8/3/2019 Measurement of Leq Using an Integrating SLM
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3SOUND AND VIBRATION/MAY 2000
The thresh old of hearing is 0 dB SPL and
the threshold of pain is 140 dB SPL, a
much more manageable range of values.
If the sou nd pressure levels h ad been A
weighted, the measured values would be
speci f ied in te rms o f Sound Levels in
dBA. The A-weighted sound level scale
provides a mu ch better estimate of hear-
ing damage risk in indu stry and anoyance
to noise compared to an overall or linear
measurement o f sound pressure level .
Figure 3 shows the RMS pressure valuesfrom Figure 2 in term s of
SPL in dB.
If I were to ask you to qu antify the SPL
over the 4-second p eriod of the measure-
ment shown in Figure 3, you would h ave
difficultly because the SPL is constantly
changing. And indeed most of the SPL
m eas u r em en t s y o u m ak e w o u l d s h o w
this type of variability and p erhaps m ore.
Because of the variability in sound lev-
els for one or more sources, a descrip tive
measurement parameter was developed
to reduce the sound level over an inter-
val in time to a single equivalent value.
This value is the Leq
or equivalent con-
t inuous sound level . This can be relatedto a constant sound
over a specific time
interval that has the same acoustic energy
as the actual varying sound over the same
interval. If the sound source being mea-
sured emitted a constant f ixed tone, the
Leq
value would s imply be the same as
the sound level . But what h appens wh en
the soun d level varies like that in Figure
3? TheLeq
is calculated usin g the follow-
ing equation:
where:
p (t) = measured sound pressure, Pap
0= reference pressure, 20 Pa
T = speci fied measurement dura t ion
time, sec
Th e Leq
is a logarithmic average of the
SL over a specified time in terval T. Fig-
ure 4 shows the o r ig inal SL measure-
ments w i th the equ ivalen t con t inuous
sound level Leq
(83.5 dB) sup erimposed .
Two other descriptive parameters are
also shown in Figure 4, the Lm ax
an d Lm in
.
Th e Lm ax
parameter is the maximum SL
that occurred in the m easurement inter-
va l T. The Lm in
is the minimum SL that
occurred in the measurement interval T.
The three parameters Leq
, Lm ax
, and Lm in
together ef fectively descr ibe th e soun d
level character is t ics over the measure-
ment interval T. An Integrat ing Sound
Level Meter is required to calcu late an Leq
value. Some SLMs give the u ser complete
flexibility in setting up the m easuremen t
interval T while others only have a pre-
def ined set of choices for the measure-
men t interval. Not all Integrating Sound
Level Meters are the same as far as the
descr ip t ive parameter s they ca lcu la te .
For example one type of meter may only
display the Leq
value where another one
may display theLeq
,Lm ax
, and Lm in
along
with a host of others. Many of the stan-
d a r d s an d o r d i n an ces u s ed f o r s o u n d
level measurements are based up on the
Leq
value measured over some specified
time interval.
Another descr ipt ive parameter that is
sometimes used as an al ternative to the
Leq
value for shor t durat ion, h igh level
sound sources i s the Sound Exposure
Level (SEL). This param eter is d efined as
a cons tan t soun d level l as t ing fo r one
s e c o n d t h a t h a s t h e s a m e a m o u n t o f
acous t ic energy as the o r ig inal soun d .This parameter is u
seful for compar ing
different types of impu lsive or short du-
ra t ion no ise even ts . For the 4 - second
measurements shown in Figure 3 the LSEL
= 89.5 dB. The equation th at defines the
LSEL
is shown below and is s imilar to the
equation for Leq
except that i t is normal-
ized to 1 second instead of the measure-
ment interval T.
W h e n y o u r m e a s u r e m e n t s r e q u i r e
more detailed information regarding thesound level over a
specified time inter-
val than what the Leq
, Lm ax
an d Lm in
p a-
rameters provide, there is another set of
descriptive statistical parameters avail-
able with some SLMs. These parameters
are called Ln
or Percentile Levels and are
based upon the cumu lative dis tribution
of soun d levels over a specified time in -
terval. For example, the L10
value is the
SL that is exceeded on ly 10% of the time.
Figure 5 illustrates the cum ulative distri-
bution of the SL of the 4-second measure-
ment shown in Figure 3 and highlights
th e L10
an d L95
values.
In this Q&A column we h ave describedsome of the fund
amental sound pressure
level and sound level param eters that are
measured and calculated using Integrat-
ing Sound Level Meters. In some sense
we have only scratched th e surface since
there are many other descr ipt ive param-
eters that can be used to help quantify a
sounds level, frequency content and time
varying natu re. Determine wh ich param-
eters are required for your specific mea-
surement circumstances and what mea-
surements are available in a specific SLM
before se lec t ing an In tegra t ing Sound
Level Meter.
Next Months Quest ion: Which S ignal
Processing Window should I be using to
analyze my data?
LT
p t
pd t
T
eq =
10
110
0
2
0
log( )
Lp t
pd t
T
SEL =
10 10 0
2
0
log( )
Figure 1. Instantaneous soun d pressure.
Figure 2. RMS sound pressure.
Figure 3. Sound pressure level, dB re 20 Pa.
Figure 4. Leq
and sou nd levels , dB re 20 Pa.
Figure 5. Cum ulative distribution of soun d lev-
els exceeded n% of the t ime, dB re 20 Pa.
