Jiancheng Tao

Sound absorption of a finite micro-perforated panel backed by a shunted loudspeaker

Deep back cavities are usually required for micro-perforated panel (MPP) constructions to achieve good low frequency absorption. To overcome the problem, a close-box loudspeaker with a shunted circuit is proposed to substitute the back wall of the cavity of the MPP constructions to constitute a composite absorber. Based on the equivalent circuit model, the acoustic impedance of the shunted loudspeaker is formulated first, then a prediction model of the sound absorption of the MPP backed by shunted loudspeaker is developed by employing the mode solution of a finite size MPP coupled by an air cavity with an impendence back wall. The MPP absorbs mid to high frequency sound, and with properly adjusted electrical parameters of its shunted circuit, the shunted loudspeaker absorbs low frequency sound, so the composite absorber provides a compact solution to broadband sound control. Numerical simulations and experiments are carried out to validate the model.

Perfect absorption of low-frequency sound waves by critically coupled subwavelength resonant system

The perfect absorption (PA) for low-frequency audible sound waves has been achieved by critically coupling the inherent loss factor to the inherent leakage factor of a system, which is constructed by attaching a deep-subwavelength lossy resonant plate (LRP) to a backed rigid wall closely. We have certified it by using the graphical method in the complex frequency plane. By coupling the LRP to an air cavity in front of the rigid wall, the high efficient (>80%) low-frequency broadband absorption is obtained from 99.1 Hz to 294.8 Hz. Here, the thickness of LRP is only 1/13.5 of the relevant wavelength at 294.8 Hz. The impedance analyses further demonstrate that the impedances are perfectly matched between the system and the surrounding background medium at PA.

Performance of a planar virtual sound barrier at the baffled opening of a rectangular cavity

This paper proposes to reduce the radiation of a sound source inside a cavity through the baffled opening by using an array of loudspeakers and microphones. The system is called a planar virtual sound barrier because it acts like a concrete sound barrier to block the transmission of sound but does not affect light and air circulation. An analytical model for the planar virtual sound barrier is developed based on the modal superposition method to calculate the sound field in and outside a rectangular cavity with a baffled opening. After the model is verified with numerical simulations, a performance study of the planar virtual sound barrier is carried out based on the proposed analytical model, and then the results are confirmed by experiments. The mechanisms of the planar virtual sound barrier are investigated and it is found that three mechanisms work together in the system, including changing the impedance of the primary source, modal control, and modal rearrangement. It is also found that there exist some frequencies where the sound cannot be controlled if all the secondary sources are on the same plane parallel to the opening, and the reasons behind the phenomenon are explained.

Spatial filtering of audible sound with acoustic landscapes

Acoustic metasurfaces manipulate waves with specially designed structures and achieve properties that natural materials cannot offer. Similar surfaces work in audio frequency range as well and lead to marvelous acoustic phenomena that can be perceived by human ears. Being intrigued by the famous Maoshan Bugle phenomenon, we investigate large scale metasurfaces consisting of periodic steps of sizes comparable to the wavelength of audio frequency in both time and space domains. We propose a theoretical method to calculate the scattered sound field and find that periodic corrugated surfaces work as spatial filters and the frequency selective character can only be observed at the same side as the incident wave. Maoshan Bugle phenomenon can be well explained with the method. Finally, we demonstrate that the proposed method can be used to design acoustical landscapes, which transform impulsive sound into famous trumpet solos or other melodious sound.

Controlling sound radiation through an opening with secondary loudspeakers along its boundaries

We propose a virtual sound barrier system that blocks sound transmission through openings without affecting access, light and air circulation. The proposed system applies active control technique to cancel sound transmission with a double layered loudspeaker array at the edge of the opening. Unlike traditional transparent glass windows, recently invented double-glazed ventilation windows and planar active sound barriers or any other metamaterials designed to reduce sound transmission, secondary loudspeakers are put only along the boundaries of the opening, which provides the possibility to make it invisible. Simulation and experimental results demonstrate its feasibility for broadband sound control, especially for low frequency sound which is usually hard to attenuate with existing methods.

Mechanisms of active control of sound radiation from an opening with boundary installed secondary sources

Previous work has demonstrated that installing secondary sources at the edge of a cavity opening can reduce sound radiation through it, but the mechanisms are not clear, which is investigated in this paper by using the modal decomposition method. It is found that a double layer edge system achieves better performance than a single layer system because secondary sources at the edge of the same layer cannot excite some modes effectively and those at different heights compensate this. There exists an upper limit frequency for the systems with boundary installed secondary sources, which is mainly decided by the length of the short side of the opening. More secondary source layers at the edge will increase the upper limit frequency.

Controlling 3D sound field generated by a 2D surface from its 1D boundary

As described by the Kirchhoff-Helmholtz integral equation, for a 3D space without internal sources, the inside sound field is completely determined by the sound pressure and its normal gradient on the boundary, so if only a portion of the boundary surface vibrates as the primary sound source while all the remaining portion of the surface is acoustically rigid, a sufficiently large number of secondary sound sources can be located evenly on the primary sound surface to control the 3D sound field completely. The question to be investigated in this research is whether it is possible to control the 3D sound field by only applying secondary sound sources along the 1D boundary of the 2D primary sound surface, for example, controlling the sound transmission from a door with secondary sound sources only on its frame. Simulation and experimental results will be presented to demonstrate the feasibility of the idea.

A composite sound absorber with micro-perforated panel and shunted loudspeaker

Micro-perforated panel (MPP) backed by a rigid cavity is a widely used clean sound absorber; however, its application at low frequency is limited because a deep cavity is required to achieve good sound absorption in the low frequency range. In the present paper, a composite absorber composed by an MPP and a shunted loudspeaker is proposed. The loudspeaker is installed at the back wall of the air cavity and the acoustic impendence can be optimized by adjusting the parameters of the loudspeaker and the electronic components in the shunt to improve the sound absorption performance. The prediction model of such a finite-sized composite absorber is established based on the mode analysis solution and the equivalent circuit of the loudspeaker. Both numerical simulations and experiments show that the thickness of the proposed composite absorber can be much smaller than that of traditional MPP constructions.

Active noise control in the exhaust port of a vacuum cleaner

Exhaust port is one of the main sound transmission paths for noise radiation of a vacuum cleaner. Porous sound absorption materials are usually stuck at the inner surface of the exhaust port to absorb the noise; however the noise reduction performance, especially at low frequency, usually is limited due to the limited transmission path length to the exhaust port for airflow and the low acoustical absorption coefficients of porous materials at low frequency. In order to reduce the low-frequency noise which radiates outside through the exhaust port, active noise control technologies are applied in exhaust port, where the configurations of the active control units including the reference sensor, the control loudspeaker and the error microphone are investigated. Finally, experiments are carried out for performance validation.

Performance of active noise barrier with a moving sound source

Active noise control is an effective technology to improve noise reduction performance of traditional passive noise barriers at low-frequencies. Previous researches on active noise barriers are mainly on the assumption that the primary sound source is stationary. Effects of source motion on the performance of an active noise barrier are investigated in this paper. First, an analytical model of a passive barrier with a moving sound source is introduced, which can be used to calculate the primary sound field distribution in time domain. Then, the performance of applying active noise control on such a barrier is investigated numerically. Finally, the experimental results with a practical prototype of active noise barrier will be reported and be compared with the numerical results.