Measurement Apparatus and Modelling Techniques of Ear Canal Acoustics

93 Pages·2008·3.85 MB·English

by  Â© Marko Hiipakka

HELSINKIUNIVERSITYOFTECHNOLOGY
FacultyofElectronics,CommunicationsandAutomation
DepartmentofSignalProcessingandAcoustics
MarkoHiipakka
Measurement Apparatus and Modelling
Techniques of Ear Canal Acoustics
Master’s Thesis submitted in partial fulfilment of the requirements for the degree of Master of
ScienceinTechnology.
Espoo,24Nov,2008
Supervisor: ProfessorMattiKarjalainen
Instructors: D.Sc. (Tech.) AnttiKelloniemi
HELSINKIUNIVERSITY ABSTRACTOFTHE
OFTECHNOLOGY MASTER’STHESIS
Author: MarkoTapioHiipakka
Nameofthethesis: MeasurementApparatusandModellingTechniquesofEarCanalAcoustics
Date: 24Nov,2008 Numberofpages: vii+85
Faculty: FacultyofElectronics,CommunicationandAutomation
Department: DepartmentofSignalProcessingandAcoustics
Professorship: S-89
Supervisor: Prof. MattiKarjalainen
Instructors: D.Sc. (Tech.) AnttiKelloniemi
Each one of us perceives the sounds around us differently. The acoustic characteristics of the
outerearhaveasignificantinfluencetothesensationofhearing. Thetendencyoftheearcanal
to shape the spectral structure of sounds has not been studied systematically and the role of
the ear canal as a part of the outer ear is not yet completely known. Our research focused on
scrutinizing the effects of varying physical dimensions of the outer ear and particularly those
of the ear canal to the pressure at the eardrum. The main emphasis was on determining the
influenceofthelengthoftheearcanaltothefrequencyresponseattheeardrumandtherebyto
thesensationofhearing. Inaddition,theeardrum’sdampingoftheresonancefrequencieswas
studied.
Twoearcanalsimulatorsandonedummyheadweremanufacturedforthepressurefrequency
response measurements. The target was to physically model the acoustic behaviour of the
humanhead,pinna,andearcanalasaccuratelyaspossible. Inaddition,thefrequencyresponses
were measured from the ear canals of human test subjects in various acoustical conditions. A
physics-basedcomputationalmodelandequationsofthephysicsofsoundwereusedtovalidate
theresultsobtainedfromthemeasurements.
Thebehaviouroftheearcanalwasinvestigatedinnormallisteningconditionsaswellasinfree
fieldconditions. Inaddition,thebehaviourwasstudiedinasituationwherethecanalentrance
was blocked with an insert earphone. A special pair of earphones with in-ear microphones
was constructed for this purpose. The earphone was also used for studying two important
phenomenarelatedtoinsertearphones,namelytheocclusioneffectandsoundleakage.
Fromourresearchwemayconcludethatthephysicaldimensionsandespeciallythelengthof
the ear canal have a considerable effect to the pressure frequency response at the eardrum. In
addition,wemayconclude,thatitispossibletoaccuratelymodeltheacousticbehaviourofthe
humanouterearwithphysicalsimulators.
Keywords: Earcanal,Outerear,Eardrum,Frequencyresponse.
i
TEKNILLINENKORKEAKOULU DIPLOMITYÖNTIIVISTELMÄ
Tekijä: MarkoTapioHiipakka
Työnnimi: Korvakäytävänakustistenominaisuuksienmittausjamallinnus
Päivämäärä: 24.11.2008 Sivuja: vii+85
Tiedekunta: Elektroniikan,tietoliikenteenjaautomaationtiedekunta
Laitos: Signaalinkäsittelynjaakustiikanlaitos
Professuuri: S-89
Työnvalvoja: Prof.MattiKarjalainen
Työnohjaajat: TkTAnttiKelloniemi
Ulkokorvan akustiset ominaisuudet vaikuttavat merkittävästi ihmisen kuuloaistimukseen. Yk-
silölliseterotulkokorvanrakenteissaovatyksisyysiihen,ettäkuulemmemeitäympäröivätää-
net hieman eri lailla. Korvakäytävän taipumusta muokata kuulemiemme äänten spektriraken-
nettaontutkittuvähän,eikäsenakustisiaominaisuuksiatunnetavielätäysin.Tutkimuksessam-
mekeskityttiinkartoittamaanulkokorvanjaerityisestikorvakäytävänfyysistenulottuvuuksien
vaikutustatärykalvollamitattavaanäänipaineeseen.Tutkimuksenpääpainonaoliselvittääkor-
vakäytävänpituudenvaikutustaajuusvasteeseentärykalvollajasitäkauttakuuloaistimukseen.
Lisäksitutkittiintärykalvonaiheuttamaavaimennustakorvakäytävänresonansseihin.
Taajuusvasteidenmittauksiavartenrakennettiinkaksiuudentyyppistäkorvakäytäväsimulaatto-
riasekäyksikeinopää.Tavoitteenaolimallintaafyysisestiihmisenpään,korvalehdenjakorva-
käytävänakustinenkäyttäytyminenmahdollisimmantarkasti.Lisäksitutkimuksessakäytettiin
koehenkilöitä, joiden korvakäytävien taajuusvasteita mitattiin erilaisissa olosuhteissa. Korva-
käytäväsimulaattorinlaskennallinenmallisekääänenfysiikanlaskukaavattoimivatmittaustu-
lostenteoreettisinaperusteluina.
Korvakäytävänkäyttäytymistätutkittiinnormaaleissakuunteluolosuhteissajavapaakenttäolo-
suhteissa. Lisäksi tutkittiin tilannetta, jossa korvakäytävään on asetettu korvakäytäväkuuloke
(engl. “insert earphone”). Tätä käyttötilannetta ja sen tutkimista varten valmistettiin korva-
käytävämikrofonit sisältävä korvakäytäväkuulokepari, jolla tutkittiin myös kuulokekuuntelun
kannaltatärkeitäokkluusioilmiötäjaäänenvuotoa.
Tutkimuksenlopputuloksenavoidaantodeta,ettäulkokorvanulottuvuuksillajaerityisestikor-
vakäytävänpituudellaonsuurimerkitystaajuusvasteeseentärykalvolla.Lisäksivoidaantodeta,
että fyysisillä simulaattoreilla on mahdollista tarkasti mallintaa ihmisen ulkokorvan akustinen
käyttäytyminen.
Avainsanat:Korvakäytävä,Ulkokorva,Tärykalvo,Taajuusvaste.
ii
Acknowledgements
Research for this Master’s thesis was conducted in the Department of Signal Processing and
AcousticsattheHelsinkiUniversityofTechnologywithfundingfromNokiaOyj.
I want to thank Professor Matti Karjalainen for his enthusiastic participation in the planning,
researchandpracticalrealizationsneededforthisThesistobecompleted. Inaddition,hisindis-
pensableguidanceandsupervisionhavehelpedinsolvingtheoreticalandpracticalproblems.
IwishtothankDr. AnttiKelloniemiforsharinghisexpertiseandforinstructionsastoresearch
directions. My gratitude also goes to Dr. Jouni Knuuttila, Jarkko Kuntanen, Heidi Linden and
TimoToivanenforasuccessfulcooperation.
I would also like to thank my co-workers Miikka Tikander, Jussi Rämö and Ville Riikonen for
cooperation and shared efforts within our research. I also have the staff and co-researchers of
the Department of Signal Processing and Acoustics to thank for help and support, and also for
thegreatspirityouhavebuiltupatthe‘laboratory’.
I am deeply indebted to friends, family and relatives who have supported me on my path to
graduation.
Finally, I would like to thank the Polytech Choir as well as Driving School’s principal Veikko
Sompaandhispupilsfortheirirrecoverablehelpintheavoidanceofanoverhastygraduation.
Otaniemi,24November,2008
Marko“Magge”TapioHiipakka
iii
Contents
Abbreviations vii
1 Introduction 1
2 Acousticsofhearing 3
2.1 Soundandhearing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3
2.2 Outerear . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5
2.2.1 Pinna . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5
2.2.2 Earcanal . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5
2.2.3 Eardrum. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7
2.3 Middleear . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8
2.4 Innerear . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9
2.4.1 Innerearstructure . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9
2.4.2 Innerearfunction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11
2.5 Soundsourcelocalization . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11
2.5.1 Binauralcues . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12
2.5.2 Monauralcues,HRTF’s . . . . . . . . . . . . . . . . . . . . . . . . . 13
2.5.3 Theprecedenceeffect . . . . . . . . . . . . . . . . . . . . . . . . . . 14
3 Headphoneandmicrophonetechnology 16
3.1 Headphones . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16
3.1.1 Headphonecategories . . . . . . . . . . . . . . . . . . . . . . . . . . 16
3.1.2 Headphonetransducers . . . . . . . . . . . . . . . . . . . . . . . . . . 17
3.1.3 PressureChamberEffect . . . . . . . . . . . . . . . . . . . . . . . . . 19
3.2 Microphones . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20
iv
3.2.1 Microphonecategories . . . . . . . . . . . . . . . . . . . . . . . . . . 20
3.2.2 Microphonesensitivity . . . . . . . . . . . . . . . . . . . . . . . . . . 22
3.2.3 Microphonedirectionalsensitivity . . . . . . . . . . . . . . . . . . . . 23
4 Outerearmodels 24
4.1 Physicalsimulators . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 24
4.1.1 Dummyheads . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 24
4.1.2 Earcanalsimulators . . . . . . . . . . . . . . . . . . . . . . . . . . . 26
4.1.3 Tubesasearcanalsimulators. . . . . . . . . . . . . . . . . . . . . . . 27
4.1.4 Adjustableearcanalsimulator(Adecs) . . . . . . . . . . . . . . . . . 27
4.1.5 Multi-adjustableearcanalsimulator(Madecs). . . . . . . . . . . . . . 28
4.1.6 Artificialpinnas . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 30
4.1.7 Dummyheadwithadjustableearcanal(Dadec) . . . . . . . . . . . . . 30
4.2 Computationalmodelling . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 31
4.2.1 Lumpedelementandtransmissionlinemodelling . . . . . . . . . . . . 31
4.2.2 Estimationofpressureateardrum . . . . . . . . . . . . . . . . . . . . 36
5 Measurements 38
5.1 Measurementequipmentandtechnology . . . . . . . . . . . . . . . . . . . . . 38
5.1.1 Equipmentandsoftware . . . . . . . . . . . . . . . . . . . . . . . . . 38
5.1.2 Earphonewithfittedin-earmicrophone(Efim) . . . . . . . . . . . . . 39
5.1.3 Measurementenvironments . . . . . . . . . . . . . . . . . . . . . . . 41
5.2 Acousticpropertiesofopenearcanal . . . . . . . . . . . . . . . . . . . . . . . 41
5.2.1 Frequencyresponseattheearcanalentrance . . . . . . . . . . . . . . 41
5.2.2 Frequencyresponsesalongthecanal . . . . . . . . . . . . . . . . . . . 46
5.2.3 Frequencyresponseattheeardrum. . . . . . . . . . . . . . . . . . . . 49
5.2.4 Effectofdifferentouterearshapes . . . . . . . . . . . . . . . . . . . . 53
5.2.5 Effectofdrumimpedance . . . . . . . . . . . . . . . . . . . . . . . . 57
5.2.6 Acomputationalmodel . . . . . . . . . . . . . . . . . . . . . . . . . . 59
5.3 Acousticpropertiesofblockedearcanal . . . . . . . . . . . . . . . . . . . . . 60
5.3.1 Frequencyresponsesofblockedearcanalsimulators . . . . . . . . . . 63
5.3.2 Frequencyresponsesofhumanears . . . . . . . . . . . . . . . . . . . 65
5.3.3 Effectofeardrumimpedance . . . . . . . . . . . . . . . . . . . . . . . 67
v
5.3.4 Effectofcanalshape . . . . . . . . . . . . . . . . . . . . . . . . . . . 70
5.3.5 Leakage . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 71
5.3.6 Occlusioneffect . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 71
6 ConclusionsandFutureWork 73
6.1 Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 73
6.2 Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 74
6.3 FutureWork . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 75
A Testsubjects 80
B Figures 81
vi
Abbreviations
Adecs Adjustableearcanalsimulator
B&K Brüel&Kjær
Dadec Dummyheadwithadjustableearcanals
Efim Earphonewithfittedin-earmicrophone
FFT FastFourierTransform
HATS HeadandTorsoSimulator
HRTF Head-relatedtransferfunction
IEC InternationalElectrotechnicalCommission
ILD Interauralleveldifference
ITD Interauraltimedifference
ITU InternationalTelecommunicationUnion
Madecs Multi-adjustableearcanalsimulator
SPL Soundpressurelevel
vii
Chapter 1
Introduction
It is well known that the outer ear contributes to the spectral shaping of sounds we hear in
everyday life. People have different ears and different ear canals, hence the sound pressure at
people’seardrumsarenotsimilarlydistributedinthefrequencydomain. Inparttherefore,people
perceivesoundsdifferently. Themiddleear,theinnerearandthenervoussystemalsoaffectthe
way we hear. The research for this thesis, though, was limited to, and focused on the spectral
shapingcharacteristicsoftheouterearandespeciallytheearcanal,whichistheinnermostpart
oftheouterear.
Innormallisteningsituationsthewholeouterearcontributestothespectralshapingofsound
before they reach the eardrum. The ear canal acts like a quarter-wave resonator and hence am-
plifiestheresonancefrequencies. Thelocationsoftheseresonancefrequenciesinthefrequency
domain depend mainly on the length of the ear canal. The shape and size of the pinna, and the
curvatureoftheearcanalalsohaveaneffectonthepressurefrequencyresponseattheeardrum.
Insert type earphones are commonly used when listening to music and together with mobile
phones etc. The sound transmission path from the insert earphone to the eardrum is different
fromthecasewhenaloudspeakerisusedasthesoundsource. Thesoundwavetravelsthrough
theearcanalonly, anearcanalthatissuggestiveofahalf-waveresonator. Thehalf-wavereso-
nancefrequenciesarepronouncedattheeardrum,andthelocationsofthesefrequenciesdepend
once again on the length of the ear canal. In addition, the overall structure of the ear canal has
aneffectontheshapeofthefrequencyresponseateardrum. Furthermore,thepressurechamber
effectandtheocclusioneffectareimportantfactorsregardinginsertearphones.
Objectives
Itistheobjectiveofthisthesistostudytheacousticalbehaviouroftheearcanal. Thetargetisto
gainknowledgethatwillbeusefulinfutureresearchprojectsrelatedtoe.g.soundreproduction
with hearing aids or headphones. The behaviour is investigated in normal listening acoustics
and in free field conditions. In addition, a thorough study on the ear canal behaviour when it is
blockedwithaninserttypeearphoneisperformed. Itisinourspecialinteresttostudythevarying
physicaldimensionsoftheearcanalastowhateffecttheyhaveonthefrequencyresponseatthe
eardrum.
1
CHAPTER1. INTRODUCTION 2
Furthermore, the goal of the research related to this thesis is to model the ear canal and the
wholeouterearasaccuratelyaspossible. Twoapproachesareapplied: thebuildingofphysical
simulators and physics-based computational modelling, the emphasis being on simulators. The
aimistovalidatetheuseofbothapproachesasparallelmodellingtechniques.
Finally, we hope that the results obtained in our research can help focusing future research
projects.
Outline
This thesis consists of three main parts; first, the basics of hearing and technologies related to
this thesis are presented. In the second part, a look at physical simulation and physics-based
computational modelling is taken. In the third part, the acoustic behaviour of the outer ear and
theearcanalisviewedthroughmeasurementresults.
ThebasicsofsoundandhearingareoverviewedinChapter2. Thedifferentpartsofthehuman
auditorysystemfromthepinnatotheinnereararepresented. Inaddition,thebasicsofbinaural
hearingarebrieflypresented. HeadphoneandmicrophonetechnologiesarepresentedinChapter
3.
Earcanalsimulators,dummyheadsandphysics-basedcomputationalmodellingarepresented
inChapter4. Thephysicalsimulatorsusedintheresearchforthisthesiswereself-madeandthey
wereintendedtocopytheacousticalbehaviouroftheouterear.
InChapter5theacousticbehaviouroftheearcanalinnormallisteningconditionsandwhen
thecanalentranceisblockedwithaninsertearphoneisstudied. Thenovelearcanalsimulators
and a dummy head were used for measuring the effects of varying physical parameters such as
theearcanallengthandtheoverallshapeoftheouterear.
ConclusionsandfutureworkscenariosarediscussedinChapter6.