Flat Loudspeakers Research Articles

Virtual bass preprocessing and carrier frequency optimization for the parametric array loudspeaker

The parametric array loudspeaker (PAL) is a novel type of loudspeaker that can project a directional sound beam. It is usually used in creating personal sound zone and projecting private messages to a targeted audience. However, the PAL has a very poor low-frequency response due to the inherent nonlinear acoustic principle generating sound from ultrasound in air. A psychoacoustic signal processing method known as the virtual bass (VB) has been proved to be an effective method to improve the bass quality of consumer electronics with miniature or flat loudspeaker unit. This paper proposes the VB processing based on the phase vocoder (PV) for the bass enhancement of the PAL that adopts a vestigial sideband modulation method. The harmonics generated by the VB processing are presented in the partial sideband, while the audio input without the bass component is embedded in the full sideband. A measure, namely the in-band peak flatness, is thereafter proposed in this paper to determine the optimal carrier frequency, given a practical uneven frequency response of the ultrasonic transducer. The subjective testing results validate that the proposed VB processing together with the optimal carrier frequency can finally realize the improvement of bass sound quality of the PAL.

Structure-Integrated Loudspeaker Using Fiber-Reinforced Plastics and Piezoelectric Transducers—Design, Manufacturing and Validation

In the present study, it could be shown that by integration of a piezoceramic transducer in a fiber-reinforced door side panel, a flat loudspeaker can be realized. Taking into account the given restrictions, the integration position has been identified, where the geometry decouples the vibrating membrane from the supporting surface. With the help of an acoustic finite-element simulation, the main design variables of the integration position were found and the relevant effects for sound radiation were made visible. The manufacturing of the test specimen with piezoceramic transducers was performed using vacuum-assisted resin infusion and the long fiber injection procedure. The effect on the real sound radiation behavior of the door side panel with a material-immanent loudspeaker was experimentally determined using laser scanning vibrometry and sound pressure measurements. The presented work shows, for the first time, the high potential of acoustic functionalization of lightweight structures during the manufacturing process for the realization of lightweight and space-saving loudspeakers in a production-ready process.

Experimental Characterisation of a Flat Dielectric Elastomer Loudspeaker

Conventional loudspeakers are often heavy, require substantial design spaces and are hard to integrate into lightweight structures (e.g., panels). To overcome these drawbacks, this paper presents a novel extremely flat loudspeaker which uses dielectric elastomer actuators with natural rubber for the elastomeric layers and metal electrodes as transduction mechanism. To facilitate the deformation of the elastomer, the electrodes are perforated. The microscopic holes lead to a macroscopically compressible stack configuration despite the elastomer incompressibility. The design is developed and the materials are chosen to guarantee low mechanical and electrical losses and a high efficiency in the entire frequency range up to several kilohertz. The loudspeaker was designed, built and afterwards experimentally investigated and characterised. Laser measurements of the surface velocity were performed to find dynamic effects present at the diaphragm. To further characterise the device, a semi anechoic chamber as used. Sound pressure levels emitted by the device were recorded at different bias and alternating voltages to study their influence. The nonlinearity of the loudspeaker, which is inherent for this kind of actuators, was quantified considering the total harmonic distortion. Here, a dependence on the amplitude of the alternating voltage is observed. Further, the distortion decreases rapidly the higher the frequency is, which qualifies the loudspeaker concept to properly work at high frequencies. Transfer functions between supplied voltage and on-axis sound pressure were measured and showed in principle potential for high frequency application. Further, the behaviour of the diaphragm changing from rigid piston to resilient disk with respect to frequency for different configurations was observed. Additionally, the directivity of the loudspeaker was investigated at several frequencies, and was in accordance with previously found research outcomes. The results, especially in the high frequency range, prove the usability of this design concept for practical applications.

A low density, high stiffness flat loudspeaker with combined feedback-feedforward response correction

This paper presents a novel perforated flat loudspeaker with improved sound frequency response by applying velocity feedback control. Flat loudspeakers provide advantages of compact dimensions and high durability. Known flat loudspeaker technology is based on high model density. However, the resonances in the panel are complex and difficult to control, which often leads to complicated computations and insufficient low frequency response. The flat loudspeaker in this paper comprises a novel panel structure, which offers low density, high stiffness, and efficient space utilization. Direct velocity feedback control provides a simple and stable control loop. We apply the multiple direct velocity feedback control method to obtain flat sound frequency response. Furthermore, a Linkwitz filter is applied to our system to increase response at very low frequencies. Experimental results show that a multiple combined feedback-feedforward control method effectively improves the performance of the flat loudspeaker with extended low-frequency response.

Efficient equalization of multi-exciter distributed mode loudspeakers

An efficient digital equalization method is applied successfully to the problem of spectral equalization of multi-exciter distributed mode loudspeakers (DML). It is based on a chain of second-order sections of infinite impulse response parametric filters with very low computational cost. The method compensates for the measured multi-exciter DML response in order to achieve a desired frequency response. The sound radiation of these flat loudspeakers is a complex superposition of excited modes that vary strongly with frequency. Therefore, the characteristic multi-exciter DML spectrum is very irregular and is equalized with the method presented here for a natural, uncolored response. In multichannel systems, such as wave field synthesis (WFS), the use of efficient filters to equalize a large amount of drivers is an advantageous approach. The equalization process has been applied to two multi-exciter DML prototypes, comprising three and five exciters per panel. Both panel and exciter equalization have been addressed, which consequences on the filtered responses are discussed. Finally, some subjective assessments are carried out to optimize the order of the filter while maintaining the perceived quality of the equalization.

The research of the DML loudspeakers properties

Some properties of the flat loudspeakers are examined, the principle of work of which is based on the excitation of the standing waves in the flat rigid panel. These devices are called distributed mode loudspeakers (DML). The frequency range of response and sound pressure level in rage was measured. The results are given of simulation and measurement for polystyrene and glass panel. In this paper Ill. 10, bibl. 6 (In English; summaries in English, Russian and Lithuanian).

Influence of material properties on sound radiation for flat loudspeakers

This work aims to analyze numerically, analytically and experimentally, the influence on sound power radiated, frequency response, and directionality of different materials in flat loudspeakers. Concerned experimental setup, the excitation force consists in a shaker which his mechanical characteristics are completely known. Finite Element Method has been used to simulate this configuration .The analytical model used couples the electrical, mechanical and acoustical system of the panel. The parameters of the panel material which we are focused are elastic modulus and loss factor. The analytical formalism serves as the basis to study the structural and vibrational behaviour of the system that also is analyzed with experimental results. The results obtained from different materials and different excitation position serves as basis for optimum configuration in this type of speakers.

Numerical modeling of Panphonics's G1 flat loudspeaker

The basic idea behind AABC (Active Acoustic Barrier Control) is to reduce the sound radiation of a structure acoustically without influencing the vibration behavior of the structure. The large surface acoustic polymer material actuator/sensor systems are primarily meant to form the actuator/sensor system for the AABC concept. The main component of this package is a special active device: the rEMA - revised Elastic Mass Actuator based on the Panphonics' G1 panel loudspeaker element. This paper deals with the modeling and the low-frequency performance issues of the G1 flat loudspeaker. The aim was to create an accurate mathematical model to understand the operational principles of the loudspeaker, and then this model was used to optimize its low-frequency performance. Due to the special structure and the optimization tasks, a new numerical method was developed to model special multi-layer coupled vibro-acoustics systems based on the Finite Difference and Boundary Element Method. The investigation focuses on the mechanical behavior of the panel and describes the sound radiation properties also. The performed work was a part of the research of the project InMAR (Intelligent Materials for Active Noise Reduction) which was funded by the European Union.

Mobile communications terminal with flat loudspeaker disposed in the terminal housing

A mobile communications terminal is provided which is designed to transmit speech signals and to receive and reproduce speech signals, and which has a speech-reproduction device, wherein improved speech quality is achieved in that the speech-reproduction device has a sound transducer, which is disposed inside a housing of the mobile communications terminal, and at least one sound transmission opening provided in the housing, which forwards the sound generated by the sound transducer to the outside of the mobile communications terminal.

Novel heat durable electromechanical film: processing for electromechanical and electret applications

New ferroelectrets were developed on the basis of foams from cyclo-olefin polymers and copolymers. The results obtained on the cyclo-olefin polymer foam demonstrate a significant improvement of the service temperature for ferroelectret transducer materials. Suitable compounding and preparation led to cyclo-olefin ferroelectrets with an electromechanical activity of around 15 pC/N, which is thermally stable at least up to 110degC. The properties in sensor and actuator applications are strongly dependent on the processing parameters related to film-making, sensor and actuator preparation, gas content and electric charging. The processing window for the film stretching was very narrow compared to the earlier developed polypropylene ferroelectrets. The film porosity, softness and thus the electromechanical activity are adjusted by gas-diffusion expansion. The activity of the electromechanically operating sensors and actuators was increased by stacking several layers of cellular cyclo-olefin film. For applications such as flat loudspeakers, the foamed films are tuned by tensioning them on a support frame. Correct tensioning was essential also for reducing the distortion levels

Acoustic Excitation of Dual Diffusion Flames with Coherent Structure Behind a Rear-Facing Semicircular Cylinder

In this paper two types of dual diffusion flames formed behind a rear-facing semicircular fuel nozzle are experimentally investigated; one is the plane diffusion flame consisting of an ordinary laminar diffusion flame on the lower-velocity side of the fuel nozzle and a lifted flame on the higher-velocity side, the other the bluffbody stabilized wake flame composed of two lifted flames on both sides of the fuel nozzle. The CAPLA(computer-aided phase-locked averaging) method is combined with the acoustic excitation to examine not only effects of the acoustic excitation on the flame structure, but also phase characteristics of the formation processes of organized eddies and eddy-flames. The acoustic excitation at a constant frequency of 300 Hz is applied by a flat loudspeaker flush-mounted on the higher-velocity side wall of the combustion tunnel. According to flash shadowgraphs of the flames the acoustic excitation is found to make the outline of eddy-flames clear with increasing the excitation intensity due to the rapid and clear formation of organized eddies, leaving general features of the flames almost unvaried. Frequency spectra of fluctuating temperature for two kinds of flames show that, at the maximum excitation intensity applied, only a fundamental peak at 300 Hz and some of its harmonics can be observed without exhibiting any white noise character. Therefore, it is confirmed that the acoustic excitation is very useful in the phase-locked-averaging measurements of turbulent flames. The phase-averaged two-dimensional temperature profiles at eight consecutive phases obtained for two kinds of flames demonstrate clearly the entire process from the formation of an initial organized eddy to the establishment of an eddy-flame. As a result, the organized eddy-flame in the plane diffusion flame is found to have an average lifetime of about three times the period of the organized eddy formation of 1/300 sec, while the lifetime of the organized eddy-flame in the bluflbody stabilized wake flame is about twice the period of the organized eddy formation.

The theory of maximally flat loudspeaker systems

A convenient mathematical formulation is given for the absolute low-frequency response of an electrodynamic direct radiator loudspeaker mounted in a vented or unvented enclosure. The combined effects of speaker <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">Q</tex> (ratio of reactance to resistance), enclosure size, and enclosure tuning are shown to control the convergence of the response curve towards its asymptotic value. Mathematical criteria are presented graphically, which enable the designer to find enclosure characteristics that give a frequency response curve flat through fourth order in reciprocal frequency, for a given loudspeaker <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">Q</tex> . In addition, it is shown that for <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">Q = 0.383</tex> the system can be made flat through sixth order by choosing the correct enclosure size and vent tuning. This "triple point" design represents the flattest possible response for direct radiator systems.