in situ measurement of absorption of acoustic material with a parametric source in air

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 ?

in situ measurement of absorption of acoustic material with a parametric source in air

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