Acoustic Impedances of Audiometric Earphones Coupled to Different Loads

Transcription

1 Acoustic Impeances of Auiometic Eaphones Couple to Diffeent Loas Dejan Ćiić, Dote Hammeshøi Depatment of Acoustics, Aalbog Univesity, DK-9 Aalbog, Feeik Bajes Vej 7B 5, Denmak, {c, The acoustic impeance of an auiometic eaphone is one of the factos that affect the soun tansmission in the human ea uing the test of heaing sensitivity. A simila situation exists uing the calibation of an eaphone whee its impeance affects the soun tansmission in a couple. In oe to etemine the acoustic impeances of five auiometic eaphones, the stanaize metho fo measuement of complex impeances is applie, which is base on the tansfe function between two micophone locations in an impeance tube. Fo that pupose, a special measuing tube containing fou micophones is evelope. The auiometic eaphones ae couple to iffeent loas, i.e., a metal plate, an atificial ubbe pinna, an the uppe pat of the stanaize couple uing measuements. The esults show that the impeances of the eaphones iffe, but they also geneally iffe fom the aiation impeances of the teminations use (seen fom insie the tube without the loa of an eaphone). In aition, thee is an inication of iffeences among the eaphones egaing the influence of the loa on thei impeances. Intouction In the test of the heaing sensitivity, only the input voltage to an auiometic eaphone is contolle in the calibation poceue assuming that a cetain efine soun pessue level is obtaine in the ea of the paticula subject. Howeve, the levels geneate iffe consieably fom one subject to anothe. This vaiability has been investigate obseving the soun tansmission in both human eas an the couple, that is, coupling of the eaphones to human eas an the couple [, ]. It is escibe by the pessue ivision efine as the atio of the pessues at ) the open an ) the blocke ea canal entance. This pessue atio is elate to the atio of the impeance of the ea canal (looking into the ea canal) an the impeance of the eaphone (looking out into the eaphone). Thus, the eaphone impeance is one of the factos affecting the soun tansmission in human eas an equivalently in the couple, but its significance epens upon the contibution of othe elements involve in the tansmission such as the ea canal impeance. In oe to etemine acoustic impeances of auiometic eaphones, the stanaize metho fo measuement of complex impeances using the impeance tube is applie [3]. Fo that pupose, the poceue name tansfe function metho is use. It epesents the acoustic tansmission line metho elying on plane soun wave popagation insie the tube with the temination of unknown impeance o eflectance [4-6]. The impeance etemination is base on the soun pessue vaiation along the tube. Fo the metho implementation, the special measuing tube containing fou micophones is evelope, whee iffeent pais of micophones ae use fo iffeent fequency anges. The tube iamete is close to the iamete of human ea canals at the entance. Since the aim is to etemine an analyse impeances of auiometic eaphones elate to soun tansmission, both in human eas an in the couple, iffeent teminations of the tube, that is, loas of eaphones ae use. Thus, the eaphones ae couple to a metal plate, to a ubbe pinna, an to the uppe pat of the stanaize couple. The eaphone impeances etemine, togethe with the loas ae mutually compae, but they ae also compae to the aiation impeances of the loas use (seen fom insie the tube without eaphone). Besies, the influence of the loa on the eaphone impeance is examine. These esults ae to be futhe analyse togethe with the coesponing ones obtaine in the investigations of coupling of the auiometic eaphones to human eas an to the same couple. Measuements of impeances. Tansfe function metho The soun tansmission in an ea, with an eaphone place on it, can be epesente by the analogue moel, Figue (a). The etails of the moel ae given in [5]. The thick ashe line epesents the ea canal entance plane. It sepaates eveything outsie the ea canal, moelle by a Thévenin equivalent (geneato soun pessue an impeance), an the pat seen fom the entance insie. A simila moel can be use fo the soun tansmission in the couple when an eaphone is place on it []. The pessue ivision is efine as the atio of the pessue at the ea canal entance p EP,OE an Thévenin geneato soun pessue p EP,BE, which is equivalent to the atio of the ea canal impeance Z EC an the sum of this impeance an Thévenin geneato impeance, that is, the eaphone impeance Z EP. 5

2 Foum Acusticum 5 Buapest Fo the measuement of eaphone impeances, the fou micophone measuing tube is evelope, Figue (b). The stationay soun wave is geneate by the louspeake place at one en of the tube, which is consiee to be insie the hea. Thus, the measuement iection is towa the outsie. The othe en of the tube epesents the measuement plane. p EP BE, + - Z meas Z EP Z EC + - p EP OE, l 4 p ED l 3 l x l l M M M 3 M 4 Figue : Analogue moel escibing soun tansmission in human ea (a) an fou micophone tube fo measuements of acoustic impeances (b) The tansfe function metho use fo etemination of the eflection facto an eaphone impeance is base on the funamental tansmission line equations. The soun pessue p ( x, t) of a plane wave popagating along the tube ( x axis) can be given as the sum of the incient (iect) p an eflecte wave p jkx ( x t) = ( p e p e ) jkx e jwt + - Z ED (a) (b) p, +. () Accoing to Equation (), the soun pessues at the two micophone positions M an M i, whee i can be, 3 o 4, ae p = p e jkl + p e jkl jkl jk li jkl jk li p p e e + p e e i, () =. (3) The pessues of the iect an eflecte waves can be epesente espectively by jk li p p e jkl i p = e jk li jk li e e, (4) jk li p e p jkl i p = e jk li jk li e e. (5) The eflection facto, efine as the atio of the eflecte an iect wave, becomes jk li p e p jk li jkl i jkl e H i = e = e, (6) jk li jk li p p e H e i whee H i is the tansfe function of the pessues measue by the two micophones. The impeance of the tube temination ( Z ) an its nomalize value ( Z n ) can be etemine fom the eflection facto + + Z = ρ c = ρ czn, Z n =. (7). Impeance tube esign Since the impeances of the auiometic eaphones couple to iffeent loas wee to be measue, the tube constuction matches the geometical conitions of the eaphones, pinna, an ea canal. The measuing tube consiste of two pats, the inne an extenal tube. The inne tube of iamete of about 7.8 mm was mae of bass of thickness of about mm. This bass tube was insete in the extenal nylon tube, whose pupose was to enable easie an contolle placement of the micophones. Fo that task, the housings fo the micophones wee ille in the nylon tube. One of the main isavantages of the tansfe function metho is the lowe fequency limit [7]. Thus, in oe to have eliable esults, the tube must have a length of at least λ/. The length of the evelope tube was about 3 mm. Besies, impotant facto etemining lowe an uppe fequency limit fo the tansfe function metho is the micophone spacing. The micophone positions have to be chosen in a way that thee is at least one pai of the micophones whose istance is not close to multiples of a half wavelength ove the whole fequency ange. The eason is inaccuate evaluation of the unknown impeance fo such a istance between the micophones. To cove the fequency ange fom Hz to 6 khz with sufficient accuacy in the tansfe function metho, it is necessay to have fou micophones [7]. Since the istances between the micophones wee: l 4 = 9 mm, l 3 = 4 mm an l = 8 mm, pais of the micophones 4-, 3- an - wee intene to be use fo the following fequency anges espectively: - 75 Hz, Hz, an 35 6 Hz. The fist micophone was positione at the istance of mm fom the tube en. Cae was given to mount the micophones so that they o not istub the soun popagation insie the tube. Thus, each micophone (Sennheise KE-4-) was couple to the tube via a small passage of a iamete of appoximately mm, simila as in [4]. The micophones wee assemble in tansisto-like i 6

3 Foum Acusticum 5 Buapest housings having small holes in the mile of the font caps. The soun was geneate by the louspeake Auax TWF esigne as a high fequency tweete, whose membane was small enough to efficiently aiate soun into the tube. In oe to euce the effects of eflections an tube moes, which incease the eo sensitivity aoun the esonant fequencies, absobing mateial was ae nea the louspeake. The length of the mateial was chosen to euce easonably the moe effects, but not to ecease the soun pessue at high fequencies too much..3 Measuement poceue The soun pessues necessay fo etemination of the eflection facto an the impeance of the tube temination wee measue using PC-base maximum length sequence (MLS) technique. WinMLS softwae was applie an the sampling fequency was 48 khz. Sequences of iffeent oes coul be use as excitation signals, but an th oe MLS with 6 aveages gave a sufficiently lage ynamic ange. The measuements wee caie out in a listening oom with a low level of backgoun noise, although it has been shown that the measuements coul be one in any othe oinay oom [5]. The measuing tube was vetically positione an fixe to the table by a mechanical aangement, Figue. This aangement enable applying the foce of the eaphone heaban on the measue capsule of each of the eaphones simila to the foce foun in the nomal listening conition, when an eaphone is place on an ea. Special cae was taken to place the eaphones in such a way that they wee not incline in any iection. The micophones wee coectly positione by inseting them into the coesponing holes in the nylon tube. Afte that, they wee fixe with the ubbe suouning the micophone caps. In that way, the micophone positions wee stable uing the measuements. Besies, it was necessay to have the accuate elative calibation of the micophones in each pai. The calibation of the micophone's sensitivities was one befoe fixing the micophones to thei positions. Fo that pupose, the micophone esponses wee measue, when each of them was place in the same position an in the same way. The measuement metho was veifie using two known impeances, a igi temination an open tube. Simila esults wee obtaine as in [5]. The impeances of five auiometic eaphones use in the pevious investigations [, ] wee measue hee. Fou of them wee supa-aual eaphones (Telephonics TDH39, Telephonics TDH39 with noise capsules an cushions type ME7 (efee as TDH39C), Beyeynamics DT48 an Holmbeg 95-), an one was cicum-aual (Sennheise HDA). They wee couple to thee iffeent loas, the metal cicula plate, the atificial ubbe pinna (fom hea an toso simulato Büel & Kjæ type 48) mounte on the cicula plate an the uppe pat of the ea simulato Büel & Kjæ type 453 hee esignate couple, which was also use in pevious investigations. Fo the metal plate, the tube en (epesenting the measuement plane) was flush with the plate suface, while fo the ubbe pinna, the measuement plane was appoximately at the entance of the ea canal. Two measuement planes wee use fo the uppe pat of the couple, the fist one appoximately at the plane of the couple micophone membane (the extension was attache to the tube) an the secon one appoximately 5 mm below the fist one (oiginal fom of the tube). Figue : Measuing tube with ubbe pinna as the temination (above) an uppe pat of the couple (below) an Holmbeg 95- eaphone Fo all eaphones, the impeances of both capsules (left an ight) wee measue 5 times emoving an epositioning the eaphone afte each measuement. Aitionally, fo each eposition, the measuements wee epeate 3 times. Fo each of them, the soun pessues wee measue by all fou micophones (fom the fist to the last one) an stability of the measuement aangement was checke by an aitional contol measuement using the fist micophone an compaing the esult with the oiginal one. If thee was any iffeence, the measuements fo that epositioning wee epeate. In aition, the impeances of the loas without eaphones (the aiation impeances) wee also measue. Duing the 7

4 Foum Acusticum 5 Buapest measuements, the metal plate an ubbe pinna aangements wee completely the same fo all five eaphones, while the couple as a loa was iffeent fo supa-aual an cicum-aual eaphones. Thus, the plastic ing use as a est fo supa-aual eaphones was exchange with the special metal plate use as a est fo the cicum-aual eaphone. Howeve, the influence of this change on the aiation impeance esults was almost negligible. Besies, the eliability of the measuements was checke fom time to time measuing the efeence impeance (the igi temination of the tube). 3 Measue impeances 3. aiation impeances The aiation impeances ae taken as the efeence ones. Thei nomalize values fo all thee loas ae shown in Figue 3. Thee is almost no iffeence between the pesente cuves at fequencies up to khz. Above that fequency, the impeance of the metal plate inceases almost without fluctuations while othe two aiation impeances show some fluctuations. Thus, the impeance of the ubbe pinna has a boa peak whose maximum of appoximately.6 is locate aoun 4 khz. This is simila to the esults pesente in [5] except that the maximum value is smalle hee. Anothe peak with geate maximum value (somewhat below ) is locate aoun khz. The fist mentione peak appeas also fo two couple aiation impeances measue at two iffeent planes with slightly iffeent maximum values an locations fom the coesponing ones fo the ubbe pinna. Z n aiation impeance k k Figue 3: Nomalize aiation impeances of use loas: metal plate (thin ), ubbe pinna (thin ---), couple (thick - -) an couple 5 mm below (the explanation is given in the text) ( ) 3. Influence of epositioning The influence of the eaphone epositioning is not the same fo iffeent loas. Thus, the epositioning of the eaphones on the metal plate an couple oes not cause any significant iffeence of the eaphone impeances. Howeve, it leas to some iffeences in almost the whole fequency ange, when the ubbe pinna is use as the loa. These iffeences ae not the same fo iffeent eaphones. The geatest iffeence exists fo Telephonics TDH39C, Figue 4, while the smallest iffeence exists fo Sennheise HDA (not shown). Besies, in most of the combinations of the eaphones an loas, thee is some iffeence between the impeances of left an ight capsule of a paticula eaphone, which is usually elatively small. In oe to take into account the influence of the epositioning an the iffeence between the eaphone capsules, the impeances fom five epositions fo left an ight capsules ae aveage to give the mean impeance fo each of the eaphone an loa. Z n Telephonics TDH39C left capsule ight capsule k k Figue 4: Diffeences of the eaphone nomalize impeance cause by epositioning fo left an ight capsule of one of the eaphones couple to ubbe pinna (ashe line epesents aveage value) 3.3 Impeances of eaphones The nomalize mean impeances of each of the eaphones ae iffeent fo iffeent loas, Figue 5. This is the case even at low fequencies except fo the cicum-aual eaphone whee the impeances ae almost the same up to khz. Fo almost all eaphones an loas, the eaphone impeances show significant fluctuations at highe fequencies vaying the value between.5 an o even moe. The smallest values of the impeances of the supa-aual eaphones an almost all loas appea between an khz. Also, it is notice that the impeances of the eaphones couple to the couple ae moe simila to the impeances of the eaphones couple to the metal plate than to the ubbe pinna. 8

5 Foum Acusticum 5 Buapest Holmbeg 95- ubbe pinna. Fo the couple as a loa, the eaphone impeances at low fequencies have lage values than the aiation impeance an also the eaphone impeances fo the othe two loas. The eviation of the eaphone impeances fom the aiation impeance is the geatest fo the couple when the measuement plane is 5 mm below the couple micophone membane. Beyeynamic DT48 Z n Metal plate Telephonics TDH39 Z n ubbe pinna Telephonics TDH39C Couple Sennheise HDA k k Figue 5: Nomalize impeances of the eaphones couple to iffeent loas: metal plate (thin ), ubbe pinna (---), couple (thick - -) an couple 5 mm below ( ) The eaphone impeances now plotte togethe fo the same loa ae pesente in Figue 6. In aition, the aiation impeance fo each of the loas is given togethe with the eaphone impeances. The iffeences between the eaphone impeances an the iffeences between the aiation an eaphone impeances ae not the same fo iffeent loas. Thus, these iffeences ae the smallest fo the metal plate. It is inteesting that they ae elatively small even at high fequencies in this case. Fo the ubbe pinna as a loa, thee is a peak at fequencies up to khz fo each of the eaphones, while this peak oes not exist in the aiation impeance. The mentione peak is the unique popety of the eaphone impeances only fo the Couple 5 mm below k k Figue 6: Nomalize impeances of the eaphones Holmbeg 95- (thin ), Beyeynamics DT48 (thin ---), Telephonics TDH39 (- -),Telephonics TDH39C (thick ) an Sennheise HDA (---) given togethe fo paticula loa incluing nomalize aiation impeance (thick ) The aitional (nomalize) impeance that only an eaphone intouces without a loa can be obtaine by subtaction of the nomalize aiation impeance fom the nomalize eaphone impeance Z nepa = Z nep Z na. (8) This aitional impeance can be intepete as an acoustic loa cause only by the eaphone. eal an imaginay pats of the iffeence of the impeances fo 9

6 Foum Acusticum 5 Buapest two loas ae pesente in Figue 7. The aitional impeance cause only by an eaphone iffes fo the iffeent eaphones couple to the same loa. Also, it is not the same fo the iffeent loas an paticula eaphone. The iffeences exist in both eal an imaginay pats. Nevetheless, the ange of values of the aitional impeances is simila to the one fom [5], although each eaphone shows its specific featues. Compaing the aitional impeances intouce by use eaphones, it is notice that the smallest impeance is obtaine fo Sennheise HDA. e(z nepa ) Im(Z nepa ) ubbe pinna Couple ubbe pinna Couple k k k (a) k (b) Figue 7: eal (a) an imaginay (b) pats of the iffeences of nomalize eaphone impeances (Holmbeg 95- (thin ), Beyeynamics DT48 (thin ---), Telephonics TDH39 (- -), Telephonics TDH39C (thick ), Sennheise HDA (thick )) an aiation impeances of two loas 4 Conclusions stanaize metho applie to the measuing tube evelope. Both the impeances of the eaphones togethe with the loas an the aiation impeances of the loas only ae etemine. Base on these esults, the aitional impeances intouce only by the eaphones consiee as the acoustic loas of the eaphones ae estimate. It is note that the impeances of the auiometic eaphones ae iffeent fo a paticula loa, but also the impeances of a paticula eaphone iffe, when couple to iffeent loas. The smallest acoustic loa intouce by an eaphone is obtaine fo the cicumaual eaphone Sennheise HDA. The esults obtaine hee ae to be use fo futhe analysis of the influence of the eaphones an thei impeances on soun tansmission in both human eas an the couple, that is, on coupling of auiometic eaphones to human eas an the couple. efeences [] D. Ćiić, an D. Hammeshøi, Coupling of Eaphones to Human Ea, Poc. Int. Conf. Euonoise 3, Naples (May 3), Abstact in Acta Acustica unit. with Acustica, Vol. 89. Suppl.. p. S 74 (May/June 3) [] D. Ćiić, an D. Hammeshøi, Coupling of Eaphones to Human Ea an to Couple, 47 th Meeting of Acoust. Soc. Am., New Yok (May 4), Abstact in J. Acoust. Soc. Am., Vol. 5. No. 5. p. 499 (pape ppp). (May 4) [3] ISO 534-: Detemination of Soun Absoption Coefficient an Impeance in Impeance Tubes Pat : Tansfe Function Metho, (996) [4] H. Hue, A. Lowig, an A. Engel, A Wie Ban Pecision Acoustic Measuing Hea, Acustica, Vol. 8. pp (996) [5] M. Voläne, Acoustic Loa on the Ea Cause by Heaphones, J. Acoust. Soc. Am., Vol. 7. pp () [6] M. Cuzao, C. Gemán, L. amiez Salvao an J. Klebe, Methoical Measuement of Acoustical Impeance an eplacement Sensiblity, Poc. Int. Conf. Foum Acusticum, Sevilla (Septembe ) [7] H. Hue, an A. Engel, Measuing an Moeling Basic Popeties of the Human Mile Ea an Ea Canal Pat I: Moel Stuctue an Measuing Techniques, Acoustica acta acustica, Vol. 84. pp (998) The impeances of the auiometic eaphones couple to fou iffeent loas ae measue using the