David A. Collings PE
(Doing business as SOUND ENGINEERING SERVICES)
Specializing in the engineering of NOISE CONTROL SOLUTIONS.
(We guarantee this site to be free of hyperbole and humbug!)
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.