A time-domain nearfield beamformer with spherical harmonic decomposition.

This paper proposes a time-domain nearfield beamformer with spherical harmonic decomposition. The beamformer design is separated into two stages: sound field measurement and beamformer coefficient design. This makes it easier for the beamformer to be implemented by different array structures. The beamformer coefficients are further separated into several parts, making it easier to design a beamformer with different characteristics. The time-domain implementation minimizes the latency between the array input and the beamformer response, and the nearfield focusing improves the farfield noise suppression ability of the beamformer. The proposed beamformer could be useful for emerging acoustic applications such as virtual reality and drones, and for further development of more advanced real-time nearfield beamformers. Simulations and experiments examine the performance of the proposed beamformer.

Secondary channel estimation in spatial active noise control systems using a single moving higher order microphone.

Spatial active noise control (ANC) systems focus on minimizing unwanted acoustic noise over continuous spatial regions by generating anti-noise fields with secondary loudspeakers. Conventionally, error microphones are necessary inside the region to measure the channels from the secondary loudspeakers to the error microphones and record the residual sound field during the noise control. These error microphones highly limit the implementation of spatial ANC systems because of their impractical geometry and obstruction to the users from accessing the region. Recent advances, such as virtual sensing, focus on ANC with microphones placed away from the region. While these techniques relax the usage of error microphones during the noise control, an error microphone array remains necessary during the secondary channel estimation. In this paper, we propose a method to estimate secondary channels without using an error microphone array. Instead, a moving higher order microphone is applied to obtain the secondary channels from the secondary loudspeakers to the region of interest, which includes all desired error microphone locations. By simulation, we show that the proposed method is robust against various measuring errors introduced by the movement of the microphone and is suitable for the secondary channel estimation in spatial ANC systems.

Coherence-based performance analysis on noise reduction in multichannel active noise control systems.

Active noise control (ANC) over an extended spatial region using multiple microphones and multiple loudspeakers has become an important problem. The maximum noise reduction (NR) potential over the control area is a critical evaluation variable as it indicates the fundamental limitation of a given ANC system. In this paper, a method to mathematically formulate the NR potential for any given multichannel ANC systems is developed. First, the residual error in the multichannel feedforward ANC system is formulated, and then the multiple-input-multiple-output problem is decomposed into the parallel-channel problem. The total energy of the residual error is further decomposed into three different terms representing (i) the signal coherence between the reference signals and error signals, (ii) the filter, and (iii) the system null space. The experimental results validate the proposed evaluation method and illustrate the effectiveness on the maximum NR performance evaluation for given systems. Using the proposed analyzing method, more insight into the contribution of each component to the NR potential can be achieved.

Parameterization of the binaural room transfer function using modal decomposition.

Binaural room responses are normally measured on a listening subject in a room. The measurements, however, rapidly change with the source and receiver position. In addition, the measurements taken in a room can only be used to simulate scenes of that environment. In this work, an efficient parameterization of the binaural room transfer function is proposed, which has separable representations for the direct-path component and the reverberation component, thus providing a flexible way to generate binaural room responses for different environments and listeners. In addition, this parameterization uses wave equation solutions as basis functions and is continuous in space.

Spatial sound intensity vectors in spherical harmonic domain.

Sound intensity is a fundamental quantity describing acoustic wave fields and it contains both energy and directivity information. It is used in a variety of applications such as source localization, reproduction, and power measurement. Until now, intensity is defined at a point in space, however given sound propagates over space, knowing its spatial distribution could be more powerful. This paper formulates spatial sound intensity vectors in spherical harmonic domain such that the vectors contain energy and directivity information over continuous spatial regions. These representations are derived with finite sets of closed form coefficients enabling ease of implementation.

Spherical harmonics based generalized image source method for simulating room acoustics.

Allen and Berkley's image source method (ISM) is proven to be a very useful and popular technique for simulating the acoustic room transfer function (RTF) in reverberant rooms. It is based on the assumption that the source and receiver of interest are both omnidirectional. With the inherent directional nature of practical loudspeakers and the increasing use of directional microphones, the above assumption is often invalid. The main objective of this paper is to generalize the frequency domain ISM in the spherical harmonics domain such that it could simulate the RTF between practical transducers with higher-order directivity. This is achieved by decomposing transducer directivity patterns in terms of spherical harmonics and by applying the concept of image sources in spherical harmonics based propagation patterns. Therefore, from now on, any transducer can be modeled in the spherical harmonics domain with a realistic directivity pattern and incorporated with the proposed method to simulate room acoustics more accurately. We show that the proposed generalization also has an alternate use in terms of enabling RTF simulations for moving point-transducers inside pre-defined source and receiver regions.

Multichannel active noise control for spatially sparse noise fields.

Multi-channel active noise control (ANC) is currently an attractive solution for the attenuation of low-frequency noise fields, in three-dimensional space. This paper develops a controller for the case when the noise source components are sparsely distributed in space. The anti-noise signals are designed as in conventional ANC to minimize the residual errors but with an additional term containing an ℓl norm regularization applied to the signal magnitude. This results in that only secondary sources close to the noise sources are required to be active for cancellation of sparse noise fields. Adaptive algorithms with low computational complexity and faster convergence speeds are proposed.