ANC (Active Noise Control) is done by outputting an inverse phase sound
from control speakers to cancel the noise sound. The noise could change
its frequency and level, so we need to observe the noise signals at the
closer point to the noise source and the control points, and calculate
the control output signal tracking the noise sound inversely.
To adjust to the change of the noise behavior, an adaptive processing method
is used. The cocntroller learns how to minimize the noise power at the
In case of a vibration problem, we can solve it by replacing microphones and speakers with acceleration sensors and shakers.
Feed-forward noise control system for Air ventilation duct
The system inputs the reference signal at the upper streqam, and outputs the inverse phase control sound from the control speakers to minimize the sound powert at the downstream control point.
When the noise is random signal, the reference microphone is installed
near the noise source, and the noise is canceled by the inverse phase sound
at the downstream over the fence with an advantage of electric speed which
is faster than sound. This kind of control is called feedforward systems
and used for air ventilation ducts or acoustic barriers.
When the noise is periodic, the control speakers are installed near the
noise source, and the noise is canceled by the inverse phase sound with
one wave-length delay. This kind of control is called feedback control and used for diesel engine exhaust outlets.
The control sound should be toward downstream (from upper to down stream).
When you try to control for a different direction, an interference pattern is made and increasing sound power points would appear.
Adjust the saound wave curvatures of the noise and control sounds.
(Adjust the both wave surfaces.)
When the control speakers are away from the noise source, much more speakers are needed to control wider area.
Control only direct sound waves
When you try to control the reflected sound, it would make you control the wave in a defferecnt direction, so an interference pattern would appear and it would make the result worse.
Installed ANC, but...
You will measure the noise level at your yards and setbacks after you
have installed an ANC system. You might find the effect of the ANC is only
arround one or two dB(A). You might not be able to hear the difference.
Than happens sometimes.
Do you really think the ANC system is only expensive but worthless?
The measuring point stays off from the ANC system and there are some other
noise source around the measuring point, so the noise level at those yards
and setbacks is increased by the background noise. Noise masking effect
would hide the effect of the ANC.
We will check out whether the ANC effect is good enough or not by the following
Meaning of 1 dB(A) result
We will check out the meaning of 1 dB(A) result at boundaries of sites.
(1) The method of experiment
a) Output random sound noise from a speaker, and ajust the sound level
to 60dB(A) as background noise.
b) Output a 60Hz sine wave mixed to the background noise and adjust the
sine wave to become the total sound level to 61dB(A), incleasing by just
(2) Difference of the sound level
The 63Hz frequency band is slightly incleased.
(3) Time domain signal
We feel the sound is really annoying, but why is it only 1dB(A) difference?
Just see the below chart in time domain, and you will find the big difference.
(4) Measuring in A-weighting scale
We measure noise levels in A-weighting scale which is the same level as a human feels. We subtract the following conpensating value from measured levels in A-weighting scale.
| A-weighting correction value (JIS)
We need to subtract 26.2dB at 63Hz low frequency band. That is why the
measured level does not show 60Hz sound is standout.
(5) Sound level without A-weiting
When we measure the sound level without A-weiting, the chart is like as
follows. The difference is 20dB.
Judjing by A-weighting scale makes the ANC effect small. Only 1dB in A-weight
scale, but 20dB in flat scale.
* Caution) In this experiment, the background noise spreads between 40-1000Hz, and the ANC result would change depending on the background noise.
(6)Inside our rooms
The noise sound level should be damped when it goes through walls and windows.
But remember the damped noises are only high-frequency band and low-frequency
band noises go through without attenuation.
Here assumed that the high-frequency band of the background noises are
attenuated when the noises come into the room, the ANC effect becomes really
| Background noise(Hz)
|40 - 1000
|40 - 500
|40 - 200
In a room where figh-frequency band is already attenuated, you can feel
the conspicuous difference whether an ANC system has been installed or
not. When the backgroud noises are only under 200Hz, it is 42dB(A) in the
room. But when 60Hz is added to it, the sound level rises to 59dB(A). And
60Hz sounds are so annoying that they destroy the environment.