Acoustical engineering is the branch of engineering dealing with sound and vibration. It is the application of acoustics, the science of sound and vibration, in technology. Acoustical engineers are typically concerned with the manipulation and control of sound.
The primary goal of acoustical engineering is the reduction of unwanted sounds, which is referred to as noise control. Sound can have significant impacts on human health and well being, and is therefore important to control. Noise control principles are implemented into technology and design in a variety of ways. Applications include the design of noise barriers, sound absorbers, silencers, and buffer zones. The implementation of noise control technology differs in indoor and outdoor environments.
In addition to reducing unwanted sounds, acoustical engineers sometimes produce useful sounds or analyze sound waves to collect information. Examples of this include applications of ultrasonics and infrasonics, which make use of sound that cannot be heard by humans. Ultrasonic waves are acoustic waves with frequencies above the audible range (approximately 20 kHz). Applications of ultrasonics include sonar and medical imaging. Infrasonic waves are acoustic waves with frequencies below the audible range (approximately 20 Hz). Applications of infrasonics include the detection of earthquakes and volcanic eruptions.[1] Although acoustical engineering most commonly involves reducing noise, it also applies to these other important applications as well.
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Although the way in which sound interacts with its surroundings is often complex, there are a few ideal sound wave behaviors that are fundamental to understanding acoustical design. Basic sound wave behaviors include absorption, reverberation, diffraction, and refraction. Absorption is the loss of energy that occurs when a sound wave reflects off of a surface. Just as light waves reflect off of surfaces, sound waves also reflect off of surfaces, and every reflection results in a loss of energy. Absorption refers both to the sound that transmits through and the energy that is dissipated by a material.[2] Reverberation is the persistence of sound that is caused by repeated boundary reflections after the source of the sound stops. This principle is particularly important in enclosed spaces. In addition to reflecting off of surfaces, sound waves also bend around surfaces in the path of the waves. This bending is known as diffraction. Refraction is another kind of sound wave bending. This type of bending, however, is caused by changes in the medium through which the wave is passing and not the presence of obstacles in the path of a sound wave. Temperature gradients, for example, cause bending in sound waves.[3] Acoustical engineers apply these fundamental concepts, along with complex mathematical analysis, to control sound for a variety of applications.
Architectural acoustics refers to the control of sound and vibrations within buildings. Although architectural acoustics was first applied to opera houses and concert halls, this branch of acoustical engineering applies to any enclosed area, whether concert halls, office spaces, or ventilation ducts.
The acoustics of rooms are often considered to ensure speech intelligibility and privacy. One thing that can affect speech intelligibility is standing waves. A standing wave results from a sound wave reflected 180 degrees out of phase with its incident wave, which often occurs for at least one specific frequency when two walls are placed parallel to each other. To avoid this, many rooms are designed with angled walls.[4] A second potential cause of poor speech intelligibility is reverberation. This effect can be reduced through porous absorbing materials. Examples of these include glass or mineral fibers, textiles, and polyurethane cell foams.[5] Since the absorption of each material is different for different frequencies of sound, the materials used often vary based on the intended purpose of the room, though compound partitions, or layered combinations of different materials, make more effective absorbers.[6] A third common technique for room acoustics is the use of masking. Masking is the canceling or drowning out of other sounds. Although this raises the overall sound pressure, masking can make irritating noises less distracting and add speech privacy.[7] As these examples highlight, room acoustics are a regular part of architectural design.
Reducing ventilation noise serves as another example of applied architectural acoustics. Many heating, ventilation, and air conditioning systems have silencers. Silencers can actively cancel noise by electronic feed forward and feedback techniques, or muffle the sound by either having sudden changes in cross section or walls with absorbent linings.[8][9] Architectural acoustics involves the control of sound for ventilation, rooms, and anything else indoors.
Environmental acoustics are concerned with the control of sound and vibrations in an outdoor environment. This includes sounds generated by traffic, aircraft, industrial equipment, and anything else that might be considered a nuisance or a safety concern. Engineers concerned with environmental acoustics face the challenge of determining an acceptable level of noise and how noise can be controlled.
Accurately determining appropriate criteria for evaluating noise is often difficult due to the unsteadiness in level and spectral content of most environmental noises. Although qualitative assessments such as speech interference evaluations by trained speakers and listeners can be useful, accurate quantitative measurements are often more difficult to obtain.[10] To address this problem, a wide variety of rating systems that simplify analysis are used for different applications. Common systems include:
Most systems simplify analysis by taking mean sound levels and by relying on what is known as the A-weighted sound level, which assigns to each frequency a weight that is related to the sensitivity of the human ear.[11] These standardized measurements are often sufficient. Otherwise the analysis specific to a given situation is usually complex.
Environmental noise control often involves the creation of noise barriers and use of buffer zones. An Example of noise barriers are the walls built between highways and residential areas. Barriers are less effective in reducing high frequency noise due to the way sound diffracts around walls, but can reduce the overall loudness of traffic noise by as much as one half.[12] Noise buffer zones are also an effective and simple method of noise control. Buffer zones group areas of high-level noise together and surround them with areas of lower-level noises. For example, an airport would not be surrounded immediately by residential properties, but rather by industrial and commercial properties.[13] These two methods are perhaps the most common examples of outdoor noise control.
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