Page 29 - Prosig Catalogue 2005

HARDWARE PRODUCTS
WHAT IS THE DIFFERENCE BETWEEN MICROPHONE TYPES?

In summary, differential inputs reduce noise and allow for potentially
longer cabling. They can be short circuited to be used as single ended
inputs if required. Differential inputs can be used for floating signals, but
in such cases a reference should be provided to the instrumentation.

What Is The Difference

Between Microphone Training & Support

Types?

We are often asked what is the difference between free field microphones,
diffuse field microphones and pressure microphones.
For a run-of-the-mill ½ inch microphone the short answer is nothing. The differences between the three types of microphone generally occur
However, the long answer is a bit more involved. at higher frequencies as we stated previously, below about 16 kHz the
Basically, if the frequency range is below 16 kHz and an accuracy of ±2dB response from each will be similar. This is partly to do with the physical
is acceptable then there is no real difference between the types. size of the microphone with respect to the wavelength of the sound being
So, what is the difference between these types of microphone? measured.
Well, first you have to understand the different types of sound field Free field microphones are designed to compensate for the effect of Condition Monitoring
involved. the microphone itself in the field. So you measure the sound as though
the microphone was not there. They are designed this way as, of course,
In theory a free field is a sound field where the sound waves are free to the presence of the microphone will affect the propagation of the sound
expand outwards forever from the source. That is, we assume that there wave. A free field microphone should be pointed towards the sound
are no reflections or reverberations. In practice, we would consider an source at a 0° angle of incidence.
anechoic chamber to be a free field or an outdoor measurement, provided
we measured at a sufficient distance from the ground. Diffuse field microphones are designed to respond in a uniform
manner to any signal arriving on its measuring surface from any angle.
A diffuse field or a random incidence field, is a sound field where the Generally, these tend to be oriented at about 75° to the direction of the
sound waves arrive equally from all directions. Another way to think of a sound wave propagation in a free field.
diffuse field is that you have sounds coming from different directions in
succession with no time in between their arrival. For example, in a diffuse Pressure microphones are designed to respond to a uniform frequency
field the sound waves that arrive at a person’s ears are so completely response to the sound level itself. When used in a free field a pressure
different that the brain finds it impossible to work out where the sound microphone should be mounted at 90° to the direction of the sound wave
came from. propagation, effectively the sound passes the measuring face of the Software
microphone.
A pressure field is a sound field where the sound pressure has the same
magnitude and phase at any position in the sound field.
It is important to understand that all of these microphone types are
fundamentally the same. They are transducers that are designed to sense
pressure levels in air. The differences between them are in the designs of
the microphone heads. Each type of microphone will normally be supplied
with a calibration data sheet that will show the frequency response
against sound pressure. Comparing the frequency response against sound
pressure for the three microphone types will show the differences clearly.
Hardware

System Packages

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