TUNED ACOUSTIC ABSORBERS

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 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 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.

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 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.