of 44-BX mics equipped with the now-rare hanging ﬁxtures.
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.
You accomplished a lot of good work, Elvis.
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, http://www.otr.com/index.shtml Mr. Kaltenborn, Ms. McBride courtesy of the Columbia Broadcasting System Ms. Smith courtesy of Guideposts magazine Mr. Carmichael courtesy of Indiana University