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 a 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 the 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.

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

Passive Tuned Acoustic Absorbers

Helmholtz Resonators
Quarter Wave Tubes
Perforated Acoustic Liners
Passive Acoustic Radiators

Helmholtz Resonators

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.

Quarter Wave Tubes

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.

Perforated Acoustic Liners

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.

Passive Acoustic Radiators

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 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.

Flame Instability (Combustion-driven Oscillation) Mitigation

Active Tuned Acoustic Absorbers

Active Tuned Acoustic Absorbers have higher power density than passive tuned devices and can be re-tuned readily and abate a narrow-band noise with time-varying frequency.

Contrary to passive tuned acoustic absorbers that use materials and the acoustics associated with a certain geometry to absorb narrowband sound, active tuned absorbers use electro-acoustical sound field modification to abate unwanted sound. The make up of an active system consists of control algorithm, sensors (microphones and accelerometers), actuators (loudspeakers and/or vibration shakers). In a typical active noise control system, sensors are used to detect unwanted sound and speakers under the command of the control algorithm are used to create a sound field capable of abating the unwanted tonal noise.

DEICON’s active acoustic damper/absorber which is highly effective in abating undesirable low-frequency tonal noise, is an example of such active systems. Applications of this acoustic treatment include, but not limited to, dampening a resonating standing wave (acoustic mode) in a small room (such as a recording studio or a cabin in a yacht) or mitigating acoustic instability in an industrial combustor (boiler, heater, gas turbine, etc.).

The controller which is realized by either a tunable, op-amp circuit or a high-speed computer (digital signal processor) can be tuned to:

one or multiple, low-frequency resonant mode(s) of an enclosed acoustic environment to add damping to that (those) modes and thus abate the boominess and rumble of sound at that (those) resonant frequencies, or
the frequency(ies) corresponding to a driving perturbation (such as the blade passage frequency of a fan) and absorb sound at that (those) perturbation frequencies.

The actuation is provided by a loudspeaker and the sensory information is supplied by either a microphone or a dynamic pressure sensor.

The size of an active low-frequency tuned acoustic damper/absorber is similar to that of a loudspeaker which is smaller and lighter than comparable passive low-frequency dampers/absorbers targeting the same low frequencies.

The application of this technology in adding modal damping to small rooms (listening rooms, recording studios, home theaters) is licensed to Modular Sound (the reputable manufacturer of Bag End professional grade speakers) which makes active bass traps, based on this technology.

Industrial applications of this technology, due to their uniqueness and need for customization, are implemented by DEICON.

Learn About Tuned Mass Dampers

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