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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
149th 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
149th 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.
149th Meeting of the Acoustical Society of America, Vancouver, May 2005
Oldenburg University, acoustics group
Parametric source:Airmar AT75 transducer
3cmPiezo ceramic
Porous epoxy
149th Meeting of the Acoustical Society of America, Vancouver, May 2005
Oldenburg University, acoustics group
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)
149th Meeting of the Acoustical Society of America, Vancouver, May 2005
Oldenburg University, acoustics group
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
149th Meeting of the Acoustical Society of America, Vancouver, May 2005
Oldenburg University, acoustics group
Parametric source:Generation of audio sound in air I
149th Meeting of the Acoustical Society of America, Vancouver, May 2005
Oldenburg University, acoustics group
Parametric source:Generation of audio sound in air II
149th Meeting of the Acoustical Society of America, Vancouver, May 2005
Oldenburg University, acoustics group
Parametric source:Audio sound frequency response
149th Meeting of the Acoustical Society of America, Vancouver, May 2005
Oldenburg University, acoustics group
Parametric source:Audio sound directivity pattern
149th Meeting of the Acoustical Society of America, Vancouver, May 2005
Oldenburg University, acoustics group
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
149th Meeting of the Acoustical Society of America, Vancouver, May 2005
Oldenburg University, acoustics group
Reflection coefficient:Measurement set-up
149th Meeting of the Acoustical Society of America, Vancouver, May 2005
Oldenburg University, acoustics group
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
149th Meeting of the Acoustical Society of America, Vancouver, May 2005
Oldenburg University, acoustics group
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
149th 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.
149th 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 ?