sound pressure difference, in given medium, between average local pressure , pressure in sound wave. square of difference (i.e., square of deviation equilibrium pressure) averaged on time and/or space, , square root of average provides root mean square (rms) value. example, 1 pa rms sound pressure (94 dbspl) in atmospheric air implies actual pressure in sound wave oscillates between (1 atm
−
2
{\displaystyle -{\sqrt {2}}}
pa) , (1 atm
+
2
{\displaystyle +{\sqrt {2}}}
pa), between 101323.6 , 101326.4 pa. human ear can detect sounds wide range of amplitudes, sound pressure measured level on logarithmic decibel scale. sound pressure level (spl) or lp defined as
l
p
=
10
log
10
(
p
2
p
r
e
f
2
)
=
20
log
10
(
p
p
r
e
f
)
db
{\displaystyle l_{\mathrm {p} }=10\,\log _{10}\left({\frac {{p}^{2}}{{p_{\mathrm {ref} }}^{2}}}\right)=20\,\log _{10}\left({\frac {p}{p_{\mathrm {ref} }}}\right){\mbox{ db}}\,}
where p root-mean-square sound pressure ,
p
r
e
f
{\displaystyle p_{\mathrm {ref} }}
reference sound pressure. commonly used reference sound pressures, defined in standard ansi s1.1-1994, 20 µpa in air , 1 µpa in water. without specified reference sound pressure, value expressed in decibels cannot represent sound pressure level.
since human ear not have flat spectral response, sound pressures frequency weighted measured level matches perceived levels more closely. international electrotechnical commission (iec) has defined several weighting schemes. a-weighting attempts match response of human ear noise , a-weighted sound pressure levels labeled dba. c-weighting used measure peak levels.
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