Contents • • • • • • • • Introduction [ ] Many engineering systems create unwanted acoustic noise. Noise may be reduced using engineering noise control methods. One noise control method popular in mufflers is the Helmholtz resonator, see. It is comprised of a cavity connected to the system of interest through one or several short narrow tubes.
Download Boney M Greatest Hits Rar Software. Engineering Acoustics/Noise control with self-tuning Helmholtz. One noise control method popular in mufflers is the Helmholtz resonator. Download as PDF. Becca Stevens Weightless.
The classical examples are in automobile exhaust systems. By adding a tuned Helmholtz resonator, sound is reflected back to the source.
Helmholtz resonators have been exploited to enhance or attenuate sound fields at least since ancient Greek times where they were used in ancient amphitheaters to reduce reverberation. Since this time, Helmholtz resonators have found widespread use in reverberant spaces such as churches and as mufflers in ducts and pipes. The Helmholtz resonator effect underlies the phenomena of sunroof buffeting seen.
One advantage of the Helmholtz resonator is its simplicity. However, the frequency range over which Helmholtz resonators are effective is relatively narrow. Consequently these devices need to be precisely tuned to the noise source to achieve significant attenuation. Noise and vibration control [ ] There are four general categories for noise and vibration control: • Active systems: load or unload the unwanted noise by using actuators such as loudspeakers and • Passive systems: achieve sound attenuation by using 2.1.
Reactive devices such as Helmholtz resonators and expansion chambers. Resistive materials such as acoustic linings and porous membranes • Hybrid systems: use both active and passive elements to achieve sound reduction • Adaptive-passive systems: use passive devices whose parameters can be varied in order to achieve optimal noise attenuation over a band of operating frequencies. Lumped element model of the Helmholtz resonator [ ] The Helmholtz resonator is an acoustic filter element. If dimensions of the Helmholtz resonator are smaller than the acoustic wavelength, then dynamic behavior of the Helmholtz resonator can be modelled as a lumped system see. It is effectively a mass on a spring and can be treated so mathematically.
The large volume of air is the spring and the air in the neck is the oscillating mass. Damping appears in the form of radiation losses at the neck ends, and viscous losses due to friction of the oscillating air in the neck. Figure 1 shows this analogy between Helmholtz resonator and a vibration absorber. Open duct system with a side branch Helmholtz resonator with electrical circuit analogy near junction point A 1- Effect of Resonator Volume on sound attenuation Figure 3 shows the frequency response of the above duct system without Helmholtz resonator, and with two different volume Helmholtz resonators with the same natural frequency. The excitation frequency axis is normalized with respect to the fundamental frequency of the straight pipe system, which was also chosen as the natural frequency of the resonator. The maximum attenuation of sound pressure for duct systems with side branch Helmholtz resonators occurs when the natural frequency of the resonator is equal to the excitation frequency.
By comparing two curves with different colors, blue and gray, it can be seen that to increase the effective bandwidth of attenuation of a Helmholtz resonator, the device should be made as large as possible. It should be mention that in order to minimize the effects of standing waves within the device, the dimensions do not exceed a quarter wavelength of the resonator natural frequency. Effect of Resonator Volume on sound attenuation 2- Effect of Resonator Damping on sound attenuation The effect of Helmholtz resonator damping(Resulting from radiation resistance and viscous losses in the neck) on the frequency response of the duct system is shown in Figure 5.