The Ribbon or Velocity Microphone

The ribbon of Fig. 14 is of thin, corrugated aluminum alloy and is loosely held between the poles of a strong permanent magnet. Sound waves approaching from the front, in a direction at right angles to the plane of the ribbon, can reach the back of the ribbon by flowing through the small slits between each side of the ribbon and the adjacent pole piece, or by flowing around the pole pieces to the back side.

Figure 14. A ribbon, or velocity, microphone. Output impedance from secondary of transformer, 50, 250, and 15,000 ohms. Output voltage 1425 X 10-6 volt open circuit across 250-ohm transformer taps with sound pressure of 10 dynes per square centimeter. (Courtesy Radio Corporation of America.)

The ribbon is moved by a difference in pressure, or pressure gradient. The difference in pressure between the two sides of the ribbon is caused by a difference in phase of the sound waves on the two sides of the ribbon.¹⁵ These phase differences are caused by the difference in the length of the acoustic path from one side of the ribbon to the other side. Because the operation of the ribbon microphone depends on air-particle motion and velocity, it is often called a velocity-operated microphone.

Figure 15. Directional characteristics of a typical ribbon, or velocity, microphone. Front and back refer to the microphone which is assumed to be at the center of the figure. (Courtesy Radio Corporation of America.)

The motion of the ribbon in the magnetic field causes a signal voltage to be induced in the ribbon. Both the voltage and the internal impedance are low. These are increased by a transformer located beneath the pole pieces. Transformer output impedances are either 50 or 250 ohms.

The ribbon or velocity microphone is directional, as shown in Fig. 15. A sound coming parallel to the plane of the ribbon will produce no pressure difference on the two sides and accordingly no motion. This directional characteristic may be very useful for certain work such as sound pickup in highly reverberant rooms or in the presence of noise, especially if it is localized. By suitably enclosing one side of the ribbon microphone, it can be made directional.