Supervision: Etienne Rivet

Project type: Master thesis

Finished

Loudspeaker systems are a combination of a driver and an acoustic enclosure, that may present different complexity. The most used enclosures are "closed-boxes" and the "vented-boxes". In the first category, the main effect of the enclosure is to modify the total stiffness of the loudspeaker diaphragm, resulting in higher resonance frequency and quality factor. When the cabinet volume is imposed (eg. compact speakers), a closed-box system can be prejudicial to the loudspeaker frequency response, as it shortens the bandwidth and may create artificial elongation resonance, limiting the dynamic range of use.

Vented-box loudspeakers, also known as "bass-reflex" loudspeakers, are loudspeaker systems comprising a loudspeaker, and a cabinet that is open over a certain surface, generally through a duct/slot. Thus the loudspeaker membrane is coupled with a volume of air and a second radiating surface (Helmholtz resonator), behaving as coupled resonators. This coupling is known to favour the extension of the loudspeaker response towards low-frequencies with a given cabinet volume, thus deserving the "bass-reflex" denomination. However, the synthesis of such systems consisting in aligning the frequency response to given 4th order high-pass filters with purely acoustic and mechanical elements is somehow tricky.

Recently, the Acoustic Group at EPFL developped a concept of Electroacoustic Resonator, allowing to modify the electromechanical dynamics of a loudspeaker dipahragm. Although this has been developped with a view to employing the membrane as a sound absorbing surface, the same concept can be used in turn to modify the dynamics of the loudspeaker used as a sound diffuser. This could prevent, eg. the use of bulky closed cabinet, or even the use of complex acoustic filters such as vented-cabinet, to extend the loudspeaker response towards the low frequency.

In this project, the student will perform the following tasks:

• study the literature on closed-box and vented-box loudspeaker design, and learn the electroacoustic resonator concept
• develop the simulation platform to design an "all-electronic" closed-box and vented-box loudspeaker system
• identify baseline components (loudspeaker, closed box) and define the synthetic acoustic impedance to assign to the loudspeaker through the electroacoustic resonator technique
• perform experimental assessment on a prototype

Profile: Electrical engineering, Micro-engineering, Physics, Mechanics

Prerequisites: Electroacoustique (BA5) or Audio Engineering (MA1)

Context: Theory (40%), design (40%), measurement (20%)