Digital acoustic projector – A proof of concept
Introduction
Digital loudspeaker is defined as an electroacoustic system in which the Digital/Analogue (D/A) conversion is performed either in the mechanical or acoustic part of the system. The use of an electronic D/A converter is no longer necessary and the drawbacks associated to its use are thus avoided. J. L. Flanagan [1] was the first to explore the concept of a direct digital to analogue conversion in the 1980s. Since then, many publications have addressed the digital loudspeaker and its application [2], [3], [4], [5], [6], [7], [8]. The physical principles described in this non-exhaustive list of references are different from those described here and their content is not addressed in this paper. It should be noted that some may use the term digital loudspeaker in a different way, mostly for an assembly of an electrodynamic loudspeaker with an advanced digital signal processing unit. To distinguish between different uses, some use the adverb “true” for the system with the direct digital-to-acoustic conversion, resulting in the term “true digital loudspeaker”. A digital loudspeaker composed of an array of individual transducers can be implemented using one of the following arrangements. In a first arrangement, the individual transducers are connected in groups, in which the number of transducers is proportional to the weight of the bit signal feeding it. In a second arrangement, the number of individual transducers corresponds to the number of bits the system uses, and the respective size of their membranes is proportional to the respective bit signal weight. In the particular case of a digital loudspeaker using an electrodynamic transducer, a third arrangement called “multiple voice coil” can be used [2]. Here the voice coil of the electrodynamic transducer is partitioned into sections. The number of voice coil turns in each section is again proportional to the weight of the bit signal it is connected to. The forces acting on the membrane are summed and in this way the D/A conversion is performed in the mechanical part of the system. There are several other ways to embody the digital transducer, including arrangements suited for non conventional electroacoustic transduction principles, e.g. optoacoustic [9]. In this paper, we will concentrate on the first arrangement, i.e. an array of transducers with equal membrane areas.
The aim of this paper is to describe a Digital Acoustic Projector (DAP) [10], which is a hybrid sound system combining both digital loudspeakers and a parametric array to convert directly a digital electrical signal into sound. The principle and design of this acoustic projector are given in Sections 2 Principle of the digital acoustic projector, 3 Design of the digital acoustic projector. The last section is dedicated to experimental results obtained with a simplified DAP designed to convert 4-bit electrical digital signals into sound.
Section snippets
Principle of the digital acoustic projector
The principle of a DAP is illustrated in Fig. 1. For the sake of clarity and because of limited size of illustrating figures, we will explain the system by its 2-bit version.
The initial analogue electrical signal to be converted into sound is first sampled and quantized so that the electrical input signal consists of binary words having a length N ( in Fig. 1). Each bit of this binary word drives a group of ultrasonic transducers . When the value of the bit is equal to zero, the
Design of the digital acoustic projector
As a proof of concept, a Digital Acoustic Projector is designed to convert a 4-bit digital electrical signal into sound. It is a simplified version of a DAP using 15 ultrasonic sources divided into four groups. The first group has only one source and is controlled by the least significant bit. The other groups consist of two, four and eight sources respectively and are controlled by the other three bits (the most significant bit driving the group of eight sources).
The experimental setup
Experimental results and discussion
As a proof of concept, the DAP is first tested by generating a 1 kHz sound wave. Fig. 5 shows the electrical signals applied to each of the four transducer groups to reproduce one period of the sound wave. Each voltage signal is an on/off modulated 40 kHz carrier with a peak amplitude of 10 V. The modulation of each signal is controlled by the value of the corresponding bit.
If the transmitters were wide-band, and if the self-demodulation effect were negligible, the sound pressure signal
Conclusion
A new type of digital acoustic projector has been developed. It is a hybrid sound system that combines the principles of both a digital loudspeaker and a parametric array. Digital-to-analog conversion is performed in the air, not in an electronic D/A converter, by adding ultrasonic acoustic waves modulated by binary signals encoding the audio signal. The system uses the self-demodulation effect by the air to generate the low-frequency audio signal at the focal point. A first simplified
Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgement
The authors would like to thank Hervé Mézière for his technical advice and a critical reading of the manuscript by Oliver Munroe. Also we would like to thank anonymous reviewers for their valuable comments.
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