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Principles of Noise Control
Published in Junbo Jia, Jeom Kee Paik, Engineering Dynamics and Vibrations, 2018
Unlike the expansion chamber filter, the sider branch resonator type silencer acts more like a band pass filter. It can provide quite effective attenuation around the resonant frequency as shown in Fig. 10.28. However, above and below this frequency the amount of attenuation drops off. One can tune the Helmholtz resonator to have broad or sharper response. As shown in Fig. 10.28, as the factor (V/Sb)* l can be treated as an approximate damping factor. As this term gets larger the response is broader. One trades off a larger attenuation at the resonant frequency for a broader frequency range of attenuation. Note that to increase this factor decreasing Sb or increasing l are the common approaches. It can also be seen intuitively that, if there is a column of air moving back and forth in a narrower or longer tube between the volume and the main pipe, there would be greater losses. Typically, increasing volume is not a good solution since the space required becomes a problem.
Architectural Acoustics
Published in Malcolm J. Crocker, A. John Price, Noise and Noise Control, 2018
Malcolm J. Crocker, A. John Price
A Helmholtz resonator, in its simplest form, consists of an acoustic cavity contained by rigid walls and connected to the outside by a small opening called the neck as shown in Figure 5.12. Incident sound causes air molecules to vibrate back and forth in the neck section of the resonator like a vibrating mass while the air in the cavity behaves like a spring. As shown in the previous section, such an acoustic mass spring system has a particular frequency at which it becomes resonant. At this frequency, energy losses in the system due to frictional and viscous forces acting on the air molecules in and close to the neck become maximum, and so the absorption characteristics also peak at that frequency. Usually there will be only a very small amount of damping in the system, and hence the resonance peak is usually very sharp and narrow, falling off very quickly on each side of the resonance frequency. This is observed in practice for if we blow across the top of the neck of a bottle we hear a pure tone developed rather than a broad resonance.
Sound transmission properties of a porous meta-material with periodically embedded Helmholtz resonators
Published in Mechanics of Advanced Materials and Structures, 2023
Dario Magliacano, Giuseppe Catapane, Giuseppe Petrone, Kevin Verdière, Olivier Robin
A Helmholtz resonator (HR) is a tunable device whose geometry is usually represented by a neck followed by a cavity, its structure being made of walls considered rigid and filled with fluid. A HR is usually assimilated to a mass-spring system, in which the air volume acts as a spring, and the fluid in the neck represents the mass. As a one degree-of-freedom system, it exhibits a single resonance frequency defined as: where is the sound speed in air, is the area of the section of the neck, is the volume of the cavity and is the effective length of the neck out of the section plane. The equations reported in this sub-section are theoretically valid for a Helmholtz resonator with perfectly rigid walls. In this context, it is assumed that they are still valid in case of walls made of a real material from empirical considerations made in some of our previous works [48], as long as there is a strong impedance mismatch between the cavity fluid and the wall material itself.
Control of a pulse combustion reactor with thermoacoustic phenomena
Published in Instrumentation Science & Technology, 2018
Gregor Križan, Janez Križan, Ivan Bajsić, Miran Gaberšček
The frequency and amplitude of pressure in the reactor were controlled using an adjustable Helmholtz resonator. The latter had a moveable piston which was used for adjustment of the volume of the resonator and hence its natural frequency. A higher amplitude means a higher efficiency of fuel combustion, which is desirable to a certain degree, because it lowers the oxygen levels in the hot reactor zone but, at the same time, also lowers the temperature in the reactor. The Helmholtz resonator reflects the acoustic oscillations back at the source (the combustor); if the reflected waves are in phase with the incoming waves it raises the amplitude whereas if the reflected waves are out of phase the amplitude is lowered. This can also be explained with Rayleigh’s theory;[29] if energy is added in phase with the highest energy release, the oscillations are encouraged, whereas if it is added out of phase or removed in phase, the oscillations are discouraged. By lowering the amplitude of one frequency, another one is attenuated.
Active Control of Combustion Noise by a Twin Resonator Trim Adjustment System
Published in Combustion Science and Technology, 2022
Varghese M. Thannickal, T. John Tharakan, Satyanarayanan R. Chakravarthy
Variable geometry Helmholtz resonators have been widely used for the active control of acoustic noise (De Bedout et al. 1997; Estève and Johnson 2005; Liu et al. 2007; Nagaya, Hano, Suda 2001). The characteristic frequency of the resonator is tuned to the frequency of the pressure perturbations by varying the dimensions. This makes it possible to precisely achieve optimal damping over a range of operating conditions. The same principle has been used for the trim adjustment active control of thermoacoustic instability. While varying the neck diameter has been investigated (Zhao and Morgans 2009), resonators with variable cavity volume (Thannickal, Tharakan, Chakravarthy 2019; Zhang et al. 2015; Zhao and Li 2012) are easier to realize in practice.