Nagra PORTABLE ANALOGUE AUDIO TAPE RECORDER Instruction Manual - page 45

which the sound comes does not affect the pressure, except at very high frequencies, when the 

microphone makes its own shadow. 

On the other hand, the velocity of the air molecules can be used in a microphone. The word velocity 

implies a combination of speed and direction. A velocity microphone consists of  a very light loose 

diaphragm, which follows the displacement of the air. It will be sensitive to waves, which strike the 

diaphragm perpendicularly whether they come from in front of, or behind it. Waves coming from the 

side will have no effect. This is the principle of velocity of bi directional microphones. Such a 

microphone eliminates an important fraction of the reverberation and if the source of undesirable 

noise is well localized, it can be placed in the dead zone of the microphone. 

In combining a pressure microphone with a velocity microphone, a unidirectional, or cardioid 

microphone is obtained. The two elements are, of course, mounted in a common casing and 

electrically interconnected. 

Secondary Characteristics Related to Directional Characteristics 

Omni-directional microphones (pressure) are much less affected by the wind than bi-directional 

(velocity) or cardioid microphones (because of their velocity element). The light diaphragms of 

velocity microphones have a tendency to float in the wind. It has been shown that the velocity 

microphones are easily damaged by a sudden air displacement (explosion). The response curve of 

an omni-directional microphone is reasonably independent of the direction. However, sounds 

coming from behind will have a tendency to become muffled. Bi-directional microphones attenuate 

the lateral sounds in a relatively uniform manner, but cardioid microphones, and above all, dynamic 

ones, can have a very bad frequency response in the null directions. In other words, the attenuation 

varies greatly according to the frequency. If a cardioid microphone is used to eliminate undesired 

noises, this phenomena is not of great importance. If such a microphone is used to balance the 

sound, when a very loud source is placed around the null area of the microphone, it is advisable to 

check the results. The internal impedance of omni-directional dynamic microphones is reasonably 

constant. They can therefore be used to feed their preamplifier either by voltage or current. On the 

other hand, the majority of cardioid microphones have an impedance varying greatly with the 

frequency. In this case only a voltage feed is recommended. Directional microphones only function 

well if they are sufficiently far from other objects, which can disturb the acoustic field, because an 

obstacle disturbs the pressure less than the velocity. 

PRACTICAL ADVICE ON THE CHOICE OF THE MICROPHONES 

Omni-directional Microphones (pressure) 

Robust, with low sensitivity to the wind, reproducing ambient sounds well-their price is lower than 

that of directional. 

Principal Use: reporting 

Special Uses: Lavalier microphone. For this use, special units have been created whose frequency 

response compensates for the perturbation of the body, and which takes into account the very low 

frequency sounds radiated directly from the chest. Recording music in the open air. Reverberation 

is non-existent and there are good microphones available  - also very robust of low sensitivity, 0.1 

mV/bar, which is acceptable as the sound level is reasonably high in these cases. Recording when 

the microphone is placed in the middle of a sound source (e.g. in the middle of an orchestra). 

Bi directional Microphones (velocity) 

These give a very good attenuation of reverberation, and a good fidelity for sounds coming from the 

null direction. They are very sensitive to wind noise, and they accentuate the low frequencies if the 

sound source is very close. This phenomenon gives a very "Warm" effect, which is exploited by 

certain "charm" singers. Principal uses: music. Dialogue in the case where the microphone is 

placed between two speakers. Remarks: Dynamic bi-directional microphones, i.e. ribbon 

microphones, are either of very low sensitivity, or very bulky. They radiate a magnetic field, which is 

capable of erasing a tape if placed close to it. Condenser microphones have a normal sensitivity.