In situ measurement of absorption of acoustic material with a parametric source in air. Roland Kruse, Bastian Epp, Volker Mellert
Overview • Objective and motivation • The parametric source • Ultrasound characteristics • Generation of audio sound • Audio sound characteristics • Measurement of the reflection coefficient • Summary • Outlook
Objective and motivation The in situ measurement of the reflection coefficient (with an arbitrary angle of incident) is desirable, e.g. for • room acoustics • outdoor wave propagation (ground impedance) Pulse echo methods suffer from the interaction of direct sound, wanted and unwanted reflections in confined locations. A highly focused sound source is capable of reducing this problem.
Parametric source:Airmar AT75 transducer Piezo ceramic 3cm Porous epoxy
Parametric source:Ultrasound characteristics Directivity pattern (160cm)
× W 2 W W 1 1 - = - a + y + y 1 2 4 1 p ( x ) exp( x ) [ ln( 1 )]² [tan ²]² p × y 3 2 2 c A x 2 0 Generation of audio sound The non-linearity of air generates sum and difference frequencies when two signals are superimposed (concentric, conical radiation). p Far-field sound pressure of the differential frequency W1,2 Transmitted power of primary waves Differential frequency AAttenuation coefficient: 1 + 2 + x Distance from source ’/ (Cone width 2’, Diff. frequency 3dB bandwidth 2) Berktay, Possible Exploitation of Non-Linear Acoustics in Underwater Transmitting Applications, J.Sound Vib. (1965) 2 (4), 435-461
Summary • The investigated parametric source generates audio sound with a beam width comparable to the ultrasound directivity pattern. • The produced audio sound pressure is sufficiently high for frequencies of 2 kHz and above. The sound pressure at 1 kHz and below is too low for most applications (in the present set-up). • No more audio sound is generated at distances higher than 1 m. • The sound source is generally suited for the measurement of the reflection coefficient by “simple” pulse echo methods.
Outlook • The ultrasound level should be increased to obtain higher levels at 1 kHz and below ( p(audio) ~ p(US)² ). • Higher driving voltage • Transducer array • An even smaller beam width could be desirable for the measurements at small incident angles. • Wavefront shape ? • Interaction of ultrasonic wave with sample surface ?