Acoustic quieting

Acoustic quieting is the process of making machinery quieter by damping

One of the major reasons for the development of acoustic quieting techniques was for making

• 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.

• Noise absorption: In
  acoustical tiles in modern office buildings with high ceilings. Submarine hulls have special coatings
  that absorb sound.
• Acoustic damping:
  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:
  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

• sound channel axis, where the speed of sound in water is the lowest, a submarine can prevent detection by surface ships, unless these ships use equipment like a towed array and/or an underwater drone
  to place hydrophones below the sound channel axis.
• 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
  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.
• firing range or an airport
  .