Deep sound-field denoiser: optically-measured sound-field denoising using deep neural network.

This paper proposes a deep sound-field denoiser, a deep neural network (DNN) based denoising of optically measured sound-field images. Sound-field imaging using optical methods has gained considerable attention due to its ability to achieve high-spatial-resolution imaging of acoustic phenomena that conventional acoustic sensors cannot accomplish. However, the optically measured sound-field images are often heavily contaminated by noise because of the low sensitivity of optical interferometric measurements to airborne sound. Here, we propose a DNN-based sound-field denoising method. Time-varying sound-field image sequences are decomposed into harmonic complex-amplitude images by using a time-directional Fourier transform. The complex images are converted into two-channel images consisting of real and imaginary parts and denoised by a nonlinear-activation-free network. The network is trained on a sound-field dataset obtained from numerical acoustic simulations with randomized parameters. We compared the method with conventional ones, such as image filters, a spatiotemporal filter, and other DNN architectures, on numerical and experimental data. The experimental data were measured by parallel phase-shifting interferometry and holographic speckle interferometry. The proposed deep sound-field denoiser significantly outperformed the conventional methods on both the numerical and experimental data. Code is available on GitHub (https://github.com/nttcslab/deep-sound-field-denoiser).

Speckle holographic imaging of a sound field using Fresnel lenses.

In this Letter, we propose to use Fresnel lenses for holographic sound-field imaging. Although a Fresnel lens has never been used for sound-field imaging mainly due to its low imaging quality, it has several desired properties, including thinness, lightweight, low cost, and ease of making a large aperture. We constructed an optical holographic imaging system composed of two Fresnel lenses used for magnification and demagnification of the illuminating beam. A proof-of-concept experiment verified that the sound-field imaging with Fresnel lenses is possible by using the spatiotemporally harmonic nature of sound.

Low-noise optical measurement of sound using midfringe locked interferometer with differential detection.

A midfringe locked interferometer with differential detection is proposed for non-contact optical sound measurement, and the equivalent noise level of approximately 0 dB SPL/Hz is achieved. The noise level of the proposed method is 30 dB lower than that of a very recent laser Doppler vibrometer and close to that of a quarter-inch measurement microphone. The midfringe locking stabilizes the optical interferometer against slow environmental fluctuations and enables detection of the acoustic signal directly from optical intensity. The differential detection method eliminates laser intensity noise, which is a dominant noise source in optical interferometers. The noise level of the constructed system was approximately 10 dB above the optical shot-noise (the classical detection limit attributed to the quantum nature of light) at frequencies higher than 2 kHz. Further noise reduction by several available methods could lead to optical measurements that are more sensitive than measurements by microphones. In addition, the constructed interferometer is used to reconstruct sound fields generated by a half-inch laboratory standard microphone used as a transmitter. The proposed method will be a powerful tool for measuring small-amplitude sound fields where it has been challenging to use existing methods.

Spurious-sound-free measurement of parametric acoustic array using optical interferometry.

When measuring a parametric acoustic array (PAA) with a microphone, the sound often suffers from so-called spurious sound, the noise generated by the nonlinearity of the microphone. This paper proposes a spurious-sound-free measurement method for a PAA using optical interferometry and Gaussian beam expansion. Comparison between the proposed method and numerical simulation by the finite element method confirmed that the proposed method measures the demodulated audio sound without any effect from spurious sound.

Respiration rate change induced by controlling the phasic relationship between melodic sound and respiration.

It has been suggested that the phasic relationship between sound and respiration (PRSR) is important. However, the effect that the relationship has on respiration rate is rarely studied. Here, by using our invented interface, we have developed a method of systematically controlling the PRSR and show that the respiration rate can be changed by controlling the PRSR. In concrete, when realized error of PRSR is large, respiration rate significantly increased. Since the effect appears with participants listening passively to melodic sounds without any instruction, the result could be utilized as a tool for managing everyday stress.

Presenting changes in acoustic features synchronously to respiration alters the affective evaluation of sound.

Synchronization of respiration to cyclic auditory stimuli is a well-observed phenomenon and known to have an effect on affective evaluation of the presented sound. However, no studies have separated the effect of the change in respiratory movement itself and that when there is synchrony between respiration and sound. In this study, we used a system that can change the acoustic features synchronously with the respiration phase and directly investigated the effect the synchrony has on affective ratings without changing respiratory movements. An acoustic stimulation was presented where the sound intensity (SI) or fundamental frequency (F0) was modulated in response to the participant's respiration phase. Affective evaluations of the acoustic stimuli were made by using the Self-Assessment Manikin (SAM). The experiments compared synchronous and asynchronous conditions. In the synchronous condition, SI (or F0) was increased with inhalation (decreased with exhalation) or decreased with inhalation (increased with exhalation). In the asynchronous condition, a sound identical to that presented in the synchronous condition was replayed. The participants evaluated sounds that were acoustically the same but where the temporal relationship differed between respiration and the acoustic features. In our results, significantly higher arousal ratings were observed when the change in SI and respiration (inhalation or exhalation) was synchronous and when the increase in F0 and inhalation was synchronous. This suggests that the synchronous phenomenon between respiration and auditory stimuli can play a critical role in affective evaluation.

Tactile phantom sensation for coaching respiration timing.

Respiration coaching is one of the key factors for radiation therapies. However, there are relatively few studies relating to respiration coaching, and most of them use audio or visual cues. In this paper, we show that a tactile phantom sensation moving continuously on the back can be used to adequately coach respiration timing. By using the tactile modality, the device rarely interferes with other communication channels used by therapists. The phantom sensation simplifies the mechanical structure. Several parameters were studied to obtain optimal performance when utilizing the phantom sensation. In a series of experiments, we determined the proper position and duty ratio for the actuators. To evaluate the device performance, we conducted an interference test (Kraepelin test), and the results suggest that the developed device interferes little with cognitive tasks. The experiments suggest that participants can easily understand stimulation on the back in terms of respiration guidance and properly follow changes in the cycle period with changes in respiration activity.