Dorte Hammershøi

Sound transmission to and within the human ear canal

Sound transmission to the eardrum from various points in the external ear was measured by means of probe microphone technique. Twelve human subjects participated, and three directions of sound incidence were included. For the major part of the audio frequency range the transmission to the eardrum proved independent of direction from points at the centerline of the ear canal, including the entrance (open or blocked). The results further suggested that the region with independent transmission extends some millimeters outside the entrance plane. The transmission from the free field to the eardrum was divided into a directional-dependent part and two directional-independent parts: (1) the transmission from the free field to the blocked entrance, (2) a pressure division between the radiation impedance and the ear-canal input impedance, and (3) the transmission along the ear canal. All parts of the transmission were seen to be highly individual. The first part was shown to be uncorrelated with any of the other parts, whereas mutual dependence of parts (2) and (3) resulted in a smaller variation in the combined transmission than for the parts in separate. The standard deviation between subjects for head-related transfer functions (HRTFs) measured at the eardrum, the open entrance, and the blocked entrance was studied, and the lowest values were found for the blocked-entrance HRTFs. It is concluded, that the blocked entrance is the most suitable point for measurements of HRTFs and for binaural recordings, since sound at this point includes the complete spatial information, and in addition to that the minimum amount of individual information.

Distortion product otoacoustic emission fine structure analysis of 50 normal-hearing humans

When distortion product otoacoustic emissions (DPOAEs) are measured with a high-frequency resolution, the DPOAE shows quasi-periodic variations across frequency, called DPOAE fine structure. In this study the DPOAE fine structure is determined for 50 normal-hearing humans using fixed primary levels of L1/L2 = 65/45 dB. An algorithm is developed, which characterizes the fine structure ripples in terms of three parameters: ripple spacing, ripple height, and ripple prevalence. The characteristic patterns of fine structure can be found in the DPOAE of all subjects, though the DPOAE fine structure characteristics are individual and vary from subject to subject. On average the ripple spacing decreases with increasing frequency from 1/8 oct at 1 kHz to 3/32 oct at 5 kHz. The ripple prevalence is two to three ripples per 1/3 oct, and ripple heights of up to 32 dB could be detected. The 50 normal-hearing subjects were divided into two groups, the subjects of group A having slightly better hearing levels than subjects of group B. The subjects of group A have significantly higher DPOAE levels. The overall prevalence of fine structure ripples do not differ between the two groups, but are higher and narrower for subjects of group B than for group A.

Binaural Technique — Basic Methods for Recording, Synthesis, and Reproduction

The term “binaural technique” is used as a cover label here for methods of sound recording, synthesis and reproduction, where the signals in focus are the acoustic signals at the eardrums. If these are presented authentically to listeners, the listeners will obtain acoustic cues which are deemed sufficient for authentic auditory experience — including its spatial aspects. This chapter reviews the basic principles of binaural technique - putting a special focus on results of investigations which have been performed at Aalborg University. These basic principles form the foundation for current utilization of binaural technique at large. They include basic theory, investigations on sound transmission in the ear canal, measurements and post-processing of head-related transfer functions, HRTFs, transfer functions of headphones and their adequate equalization, and results from localization experiments in real life as well as with binaural recordings from real heads and artificial heads. Numerous applications to these methods exist. Some of them will be introduced exemplarily.

Distortion product otoacoustic emission of symphony orchestra musicians before and after rehearsal

The 2f1-f2 distortion product otoacoustic emission (DPOAE) and hearing levels are obtained for 12 normal-hearing symphony orchestra musicians both before and after their rehearsal. The DPOAE fine structures are determined and analyzed according to the character and prevalence of ripples. Hearing levels, DPOAE levels, and DPOAE fine structures before and after rehearsal are similar, indicating that no or marginal temporary change of the state of hearing were caused by the exposure. The data were further compared to similar data for occupationally nonexposed subjects, one group which was age and gender matched, and other two groups of younger individuals (one group with better hearing levels than the other). The data for the age and gender matched group compared well with the musicians data (and the data for the group of better-hearing younger individuals). In general, the analyses of hearing thresholds and DPOAE data thus lead to the same conclusions concerning the state of hearing.

Overexposure effects of a 1-kHz tone on the distortion product otoacoustic emission in humans

The effects of overexposure on the properties of distortion product otoacoustic emissions (DPOAEs) are investigated. In total, 39 normal-hearing humans were monaurally exposed to a 1-kHz tone lasting for 3 min at an equivalent threshold sound-pressure level of 105.5 dB. The effects of overexposure were studied in two experiments (1) on the broadband DPOAE and (2) on the DPOAE fine structure, measured using a higher frequency resolution in a narrower frequency range. The obtained DPOAE shifts were compared to temporary threshold shift (TTS) obtained after a similar exposure. Similarities between DPOAE shifts and TTS were found in the affected frequency range and the time course of recovery. The amount of TTS was higher in the early recovery time (1-4-min postexposure), but similar to the DPOAE shift (even in absolute terms) at later recovery times (5-20-min postexposure). The DPOAE fine structure was not systematically changed after the exposure.

Determination of noise immission from sound sources close to the ears

When a sound exposure stems from a sound source that is close to the ear of the exposed person, the noise is described in terms of the free-field related or diffuse-field related sound pressure level, i.e. the level of a free or a diffuse-sound field that would result in the same exposure of the persons ear as that stemming from the close sound source. The at-ear sound exposure level is measured either by MIRE technique (microphones in real ears) or by a manikin, and the free-field related or diffuse-field related sound pressure levels are obtained by subtracting the free-field-front or the diffuse-field head-related transfer function (HRTF) expressed in dB. The use of either, the eardrum, the open entrance, or the blocked entrance as measurement point for the MIRE-technique is evaluated. The results are the same, wherever in the ear canal the measurements are made. There is good agreement between human HRTFs measured at different laboratories, and for eardrum, open-entrance and blocked entrance, standard HRTF data have been derived, which may be used instead of HRTFs measured for each subject. The resulting statistical uncertainty depends on the choice of measurement point, and whether individual or standard HRTF data are used. Generally, measurements at the blocked entrance are practical and produce results with low statistical uncertainty. The results from manikin-measurements do not agree well with results from humans (MIRE), when HRTFs from the actual manikin or from the manikin standards (IEC 60959 and ITU-T P.58) are used. A better agreement is obtained with HRTFs constructed by multiplying human blocked-entrance data with the transfer function of the standardized coupler for manikins. This method is therefore described (and data tabled) in ISO 11904-2. A comparison between humans, manikins, and the manikin standards suggest, that standards do not specify an average human and should be revised.

Stimulus ratio dependence of low-frequency distortion-product otoacoustic emissions in humans

Active amplifiers within the cochlea generate, as a by-product of their function, distortion-product otoacoustic emissions (DPOAEs) in response to specific two-tone stimuli. Focus has been on invoking emissions in a mid-frequency range from ∼0.5 to 4 kHz. The present study investigates stimulus parameters of the DPOAE at 2f1-f2 frequencies below 0.5 kHz. Eighteen out of 21 young human adults screened had audiometrically normal hearing for inclusion in the experiment. DPOAEs were measured with pure-tone stimuli in four configurations: f2 fixed around 2.13 kHz, f2 fixed around 0.53 kHz, 2f1-f2 fixed at 1.23 kHz and 0.25 kHz. Eight stimulus ratios, f2/f1, and three stimulus sound pressure levels, L1/L2, were measured in each configuration. Trends in ratio-magnitude responses for the mid-frequency DPOAE agree with those reported in previous literature. DPOAEs are not limited to distortion frequencies >0.5 kHz, but the stimulus ratio invoking the largest DPOAE in the mid-frequency range does not do so in the low-frequency range. Guiding the ratio according to the equivalent rectangular bandwidth of auditory filters maintains the DPOAE level.

Coupling of earphones to human ear and to coupler

The use of a standardized acoustical coupler should enable a calibration of audiometric earphones which ensures that the thresholds determined in the audiometry will be independent of the earphone type. This requires that the coupler approximates the average human ear closely. Nevertheless, the differences among earphones as well as between human ears and the coupler affect the results of audiometric measurements inducing uncertainty. The influence of these differences is examined by investigating the coupling of different earphones to human ears and to the standardized coupler. This is done by measurement of the transfer functions from input voltage of the earphone terminals to the entrance of the ear canal in two situations: (1) open, and (2) blocked. Similar measurements were carried out with the coupler, but since the ‘‘ear‐canal entrance’’ is not well‐defined for the coupler, the mentioned measurements were done at different depths in the coupler. The earphone’s coupling to (i) human ears and to (ii)...

Separation and characterization of maternal cardiac and vascular sounds in the third trimester of pregnancy

To characterize the vascular sounds of the uteroplacental blood flow obtained by microphones.

Quantitative assessment of spatial sound distortion by the semi-ideal recording point of a hear-through device

A hear-through device combines a microphone and earphone in an earpiece so that when worn, one per ear, it can work as an acoustically transparent system allowing for simultaneous individual binaural recording and playback of the real sound field at the ears. Recognizing the blocked entrance to the ear canal as the ideal recording point - i.e. all directional properties of the incident sound field are recorded without distortion - it is critical for such device to be sufficiently small so that it can be completely inserted into the ear canal. This is not always feasible and the device may stretch out from the ideal position and thus distort the captured spatial information. Here we present measurements that quantify by how much the directional properties of the sound field are distorted by semi-ideal hear-through prototypes built by mounting miniature microphones on the outer part of selected commercial insert earphones. This includes an analysis of the magnitude by which spatial information is distorted ...