Reducing Loudspeaker Systems Panel Vibrations

D. Cinanni [1],
[1] ASK Industries Spa, subject to direction and coordination of JVCKENWOOD Corporation, Italy
Published in 2018

One of the main design questions in loudspeaker enclosures concern vibrations reduction. An ideal cabinet would be infinitely rigid, so a section of an engineer work is focused on this goal.

Solutions depend by loudspeaker system audio field: Line Arrays, Automotive environments or hi-fi applications have different approaches to cabinets design, due to context and layout constraints.

Kinetic energy transmitted to panels by a loudspeaker driver is caused by acoustic radiation or mechanical transmission, particularly: •vibration from the air pressure variation in the cabinet (sound pressure inside the cabinet is generated from the rear loudspeaker membrane and it is in anti-phase compared to front membrane radiation; the sound transmitted by the panels where loudspeaker driver is fixed interferes with that from the membrane causing irregularities in both the steady-state axial frequency response curve and in the polar diagram); •vibration through the reactive force from the loudspeaker unit (loudspeaker moving assembly mass oscillations are transferred to panels through the basket by mechanical transmission); •vibration from movement of other panels (it happens widely in automotive environments).

Generally remedial treatments used to reducing resonances problems caused by baffle vibrations are bracing, damping and/or density increasing of vibrating panels. Other ingenious patented solutions are related to: •Active damping with drivers mounted in a “back to back” compound configuration (for example Bose, Kef), using also passive radiators (for example Matsushita, Bose); • Active damping with masses (for example Sony, Hikida, ASK); • Passive damping with masses (for example Genelec, Fujitsu) or decoupling air volume of the loudspeaker rear emission (for example B&W).

This study deals with a patent pending solution based on an active anti-rattle system vibrating assembly generating an inertia force, moving co-axially and in phase opposition, with the loudspeaker moving assembly. The anti-rattle system vibrating assembly uses the loudspeaker’s magnetic circuit and is constituted by a moving coil attached to a solid part and to a spring, integrated in the loudspeaker driver. The vibrating assembly study is based on the Tuned Mass Damper (TMD); starting on 2 Degree of Freedom TMD differential equation of motion, analytical solution of the ratio between mass and excursion is calculated and presented. A new loudspeaker woofer 3D virtual prototype is arranged and the project is imported on COMSOL Multiphysics® in order to study interactions between loudspeaker driver and the anti-rattle system vibrating assembly. Magnetic assembly simulations are carried out optimizing pole plates sizes and also the spring element using Von Mises stress. Then a physical woofer prototype is implemented and measured in anechoic room. Acoustical measurements on IEC panel and on a closed box with the vibrating assembly ON and OFF are compared and the real electric impedance is matched with the simulated impedance. The woofer prototype is also mounted on a baffle, loaded with a Gaussian filtered noise on its terminals and a laser scanning vibrometer system is used to measure mechanical baffle oscillations due to vibrations transmitted by the transducer. Then displacement magnitude is displayed at different frequencies with anti-rattle system ON, to evaluate panel excursion reduction.