- Acoustic quieting
:"This article primarily discusses mechanical and acoustic noise. See
Noise reductionfor electronic noise.":"For noise masking by saturation, see Sound maskingor pink noise.Acoustic quieting is the process of making machinery quieter by damping vibrationsto prevent them from reaching the observer. Machinery vibrates, causing sound wavesin 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
submarinesdifficult 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 drivetechnology), automobiles(e.g. motor mounts), and even sporting goods (e.g. golf clubs[ [http://www.google.com/patents?hl=en&lr=&vid=USPAT5692968&dq=acoustic+damping U.S. Patent 5,692,968] ] ).
Aspects of Acoustic Quieting
When the goal is acoustic quietening, 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 scientist can provide many ways to quieten the machine. The challenge is to do this in a practical and inexpensive way. The scientist 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.
* 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 proofingis accomplished with bass traps and anechoic chambers. Wallace Sabine, an American physicist, is credited with studying sound reverberations in 1900, and Carl Eyringrevised his equations in 1930 ["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. Retrieved from [http://links.jstor.org/sici?sici=0021-1753%28199712%2988%3A4%3C597%3ADRALWH%3E2.0.CO%3B2-D&size=LARGE&origin=JSTOR-enlargePage#abstract] in November 2007.] for Bell Labs. Another example is the ubiquitoususe of dropped ceilings and acoustical tilesin modern office buildings with high ceilings. Submarine hulls have special coatings that absorb sound.
* Acoustic damping: Isolating vibrations to prevent them 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. [ [http://www.patentstorm.us/patents/6386134-fulltext.html U.S. Patent 6,386,134] ] Acoustic damping also has uses in seismic shockprotection 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:
Volkswagenhas registered a patentfor an "acoustically decoupled underbody for a motor vehicle." [ [http://www.freepatentsonline.com/5090774.html?highlight=decoupl,acoust&stemming=on U.S. Patent 5,090,774] ] . Another example: Western Digitalhas registered a patent for an "acoustic vibration decoupler for a disk drivepivot bearingassembly." [ [http://www.freepatentsonline.com/5675456.html?highlight=decoupl,acoust&stemming=on U.S. Patent 5,675,456] ] .
* 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
hydraulicsand plumbing, water hammeris a known cause for the failure of piping systems. It also generates considerable noise. A valvethat abruptly opens or shuts is the most common cause for water hammer.
* Shock absorption: Just as automotive
shock absorbersare 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 like the infamous Galloping Gertie(the first Tacoma Narrows Bridge) would make a tremendous noise. Resonance also occurs in enclosures, such as when echoes reverberate in an ocarinaor 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
vacuuminside becoming a barrier to sound transmission.
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 soundin 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 signaturefrom surface vessels, the same principle of sound refractioncan 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 engineand make sound power levelobservations 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 plugsin 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 rangeor an airport.
* 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 cancellationwhich matches the sound amplitudewith a wave of the opposite polarity. This method employs the use of an active sound generating device, such as a loudspeakerto 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 noiseor 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. Helicopterpilots rely on this technology to speak on the radio.
Helicopter noise reduction
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