The RCA Type 44-BX
bi-directional velocity microphone (1932)
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 microphone case. The transformer is well shielded against stray magnetic fields by multiple shields of mu-metal and copper.
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.
The directional pattern also makes it possible to eliminate acoustic feedback from loudspeakers, which occurs frequently 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 microphone.
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. Speakers and singers are often required to stand close to the microphone, and the low-frequency accentuation which occurs is 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. Seen here with its terminal board cover removed, jumper connections are (from left to right) M, V1, and V2. Refer to Fig. 4 for corresponding frequency responses.
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).”
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.”
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.
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.
Click photo to learn more about Ms. Smith.
A fine old pair of 44-BX mics equipped with the now-rare hanging fixtures.
This item was known as a Suspension Hanger or Mounting, Type UP-4212 (Stock No. MI-4071) or Type UP-4212-A (Stock No. 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.
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