149 th Meeting of the Acoustical Society of America, Vancouver, May 2005 Oldenburg University, acoustics group In situ measurement of absorption of acoustic

149th 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



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

3cmPiezo ceramic

Porous epoxy



Parametric source:Ultrasound characteristics

Frequency response (70cm, 100V)

80

90

100

110

120

130

60 70 80 90

Frequency [Hz]

Sou

nd p

ress

ure

[dB

re.

20u

Pa]

Directivity pattern (160cm)



Generation of audio soundThe non-linearity of air generates sum and differencefrequencies 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

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 UnderwaterTransmitting Applications, J.Sound Vib. (1965) 2 (4), 435-461

²]²[tan)]²1ln(2

1[

1)exp(

2)( 14

230

221

xxAc

WWxp



Parametric source:Generation of audio sound in air I



Parametric source:Generation of audio sound in air II



Parametric source:Audio sound frequency response



Parametric source:Audio sound directivity pattern



Parametric source: Distance dependency of audio sound

Sound level vs. distance

0.0

0.5

1.0

1.5

2.0

2.5

3.0

3.5

0 0.5 1 1.5 2 2.5 3

Distance [m]

Sou

nd p

ress

ure

[rel

.]

2kHz

4kHz

8kHz

Sphere



Reflection coefficient:Measurement set-up



Reflection coefficient:Results I

Carpet

0.0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1.0

0 2 4 6 8 10

Frequency [kHz]

Ref

lect

ion

coef

f.

90°

45°

Imp.tube



Reflection coefficient:Results II

Two layer melamine foam with textile cover

0.0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1.0

0 1 2 3 4 5 6 7 8 9

Frequency [kHz]

Ref

lect

ion

coef

f.

90°

45°

Imp.tube



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 ?

Documents

149 th Meeting of the Acoustical Society of America, Vancouver, May 2005 Oldenburg University, acoustics group In situ measurement of absorption of acoustic