Acoustic quieting is the process of making machinery quieter by damping
vibrations
Vibration is a mechanical phenomenon whereby oscillations occur about an equilibrium point. The word comes from Latin ''vibrationem'' ("shaking, brandishing"). The oscillations may be periodic, such as the motion of a pendulum—or random, such ...
to prevent them from reaching the observer. Machinery vibrates, causing
sound waves
In physics, sound is a vibration that propagates as an acoustic wave, through a transmission medium such as a gas, liquid or solid.
In human physiology and psychology, sound is the ''reception'' of such waves and their ''perception'' by the ...
in air,
hydroacoustic waves in water, and
mechanical stress
In continuum mechanics, stress is a physical quantity. It is a quantity that describes the magnitude of forces that cause deformation. Stress is defined as ''force per unit area''. When an object is pulled apart by a force it will cause elonga ...
es in solid matter. Quieting is achieved by absorbing the vibrational energy or minimizing the source of the vibration. It may also be redirected away from the observer.
One of the major reasons for the development of acoustic quieting techniques was for making
submarines
A submarine (or sub) is a watercraft capable of independent operation underwater. It differs from a submersible, which has more limited underwater capability. The term is also sometimes used historically or colloquially to refer to remotely o ...
difficult to detect by
sonar
Sonar (sound navigation and ranging or sonic navigation and ranging) is a technique that uses sound propagation (usually underwater, as in submarine navigation) to navigation, navigate, measure distances (ranging), communicate with or detect o ...
. This military goal of the mid- and late-twentieth century allowed the technology to be adapted to many industries and products, such as
computers
A computer is a machine that can be programmed to carry out sequences of arithmetic or logical operations (computation) automatically. Modern digital electronic computers can perform generic sets of operations known as programs. These programs ...
(e.g.
hard drive
A hard disk drive (HDD), hard disk, hard drive, or fixed disk is an electro-mechanical data storage device that stores and retrieves digital data using magnetic storage with one or more rigid rapidly rotating platters coated with magnet ...
technology),
automobiles
A car or automobile is a motor vehicle with wheels. Most definitions of ''cars'' say that they run primarily on roads, seat one to eight people, have four wheels, and mainly transport people instead of goods.
The year 1886 is regarded as ...
(e.g.
motor mounts), and even sporting goods (e.g.
golf clubs
A golf club is a club used to hit a golf ball in a game of golf. Each club is composed of a shaft with a grip and a club head. Woods are mainly used for long-distance fairway or tee shots; irons, the most versatile class, are used for a variety ...
).
Aspects of acoustic quieting
When the goal is acoustic quieting, a number of different aspects might be considered. Each aspect of acoustics can be taken alone or in concert so that the end result is that the reception of noise by the observer is minimized.
Acoustic quieting might consider...
* Noise generation: by limiting the noise at its source,
* Sympathetic vibrations: by acoustic decoupling,
* Resonations: by acoustic damping or changing the size of the resonator,
* Sound transmissions: by reducing
transmission
Transmission may refer to:
Medicine, science and technology
* Power transmission
** Electric power transmission
** Propulsion transmission, technology allowing controlled application of power
*** Automatic transmission
*** Manual transmission
*** ...
using many methods (depending whether the transmission is through air, liquid, or solid), or
* Sound reflections: by limiting the reflection using many methods, e.g. by using acoustic absorption (deadening) materials,
trapping
Animal trapping, or simply trapping or gin, is the use of a device to remotely catch an animal. Animals may be trapped for a variety of purposes, including food, the fur trade, hunting, pest control, and wildlife management.
History
Neolithic ...
the sound, opening a "window" to let sound out, etc.
By analyzing the entire sequence of events, from the source to the observer, an acoustic engineer can provide many ways to quieten the machine. The challenge is to do this in a practical and inexpensive way. The engineer might focus on changing materials, using a damping material, isolating the machine, running the machine in a vacuum, or running the machine slower.
Methods of quieting
Mechanical acoustic quieting
* Sound isolation: Noise isolation is isolating noise to prevent it from transferring out of one area, using barriers like deadening materials to trap sound and vibrational energy. Example: In home and office construction, many builders place sound-control barriers (such as
fiberglass batting
Building insulation materials are the building materials which form the thermal envelope of a building or otherwise reduce heat transfer.
Insulation may be categorized by its composition (natural or synthetic materials), form (batts, blankets, loo ...
) in walls to deaden the transmission of noise through them.
* Noise absorption: In
architectural acoustics
Architectural acoustics (also known as building acoustics) is the science and engineering of achieving a good sound within a building and is a branch of acoustical engineering. The first application of modern scientific methods to architectura ...
, unwanted sounds can be absorbed rather than reflected inside the room of an observer. This is useful for noises with no point source and when a listener needs to hear sounds only from a point source and not echo reflections. Example: In a
recording studio
A recording studio is a specialized facility for sound recording, mixing, and audio production of instrumental or vocal musical performances, spoken words, and other sounds. They range in size from a small in-home project studio large enoug ...
,
sound proofing
Soundproofing is any means of impeding sound propagation. There are several basic approaches to reducing sound: increasing the distance between source and receiver, decoupling, using noise barriers to reflect or absorb the energy of the sound wav ...
is accomplished with
bass trap
Bass traps are acoustic energy absorbers which are designed to damp low frequency sound energy with the goal of attaining a flatter low frequency (LF) room response by reducing LF resonances in rooms. They are commonly used in recording studio ...
s and
anechoic chamber
An anechoic chamber (''an-echoic'' meaning "non-reflective") is a room designed to stop reflections of either sound or electromagnetic waves. They are also often isolated from energy entering from their surroundings. This combination means ...
s.
Wallace Sabine, an American physicist, is credited with studying sound reverberations in 1900, and
Carl Eyring
Carl Ferdinand Eyring (August 30, 1889 – January 3, 1951) was an American acoustical physicist. He was the dean of the College of Arts and Sciences at Brigham Young University (BYU) for 26 years and was also the vice president of the Acou ...
revised his equations in 1930 for
Bell Labs
Nokia Bell Labs, originally named Bell Telephone Laboratories (1925–1984),
then AT&T Bell Laboratories (1984–1996)
and Bell Labs Innovations (1996–2007),
is an American industrial research and scientific development company owned by mult ...
. Another example is the
ubiquitous
Omnipresence or ubiquity is the property of being present anywhere and everywhere. The term omnipresence is most often used in a religious context as an attribute of a deity or supreme being, while the term ubiquity is generally used to describe ...
use of
dropped ceiling
A dropped ceiling is a secondary ceiling, hung below the main (structural) ceiling. It may also be referred to as a drop ceiling, T-bar ceiling, false ceiling, suspended ceiling, grid ceiling, drop in ceiling, drop out ceiling, or ceiling til ...
s and
acoustical tiles
A dropped ceiling is a secondary ceiling, hung below the main (structural) ceiling. It may also be referred to as a drop ceiling, T-bar ceiling, false ceiling, suspended ceiling, grid ceiling, drop in ceiling, drop out ceiling, or ceiling tile ...
in modern office buildings with high ceilings. Submarine hulls have
special coatings that absorb sound.
* Acoustic damping:
Vibration isolation Vibration isolation is the process of isolating an object, such as a piece of equipment, from the source of vibrations.
Vibration is undesirable in many domains, primarily engineered systems and habitable spaces, and methods have been developed to p ...
prevents
vibration
Vibration is a mechanical phenomenon whereby oscillations occur about an equilibrium point. The word comes from Latin ''vibrationem'' ("shaking, brandishing"). The oscillations may be periodic function, periodic, such as the motion of a pendulum ...
from transferring beyond the device into another material. Damping mounts have progressed in the industry to offer vibrational resistance in many
degrees of freedom
Degrees of freedom (often abbreviated df or DOF) refers to the number of independent variables or parameters of a thermodynamic system. In various scientific fields, the word "freedom" is used to describe the limits to which physical movement or ...
. Recent advances include shock isolators damping in at least six degrees of freedom. Acoustic damping also has uses in
seismic shock
A seismic wave is a wave of acoustic energy that travels through the Earth. It can result from an earthquake, volcanic eruption, magma movement, a large landslide, and a large man-made explosion that produces low-frequency acoustic energy. S ...
protection of buildings. Motors and rotating shafts are commonly fitted with these mounts at the points where they contact the building or the chassis of a large machine.
* Acoustic decoupling: certain parts of a machine can be built to keep the frame, chassis, or external shafts from receiving unwanted vibrations from a moving part. Example:
Volkswagen
Volkswagen (),English: , . abbreviated as VW (), is a German Automotive industry, motor vehicle manufacturer headquartered in Wolfsburg, Lower Saxony, Germany. Founded in 1937 by the German Labour Front under the Nazi Party and revived into a ...
has registered a
patent
A patent is a type of intellectual property that gives its owner the legal right to exclude others from making, using, or selling an invention for a limited period of time in exchange for publishing an enabling disclosure of the invention."A p ...
for an "acoustically decoupled underbody for a motor vehicle.". Another example:
Western Digital
Western Digital Corporation (WDC, commonly known as Western Digital or WD) is an American computer drive manufacturer and data storage company, headquartered in San Jose, California. It designs, manufactures and sells data technology produc ...
has registered a patent for an "acoustic vibration decoupler for a
disk drive
Disk storage (also sometimes called drive storage) is a general category of storage mechanisms where data is recorded by various electronic, magnetic, optical, or mechanical changes to a surface layer of one or more rotating disks. A disk drive is ...
pivot
bearing assembly.".
* Preventing stalls: Whenever a machine undergoes an aerodynamic
stall, it will abruptly vibrate.
* Preventing cavitation: When a machine is in contact with a fluid, it may be susceptible to
cavitation
Cavitation is a phenomenon in which the static pressure of a liquid reduces to below the liquid's vapour pressure, leading to the formation of small vapor-filled cavities in the liquid. When subjected to higher pressure, these cavities, cal ...
. The sounds of gas bubbles
imploding is the source of the noise. Ships and
submarine
A submarine (or sub) is a watercraft capable of independent operation underwater. It differs from a submersible, which has more limited underwater capability. The term is also sometimes used historically or colloquially to refer to remotely op ...
s which have
screws
A screw and a bolt (see '' Differentiation between bolt and screw'' below) are similar types of fastener typically made of metal and characterized by a helical ridge, called a ''male thread'' (external thread). Screws and bolts are used to fa ...
that ''cavitate'' are more vulnerable to detection by
sonar
Sonar (sound navigation and ranging or sonic navigation and ranging) is a technique that uses sound propagation (usually underwater, as in submarine navigation) to navigation, navigate, measure distances (ranging), communicate with or detect o ...
.
* Preventing water hammer: In
hydraulics
Hydraulics (from Greek: Υδραυλική) is a technology and applied science using engineering, chemistry, and other sciences involving the mechanical properties and use of liquids. At a very basic level, hydraulics is the liquid counter ...
and
plumbing
Plumbing is any system that conveys fluids for a wide range of applications. Plumbing uses pipes, valves, plumbing fixtures, tanks, and other apparatuses to convey fluids. Heating and cooling (HVAC), waste removal, and potable water delivery ...
,
water hammer
Hydraulic shock (colloquial: water hammer; fluid hammer) is a pressure surge or wave caused when a fluid in motion, usually a liquid but sometimes also a gas is forced to stop or change direction suddenly; a momentum change. This phenomenon com ...
is a known cause for the failure of piping systems. It also generates considerable noise. A
valve
A valve is a device or natural object that regulates, directs or controls the flow of a fluid (gases, liquids, fluidized solids, or slurries) by opening, closing, or partially obstructing various passageways. Valves are technically fittings ...
that abruptly opens or shuts is the most common cause for water hammer.
* Shock absorption: Just as automotive
shock absorbers
A shock absorber or damper is a mechanical or hydraulic device designed to absorb and damp shock impulses. It does this by converting the kinetic energy of the shock into another form of energy (typically heat) which is then dissipated. Most sh ...
are used to prevent mechanical shocks from reaching the passengers in a car, they are also important for quieting shocks.
* Reduction of
resonance
Resonance describes the phenomenon of increased amplitude that occurs when the frequency of an applied periodic force (or a Fourier component of it) is equal or close to a natural frequency of the system on which it acts. When an oscillatin ...
: Essentially any piece of metal or glass has certain frequencies to which it is susceptible to resonate. A machine that resonates would make a tremendous noise. Resonance also occurs in enclosures, such as when echoes reverberate in an
ocarina
The ocarina is a wind musical instrument; it is a type of vessel flute. Variations exist, but a typical ocarina is an enclosed space with four to twelve finger holes and a mouthpiece that projects from the body. It is traditionally made from c ...
or the pipe of a
pipe organ
The pipe organ is a musical instrument that produces sound by driving pressurized air (called ''wind'') through the organ pipes selected from a keyboard. Because each pipe produces a single pitch, the pipes are provided in sets called ''ranks ...
.
* Material selection: By choosing nonmetallic components, the transmission of sound and vibrations can be minimized. For example: instead of using rigid brass fittings, a machine using flexible plastic pipe fittings may be much quieter. In some cases air can be evacuated from a machine and
sealed hermetically, the
vacuum
A vacuum is a space devoid of matter. The word is derived from the Latin adjective ''vacuus'' for "vacant" or "void". An approximation to such vacuum is a region with a gaseous pressure much less than atmospheric pressure. Physicists often dis ...
inside becoming a barrier to sound transmission. In cases where porous plastic materials are used in acoustic applications, the porosity of the plastic is adjusted to either dampen specific wavelengths or for minimal sound loss in a speaker grill cover.
Quieting for specific observers
*
Underwater acoustics
Underwater acoustics is the study of the propagation of sound in water and the interaction of the mechanical waves that constitute sound with the water, its contents and its boundaries. The water may be in the ocean, a lake, a river or a tank. Typ ...
: All of the above types of acoustic quieting apply to submarines. Additionally, a submarine may employ a tactic that prevent sounds from reaching a listener at a particular ocean depth. Operating below the depth of the
sound channel axis
The SOFAR channel (short for sound fixing and ranging channel), or deep sound channel (DSC), is a horizontal layer of water in the ocean at which depth the speed of sound is at its minimum. The SOFAR channel acts as a waveguide for sound, and low ...
, where the
speed of sound
The speed of sound is the distance travelled per unit of time by a sound wave as it propagates through an elastic medium. At , the speed of sound in air is about , or one kilometre in or one mile in . It depends strongly on temperature as w ...
in water is the lowest, a submarine can prevent detection by surface ships, unless these ships use equipment like a
towed array
A towed array sonar is a system of hydrophones towed behind a submarine or a surface ship on a cable. Trailing the hydrophones behind the vessel, on a cable that can be kilometers long, keeps the array's sensors away from the ship's own noise sou ...
and/or an
underwater drone
Unmanned underwater vehicles (UUV), sometimes known as underwater drones, are submersible vehicles that can operate underwater without a human occupant. These vehicles may be divided into two categories: remotely operated underwater vehicles (RO ...
to place hydrophones below the sound channel axis.
*
Sound refraction: Just as a submarine can use refraction to hide its
acoustic signature
The term acoustic signature is used to describe a combination of acoustic emissions of sound emitters, such as those of ships and submarines. In addition, aircraft, machinery, and living animals can be described as having their own characteristic ...
from surface vessels, the same principle of sound
refraction
In physics, refraction is the redirection of a wave as it passes from one medium to another. The redirection can be caused by the wave's change in speed or by a change in the medium. Refraction of light is the most commonly observed phenomeno ...
can be used to prevent certain observers from hearing the noise. For example, an outdoor observer close to the ground will have sound waves refracted toward him when the ground is ''cooler'' than the ambient air and away from him when the ground is ''hotter'' than the air.
* Sound redirection: One of the obvious ways to reduce the received sound level of an observer is to place the observer out of the path of the highest amplitude sounds. For example, if we mark off a circle around a
jet engine
A jet engine is a type of reaction engine discharging a fast-moving jet of heated gas (usually air) that generates thrust by jet propulsion. While this broad definition can include rocket, Pump-jet, water jet, and hybrid propulsion, the term ...
and make
sound power level
Sound power or acoustic power is the rate at which sound energy is emitted, reflected, transmitted or received, per unit time. It is defined as "through a surface, the product of the sound pressure, and the component of the particle velocity, at ...
observations along that circle, we would expect that the sound is loudest directly in line with the jet's exhaust. Observations perpendicular to the exhaust would be significantly quieter.
*
Hearing protection
A hearing protection device, also known as a HPD, is an ear protection device worn in or over the ears while exposed to hazardous noise and provide hearing protection to help prevent noise-induced hearing loss. HPDs reduce the level of the no ...
: An observer may be forced to wear
ear plugs
An earplug is a device that is inserted in the ear canal to protect the user's ears from loud noises, intrusion of water, foreign bodies, dust or excessive wind. Since they reduce the sound volume, earplugs are often used to help prevent hearing ...
in areas of high
ambient noise level
In atmospheric sounding and noise pollution, ambient noise level (sometimes called background noise level, reference sound level, or room noise level) is the background sound pressure level at a given location, normally specified as a reference l ...
s. This may be the only quieting method available in areas of
noise pollution
Noise pollution, also known as environmental noise or sound pollution, is the propagation of noise with ranging impacts on the activity of human or animal life, most of them are harmful to a degree. The source of outdoor noise worldwide is main ...
, such as an open-air
firing range
A shooting range, firing range, gun range or shooting ground is a specialized facility, venue or field designed specifically for firearm usage qualifications, training, practice or competitions. Some shooting ranges are operated by military ...
or an
airport
An airport is an aerodrome with extended facilities, mostly for commercial air transport. Airports usually consists of a landing area, which comprises an aerially accessible open space including at least one operationally active surface ...
.
Electronic quieting
*
Electronic vibration control: Electronics, sensors, and computers are now employed to reduce vibration. Using high speed logic, vibrations can be damped quickly and effectively by counteracting the motion before it exceeds a certain threshold.
*
Electronic noise control: Electronics, sensors, and computers are also employed to cancel noise by using
phase cancellation
In physics, interference is a phenomenon in which two waves combine by adding their displacement together at every single point in space and time, to form a resultant wave of greater, lower, or the same amplitude. Constructive and destructive ...
which matches the sound
amplitude
The amplitude of a periodic variable is a measure of its change in a single period (such as time or spatial period). The amplitude of a non-periodic signal is its magnitude compared with a reference value. There are various definitions of amplit ...
with a wave of the opposite
polarity
Polarity may refer to:
Science
*Electrical polarity, direction of electrical current
*Polarity (mutual inductance), the relationship between components such as transformer windings
* Polarity (projective geometry), in mathematics, a duality of ord ...
. This method employs the use of an active sound generating device, such as a
loudspeaker
A loudspeaker (commonly referred to as a speaker or speaker driver) is an electroacoustic transducer that converts an electrical audio signal into a corresponding sound. A ''speaker system'', also often simply referred to as a "speaker" or " ...
to counteract ambient noise in an area. See
noise-canceling headphone. Workers in noisy environments may favor this method over ear plugs.
*
Noise reduction
Noise reduction is the process of removing noise from a signal. Noise reduction techniques exist for audio and images. Noise reduction algorithms may distort the signal to some degree. Noise rejection is the ability of a circuit to isolate an und ...
: In sound and video equipment, noise reduction is the process of removing noise from a signal. This is strictly for
electronic noise
In electronics, noise is an unwanted disturbance in an electrical signal.
Noise generated by electronic devices varies greatly as it is produced by several different effects.
In particular, noise is inherent in physics, and central to the ...
or noise which has been detected and put into electronic form.
*
Noise canceling
Active noise control (ANC), also known as noise cancellation (NC), or active noise reduction (ANR), is a method for reducing unwanted sound by the addition of a second sound specifically designed to cancel the first. The concept was first develop ...
: If both the noise and the signal are received by an electronic or digital medium, noise can be filtered from the signal electronically and retransmitted without the noise. See
noise-canceling microphone
A noise-canceling microphone is a microphone that is designed to filter ambient noise.
Technical details
The development is a special case of the differential microphone topology most commonly used to achieve directionality. All such microphones ...
.
Helicopter
A helicopter is a type of rotorcraft in which lift and thrust are supplied by horizontally spinning rotors. This allows the helicopter to take off and land vertically, to hover, and to fly forward, backward and laterally. These attributes ...
pilots rely on this technology to speak on the radio.
See also
*
Acoustic signature
The term acoustic signature is used to describe a combination of acoustic emissions of sound emitters, such as those of ships and submarines. In addition, aircraft, machinery, and living animals can be described as having their own characteristic ...
*
Noise reduction
Noise reduction is the process of removing noise from a signal. Noise reduction techniques exist for audio and images. Noise reduction algorithms may distort the signal to some degree. Noise rejection is the ability of a circuit to isolate an und ...
, for
electronic noise
In electronics, noise is an unwanted disturbance in an electrical signal.
Noise generated by electronic devices varies greatly as it is produced by several different effects.
In particular, noise is inherent in physics, and central to the ...
*
Sound masking
Sound masking is the inclusion of generated sound (commonly, though inaccurately, referred to as "white noise" or "pink noise") into an environment to mask unwanted sound. It relies on auditory masking. Sound masking is not a form of active noise ...
, for noise masking by saturation
*
Pink noise
Pink noise or noise is a signal or process with a frequency spectrum such that the power spectral density (power per frequency interval) is inversely proportional to the frequency of the signal. In pink noise, each octave interval (halving ...
*
Stealth technology
Stealth technology, also termed low observable technology (LO technology), is a sub-discipline of military tactics and passive and active electronic countermeasures, which covers a range of methods used to make personnel, aircraft, ships, subm ...
, for signature reduction in general
*
Longitudinal wave
Longitudinal waves are waves in which the vibration of the medium is parallel ("along") to the direction the wave travels and displacement of the medium is in the same (or opposite) direction of the wave propagation. Mechanical longitudinal waves ...
*
Soundproofing
Soundproofing is any means of impeding sound propagation. There are several basic approaches to reducing sound: increasing the distance between source and receiver, decoupling, using noise barriers to reflect or absorb the energy of the sound wav ...
*
Mechanical resonance
Mechanical resonance is the tendency of a mechanical system to respond at greater amplitude when the frequency of its oscillations matches the system's natural frequency of vibration (its ''resonance frequency'' or ''resonant frequency'') close ...
*
Sound masking
Sound masking is the inclusion of generated sound (commonly, though inaccurately, referred to as "white noise" or "pink noise") into an environment to mask unwanted sound. It relies on auditory masking. Sound masking is not a form of active noise ...
*
Seismic retrofit
Seismic retrofitting is the modification of existing structures to make them more resistant to seismic activity, ground motion, or soil failure due to earthquakes. With better understanding of seismic demand on structures and with our recent exp ...
*
Helicopter noise reduction
Helicopter noise reduction is a topic of research into designing helicopters which can be operated more quietly, reducing the public-relations problems with night-flying or expanding an airport. In addition, it is useful for military applicat ...
*
Muffler
A muffler (North American and Australian English) or silencer (British English) is a device for reducing the noise emitted by the exhaust of an internal combustion engine—especially a noise-deadening device forming part of the exhaust sys ...
*
Deperming
Degaussing is the process of decreasing or eliminating a remnant magnetic field. It is named after the gauss, a unit of magnetism, which in turn was named after Carl Friedrich Gauss. Due to magnetic hysteresis, it is generally not possible to redu ...
*
Degaussing
Degaussing is the process of decreasing or eliminating a remnant magnetic field. It is named after the gauss, a unit of magnetism, which in turn was named after Carl Friedrich Gauss. Due to magnetic hysteresis, it is generally not possible to redu ...
References
{{DEFAULTSORT:Acoustic Quieting
Acoustics
Noise control
Stealth technology