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HD2010UC/A
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V4.1
A4: S
OUND LEVEL METER
The sound level meter is the instrument that measures the sound level. Usually, it is made up
by a microphone, the sound sensitive element, by an amplifier, by a signal processing unit and by a
reading and data display unit.
The microphone converts the sound signal into a corresponding electrical signal. The sensitiv-
ity of microphones for level measurements does not depend on the sound signal frequency. The
preference of the type of microphone is usually based on the condenser type that grants excellent
features like accuracy, stability and reliability.
The amplifier is necessary to bring the electrical signal to measurable amplitude and to
strengthen the signal to allow cable transmission.
The processing unit takes care of calculating all measuring parameters necessary to characterize a
sound event.
F
REQUENCY
W
EIGHTING
If you have to assess the auditory impact of a noise source, you will have to make some cor-
rections to the acoustic signal provided by the microphone in order to simulate the sound percep-
tion; this means that the microphone sensitivity will have to be adjusted to simulate the frequency
response of the human ear. Two correction curves named “A weighting” and “C weighting” have
been defined as international standard (IEC 60651, recently replaced by IEC 61672).
When the sound level is corrected by A weighting, it is referred to as L
Ap,
and simulates the
perception of low sound levels. When, on the contrary, the sound level is corrected by C weighting,
it is referred to as L
Cp
, and simulates the perception of high sound levels.
When there is no interest in sound perception, measurements are carried out using Z weighting (LIN
for IEC 60651) that represents a constant response to all frequencies in the sound field.
S
PECTRAL
A
NALYSIS
The spectral analysis by bands is used to analyze in detail the features of a complex sound.
For this analysis the range of audio frequencies (between 20 Hz and 20 kHz) is divided into bands,
typically with a constant percentage bandwidth corresponding to an octave or to a third octave. The
sound level is calculated for each band considering only the noise components with frequencies in-
cluded within the band limits: for octave bands the upper limit always equal the double of the lower
limit, while for the third octave
bands, the upper limit equals 1.26
times the lower limit, so that an
octave band is divided into three
third octave bands. For example,
the band centered at 1 kHz will
consider the sounds between 707
Hz and 1414 Hz for octave and be-
tween 891 Hz and 1122 Hz for
third octave bands. The analysis
result is usually shown in a graph
called “spectrogram” where the
sound levels are represented in
graphic format for each of the
bands in which the sound spectrum