Akustische Übertragungsfunktion des Außenohres beim Haushund

Abstract

Die akustische Funktion des Außenohres besteht in der Sammlung und Weiterleitung eines akustischen Signals, sowie dessen richtungsabhängiger spektraler Umformung. Die Zielsetzung der vorliegenden Arbeit lag in der Darstellung der akustischen Eigenschaften der Außenohren zweier Haushunde mit langen Stehohren durch die Bestimmung der akustischen Übertragungsfunktionen anhand künstlicher Außenohrmodelle mit exakten anatomischen Abmessungen. Die binauralen und monauralen Merkmale der am Trommelfell anliegenden Signale wurden als potentielle Informationsquelle für die Schalllokalisation untersucht. Zur Validierung wurde die Methodik durch den Vergleich der mit der gleichen Methode an einem Katzenohrmodell ermittelten Daten mit publizierten Ergebnissen von narkotisierten Katzen überprüft. Weiterhin wurde an den gewonnenen Abdrücken der Außenohre deren anatomischer Aufbau beschrieben und die Abmessungen festgestellt. Die Untersuchungen fanden in einem reflexionsarmen Raum (IAC-Freifeld- Halbraum) statt. Die Messungen wurden bei verschiedenen Schallquellenpositionen in der horizontalen Ebene (Azimut von 0° bis 360° in 10°-Schritten) durchgeführt. Als Stimulus wurde ein bandbegrenztes Pseudorauschen (500 Hz bis 20 kHz) mit einer Dauer von 1 s verwendet. Das Signal wurde mittels Elektretmikrofonen, die sich an der Position des Trommelfelles befanden, erfasst. Als Referenz wurde das gleiche Signal im Freifeld aufgezeichnet. Beschrieben wurde die Umformung des akustischen Signals nach der Passage durch das Außenohr (Außenohrübertragungsfunktion). Weiterhin wurden die Laufzeitdifferenzen eines 500 Hz Sinustones ermittelt. Für die Versuchsdurchführung wurden verschiedene Außenohrmodelle erstellt. Diese unterschieden sich in der Ausrichtung der äußeren Ohrmuschel (vorwärts und seitwärts gerichtete Ohrposition). Des weiteren wurden die originalen Modelle modifiziert, indem die Conchainnenstrukturen oder die äußere Ohrmuschel entfernt bzw. tierärztliche Manipulationen, z.B. eine Otitis- Operation oder das Kupieren der äußeren Ohrmuschel, simuliert wurden. Ein weiterer Punkt war die Darstellung des Einflusses und der Bedeutung des Kopfes für die Übertragungsfunktionen, das durch das Verwenden eines Kunstkopfes im Versuchsaufbau realisiert wurde. Die Daten am Beispiel von Haushunden mit langen Stehohren demonstrieren, dass der Verlauf und die spektralen Merkmale der Übertragungsfunktionen auf spezielle morphologische Strukturen im Außenohr zurückzuführen sind. Dadurch ist es möglich, richtungsabhängige Informationen durch die Umformung eines Stimulus bereitzustellen.

The acoustical function of the outer ear consists of the collection and forwarding of an acoustical signal as well as its directional spectral transformation. The objective of the present paper is the presentation of the acoustical characteristics of the outer ear of two domestic dogs with long erect ears by determinating of the acoustical transfer functions by means of artificial ear models with exact anatomical dimensions. The binaural and monaural features of the signals connected to the tympanic membrane were examined as a potential information source for sound localization. The methodological approach was verified by comparison with the data determined by the same means at a ear model of cat to those of published results using anaesthetized cats. Moreover has the anatomical construction and the measurements of the outer ear prints been described. The experiments were carried out inside an anechoic chamber (IBAC). The measurements were carried out with different acoustic source positions in the horizontal plane (azimuth of 0ý to 360ý in 10ý steps). A band limited pseudo noise (500 Hz to 20 kHz) was employed as acoutic stimuli with a duration of one second. The signal was measured by means of a miniature microphone instead of the tympanic membrane. As a reference, the same signal in the free field was recorded. The transformation of the acoustic signals from the free field to the eardrum was described (head related transfer function). Furthermore the interaural time difference of the 500 Hz sine tone was determined. For the testing procedure, different artificial outer ear models were constructed., which differed in the direction of the pinna (ear position set forward and sideways). Since the concha is characterized by small cartilaginous protuberances, we have modified the model by eliminating these special structures. Furthermore, the original models were modified when the pinna was removed or veterinary manipulations were simulated such as an Otitis operation or the reduction of the size of the pinna (cropping). A further point was the representation of the influence and the importance of the head for the transfer functions which was implemented by employing an artificial head in an experimental setup. The following results were obtained: 1) The data of the external ear models of the cat show that plastic models with realistic sizes show the same directivity as the outer ears of anaesthetized animals. 2) The outer ear of the dogs has a marked directivity and supplies binaural (interaural level differences and interaural time differences) and monaural spectral features which allow an unambiguous inference to the sound direction of the incidence. 3) Interaural time differences of the 500 Hz sine tone allow an unambiguous definition of the sound direction of the incidence. The amount of the directional interaural time differences is enlarged clearly by the head. 4) Tree frequency regions have been defined by different directional-dependent sound pressure transformations of the outer ear a) Sound signals with frequencies from the low-frequency region (500 Hz to 1000 Hz) show no appreciable monaural and interaural directional-dependent modification of their level differences. b) Sound signals with frequencies from the mid-frequency region (1 kHz to 4 kHz) are clearly amplified via 10 dB. The directional-dependent markedness of the first and most prominent broad peak in the free field transfer functions is made out. The spectral maximum which is developed by frequencies between 2,5 kHz to 4 kHz lies in this broad peak. c) Sound signals with frequencies more than 4 kHz (higher-frquency region: 4 kHz to 17 kHz) shows the highest directional behaviour in the examined frequency range. Notch and futher peaks whose concrete values strong show individual differences are developed. The central frequency of the first notch shows a marked direction dependence with both animals. The monaural and interaural level differences of higher frequency signals change clearly during modifications of the acoustic source position and supply unambiguous information on the definition of the horizontal acoustic source position. 5) The most important acoustical function of the pinna lies in the possibility of focusing on a specific sound field of the incidence. Because of the mobility of the outer ear, dogs are able to arrange the pinna at a sound event. When the pinna is oriented directly towards the source, the pressure ratio reaches a maximum. Simultaneously sound signals (noise interference) from other directions are decreased. The acoustical spectral and spatial axes are in the field of the open face of the pinna. The position of first notch varies continuously with changes of the acoustic source position in the frontal sound field. In this way, ear position sets up the set forward especially for available sound events from the frontal field direction information. 6) The small cartilaginous protuberances of the concha and the upper frontal edge of the external auditory canal produce the first notch if direct acoustic waves fall into the open face of the pinna. If the protuberances are missing, an alternation of the sound direction of incidence in the frontal sound field does not produce a change of the monaural level differences from sound signal with frequencies between 500 Hz and 10 kHz. 7) The intensity of low-frequency sound signals (lower 1 kHz) is nearly no influenced by the head. At sound with frequencies over 1 kHz, the head affects the amplification of frontal sound caused by the outer ear as well as the attenuation of the sound supporting in the case of contralateral and reverse incidence. Furthermore, the directional shift of the position of the first notch caused by the outer ear is stabilized by the head and the sound field of incidence, in which the rise of its central frequency is observed, is extended. 8) Veterinary manipulations of the outer ear modify its directivity. The consequences of an Otitis operation for directional hearing are to be hardly estimated since the effects they represent for itself in the prevention of the training of the first notch in the case of some sound directions of arrival from the frontal field. This notch embodies an important directional monaural feature there. 9) Removing a part of the pinna, as while cropping, has no dramatic effects on the directivity of the outer ear. However, focusing onto a specific sound field of the incidence is limited. In this case, the scale of influence increases with the size of the reduction. On the other hand, absence of the entire pinna produces a loss on direction information for frequencies below 9 kHz and in the range from 14 kHz to 16 kHz The data on the example of domestic dogs with long erect ears demonstrate that the process and the spectral features of the transfer functions are to be traced back to specific morphological structures in the outer ear. As a result, it is possible to provide directional information by transforming a stimulus.