David A. Collings PE
(Doing business as SOUND ENGINEERING SERVICES)
Specializing in the engineering of NOISE CONTROL SOLUTIONS.
To all who suffer
from annoying, distracting or damaging noise.
(We guarantee this site to be free of
hyperbole and humbug!)
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With a background of 26 years of service within the Noise Control industry,
I am able to share some of the secrets of Noise Control and offer my services
in applying the science of acoustics to the control of noise.
NOTE: Opinions and conclusions expressed
herein are strictly my own and do not necessarily represent the points of view
of any referenced individuals or organizations. DAC.
We spend
our lives immersed in a very shaky medium - a mixture of gases called air.
During our evolution we have developed sensory organs that enable us to detect
very small vibrations in this medium. We have learned to use these vibrations
for communication, but in the process we have become sensitive to the random,
unwanted disturbances that we call noise.
If we could slow down all activities around
us, we would eliminate much of the noise. Unfortunately, many of the machines
that make our lifestyles possible today rely upon speed to maintain
efficiency. It is impossible to reduce or eliminate many of these noise sources
and we must rely upon containment to keep the noise from reaching our
ears. Containment may be the placement of a physical barrier between us and the
sound waves or the use of some device for canceling or attenuating the
sounds close to the source.
First let's make sure we understand the nature of sound
waves. Sure everyone knows that sound travels through air just like ripples on
a pond when you toss in a stone. There are many similarities between sound
waves and water waves, but there are also some very important differences:
1. Water is incompressible. The surface waves on
the pond are formed when water bobs up and down. In this case gravity provides
an elastic restoring force. Air, on the other hand, is compressible and
a sound wave is a pressure pulse, i.e. an expansion and contraction of the air
that radiates out in all directions from the initial disturbance.
2. The speed of a surface water wave depends upon
its size. A large wave propagates faster than a small wave. All air pressure
waves on the other hand travel at essentially the same speed - the speed of
sound - which is affected only by the temperature of the air (it gets a little
slower when its cold!).
3. Waves of all types are reflected when they strike
rigid surfaces. Air pressure waves however, because they travel so rapidly, can
be reflected many times inside an enclosed space such as a building so that the
resulting wave patterns may be very complex.
The study of waves can be fascinating. Those with
the stomach (no sea sickness pun intended) for such a pastime should study
"Waves in Fluids" by the late Sir James Lighthill who at one time
held the chair of Applied Mathematics at Cambridge (a position originally
filled by Sir Isaac Newton).
Because air responds so rapidly to any disturbance, it is
possible to generate sound waves with a very short interval between each
passing wave. This interval is conveniently measured as frequency i.e.
the number of waves that pass by every second. The human ear contains a spiral
shaped cavity called the cochlea filled with tiny hair cells that respond to
the various frequencies and allow us to recognize the difference between a
"high" and a "low" note (for want of better names). Since
most of us are only interested in sounds that we can hear, the practice of noise
control is generally directed at sounds with frequencies between 45 and
11,200 cycles per second. This encompasses a range of eight octave bands.
The importance of frequency will be discussed
further when we consider the interaction between sound waves and solid
surfaces.
As a sound wave travels away from its source like an
expanding bubble, the intensity of the pressure pulse diminishes as the
energy is spread over an increasing area. In a "free" unbounded space
the level of sound drops by 6 decibels for every doubling of its distance from
the source (Heck, we all know that things get quieter as you get further away).
Unfortunately there are many situations where sounds are reflected, refracted
or are just too damn loud and we need to find an additional means of reducing
the sound level.
Now when a sound wave passes by, the air
along its path expands and contracts rapidly and moves backwards and forwards
in the direction of the sound. This movement is very small but it is important
when we are considering mechanisms for damping or attenuating the energy of
sound.
Because sound waves involve a small vibratory movement
of the air, it is possible, by slowing the motion, to reduce the energy and
thus lower the sound level. This can occur when the sound waves pass through
porous materials containing fine strands or fibers. The viscous drag of the
tiny fibers damps the motion and the sound is eventually dissipated. On a
molecular level, a fibrous material can also slow the speed of transmission
of sound. This is very important when we consider wave interference
phenomena in the next section.
Sound waves quickly become chaotic when sound energy
bounces around in a closed space. Reflected waves cross and re-cross so that at
any one instant the peaks and valleys of the sound pressure fields may combine
with or cancel each other. A sound that is short and crisp when heard outdoors
may become prolonged and distorted in a highly reflective indoor environment.
However, under certain conditions, the reflections of sound waves can help to
control and modify the sound levels experienced in a room or occupied space.
This may sound like a contradiction but read on...
We have seen how porous materials containing
fine fibers will absorb sound energy. In order to be effective however, the
small random movements of air driven by the sound waves must somehow be
concentrated within the sound absorbing material. This will happen if the
material is positioned next to a solid reflective surface and the material
thickness is at least one quarter of the wavelength of the lowest frequency of
interest. Even if the waves are travelling almost parallel to the
surface, the denser medium will slow the speed of sound and turn the sound
waves into the material by refraction. The reflective surface returns
the wave but with a different phase relative to the incident field. The
destructive interference between the fields increases the total energy
absorption of the porous material.
Some common materials that make good sound absorbers are
fiberglass insulation wool, plastic foams and dense woven fabrics. The
characteristic that makes them absorb sound is the airflow resistivity
of the material. This can be measured in the laboratory by passing a small flow
of air through a sample in a special rig described in ASTM
Standard C522. For every frequency of sound,
there is an optimum thickness and resistivity for an effective sound absorber.
The relative amount of sound energy that can be absorbed by a square foot of a
material is known as its sound absorption coefficient. To measure this
directly requires a reverberation chamber and a fairly elaborate laboratory
procedure described in ASTM
Standard C423.
Specifications for materials used for sound
absorption often use a single number rating misnamed Noise Reduction
Coefficient (NRC). This is simply an average of the material's absorption
coefficients at four different frequencies. Unfortunately, this rating tells
nothing about a material's effectiveness at low frequencies which is often more
critical.
Air-moving
devices such as fans and turbines create periodic disturbances in the air that
can result in very high noise levels over a wide range of frequencies. Any
method of containment of the sound waves must allow air to pass as freely as
possible while absorbing the tiny fluctuations of pressure that are transmitted
through the moving stream of air. One effective method is to duct the
air through a section containing parallel baffles that divert the stream into
narrow passages lined with sound absorbing materials. The sound propagation is
slowed at the surface of the porous liners so that the sound waves are turned
into the linings and then reflected back into the passages with their phases
shifted as described above. A properly designed silencer of this type
can capture much of the sound energy while being open to the passage of air. A
laboratory testing procedure for duct silencers is described in ASTM
Standard E477.
In order to provide some degree of acoustical
privacy in open plan offices, a system has been developed that provides a
broadband, steady sound signal throughout a work area. The signal has the
effect of masking quiet sounds in the speech frequencies, without itself being
noticeably distracting. A properly designed and tuned masking sound system has
found widespread acceptance as a solution to privacy and distraction concerns.
It offers a lower cost alternative and greater architectural flexibility
compared to the use of closed offices.
A subject that has attracted much attention in the last
decade is the concept of Active Noise Cancellation. This truly seductive
idea has evolved around the principle that any waveform of sound energy
can be fed back electronically as a matching, yet oppositely phased noise that
will cancel the original sound. The topic has spawned enough hyperbole to take
us to Mars and back. Academic careers have been launched on the subject and the
mathematics of the various control algorithms has been a favorite subject in
theses of students of acoustics.
To the noise control engineer however, the practical
realization of active cancellation has remained very much a pipe dream. Except
under certain, limited conditions, the majority of real world noise situations
involve chaotic sound fields that are far too complex to be precisely paired
with a synthetic copy. Adding chaos to chaos simply results in more chaos!
Exceptions to this can be found in situations where the sound field is highly
coherent (for example, a low frequency noise in a narrow duct), or when a
canceling signal can be introduced right at the ear of the listener.
"Passive" noise control treatments
are likely to remain the preferred means for controlling noise in large spaces.
We have seen however that the mechanics of sound absorption can often involve reactive
components. Electronically modifying the tuning of the reactive
part of a sound absorber might have applications in studios and music practice
rooms where it would be desirable to be able to adjust the room acoustics for
different activities.
