149 th Meeting of the Acoustical Society of America, Vancouver, May 2005 Oldenburg University, acoustics group In situ measurement of absorption of acoustic material with a parametric source in air. Roland Kruse, Bastian Epp, Volker Mellert

149 th Meeting of the Acoustical Society of America, Vancouver, May 2005 Oldenburg University, acoustics group Overview Objective and motivation The parametric source Ultrasound characteristics Generation of audio sound Audio sound characteristics Measurement of the reflection coefficient Summary Outlook

149 th Meeting of the Acoustical Society of America, Vancouver, May 2005 Oldenburg University, acoustics group 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.

149 th Meeting of the Acoustical Society of America, Vancouver, May 2005 Oldenburg University, acoustics group Parametric source: Airmar AT75 transducer 3cm Piezo ceramic Porous epoxy

149 th Meeting of the Acoustical Society of America, Vancouver, May 2005 Oldenburg University, acoustics group Parametric source: Ultrasound characteristics Directivity pattern (160cm)

149 th Meeting of the Acoustical Society of America, Vancouver, May 2005 Oldenburg University, acoustics group 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 W 1,2 Transmitted power of primary waves  Differential frequency A Attenuation 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), ²]²[tan)]²1ln( 2 1 [ 1 )exp( 2 )(          x xAc WW xp

149 th Meeting of the Acoustical Society of America, Vancouver, May 2005 Oldenburg University, acoustics group Parametric source: Generation of audio sound in air I

149 th Meeting of the Acoustical Society of America, Vancouver, May 2005 Oldenburg University, acoustics group Parametric source: Generation of audio sound in air II

149 th Meeting of the Acoustical Society of America, Vancouver, May 2005 Oldenburg University, acoustics group Parametric source: Audio sound frequency response

149 th Meeting of the Acoustical Society of America, Vancouver, May 2005 Oldenburg University, acoustics group Parametric source: Audio sound directivity pattern

149 th Meeting of the Acoustical Society of America, Vancouver, May 2005 Oldenburg University, acoustics group Parametric source: Distance dependency of audio sound

149 th Meeting of the Acoustical Society of America, Vancouver, May 2005 Oldenburg University, acoustics group Reflection coefficient: Measurement set-up

149 th Meeting of the Acoustical Society of America, Vancouver, May 2005 Oldenburg University, acoustics group Reflection coefficient: Results I

149 th Meeting of the Acoustical Society of America, Vancouver, May 2005 Oldenburg University, acoustics group Reflection coefficient: Results II

149 th Meeting of the Acoustical Society of America, Vancouver, May 2005 Oldenburg University, acoustics group 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.

149 th Meeting of the Acoustical Society of America, Vancouver, May 2005 Oldenburg University, acoustics group 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 ?