Marko Hiipakka

Estimating ressure at eardrum with pressure-velocity measurement from ear canal entrance

It is important to know the sound pressure signal at the eardrum in headphone reproduction and in audiological applications. Unfortunately, it is difficult to conduct direct measurement safely. Estimating pressure signals at the eardrum based on measurements done elsewhere in the ear canal is sensitive to positioning errors. This is particularly the case at the canal entrance. In addition, not knowing the acoustic properties of the ear canal makes estimation difficult. This study shows that when both sound pressure and velocity are measured at the canal entrance using a pressure-velocity probe, the pressure signal at the eardrum can be estimated with much higher accuracy than from the pressure-only measurement. The method is demonstrated and validated by using physical simulators and computational modeling.

An individualised acoustically transparent earpiece for hearing devices

Abstract Objective: An important and often still unresolved problem of hearing devices such as assistive listening devices and hearing aids is limited user acceptance – a primary reason is poor conservation quality of the acoustic environment. Approaching a possible solution to this problem, an earpiece prototype is presented and evaluated. The prototype is individually and automatically calibrated in situ to provide acoustical transparency, i.e., achieving an audio perception alike to the open ear. Design: A comprehensive evaluation was performed, comprising technical measurements on an advanced dummy head and listening tests, in which listeners directly compared sound perception through the prototype and a simulated open ear canal reference. Study sample: Ten normal hearing subjects, including five expert listeners, participated in the listening test. Results: The technical evaluation verified good achievement of acoustical transparency. The psychoacoustic results showed that a reliable distinction between the two conditions presented was not possible for relevant communication sounds. Conclusion: The prototype can be described as an initial realisation of an acoustically transparent hearing system, i.e. a device that does not disturb the perception of external sounds. In further developments, the device can be considered as the basis for systems integrating high sound quality, hearing support and other desired modifications.

Estimating head-related transfer functions of human subjects from pressure–velocity measurements

Direct measurements of individual head-related transfer functions (HRTFs) with a probe microphone at the eardrum are unpleasant, risky, and unreliable and therefore have not been widely used. Instead, the HRTFs are commonly measured from the blocked ear canal entrance, which excludes the effects of the individual ear canals and eardrums. This paper presents a method that allows obtaining individually correct magnitude frequency responses of HRTFs at the eardrum from pressure-velocity (PU) measurements at the ear canal entrance with a miniature PU sensor. The HRTFs of 25 test subjects with nine directions of sound incidence were estimated using real anechoic measurements and an energy-based estimation method. To validate the approach, measurements were also conducted with probe microphones near the eardrums as well as at blocked ear canal entrances. Comparisons between the different methods show that the method presented is a valid and reliable technique for obtaining magnitude frequency responses of HRTFs. The HRTF filters designed using the PU measurements are also shown to yield more correct frequency responses at the eardrum than the filters designed using measurements from the blocked ear canal entrance.

Toward a scalable, binaural hearing device: Acoustics, interaural processing, and scene decomposition

One of the aims of the focused research group “Individualized hearing acoustics” are elements of an assistive listening device that both fits the requirements of near-to-normal listeners (i.e., providing benefit in noisy situations or other daily life acoustical challenges) and, in addition, can be scaled up to a complete hearing aid for a more substantial hearing loss. A review is given on the current work on acoustically “transparent” open-fitting earpieces, algorithms for interaural cue enhancement, and for automatically detecting relevant changes in the acoustical scene that call for an altered processing strategy. The current prototype runs on the binaural, cable-connected master hearing aid (MHA) that includes earpieces that allow for approaching acoustic transparency. This can be achieved by individual calibration of the earpieces using in-situ in-ear measurements and electro-acoustic models of the earpieces and the ear canal. A binaural high-fidelity enhancement algorithm motivated by interaural m...

Individual in-situ calibration of insert headphones

An important procedure in binaural reproduction is the calibration of headphones, which is commonly achieved by first measuring the headphone transfer functions (HpTFs). The commonly used methods of measuring the HpTF are not applicable for insert headphones, since the inserts block the ear canal entrance and since the transducer ports of the inserts are inside the ear canals. Recently, an alternative technique of obtaining HpTFs of inserts using measurements with in-ear microphones, computational modeling, and electro-acoustic Norton-type source models of the inserts has been proposed. In this study, the technique is evaluated using measurements at the eardrums of six human subjects and computational modeling with normal human ear canal parameters. In addition, different methods of obtaining the electro-acoustic source model parameters are compared. It is shown that the most reliable method of obtaining the Norton source parameters of insert headphones is through measurements with a miniature-sized press...

Measuring pressure and particle velocity along the human ear canal

A non-invasive method of measuring or estimating accurately the head-related transfer functions (HRTFs) and headphone transfer functions (HpTFs), i.e., the pressure at the eardrum rather than at the blocked ear canal entrance is called for. In this work, a miniature-sized acoustic pressure-velocity sensor is used to measure both pressure and velocity along the ear canals of human test subjects. The measurements are used to study the applicability of a recently proposed method of estimating the pressure at the eardrum from pressure-velocity measurements made at the ear canal entrance. The measurement results are compared to results from computational modeling with human ear canal parameters. In addition, the effect of the PU-sensor itself on the pressure at the eardrum is studied. It is shown that the estimation method is reliable and accurate for most human subjects. The diameter and the shape of the ear canal affect the results in such a way that the best results are obtained with wide and straight ear canals. It is concluded that the estimation method facilitates the obtaining of individual HRTFs and HpTFs at the eardrum using non-invasive measurements.