Digital acoustic projector – A proof of concept

Highlights

• This article proposes a solution to a drawback inherent to the digital loudspeaker principle which is the different time delay of signals radiated by spatially distributed transducers by using a parabolic reflective surface and the ultrasonic auto-demodulation effect.

Abstract

This paper describes a Digital Acoustic Projector (DAP), which is a hybrid sound system combining both a digital loudspeaker and a parametric array to convert directly an n-bits digital electrical signal into sound. It consists of a set of ultrasonic transducers emitting a sinusoidal signal modulated by a sampled and quantized low-frequency signal. Self demodulation effect is used to demodulate the acoustic signal during its propagation in the air. As a proof of concept, a DAP is designed to convert a 4-bit digital electrical signal into sound. First experimental results are presented.

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.