DeltaOHM HD2010UC/A, Руководство по эксплуатации

Страница: 151 / 162

HD2010UC/A
- 151 -
V4.1
A7: DEFINITIONS
Frequency: is the number of oscillations per second, expressed in Hertz (Hz).
Wave Length:
is the distance between two adjacent maximum values of pressure, expressed in me-
ters (m).
Period:
is the interval of time necessary to make a complete oscillation, expressed in seconds (s).
Sound Propagation Speed:
is the distance covered by the sound wave front in the unit of time, ex-
pressed in meters/second (m/s). The propagation speed depends on the medium and, in the air, at
ambient temperature, it equals to about 344 m/s.
Decibel:
a decibel (symbol: dB) is defined by:
0
10
log 20
X
X
dB
⋅ =
where: X is the measured quantity.
X
0
is the reference value of the measurement itself (to which correspond 0 dB).
Sound Pressure: the sound pressure is the value of the change of atmospheric pressure caused by
acoustic perturbations, expressed in Pascal.
Reference Sound Pressure:
sound pressure taken as reference for the calculation of the pressure
level; it equals 20•10
–6
Pascal and corresponds to the average human audibility threshold at a fre-
quency of 1 kHz.
Effective Value:
sound pressure effective value (p
rms
) is the value of constant pressure that is ener-
getically equivalent to the instantaneous value (p) in a given time interval (T).
∫
=
2
1
) (
1
2
t
t
rms
dt t p
T
p
where:
T = t
2
– t
1
is the considered time interval.
p
2
(t) is the sound pressure squared at time t in t
1
÷ t
2
interval.
rms
means “ROOT MEAN SQUARE”, that is square root of the average of squared values.
Sound pressure effective value is expressed in Pa and takes importance in sound measurement since
the value is directly connected to the quantity of energy contained in the sound signal.
Crest Factor: is the ratio between the maximum and the effective value of a quantity, measured in
a given time interval with reference to the arithmetic average value.
Sound Pressure Level:
is defined by the expression:
0
10
log 20
p
p
L rms
p
⋅ =
where:
p
rms
= pressure effective value.
p
0
= reference sound pressure.
L
p
sound pressure level (also referred to as SPL) is expressed in dB.