Preliminary Analyses of Tympanic-Membrane Motion from Holographic Measurements

Abstract

Computer-aided, personal computer (PC) based, optoelectronic holography (OEH) was used to obtain preliminary measurements of the sound-induced displacement of the tympanic membrane (TM) of cadaver cats and chinchillas. Real-time time-averaged holograms, processed at video rates, were used to characterise the frequency dependence of TM displacements as tone frequency was swept from 400 Hz to 20 kHz. Stroboscopic holography was used at selected frequencies to measure, in full-field-of-view, displacements of the TM surface with nanometer resolution. These measurements enable the determination and the characterisation of inward and outward displacements of the TM. The time-averaged holographic data suggest standing wave patterns on the cat's TM surface, which move from simple uni-modal or bi-modal patterns at low frequencies, through complicated multimodal patterns above 3 kHz, to highly ordered arrangements of displacement waves with tone frequencies above 15 kHz. The frequency boundaries of the different wave patterns are lower in chinchilla (simple patterns below 600 Hz, ordered patterns above 4 kHz) than cat. The stroboscopic holography measurements indicate wave-like motion patterns on the TM surface, where the number of wavelengths captured along sections of the TM increased with stimulus frequency with as many as 11 wavelengths visible on the chinchilla TM at 16 kHz. Counts of the visible number of wavelengths on TM sections with different sound stimulus frequency provided estimates of wave velocity along the TM surface that ranged from 5 m s(-1) at frequencies below 8 kHz and increased to 25 m s(-1) by 20 kHz.

Figure 1

Optical probe (OP) being developed for characterising shape and deformation of samples in confined spaces by optoelectronic holography (OEH). CCD, the high speed camera; BS, the beam splitter; BO, the borescope assembly; PZT, the piezoelectric mirror; M, the steering mirror; RB and OB, the reference and object beam fibres; MI, the microphone; SP, the sound source; K₁ and K₂, the illumination and observation direction vectors

Figure 2

Time-averaged holograms in tympanic membrane (TM) of cat and chinchilla at selected frequencies showing: (A) simple, (B) complex and (C) ordered displacement patterns. The TM is outlined in (A) and the umbo is marked by ‘U’

Figure 3

Full-field-of-view stroboscopic holography measurements in cat at 8776 Hz, 110 dB sound pressure level (SPL), showing a maximum displacement on the order of 350 nm. The vertical contour crosses the manubrium and the umbo is marked by ‘U’

Figure 4

Full-field-of-view stroboscopic holography measurements in chinchilla at 15 700 Hz, 107 dB sound pressure level (SPL), showing a maximum displacement on the order of 150 nm. The horizontal contour crosses the umbo is marked by ‘U’

Figure 5

Automatic mask generation applied to data measured in a chinchilla tympanic membrane (TM) at 2800 Hz, 94 dB sound pressure level (SPL) stimulus: (A) mask without edge smoothing; and (B) mask after application of island-filling and spatial smoothing algorithms

Figure 6

Separation between displacement maxima in stroboscopic holograms from a chinchilla ear at various frequencies (A); and (B) wave speed computed from data in (A) by using Equation (14)