| Ken:
When you told me that the unit of noise level is dBA in
talking of Pascal and decibel. When I was waiting for a
bus in a station, the sign on a tower in planting in
front of the station showed noise level at that moment
with “dB”.
I first thought it was “A characteristic” because it was
a noise level but actually it was without “A” on the
sign.
Dad:
The noise level should have come with “A” because it is
“A characteristic”. “dB” reminds people of sound unit
but they may wonder what it is with “A”. “Phon” used to
be common as well but “dBA” was standardized since 1997.
The way of saying “Noise level” is distinctive to Japan.
It is called “A characteristic weighted frequency sound
pressure level”.
Ken:
How long the name is! “Noise level” is easy
to remember and use.
Dad:
Noise level is very useful indeed. A lot of noise
consists of complicated frequency ingredients and gets
higher and lower as time passing. This is excellent as
responses of human being for the complicated noise can
be shown numerically with one value.
Ken:
You told me for quite some time now that there
are frequency masking and time masking in human
perception of sound. Does noise level include those
effects as well?
Dad:
No, it doesn’t. There is an amount of estimation called
“Loudness” showing the loudness of sound. Masking is
considered with it and it treats the loudness which
human senses precisely. However, the calculation is very
complicated and equipment to measure it is not common so
far. In these days loudness has come to be used for the
noise discharged from a machine when estimating sound
from an OA equipment or inside of a car. Traffic noise
is often estimated with “noise level”.
Ken:
Why is it?
Dad:
Sometimes sound discharged from equipment tells you its
failure. You call “abnormal sound” the sound in its
working incorrectly. If you estimate with noise level,
you can not indicate loudness of sound human can hear
precisely. In short, abnormal sound is often an
outstanding sound of a certain level of frequency. By
considering frequency masking, you can express loudness
corresponding to human acoustic sense. In contrast,
traffic noise can be estimated with noise level as it
includes wide range of frequencies.
Ken: Range of
frequencies? It needs to determine by what means you estimate it
depending on kinds of noise.
Dad: Yes. When I
talked about frequency masking, I related how basal membrane vibrates in
a cochlea. The acoustic sense of human is “A characteristic” but it has
a filter to distinguish tone of sound. This filter bandpass is called
critical bandpass whose breadth is of approx.1/3 of an octave.
Ken: I don’t quite
understand. There is a filter in human’s acoustic sound distinguishing
tone of sound. You said 1/3 of an octave…what is it?
Dad: You know an
octave, don’t you? A range from the note “Do” to the next “Do” accounts
for one octave. The span of an octave accounts for twice in terms of
frequency. The sound of time tone is the note “La” but the first three
are all 440Hz and the last long one is 880Hz which is one octave higher.
Ken: Which sound is
440Hz with a guitar?
Dad: The sound at the
fifth fret of the first chord. One fret is semitone, so the next “La”
note is at the 17th fret of the first chord. There are 12 semitones in
one octave. This means a semitone accounts for 1/12 of an octave.
Ken: 1/3 of an octave
contains 4 semitones and 2 whole tones. This is between the notes “Do”
and “Mi”.
Dad: You are right.
An octave accounts for twice in terms of frequency. To analyze sound,
center frequency which standardizes 1000Hz is used instead of “sol-fa”.
63Hz, 125Hz, 250Hz, 500Hz, 1000Hz, 2000Hz, 4000Hz and 8000Hz … A while
ago, I told you that human being can hear sound ranging approx. from
20Hz to 20kHz, but actual sounds surrounding a living environment range
from 63 to 8000Hz. So it is common to estimate between these 8 bands.
Ken: What is a band?
Dad: I haven’t talked
to you about it, have I? Bandpass of frequency with one octave breadth
is called octave band. I won’t tell you in detail as calculating formula
is too difficult. An octave band whose centering frequency is 1000Hz has
a breadth from 770Hz to 1410 Hz and band whose centering frequency is
500Hz has one from 355Hz to 710Hz. Level of sound strength which is in
one octave is called Octave band level. When you analyze sound, it is
the easiest to estimate with only one value of noise level expressing
with dBA. However, you can not tell if it is low frequency sound with
high volume or high frequency sound with high volume. In order to see
octave band or see more closely, analyzing 1/3 octave band enable you to
understand the characteristic of frequencies of the noise.
Ken:
But for what do you have to analyze in such a way? You
can estimate by noise level if it does not include
abnormal sound, can’t you?
Dad: Think a
situation to take measures against noise. When you find a level at each
frequency, you find which noise of frequency you need to take care of.
For example, when measuring noise of road with not only noise level but
also measured figures in octave band, you can estimate what kind of
design of sound barrier is most effective or how extent the level is
around the residential area nearby.
Ken:
I see. Noise level is enough in estimation but
considering measurements and precaution, analyzing an
octave band is effective.
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