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The minimum rms level is widely recognised as LAmin, LCmin or LZmin
depending on the frequency weighting selected and is measured in decibels (dB).
Maximum rms Level (L
max
)
This is the highest root-mean-square sound pressure level measured over the
measurement time period.
The maximum rms level is widely recognised as LAmax, LCmax or LZmax
depending on the frequency weighting selected and is measured in decibels (dB).
It is not to be confused with peak levels, which are very different.
Microphone
A microphone is the transducer that detects sound and creates an electrical
image of it. The image created is a voltage level which is proportional to the
sound pressure at the microphone.
A condenser microphone operates like a charging / discharging capacitor and
is therefore also referred to as a capacitor microphone. The condenser
microphone has a thin diaphragm and a solid back plate both of which construct
the two plates required to form a capacitor. When a sound wave hits the
diaphragm it vibrates and the distance between the diaphragm and back plate
change. This variation in distance is realised as a capacitance change. A
decrease in distance results in a charge current as the capacitance is higher
and an increase in distance between the plates results in a discharge current as
the capacitance decreases.
An external polarisation voltage must be applied across the condenser
microphone for it to operate.
Electret condenser microphones operate using the same principle as condenser
microphones with the exception that they do not need an external power source
which is created during manufacture by using permanently charged materials.
All current Castle sound meters use electret condenser microphones.
Noise
Noise is generally regarded as unwanted sound.
See also ‘Ambient Noise’, ‘Background Noise’, ‘Pink Noise’, ‘White Noise’ and
‘Sound’.
