Nagra PORTABLE ANALOGUE AUDIO TAPE RECORDER Instruction Manual - page 43
8.0 MICROPHONES
INTRODUCTION
There are several different types of microphone pre-amplifiers available for the NAGRA 4.2, in order
to accommodate almost all of the different types of microphones presently available. Microphone
technology is not stable, and each year new models are put on to the market. Many of them require
power supplies, which exist in different forms. The output voltages can vary, according to the type,
in the ratio of 1 to 20. This means that it is not advisable to supply a tape recorder with only one
type of microphone pre-amplifier. It is preferable to have interchangeable preamplifiers.
5.2. THE MICROPHONES
A microphone converts acoustic energy into an electrical signal. Numerous physical principles have
been used to obtain this conversion and there are many different types of microphone available:
Condenser, moving coil dynamic, ribbon dynamic, microphones etc. Neither the perfect nor the
universal microphones exist. Each type has its defects and particular qualities, and the choice
depends upon the required effect.
MICROPHONE CHARACTERISTICS
Sensitivity
Placed in a given acoustic field (e.g. µbar R.M.S.), a microphone will give a signal of X mV R.M.S. X
represents the sensitivity or, in effect, its efficiency. To give this value sense, it is also necessary to
state the internal impedance of the microphone and the load impedance.
A classic dynamic microphone may have a sensitivity of 0.2 mV/ìbar from 200 Ohm internal
impedance. A model giving 0.25 mV is considered to be sensitive, whereas a model giving 0.1 mV
is unsuitable for capturing low level sounds.
Condenser microphones always have a preamplifier in the microphone casing, otherwise their high
impedance would not allow the signal to be transmitted along a cable. At the output of the
preamplifier a typical sensitivity figure is 1-4 mV/µbar with a load impedance of 200-1000 Ohm. It is
difficult to produce a very low noise preamplifier capable of receiving (without overloading) a signal
given by a condenser microphone placed in a strong acoustic field (100 µbars). For this reason, it is
better to have a special preamplifier for condenser microphones. The use of an attenuator between
a condenser microphone and a preamplifier designed for a dynamic microphone is not
recommended, as the signal-to-noise ratio will be unfavorable.
Frequency Response
The frequency response represents the sensitivity of the microphone as a function of the frequency.
It is possible that the response will be different according to the direction from which the sound
comes. This point is very important and will be dealt with in detail later.
Microphone manufacturers pay careful attentions to the frequency response, and in general, most
of the professional microphones available have a sufficiently good characteristic, at least for sound
arriving along the principal axis.
Coloration. Transient Reproduction. Reverberation
An artificial reverberation chamber may have excellent frequency response, distortion and signal-to-
noise characteristics, but it must also modify the signal, which passes it. It adds the reverberation.
This shows that the frequency response, distortion and signal-to-noise ratio are not sufficient to
describe an electro-acoustic device. A moving coil dynamic microphone makes use of resonances