USE OF THE PARAMETRIC LOUDSPEAKER IN ACOUSTIC DIAGNOSTICS OF PANEL PAINTINGS: LABORATORY CHARACTERIZATION AND ON SITE EXPERIMENTATIONS

The ACoustic Energy Absorption Diagnostic Device, ACEADD, reveals detachments of pictorial film in paintings. A transceiver unit delivers an acoustic wave towards the analysed surface, recording the incident and the reflected wave for the extrapolation of the surface impulse response and therefore of the absorption coefficient. A highly directive acoustic source was recently integrated in the measuring apparatus to overcome a limitation of the instrumental spatial resolution due to the geometrical spreading of the acoustic beam. The parametric loudspeaker generates audio frequencies in air by emitting ultrasonic waves, based on the nonlinear propagation of finite-amplitude waves, and unlike the traditional ones its directivity pattern is as narrow as the emitted ultrasonic waves.Before integrating it into the measuring apparatus, a commercial parametric loudspeaker was characterized in an anechoic environment. The frequency response was measured using an omni-directional microphone aligned to the source's central axis. The aperture of the beam was identified by mapping the sound pressure level in the vertical half plane in front of the source. The analysed audio frequency band covered the significant interval of the acoustic excitation for revealing rather thin cavities beneath the pictorial film. The experimental - 3 dB aperture 2? resulted below 12° for most of the interesting frequencies.The spatial resolution of the ACEADD system was tested on laboratory models. Moving the transceiver unit facing the model's surface, crossing both highly reflecting and absorbing materials, alternating acoustic responses resulted in a profile of the acoustic absorption indicator with rising or falling edges correlated to a spatial resolution of few centimetres.Finally the instrument was used in field experimentations on panel paintings. The first investigation, on the Annunciazione by Benozzo Gozzoli, though restricted to two absorption profiles of major interest included an evaluation of the uncertainty of the measurement. A second investigation, on the Venus and Mars by Pieter Paul Rubens, was realized during usual museum exhibition time, providing an extended acoustic image of the panel.The parametric loudspeaker showed a great potential in on site non destructive applications. Its small size, the beam directivity pattern, the non invasiveness gave evidences of great flexibility for moveable instrumentations, which can be easily used where the artefact is placed.