Description:

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.

 

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.

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.

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