Acoustic quieting

This article primarily discusses mechanical and acoustic noise. See Noise reduction for electronic noise.
For noise masking by saturation, see Sound masking or pink noise.
For signature reduction in general see Stealth technology.

Acoustic quieting is the process of making machinery quieter by damping vibrations to prevent them from reaching the observer. Machinery vibrates, causing sound waves in air, hydroacoustic waves in water, and mechanical stresses 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 difficult to detect by sonar. This military goal of the mid- and late-twentieth century allowed the technology to be adapted to many industries and products, such as computers (e.g. hard drive technology), automobiles (e.g. motor mounts), and even sporting goods (e.g. golf clubs[1]).

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 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 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) in walls to deaden the transmission of noise through them.
Anechoic chamber
A sound proof room, showing acoustic damping tiles used for noise absorption and soundproofing.
  • Noise absorption: In architectural acoustics, 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, sound proofing is accomplished with bass traps and anechoic chambers. Wallace Sabine, an American physicist, is credited with studying sound reverberations in 1900, and Carl Eyring revised his equations in 1930[2] for Bell Labs. Another example is the ubiquitous use of dropped ceilings and acoustical tiles in modern office buildings with high ceilings. Submarine hulls have special coatings that absorb sound.
  • Acoustic damping: Vibration isolation prevents vibration from transferring beyond the device into another material. Damping mounts have progressed in the industry to offer vibrational resistance in many degrees of freedom. Recent advances include shock isolators damping in at least six degrees of freedom.[3] Acoustic damping also has uses in seismic shock 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 has registered a patent for an "acoustically decoupled underbody for a motor vehicle.".[4] Another example: Western Digital has registered a patent for an "acoustic vibration decoupler for a disk drive pivot bearing assembly.".[5]
  • 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. The sounds of gas bubbles imploding is the source of the noise. Ships and submarines which have screws that cavitate are more vulnerable to detection by sonar.
  • Preventing water hammer: In hydraulics and plumbing, water hammer is a known cause for the failure of piping systems. It also generates considerable noise. A valve that abruptly opens or shuts is the most common cause for water hammer.
  • Shock absorption: Just as automotive shock absorbers are used to prevent mechanical shocks from reaching the passengers in a car, they are also important for quieting shocks.
  • Reduction of resonance: 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 or the pipe of a pipe organ.
  • 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 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.[6]

Quieting for specific observers

  • Underwater acoustics: 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, where the speed of sound in water is the lowest, a submarine can prevent detection by surface ships.
  • Sound refraction: Just as a submarine can use refraction to hide its acoustic signature from surface vessels, the same principle of sound refraction 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 and make sound power level 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: An observer may be forced to wear ear plugs in areas of high ambient noise levels. This may be the only quieting method available in areas of noise pollution, such as an open-air firing range or an airport.

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 which matches the sound amplitude with a wave of the opposite polarity. This method employs the use of an active sound generating device, such as a loudspeaker to counteract ambient noise in an area. See noise-canceling headphone. Workers in noisy environments may favor this method over ear plugs.
  • Noise reduction: In sound and video equipment, noise reduction is the process of removing noise from a signal. This is strictly for electronic noise or noise which has been detected and put into electronic form.
  • Noise canceling: 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. Helicopter pilots rely on this technology to speak on the radio.

See also


  1. ^ U.S. Patent 5,692,968
  2. ^ "Dead Rooms and Live Wires: Harvard, Hollywood, and the Deconstruction of Architectural Acoustics, 1900-1930," Emily Thompson, Isis, Vol. 88, No. 4 (Dec., 1997), pp. 597-626. JSTOR 237829.
  3. ^ U.S. Patent 6,386,134
  4. ^ U.S. Patent 5,090,774
  5. ^ U.S. Patent 5,675,456
  6. ^ "Porous Acoustic Technology & Porous Acoustic Materials". Retrieved 2017-03-24.
Acoustic homing

Acoustic homing is a system which uses the acoustic signature (sound) of a target to guide a moving object, such as a torpedo. Acoustic homing can be either Passive or Active in nature.

Using Passive, the system is designed to move either toward or away from a sound, and may also be designed to move only toward certain types of sounds to the exclusion of others, while Active is a true Sonar. The system emits a sound pulse that reflects off objects and then back to the system, where the system processes the echos to determine the proper response.

Acoustic signature

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 acoustic signatures or sound attributes, which can be used to study their condition, behavior, and physical location.

Ballistic missile submarine

A ballistic missile submarine is a submarine capable of deploying submarine-launched ballistic missiles (SLBMs) with nuclear warheads. The United States Navy's hull classification symbols for ballistic missile submarines are SSB and SSBN – the SS denotes submarine (or submersible ship), the B denotes ballistic missile, and the N denotes that the submarine is nuclear powered. These submarines became a major weapon system in the Cold War because of their nuclear deterrence capability. They can fire missiles thousands of kilometers from their targets, and acoustic quieting makes them difficult to detect (see acoustic signature), thus making them a survivable deterrent in the event of a first strike and a key element of the mutual assured destruction policy of nuclear deterrence. Their deployment has been dominated by the United States and the Soviet Union / Russia, with smaller numbers in service with France, the United Kingdom, China, and India.

Block-matching and 3D filtering

Block-matching and 3D filtering (BM3D) is a 3-D block-matching algorithm used primarily for noise reduction in images.


Bruitparif is a non-profit environmental organization responsible for monitoring the environmental noise in the Paris agglomeration. It was founded in 2004.


Hearing, or auditory perception, is the ability to perceive sounds by detecting vibrations, changes in the pressure of the surrounding medium through time, through an organ such as the ear. The academic field concerned with hearing is auditory science.

Sound may be heard through solid, liquid, or gaseous matter. It is one of the traditional five senses; partial or total inability to hear is called hearing loss.

In humans and other vertebrates, hearing is performed primarily by the auditory system: mechanical waves, known as vibrations are detected by the ear and transduced into nerve impulses that are perceived by the brain (primarily in the temporal lobe). Like touch, audition requires sensitivity to the movement of molecules in the world outside the organism. Both hearing and touch are types of mechanosensation.

INS Kamorta (P28)

INS Kamorta is the first of four anti-submarine Kamorta-class stealth corvettes which has been built for the Indian Navy. It is a significant step towards India's pursuit for self-reliance in indigenous warship building, bringing closer home Indian Navy's quest to be a true Blue-Water Navy with ships and submarines designed and built within the country. She was designed and manufactured by GRSE, launched on 19 April 2010, as part of Project 28, approved in 2003. As in INS Shivalik, high-grade steel produced in India was utilized for its construction. She was delivered to Navy on 12 July 2014. She is the first indigenous anti-submarine corvette as well as the first indigenous stealth corvette built by India. She has enhanced stealth features such as an X Form Hull and inclined sides for low Radar cross-section, Infra-red suppression, and Acoustic quieting systems. Union minister of defense, Arun Jaitley commissioned the ship on 23 August 2014. It was named after Kamorta island in Andaman and Nicobar, India .

Kölner Philharmonie

The Kölner Philharmonie is a symphonic concert hall located in Cologne, Germany. It is part of the building assemble of the Museum Ludwig and was opened in 1986. The Kölner Philharmonie is located close to the Cologne Cathedral and the Cologne Main Station. The ensemble was designed by the architects Busmann + Haberer in the 1980s.

Median filter

The Median Filter is a non-linear digital filtering technique, often used to remove noise from an image or signal. Such noise reduction is a typical pre-processing step to improve the results of later processing (for example, edge detection on an image). Median filtering is very widely used in digital image processing because, under certain conditions, it preserves edges while removing noise (but see discussion below), also having applications in signal processing.


A muffler (silencer in British English) is a device for reducing the noise emitted by the exhaust of an internal combustion engine.

Noise, vibration, and harshness

Noise, vibration, and harshness (NVH), also known as noise and vibration (N&V), is the study and modification of the noise and vibration characteristics of vehicles, particularly cars and trucks. While noise and vibration can be readily measured, harshness is a subjective quality, and is measured either via "jury" evaluations, or with analytical tools that can provide results reflecting human subjective impressions. These latter tools belong to the field known as "psychoacoustics."

Interior NVH deals with noise and vibration experienced by the occupants of the cabin, while exterior NVH is largely concerned with the noise radiated by the vehicle, and includes drive-by noise testing.

NVH is mostly engineering, but often objective measurements fail to predict or correlate well with the subjective impression on human observers. For example, although the ear's response at moderate noise levels is approximated by A-weighting, two different noises with the same A-weighted level are not necessarily equally disturbing. The field of psychoacoustics is partly concerned with this correlation.

In some cases the NVH engineer is asked to change the sound quality, by adding or subtracting particular harmonics, rather than making the vehicle quieter.

Noise-canceling microphone

A noise-canceling microphone is a microphone that is designed to filter ambient noise

Noise and vibration on maritime vessels

Noise and vibration on maritime vessels are not the same but they have the same origin and come in many forms. The methods to handle the related problems are similar, to a certain level, where most shipboard noise problems are reduced by controlling vibration.

Non-local means

Non-local means is an algorithm in image processing for image denoising. Unlike "local mean" filters, which take the mean value of a group of pixels surrounding a target pixel to smooth the image, non-local means filtering takes a mean of all pixels in the image, weighted by how similar these pixels are to the target pixel. This results in much greater post-filtering clarity, and less loss of detail in the image compared with local mean algorithms.If compared with other well-known denoising techniques, non-local means adds "method noise" (i.e. error in the denoising process) which looks more like white noise, which is desirable because it is typically less disturbing in the denoised product. Recently non-local means has been extended to other image processing applications such as deinterlacing, view interpolation, and depth maps regularization .

Occupational noise

Occupational noise is the amount of acoustic energy received by an employee's auditory system when they are working in the industry. Occupational noise, or industrial noise, is often a term used in occupational safety and health, as sustained exposure can cause permanent hearing damage.

"Twenty-two million workers are exposed to potentially damaging noise at work each year. Last year, U.S. business paid more than $1.5 million in penalties for not protecting workers from noise." - OSHAOccupational noise is considered an occupational hazard traditionally linked to loud industries such as ship-building, mining, railroad work, welding, and construction, but can be present in any workplace where hazardous noise is present.

Shrinkage Fields (image restoration)

Shrinkage fields is a random field-based machine learning technique that aims to perform high quality image restoration (denoising and deblurring) using low computational overhead.

Silent running (submarine)

Silent running is a stealth mode of operation for naval submarines. The aim is to evade discovery by passive sonar by eliminating superfluous noise: nonessential systems are shut down, the crew is urged to rest and refrain from making any unnecessary sound, and speed is greatly reduced to minimize propeller noise.

The propellers have a characteristic RPM band in which no cavitation noise arises. Since this rotation speed is usually relatively low, the first electric submarines had special "silent running" engines designed for optimum performance at reduced speed. These required less active cooling (further reducing noise), and were generally equipped with plain bearings rather than ball bearings. These engines were also acoustically decoupled from the hull, as they employed belt transmission rather than direct coupling to the propeller shaft.

Nuclear submarines can run even more quietly, at very low speeds only, by turning off active reactor cooling during silent running. The reactor is then only cooled by natural convection of the water.


Soundproofing is any means of reducing the sound pressure with respect to a specified sound source and receptor. There are several basic approaches to reducing sound: increasing the distance between source and receiver, using noise barriers to reflect or absorb the energy of the sound waves, using damping structures such as sound baffles, or using active antinoise sound generators.

Two distinct soundproofing problems may need to be considered when designing acoustic treatments - to improve the sound within a room (see reverberation), and reduce sound leakage to/from adjacent rooms or outdoors (see sound transmission class and sound reduction index). Acoustic quieting and noise control can be used to limit unwanted noise. Soundproofing can suppress unwanted indirect sound waves such as reflections that cause echoes and resonances that cause reverberation. Soundproofing can reduce the transmission of unwanted direct sound waves from the source to an involuntary listener through the use of distance and intervening objects in the sound path.

Stealth ship

A stealth ship is a ship which employs stealth technology construction techniques in an effort to ensure that it is harder to detect by one or more of radar, visual, sonar, and infrared methods.

These techniques borrow from stealth aircraft technology, although some aspects such as wake and acoustic signature reduction (Acoustic quieting) are unique to stealth ships' design. Though radar cross-section (RCS) reduction is a fairly new concept many other forms of masking a ship have existed for centuries or even millennia.

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