![]() ![]() Above 1600 Hz, however, the wavelengths become much smaller than the interaural distance (distance between the ears) itself, rendering phase delays impossible to resolve. This is based on limitations of the human auditory system in resolving the sound spectrum of frequencies and intensities: below 800 Hz, the wavelengths are larger than the distance between the ears, delivering easily detectable phase delays calculated as ITD. The Duplex Theory, published by Lord Rayleigh in 1907, prioritizes one over the other based on sound frequency: ITD for lower frequency (i.e., less than 800 Hz) and ILD for higher frequency (i.e., more than 1600 Hz) sounds, with intermediate frequency, sounds presenting an overlap between ITD and ILD dominance. It is unclear whether interaural time or volume difference is the main aspect involved in the localization of sounds. ![]() ![]() ITD and ILD would increase according to the azimuth of the sound source from 0 degrees (centered in front of the subject) up to 90 degrees and decrease from 90 to 180 degrees (centered at the back of the subject). The brain centers involved in integrating information from both ears are located in the brainstem these consist the inferior colliculus in the midbrain, and a group of pontine nuclei called the superior olivary complex, of which only the medial superior olive and superior lateral olive truly receive afferents from bilateral sources.Īrrival time and volume from each ear are registered in the brain, and the interaural time difference (ITD) and interaural level difference (ILD) are computed. The ears are located symmetrically on either side of the head, which allows for binaural sound localization in the brain regarding direction and distance away. It is homologous to the tip of the ears of pointy-eared mammals. The lobule is devoid of cartilage and contains areolar connective tissue.Ī small projection from the tragus, called Darwin’s tubercle, is present in a fraction of the population. Anteriorly to the acoustic meatus and concha is a cartilaginous projection called the tragus, and inferiorly to it hangs the lobule (earlobe). Given that the external acoustic meatus is not in perfect alignment with the external auditory canal, the pinna must be pulled backward and upward during an otoscopic examination to allow proper angling of the otoscope tip in the external auditory canal. The canal continues in the skull in sigmoid fashion until it meets the tympanic membrane. The external acoustic meatus arises from the anteriormost part of the concha and marks the beginning of the external auditory canal. Between the helix and antihelix is a shallow concavity, the scaphoid fossa. The antihelix is a convex eminence located anteriorly to the helix it curves outward from the deep concha anterior to it. The helix is the outer posterosuperior rim of the ear and curves slightly inward towards the external acoustic meatus, giving the ear its concave geometry. The external auditory canal is laden with specialized ceruminous glands, which produce cerumen (earwax) that may hinder or repel the entry of insects and debris through the external acoustic meatus. The skin of the pinna is rich in sebaceous glands, which serve to protect the ear from cracking. The external ear’s flexible structure is maintained by elastic cartilage, which is covered in skin and attached to the skull with ligaments and muscles. ![]()
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