MIcrophone Design II Reading

3. Omni vs Directional

Microphones can have different directional characteristics. Omni's that pick up the sound all around them, and cardioids that mainly pick up the sound directly in front of them. Other directional patterns are: bi-directional/figure 8, super cardioid, hyper cardioid, and wide cardioid.

Generally a cardioid sounds appealing, since in a musical setting you only want the sound source and rarely want to record or amplify the surroundings. But directionality has a price and that sometimes is not worth paying.

From a focus standpoint, omni-directional patterns are the least focused. This is because the sound source being captured has to compete with all the sounds within the recording space. Bi-directional microphones begin to focus the image as sound on the side of the polar pattern in being rolled off. Cardioid type polar patterns offer the most focus, with the image getting tighter as the cardioid pattern goes from cardioid to super to hyper.

Directional microphones need to have a much softer diaphragm than an omni. This softness results in handling, pop & wind noise which puts a limit to how close you can get to a vocalist, even when using pop-filters.

A directional also suffers from proximity effect, which means that the closer you get to the sound source the louder the low frequencies get.

Additionally the off-axis sound of a cardioid is less linear than that of an omni. It is very hard to reduce the level of sound taken in from the sides without some coloration, and some directional microphones have a notably poor off-axis response. This means that sound entering the microphone from the sides and the rear are more or less strongly colored. This effect can be seen on the microphones polar pattern as ‘spikes’.

Multi-pattern microphones with both omni, bi-directional and cardioid characteristics will always compromise the sound quality. It may be very convenient to have a 3-in-1 solution, but the drawback is reduced performance in each mode. Due to the need of a pressure gradient design, a multi pattern microphone in omni mode has many of the weaknesses of the cardioid, such as popping, handling & wind noise and a less linear off-axis sound. In fact a multi pattern microphone in the same mode can have different characteristics depending on the frequency.