Spartan 821/721 Manual Glossary C-7
Frequency Band Pass Filter
The part of certain sound level meters that divides the frequency spectrum on the sound
or vibration into a part that is unchanged and a part that is filtered out. It can be
composed of one or more of the following types:
Low Pass: A frequency filter that permits signals to pass through that have frequencies
below a certain fixed frequency, called a cutoff frequency. It is used to discriminate
against higher frequencies.
High Pass: A frequency filter that permits signals to pass through that have frequencies
above a certain fixed frequency, called a cutoff frequency. It is used to discriminate
against lower frequencies.
Bandpass: A frequency filter that permits signals to pass through that have frequencies
above a certain fixed frequency, called a lower cutoff frequency, and below a certain
fixed frequency, called an upper cutoff frequency. The difference between the two cutoff
frequencies is called the bandwidth. It is used to discriminate against both lower and
higher frequencies so it passes only a band of frequencies.
Octave band: A bandpass frequency filter that permits signals to pass through that have
a bandwidth based on octaves. An octave is a doubling of frequency so the upper cutoff
frequency is twice the lower cutoff frequency. This filter is often further subdivided in 1/
3 and 1/12 octaves (3 and 12 bands per octave) for finer frequency resolution.
Instruments with these filters have a sufficient number of them to cover the usual range
of frequencies encountered in sound and vibration measurements. The frequency
chosen to describe the band is the center frequency.
Frequency and Exponential Time-Weighted Sound Level (
L
ωτ
)
L
ωτ
corresponds to the frequency and exponential-time weighted sound level in dB. It is
sometimes referred to as the “RMS sound level”. Similarly the A, C, or Z-frequency
weighting is sometimes referred to as the “RMS frequency weighting,” where RMS is an
acronym for root-mean-square.
The Detector rate setting on the dosimeter corresponds to an exponential time constant
of SLOW (1 second), which is designated as ω in the equation. These time constants are
required by both ANSI and IEC standards.
In the
L
ωτ
equation,
ω
designates the frequency weighting (A or C). For example, on the
dosimeter, L
AS
signifies the A weighted, SLOW exponential-time weighted sound level.
L
10 log
10
1
---
p
2
e
t –
---------------
–
p
o
2
---------------------------------------
d
–
t
dB=
p = instantaneous, frequency-weighted (A or C), sound pressure in pascals.
p
o
= reference sound pressure, 20 Pa
t = time of observation
= Exponential time constant in seconds for either the S(slow)
or F(fast) time weighting
=
Frequency weighting (A, C, or Z)
To Next Page
Loading...
