Octave Band Filters
A sound level meter fitted with Octave Band Filters has the ability
to split the audible spectrum into smaller bands, identifying the frequency
content of the noise.
To help explain why Octave Band Filters are sometimes used,
first have a look at a simple noise measurement.
Simple Measurements with No Octave Band Filters
Most simple noise measurements are made using the "A"
frequency weighting and the results expressed in dB(A). When
a measurement is "A" weighted the sound level meter
is responding in a similar way to the human ear, being more "deaf" at
the lower and higher frequencies.
The image on the right shows the A weighting filter, removing
much of the lower (deep tones) and some of the higher frequency
(high pitch) noises. |
A Weighting Filter Response |
Checking The Frequency Content - Octave Bands
When you need a little more information than this, you split the spectrum
into about 10 bands. These are called the Octave Bands because there
is one octave between the bottom and top of each band. The centre frequencies
for these bands are usually:
31.5Hz , 63Hz , 125Hz , 250Hz , 500Hz , 1kHz , 2kHz
, 4kHz , 8kHz and 16kHz
The sound level meter will measure the noise in each of the
octave bands, either all at the same time (real-time or parallel
filters) or switching to one band at a time (serial filters).
The image on the right shows a typical octave band spectrum.
In this case you can clearly see that the 1kHz and 2kHz bands
are showing much higher levels than the others. If you were
trying to reduce the noise then based on this measurement you
can see where all the effort needs to go, especially when you
consider that these bands coincide with the frequencies where
the human ear is most sensitive. |
Octave Band Spectrum |
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1/3 Octave Band Filters
1/3 Octave Band Filters are very similar in nature to the
Octave Band filters described above. The difference is that
each of the Octave Bands is split into three, giving a more
detailed description of the frequency content of the noise.
Although rarely used for occupational noise measurement, 1/3
octaves are useful in many environmental, building acoustics
and noise control applications. |
Note: Different meters cover different bands |
Real Time or Serial Filters?
The answer to this question comes down to the nature of the noise
you are measuring and to the size of your budget.
| Real Time Filters - the sound level meter measures
in all the bands at the same time. In the past this one done with
multiple filters but now it tends to be done using Digital Signal
Processing techniques. The advantage of real-time octave band filters
is that measurements can be made quickly and none of the bands
miss any of the noise during the measurement period. |
 CEL620 Real-Time Octave
 CEL630 Real-Time 1/3 Octave |
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| Serial Filters - the sound level meter measures
in one band at a time. The meter will usually be fitted with a
single filter circuit, which will be electronically switched to
measure the different bands. With most modern meters you can set
the filters to scan through from 31.5Hz to 16kHz automatically.
The advantage of serial octave band filters is that the meter has
only one filter circuit, keeping the cost and power consumption
down. |
 Serial Octave Filters
 Serial
1/3 Octave Filters |
If you are measuring noise that varies considerably then you should
use real-time filters or be prepared to monitor for a much longer period
of time using serial filters. The real-time method will always allow
you to make the measurements much more quickly. If you have quite steady
state noise levels to measure and you are not doing it too often then
serial filters will be adequate.
Typical Octave and 1/3 Octave Band Filter Applications
These filters can be used for many applications, but in particular
they are clearly used to identify the frequency content of the noise
and to help you to control and reduce the noise. Here are some examples:
- Hearing protection - the Octave Band spectrum
is used for calculating the level at the ear when wearing hearing
protectors. Some of our data logging meters are provided with software
that makes these calculations for you.
- Noise reduction and control - identifying the
problem areas and then focusing the control on those levels.
- Building Acoustics
- Machine and product testing
< Acoustic Terms Index
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