Parametric Array Loudspeakers Research Articles

Visualization of Demodulated Sound Based on Sequential Acoustic Ray Tracing with Self-Demodulation in Parametric Array Loudspeakers

With the development of acoustic simulation methods in recent decades, it has become feasible to simulate the sound pressure distribution of loudspeakers before actually setting physical speakers and measuring the sound field. The parametric array loudspeaker (PAL) has attracted attention due to its sharp directivity and unique applications. However, the sound reproduced by PALs is generated by the nonlinear interactions of ultrasound in the air, which makes it difficult to simulate the reproduced sound of a PAL with low computational load. Focusing on the sharp directivity of ultrasound, we extended conventional acoustic ray-tracing methods to consider the self-demodulation phenomenon of PALs. In this study, we developed a visualization method for the demodulated sound of a PAL. Specifically, the demodulated sound pressure distribution can be simulated to estimate and visualize the area covered by the reproduced sound of PAL before setting a real PAL. In the proposed method, acoustic rays were generated sequentially to express the generation of demodulated sound. Therefore, the proposed method is expected to simulate the demodulated sound of a PAL with acceptable accuracy and low calculation complexity. Quantitative evaluation between simulation results and practical measurement has been carried out, and the results demonstrate the effectiveness of the proposed method.

A note on the audio sound power generated by a parametric array loudspeaker.

Understanding the sound power characteristics of parametric array loudspeakers (PALs) poses a unique challenge due to the nonlinear process involved. This work addresses this issue by employing the spherical convolution directivity model to obtain the far field intensity, which is then used to calculate the sound power radiated by a PAL. The results reveal that the audio sound power is approximately proportional to the square of the audio frequency and the aperture size, while inversely proportional to the ultrasound frequency. Additionally, the conversion efficiency from ultrasound to audio sound is found to be generally below 0.15% for typical configurations.

A study on improving the sound radiation capability of stepped-plate parametric array loudspeakers

It is well known that Parametric Array Loudspeakers (PALs) produce highly directional audible sounds. However, their widespread adoption is hindered by their high cost, which is primarily attributable to the use of an array of ultrasonic radiator units. To address this issue, our research group introduced stepped-plate parametric array loudspeakers (SPPALs) in 2018. SPPALs utilize a single-structure directional ultrasonic radiator, featuring steps of half-wavelength thickness on the radiating plate. This design mitigates the problem of poor directivity, especially at high frequencies, caused by destructive interference resulting from the plate's flexural mode. Moreover, SPPALs can offer cost advantages compared to commercial PALs due to their single structure. However, understanding the vibration characteristics of SPPALs in relation to sound radiation performance is quite challenging due to their various design parameters. In this study, we present an analytical approach that offers a quantitative method for enhanced sound radiation capability. We present a simplified vibration model (SVM), which enables an efficient design process based on a comprehensive analysis of vibration characteristics. By minimizing reliance on finite element method (FEM) simulations, our method expedites the design of various directional speaker systems.

A brief review of independent sound zone technology

Independent sound zone technology aims to construct multiple independent sound zones with different sound fields, which are independent of each other and do not interfere with each other, and has wide research value and application prospects in today’s increasingly complex received information. To help scholars preparing to investigate independent sound zone techniques get started quickly, this paper presents research advances in three independent sound zone construction methods based on loudspeaker arrays, sound field reconstruction and parametric array loudspeakers, including the implementation principles and optimization schemes of these methods.

Fast and accurate modeling of parametric array loudspeakers in the frequency domain

The parametric array loudspeaker (PAL) is known for its ability to generate highly directional audio beams. It has been widely used in a variety of audio applications, such as the sound field reproduction and active noise control. Fast and accurate computations of the audio sound field generated by a PAL are challenging due to the complexity of nonlinear interactions between airborne ultrasonic waves. This paper presents an overview of the commonly used computational methods in the frequency domain under the quasilinear assumption, which include the direct integration method, convolution directivity model, Gaussian beam expansions, cylindrical expansions, spherical expansions, and finite element methods. The computational efficiency and the accuracy of these methods in the near field, Westervelt far field, and the inverse-law far field are compared. Future challenges and perspectives are discussed.

Subjective evaluation for sharp sound image construction based on reverberation control with surround sound system using parametric and electro-dynamic loudspeakers

3-D sound field reproduction systems can provide a high presence. These systems commonly use electro-dynamic loudspeakers. Electro-dynamic loudspeakers tend to construct diffuse sound images due to its wide directivity and high reverberation. In contrast, parametric array loudspeakers can construct sharp sound images due to its sharp directivity and low reverberation. On the other hand, it is difficult to provide reverberation presence by parametric array loudspeakers because of the sharp directivity. We have previously proposed a sharp sound image construction based on reverberation control with surround sound system using parametric and electro-dynamic loudspeakers. In this method, the sharp sound image is rendered using parametric array loudspeakers, and the reverberation presence is provided by electro-dynamic loudspeakers, emitting reverberation signals synthesized with reverberation control filters. Through the objective experiments, we have confirmed that this method can construct the sharp sound image with reverberation presence. In this paper, we conduct subjective experiments to confirm if the listeners can perceive the reverberation presence provided by the proposed method. In particular, we evaluated the sharpness and direction of the sound image, and the reverberation presence with objective and subjective evaluation experiments. From the subjective evaluation, we confirmed that the reverberation presence can be perceived.

A parametric array loudspeaker featuring a Langevin transducer, a circular aluminum plate, and composite polymer steps

A parametric array (PA) loudspeaker generates highly directional sound beams by exploiting the PA phenomenon. Before such loudspeakers were fabricated from stepped plate transducers, arrays of small ultrasonic radiators were employed. A stepped plate transducer generates broadband PA sound by exploiting the multi-resonance characteristics of its structure, even though it has only a single radiator. However, the design process is difficult because the plate must be thicker than the steps. In this study, we present a new method for fabrication of stepped plate PA loudspeakers (SPPALs); we employ a Langevin transducer, a circular aluminum plate, and composite polymer steps. The new stepped plate is fabricated by molding the composite steps on a flat plate. This is unlike the conventional stepped plate, in which both the plate and steps are made of the same metal. Using this method, although the radiation plate is thinner than the steps, such stepping is associated with minimal changes in the natural frequency and mode shape. Therefore, the SPPAL design process is simplified. We experimentally confirmed that the new stepped plate exhibited the desired bending mode. The SPPALs exhibited a maximum sound pressure of 61 dB at 2 m and a half-power beam width of < 5°; the sound was highly directional.

Quiet zone generation in an acoustic free field using multiple parametric array loudspeakers.

This paper investigates the feasibility of remotely generating a quiet zone in an acoustic free field using multiple parametric array loudspeakers (PALs). A primary sound field is simulated using point monopoles located randomly in a two-dimensional plane, or three-dimensional (3D) space, whereas the secondary sound field is generated by multiple PALs uniformly distributed around the circumference of a circle sitting on the same plane as the primary sources, or on the surface of a sphere for 3D space. A quiet zone size is defined as the diameter of the maximal circular zone within which the noise reduction is greater than 10 dB. The size of this quiet zone is found to be proportional to 0.19λN for N secondary sources with a wavelength λ when the primary and secondary sources are in the same plane, whereas it is found to be 0.55λN1/2 for the 3D case. The size of the quiet zones generated by PALs is similar to that observed with traditional omnidirectional loudspeakers; however, the effects of using PALs on the sound field outside the target zone is much smaller due to their sharp radiation directivity and slow decay rate along the propagation distance. Experimental results are also presented to validate these numerical simulations.

Development of an Acoustic Spot Data Communication System Using Multiple Parametric Array Loudspeakers

In this letter, we develop an acoustic spot data communication system using multiple parametric array loudspeakers, which are directional loudspeakers and can send audio in a specific direction. In our system, data are modulated and transmitted through multiple parametric array loudspeakers, and complete data reception is possible only at a specific spot. We implemented our system using low‐cost commercially available products and a self‐developed Java program, and we used frequency‐shift keying to modulate the acoustic data communication between the sender and receiver modules. Through comparative experimental evaluations, we show that data reception at a specific spot is possible using our system. © 2021 Institute of Electrical Engineers of Japan. Published by Wiley Periodicals LLC.

3-D sound field reproduction with reverberation control on surround sound system by combining parametric and electro-dynamic loudspeakers

Recently, with the development of video technology, 3-D sound field reproduction systems that can provide a high presence have attracted attention. Conventional systems with electro-dynamic loudspeakers have a problem that the sound image lacks sharpness when constructing a narrow sound image. To solve this problem, we utilize a parametric array loudspeaker which has sharp directivity by the straightness of ultrasounds. Parametric array loudspeakers can produce sharper sound images due to the sharper directivity and lower reverberation compared with electro-dynamic loudspeakers. However, it is difficult to represent reverberation by parametric array loudspeakers. In this paper, we propose a method for 3-D sound field reproduction to achieve both the sharp images and reverberation presence by combing parametric array loudspeakers and electro-dynamic loudspeakers in surround sound system. In the proposed method, sharp sound images are rendered by parametric array loudspeakers and the reverberation presence is provided by electro-dynamic loudspeakers, emitting reverberation signals synthesized with a reverberation control filter. The reverberation control filter is adaptively designed to reproduce the reverberation time and the direct-to-reverberation ratio of the target sound field in other environments. The experimental results show that the proposed method can achieve the reproducing the sharp sound image with some reverberation presence.

The near field, Westervelt far field, and inverse-law far field of the audio sound generated by parametric array loudspeakers.

The near and far fields of traditional loudspeakers are differentiated by whether the sound pressure amplitude is inversely proportional to the propagating distance. However, the audio sound field generated by a parametric array loudspeaker (PAL) is more complicated, and in this article it is proposed to be divided into three regions: near field, Westervelt far field, and inverse-law far field. In the near field, the audio sound experiences strong local effects and an efficient quasilinear solution is presented. In the Westervelt far field, local effects are negligible so that the Westervelt equation is used, and in the inverse-law far field, a simpler solution is adopted. It is found that the boundary between the near and Westervelt far fields for audio sound lies at approximately a2/λ - λ/4, where a is transducer radius and λ is ultrasonic wavelength. At large transducer radii and high ultrasonic frequencies, the boundary moves close to the PAL and can be estimated by a closed-form formula. The inverse-law holds for audio sound in the inverse-law far field and is more than 10 meters away from the PAL in most cases. With the proposed classification, it is convenient to apply appropriate prediction models to different regions.

Design of Phase Control System for Parametric Array Loudspeakers Based on FPGA

In order to achieve accurate control of the audible sound of the acoustic parametric array in the air, fine phase control of the ultrasonic signal as a carrier is required. By establishing the parametric matrix mathematical model and the directivity function formula, the effects of delay accuracy on the ultrasonic carrier and the audible sound deflection angle are simulated and analyzed. The control of 16-channel phase-controlled parametric array excitation signal is realized by using the fast parallel processing speed of FPGA. The signal processing circuit is designed to D/A conversion and amplification of the parametric array excitation signal to drive the piezoelectric transducer array efficiently. The experimental results show that the system can achieve fine control of the parameter array carrier transmission delay. The control accuracy of the parametric array carrier for the 40KHz carrier is 1 ns, and the error delay is less than 1 ns.

A Critical Step to Using a Parametric Array Loudspeaker in Mobile Devices.

A parametric array (PA) loudspeaker is a highly directional audio source that might grant one's convenience if it is used with mobile devices. However, conventional PA loudspeakers is almost impossible to apply in mobile devices using a battery because of the large power consumption and large device size. In this study, a PA loudspeaker system (PALS) was fabricated and evaluated to show that those difficulties could be overcome to apply it to mobile devices. In order to construct a PALS for demonstration, a power amplifier and signal-processing unit should also be properly designed and built. The PA source transducer should also be designed and built for a mobile device application. These components were integrated into a single PALS. The PALS generated a 125-dB primary wave and 62 dB of a different frequency wave (DFW) through the PA at 0.45 m in a 3 m × 3 m × 2 m semi-anechoic chamber. We confirmed that the half-power bandwidth (HPBW) formed a 6° beam at 83 kHz of DFW and 90 kHz of the primary wave (PW), and the HPBW formed a 7.3° beam at 5 kHz of DFW and a 7.1° beam at 10 kHz of DFW, respectively. Lastly, the power required was 6.65 W without a matching circuit, and 3.25 W with such a circuit.

Virtual sound source construction based on radiation direction control using multiple parametric array loudspeakers.

Due to developments in video technology, the study of three-dimensional sound-field reproduction has recently gained attention. A virtual-sound-source (VSS) construction method using multiple parametric-array loudspeakers (PALs), which have sharper directivity, was previously proposed. With this method, multiple PAL units (called PAL array) are used and a VSS is presented with a fixed radiation direction at any point in the air. However, this method requires rearrangement of the PAL array to change the position of the VSS. Thus, in this paper, a VSS-construction method based on radiation direction control using the PAL array is proposed. The proposed method can reconstruct positions and radiation directions of VSSs without rearranging the PAL array. Specifically, the proposed method involves mounting pan and tilt units onto PAL units and electrically controlling their placement angles to focus the acoustic beams from them to the VSS positions. The proposed method also involves controlling the radiation direction of a VSS by weighting the output signal of each PAL unit to simulate the radiation direction of the real sound source. The effectiveness of the proposed method is confirmed through objective and subjective experiments.

A wide-band in-air parametric array source composed of a circular plate with steps and a Langevin transducer with a horn

Parametric array (PA) sources generate a directional sound beam, via the PA phenomenon. Practical PA sources, such as PA loudspeakers, have high power consumption, low sound quality, and high cost. The stepped-plate transducer, which consists of a Langevin transducer, a horn, and a radiating plate with steps, can provide high-intensity sound and a highly directional sound beam in the acoustic axis. This study investigated the use of a stepped-plate transducer to make a PA source with high directivity and a wide bandwidth. A stepped-plate transducer that had two different resonance frequencies (f1 = 77.5 kHz, f2 = 87.4 kHz) was designed for wide bandwidth, and the three different heights of the steps on the plate (ts1 = 2.28 mm, ts2 = 2.14 mm, ts3 = 2.02 mm) were intended to provide high directivity sound beams. The fabricated stepped-plate transducer demonstrated a 50 ± 3 dB sound pressure level for the difference frequency wave in the 4–19 kHz frequency range (with equalization), and 4.2–5.1° of HPBW was shown for the highly directional sound beam.

A “composite” stepped plate for use in highly directional parametric array loudspeakers

It has been shown that stepped plate radiators with appropriately designed steps can be used in highly directional parametric array (PA) loudspeakers [Hwang and Moon, 170th Meeting of the Acoustical Society of America, 138(3), 1801(2015)]. To improve the performance of the PA loudspeaker, a relatively thin plate with steps needs to be adopted for its higher power efficiency. However, the natural frequencies of the plate and the corresponding mode shapes depend on the mass of the steps. Hence, it can be difficult to design a stepped plate suitable for use in PA loudspeakers. In this paper, we propose a novel plate design, with steps composed of a stiff, lightweight, polymer material. The steps were fabricated with a thickness of 2.3 mm from a polymer material with an elastic modulus of 1 GPa, a Poisson ratio of 0.23, and a mass density of 540 kg/m3 on the plate with a thickness of 1 mm. Experiments and simulations were carried out to determine the acoustic characteristics of our fabricated “composite” stepped plate. [Work supported by NRF 2016R1E1A2A02945515).]

Feasibility study on a highly-directional parametric array loudspeaker applicable to mobile devices

A conventional parametric array (PA) loudspeaker has a relatively small aperture and produces a highly directional audible sound beam. Due to limitations arising from their size and power consumption, standard PA loudspeakers are not used in mobile devices. PA loudspeakers are typically larger than 25 cm and require more than 60 W. We show that piezoelectric micro-machined ultrasonic transducer (pMUT) arrays can emit highly directional ultrasonic sounds with a power efficiency of up to 80% [Je and Moon, Ultrasonics 53, 1124-1134 (2014)]. Highly directional sound beams with a frequency bandwidth of 13 kHz can be generated efficiently by pMUT arrays with two resonant frequencies [Je and Moon, JASA 173(4), 1732-1743 (2015)]. We used a signal processor, power amplifier, and pMUT array to build a loudspeaker system that is smaller than, and requires less power than, a conventional PA loudspeaker system. In our loudspeaker system, the maximum sound pressure level (SPL) is 75 dB, ± 3 dB frequency bandwidth is 12.5 kHz, and the total power consumption is 3.4 W. Hence, this loudspeaker system could potentially be used in mobile devices. [This work was supported by Samsung Research Funding Center, SRFC-IT1404-00.]

Binaural active noise control using parametric array loudspeakers

This paper reports the binaural active noise control (ANC) system developed to deal with factory noise. The control points are located in the vicinity of the left and right ears of a worker sitting along the production line. Due to the complicated safety requirements in the factory, secondary sources and error microphones are not allowed to be placed near the worker. Therefore, the proposed ANC system employs the feedforward structure and adopts the parametric array loudspeakers (PALs) as the secondary sources. The PAL is a type of directional loudspeaker that generates a much narrower sound field as compared to the conventional loudspeaker. Once the proposed ANC system has been trained offline, the error microphones can be removed. The performance of the binaural ANC system is successfully demonstrated based on a digital signal processor (DSP) implementation.

A micromachined efficient parametric array loudspeaker with a wide radiation frequency band.

Parametric array (PA) loudspeakers generate directional audible sound via the PA effect, which can make private listening possible. The practical applications of PA loudspeakers include information technology devices that require large power efficiency transducers with a wide frequency bandwidth. Piezoelectric micromachined ultrasonic transducers (PMUTs) are compact and efficient units for PA sources [Je, Lee, and Moon, Ultrasonics 53, 1124-1134 (2013)]. This study investigated the use of an array of PMUTs to make a PA loudspeaker with high power efficiency and wide bandwidth. The achievable maximum radiation bandwidth of the driver was calculated, and an array of PMUTs with two distinct resonance frequencies (f1 = 100 kHz, f2 = 110 kHz) was designed. Out-of-phase driving was used with the dual-resonance transducer array to increase the bandwidth. The fabricated PMUT array exhibited an efficiency of up to 71%, together with a ±3-dB bandwidth of 17 kHz for directly radiated primary waves, and 19.5 kHz (500 Hz to 20 kHz) for the difference frequency waves (with equalization).