It is difficult and expensive to match the sensitivity of the most sensitive vertebrate ears with off-the-shelf microphones due to the self-noise of the sensor. The extremely small apertures of microelectromechanical microphones create options to use horn waveguides to amplify sound prior to transduction without resulting in an unacceptably narrow directivity. Substantial gain can be achieved at wavelengths larger than the horn. An analytical model of an exponential horn embedded in a rigid spherical housing was formulated to describe the gain relative to a free-field receiver as a function of frequency and angle of arrival. For waves incident on-axis, the analytical model provided an accurate estimate of gain at high frequencies as validated by experimental measurement. Numerical models, using the equivalent source method, can account for higher order modes and comprehensively describe the acoustic scattering within and around the horn for waves arriving from any direction. Results show the directivity of horn receivers were adequately described by the analytical model up to a critical wavelength, and the mechanisms of deviation in gain at high frequencies and large angles of arrival were identified.
References
1.
D. T.
Blumstein
, D. J.
Mennill
, P.
Clemins
, L.
Girod
, K.
Yao
, G.
Patricelli
, J. L.
Deppe
, A. H.
Krakauer
, C.
Clark
, K. A.
Cortopassi
, and S. F.
Hanser
, “Acoustic monitoring in terrestrial environments using microphone arrays: Applications, technological considerations, and prospectus
,” J. Appl. Ecol.
48
, 758
–767
(2011
).2.
M.
Campos Cerqueira
and M. T.
Aide
, “Improving distribution data of threatened species by combining acoustic monitoring and occupancy modeling
,” Methods Ecol. Evol.
7
, 1340
–1348
(2016
).3.
M. E.
Thompson
, S. J.
Schwager
, and K. B.
Payne
, “Heard but not seen: An acoustic survey of the African forest elephant population at Kakum Conservation Area, Ghana
,” Afr. J. Ecol.
48
, 224
–231
(2010
).4.
T. A.
Marques
, L.
Thomas
, S. W.
Martin
, D. K.
Mellinger
, J. A.
Ward
, D. J.
Moretti
, D.
Harris
, and P. L.
Tyack
, “Estimating animal population density using passive acoustics
,” Biol. Rev. Cambridge Philos. Soc.
88
, 287
–309
(2013
).5.
D. J.
Mennitt
and K. M.
Fristrup
, “Influence factors and spatiotemporal patterns of environmental sound levels in the contiguous United States
,” Noise Control Eng. J.
64
, 342
–353
(2016
).6.
E.
Lynch
, D.
Joyce
, and K.
Fristrup
, “An assessment of noise audibility and sound levels in U.S. National Parks
,” Landsc. Ecol.
26
, 1297
–1309
(2011
).7.
L.
Fritschi
, L.
Brown
, R.
Kim
, D.
Schwela
, and S.
Kephalopolous
, Burden of Disease from Environmental Noise: Quantification of Healthy Life Years lost in Europe
(World Health Organization
, Bonn
, 2011
).8.
G.
Shannon
, M. F.
McKenna
, L. M.
Angeloni
, K. R.
Crooks
, K. M.
Fristrup
, E.
Brown
, K. A.
Warner
, M. D.
Nelson
, C.
White
, J.
Briggs
, and S.
McFarland
, “A synthesis of two decades of research documenting the effects of noise on wildlife
,” Biol. Rev.
91
, 982
–1005
(2015
).9.
B. C.
Pijanowski
, A.
Farina
, S. H.
Gage
, S. L.
Dumyahn
, and B. L.
Krause
, “What is soundscape ecology? An introduction and overview of an emerging new science
,” Landsc. Ecol.
26
, 1213
–1232
(2011
).10.
R.
Horonjeff
, “Overcoming instrument noise when measuring in quiet environments
,” in Proceedings of Inter-noise
(2002
).11.
D. J.
Mennitt
, K.
Fristrup
, and K.
Sherrill
, “A geospatial model of ambient sound pressure levels in the contiguous United States
,” J. Acoust. Soc. Am.
135
, 2746
–2764
(2014
).12.
E.
Lynch
, L. M.
Angeloni
, K.
Fristrup
, D.
Joyce
, and G.
Wittemyer
, “The use of on-animal acoustical recording devices for studying animal behavior
,” J. Ecol. Evol.
3
, 2030
–2037
(2013
).13.
H. R.
Goerlitz
, S.
Greif
, and B. M.
Siemers
, “Cues for acoustic detection of prey: Insect rustling sounds and the influence of walking substrate
,” J. Exp. Biol.
211
. 2799
–2806
(2008
).14.
R.
Fay
, Hearing in Vertebrates: A Psychophysics Databook
(Hill-Fay Associates
, Winnetka, IL
, 1988
).15.
S.
Wahlström
, “The parabolic reflector as an acoustical amplifier
,” J. Aud. Eng. Soc.
33
, 418
–429
(1985
).16.
N. H.
Fletcher
, “Acoustic systems in biology: From insects to elephants
,” Acoust. Austr.
33
, 83
–88
(2005
).17.
H. F.
Olson
and I.
Wolff
, “Sound concentrator for microphones”
J. Acoust. Soc. Am.
1
, 410
–417
(1930
).18.
D. M.
Donskoy
and B. A.
Cray
, “Acoustic particle velocity horns
,” J. Acoust. Soc. Am.
131
, 3883
–3890
(2012
).19.
K. M.
Fristrup
and D.
Mennitt
, “Bioacoustical monitoring in terrestrial environments
,” Acoust. Today
8
, 16
–24
(2012
).20.
E.
Eisner
, “Complete solutions of the ‘Webster’ horn equation
,” J. Acoust. Soc. Am.
41
, 1126
–1146
(1967
).21.
K. R.
Holland
, F. J.
Fahy
, and C. L.
Morfey
, “Prediction and measurement of the one-parameter behavior of horns
,” J. Aud. Eng. Soc.
39
, 315
–337
(1991
).22.
M. R.
Stinson
and G. A.
Daigle
, “Comparison of an analytic horn equation approach and a boundary element method for the calculation of sound fields in the human ear canal
,” J. Acoust. Soc. Am.
118
, 2405
–2411
(2005
).23.
D. J.
Mennitt
and K.
Fristrup
, “Gain and directivity of exponential horn receivers
,” J. Acoust. Soc. Am.
140
, 3140
(2016
).24.
25.
T. F.
Johansen
, “On the directivity of horn loudspeakers
,” J. Aud. Eng. Soc.
42
, 1008
–1019
(1994
).26.
H. F.
Olson
, Elements of Acoustical Engineering
(D. Van Nostrand Company
, New York
, 1947
).27.
L.
Beranek
and T.
Mellow
, Acoustics: Sound Fields and Transducers
(Academic Press
, New York
, 2012
).28.
V.
Salmon
, “Horns
,” in Encyclopedia of Acoustics
, edited by M. J.
Crocker
(John Wiley
, New York
, 1997
), Vol. 4, pp. 1925
–1931
.29.
W. M.
Hall
, “Comments on the Theory of Horns
,” J. Acoust. Soc. Am.
3
, 552
–561
(1932
).30.
IEC Standard 60268-4:
Sound System Equipment Part 4: Microphones
(International Electrotechnical Commission
, Geneva, Switzerland
, 2004
).31.
B.
Zhou
and D.
Green
, “Characterization of external ear impulse responses using Golay codes
,” J. Acoust. Soc. Am.
92
, 1169
–1171
(1992
).32.
G. H.
Koopmann
, L.
Song
, and J. B.
Fahnline
, “A method for computing acoustic fields based on the principle of wave superposition
,” J. Acoust. Soc. Am.
86
, 2433
–2438
(1989
).33.
M.
Johnson
, S.
Elliott
, K.
Baek
, and J.
Garcia-Bonito
, “An equivalent source technique for calculating the sound field inside an enclosure containing scattering objects
,” J. Acoust. Soc. Am.
104
, 1221
–1231
(1998
).34.
B.
Lam
, S. J.
Elliott
, J.
Cheer
, and W.
Gan
, “Regularisation of the equivalent source method for robust numerical modelling of acoustic scattering
,” in Proceedings of Inter-Noise
(2015
).35.
Y. J. R.
Gounot
and R. E.
Musafir
, “Simulation of scattered fields: Some guidelines for the equivalent source method
,” J. Sound Vib.
330
, 3698
–3709
(2011
).© 2017 Acoustical Society of America.
2017
Acoustical Society of America
You do not currently have access to this content.