Microphone Directivity

Author: N.H. Crowhurst

The velocity microphone responds equally to sound waves from the front and the back, but not at all to sound waves from the side. It is therefore bidirectional.

As well as differing between the way that they convert vibrations into electrical voltages or currents, microphones differ in the way that they pick up acoustic waves. For example, the ribbon moves with the air particles as the wave passes it. If a wave in a direction parallel to the lat surface of the ribbon, the ribbon will not move, and no sound will be picked up. This property of a ribbon microphone makes it bidirectional. This means it is sensitive to sound from two directions (back and front), but not sensitive to sound from other directions.

In most other types of microphones, the back of the diaphragm is shut off from access to outside air, so that sound waves reach it only from the front. This means that the microphone is sensitive only to pressure fluctuations, rather than to a pressure gradient or the air particle velocity. For this reason, such microphones are called pressure type. The pressure at the diaphragm varies in the same way, regardless of the direction from which the sound comes. For this reason, pressure microphones pick up sound from all directions, and are called omnidirectional.

The pressure microphone responds equally to sound waves from the front, the back, and the side. It is therefore omnidirectional.

Suppose that the lower half of a ribbon microphone is enclosed at the back so that this portion works like a pressure microphone; when sound conies from the front, both halves will move the same way and the microphone will give maximum output. Sounds from the back, however, will move the two halves of the ribbon in opposite directions, so that the resultant output cancels. Sound from the sides will only move the half of the ribbon that is enclosed at the back, but not the free, or velocity, half. This makes what is called a unidirectional or cardioid response.

Directivity of the cardioid microphone

Unidirectional signifies pickup from only one direction - the front. Cardioid describes the heart-shaped sensitivity of this microphone, when plotted on polar-coordinate paper. (To plot this kind of curve, the output from the microphone for a given sound intensity from various directions is marked along radii with corresponding directions.)