Tuned Acoustic Absorbers


Tuned Acoustic Absorbers

Helmholtz Resonators, Quarter Wave Tubes, Perforated Acoustic Liners, and Passive Acoustic Radiators are the tuned acoustic absorbers (band-reject acoustic filters) commonly used in mitigating narrow-band noise. Examples of such mitigation applications include, but are not limited to, quieting the tonal noise at the blade passage frequency of an axial fan and dampening a standing wave of an enclosure (or a duct).

Optimal design of tuned acoustic absorbers for maximum absorption requires thorough understanding of the effects of geometry variations on their performance. DEICON has developed a finite element analysis (FEA) based tool for optimally designing tuned absorbers. In addition to synthesis of such absorbers, this in-house numerical tool allows for analysis/evaluation of tuned absorption mechanisms in any acoustic environment. The FEA based design tool uses
acoustic impedance
boundary conditions to account for damping realized by the acoustic treatment.
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In low-frequency applications the size of passive tuned acoustic absorbers becomes too large. In such applications semi-active and active tuned acoustic absorbers/dampers can be used, instead. In addition to having higher power density than passive devices, active tuned acoustic absorbers can also be re-tuned readily and abate a narrow-band noise with time varying frequency.

Helmholtz Resonators

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Helmholtz resonators are used in many industrial and architectural applications including exhaust and induction systems of internal combustion engines. They are also used as passive bass traps in architectural acoustics to dampen undesirable low frequency standing waves. In addition, Helmholtz resonators are the building block of acoustic liners used for reducing the noise of aircraft engines.

Helmholtz

Helmholtz Resonator, a band-reject acoustic filter (PDF)

The make up of a Helmholtz resonator consists of a rigid cavity of volume V, communicating with the acoustic environment where the noise resides through a port (neck) of length L . The fluid in the neck is assumed to move as a unit resembling a mechanical ‘mass’ element. The pressure in the cavity changes by the influx and efflux of the fluid through the neck, acting as a ‘stiffness’ element (spring). The schematic of a typical Helmholtz resonator application is shown below.

Quarter Wave Tubes

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Quarter wave tubes are commonly used in applications such as air intake induction system on engines, pump pulsation abatement, and other narrow band noise mitigation applications. The length of a quarter wave tube is a quarter of a wavelength of the noise it is tuned to. The acoustic wave travels down the quarter wave tube and back, travelling half the wavelength which in turn experiencing 180 degree phase shift interfering with the incoming acoustic wave, destructively, abating the target noise. A 40 inch long quarter wave tube, built around a 10 inch diameter pipe, is shown below.

quarter wave tube quarter wave tube 2

Placement of Adjacent Tuned Acoustic Absorbers (PDF)

Quarter wave tube is a tube open at the end that connects with the acoustic environment where the objectionable noise is generated and closed at the other end. The frequency at which the quarter wave tube attenuates is controlled by the length of the quarter wave tube, L . The schematic of a typical quarter wave tube application is shown below.

Perforated Acoustic Liners

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Perforated acoustic liners are used in industrial mufflers, nacelle of an aircraft engine, and architectural applications for sound absorption purposes. In high temperature applications, a small amount of cooling gas (e.g., air), known as ‘bias flow’, is flown thru through the perforations to protect the liner. In addition to cooling the liner this bias flow improves the effectiveness of the liner as an absorber of sound. A perforated liner built into a transition duct piece is shown below.

acoustic liner perforated acoustic liner

A perforated liner is made up of a solid backing and a perforated sheet arranged over and spaced from the backing. The perforated sheet faces the acoustic environment where the objectionable noise is generated. The image below shows an acoustic liner placed at the bottom of an enclosure. An perforated liner, can be viewed as a collection of Helmholtz resonators spread over an area, and consequently effective over a relatively large region.

Passive Acoustic Radiators

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Helmholtz resonators, quarter-wave tubes, and perforated liners become too large and too heavy to be used for suppression of low frequency narrowband noise. On the other hand, a passive acoustic radiator can be tuned to low frequencies with no or very small weight/size penalty while maintaining high levels of absorption effectiveness. Note that the passive radiator becomes part of the structure and most of its added mass will be made up for by the mass removed from the structure to accommodate the radiator.

passive radiator

An acoustic radiator is a vibrating surface that produces acoustic waves, in response to the pressure pulsation it experiences. The pistonic motion of the passive radiator surface results in acoustic radiation. At low frequencies, a passive radiator can be viewed as a 2nd order mechanical system. The resilience (stiffness) is realized mainly by the stiffness of the suspension materials. The image below shows 3 passive radiators installed on a cylindrical enclosures.

Active Tuned Acoustic Absorbers

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Contrary to passive tuned acoustic absorbers that use materials and the acoustics associated with a certain geometry to absorb sound active tuned absorbers use electro-acoustical sound field modification to abate unwanted sound. Active systems include controllers, sensors (microphones and accelerometers), actuators (including loudspeakers), and other electro-acoustical devices for mitigating narrowband noise. In a typical active tuned absorption system, sensors are used to detect unwanted sound and a controller and speakers are used to create a sound field capable of abating the unwanted noise.

Tuned Acoustic Absorbers RESOURCE LIBRARY SEARCH BY YOUR ISSUE

DEICON designs and fabricates tuned acoustic absorbers, of various kinds, for different narrowband sound absorption applications.

In addition to analytical and semi-empirical methods for evaluating the surface impedance of acoustic absorbers, this quantity can be measured experimentally.

Acoustic Impedance of Perforated Liners (PDF)
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