The acoustics program at the Institute of Acoustics, Tongji University, China.

The acoustics research at Tongji University began in the mid-1950s. The Institute of Acoustics (IOA) of Tongji University, which is one of the earliest institutions engaged in acoustics research in China, was formally established in 1984. The IOA has decades of experience in acoustics education, including offering a master's degree program for approximately 40 years, a Ph.D. program for 35 years, and a postgraduate program for 25 years. The IOA is one of the oldest acoustic research facilities in China with outstanding acoustics laboratories. Research at the IOA is performed in many areas of acoustics, which focus on detection acoustics and marine acoustics, laser ultrasonics and photoacoustics, medical ultrasonics and bioacoustics, architectural acoustics, environmental acoustics, noise control, functional and microstructural acoustic materials, aeroacoustics and aeronautical acoustics, and vehicle acoustics. This paper presents acoustics education at the undergraduate and graduate levels of Tongji University and its outstanding acoustics research facilities in detail.

Extremely Asymmetrical Acoustic Metasurface Mirror at the Exceptional Point.

Previous research has attempted to minimize the influence of loss in reflection- and transmission-type acoustic metasurfaces. This Letter shows that, by treating the acoustic metasurface as a non-Hermitian system and by harnessing loss, unconventional wave behaviors that do not exist in lossless metasurfaces can be uncovered. Specifically, we theoretically and experimentally demonstrate a non-Hermitian acoustic metasurface mirror featuring extremely asymmetrical reflection at the exception point. As an example, the metasurface mirror is designed to have high-efficiency retroreflection when the wave comes from one side and near-perfect absorption when the wave comes from the opposite side. This work marries conventional gradient index metasurfaces with the exceptional point from non-Hermitian systems, and it paves the way for identifying new mechanisms and functionalities for wave manipulation.

Broadband acoustic skin cloak based on spiral metasurfaces.

A skin cloak based on the acoustic metasurface made of graded spiral units is proposed and numerically investigated. The presented skin cloak is an acoustical layer consisting of 80 subwavelength-sized unit cells, which provide precise local phase modulation and hence resort the disturbed sound filed in such a way to hide the object to acoustic wave. Numerical simulations show that the suggested skin cloak both work well under normal and small-angled incidences. By taking the advantage of the spiral-typed metasurface, the suggested skin cloak is rather thin with thickness in the order around 1/7 of the wavelength of target frequency, moreover, the intrinsic characteristics of modest dispersion ensure the skin cloak provides remarkable acoustic invisibility in a broad frequency ranging from 2500 Hz to 3600 Hz.

Sound trapping and dredging barriers.

When sound barriers are installed on both sides of a noise source, degradation in performance is observed. Barriers having negative-phase-gradient surfaces successfully eliminate this drawback by trapping sound energy in between the barriers. In contrast, barriers can also be designed to "dredge" the energy flux out. An extended model considering higher-order diffractions, which resulted from the interplay of the induced surface wave and barrier surface periodicity, is presented. It is found that the sound dredging barriers provide a remarkable enhancement over the trapping ones, and hence have the potential to be widely used in noise control engineering.

Effects of ceiling phase gradients on the acoustic environment on roadside balconies.

The screening effect of a roadside balcony deteriorates considerably due to sound reflection from its ceiling. Nonetheless, this drawback can be eliminated by artificially steering the reflected sound away from the building facade, which is achieved by introducing impedance inhomogeneity to the ceiling surface. This paper presents an elaborated investigation on the effect of the phase gradient of a ceiling surface on screening exterior noise, theoretically and numerically. By balancing its sound-steering capability and performance fluctuation level, such a balcony ceiling may have the potential to be widely used on roadside buildings.

Acoustic performance of boundaries having constant phase gradient.

In this paper, inhomogeneous boundaries having constant phase gradient are investigated. In principle, such a theoretically proposed boundary is dispersionless. In practice, however, when the boundary is realized by a subwavelength-structured tubes array, the impedance discretization brings about sub-reflections at high frequencies. Moreover, determined by the longest duct in the array, a realized boundary is impractically thick. Therefore, a finite-thickness boundary is further proposed by truncating and periodizing the tubes in the array. In this paper, the theoretical analysis agrees well with the numerical simulations. By appropriately choosing its phase gradient and target frequency, the finite-thickness boundaries have potential applications in noise control.

A theoretical model of barriers having inhomogeneous impedance surfaces.

When barriers are placed in parallel on opposite sides of a source, their performance deteriorates markedly. However, barriers made from materials of inhomogeneous impedance eliminate this drawback by altering the behavior of sound as it undergoes multiple reflections between the barriers. In this paper, a theoretical approach is carried out to estimate the performance of the proposed barriers. By combining the ray-tracing method and sound diffraction theory, the existence of different ray paths between the proposed barriers is revealed. Compared to conventional rigid-walled barriers, barriers having inhomogeneous surfaces may have the potential to be widely used in environmental noise control.

The effect of stimulus bandwidth on binaural loudness summation.

Binaural loudness summation is an important property of the human auditory system. This paper presents an experimental investigation of how binaural loudness summation varies with stimulus bandwidth. Loudness matches were obtained between dichotic stimuli, with interaural level differences (ILDs) of 2-12 dB, and diotic stimuli. The stimuli were noise bands with seven center frequencies and four bandwidths. Results showed that the loudness of dichotic stimuli increased nonlinearly with ILD, the increase being slightly less with broader bandwidths. There was a bandwidth-dependent difference between the listening tests results and the predictions of Moore and Glasberg's [(2007) J. Acoust. Soc. Am. 121, 1604-1612] loudness model. The size of the difference was, however, small. A characteristic function was derived describing how overall loudness depends on stimulus bandwidth and ILD.

Sound barriers from materials of inhomogeneous impedance.

Sound barriers are extensively used in environmental noise protection. However, when barriers are placed in parallel on opposite sides of a sound source, their performance deteriorates markedly. This paper describes a barrier made from materials of inhomogeneous impedance which lacks this drawback. The nonuniform impedance affects the way sound undergoes multiple reflections, and in the process traps acoustic energy. A proposed realization of the barrier comprises a closely spaced array of progressively tuned hollow narrow tubes which create a phase gradient. The acoustics of the barrier is theoretically examined and its superiority over conventional barriers is calculated using finite element modeling. Structural parameters of the barrier can be changed to achieve the required sound insertion loss, and the barrier has the potential to be widely used in environmental noise control.