Volume 125, Issue 5, May 2009
Index of content:
- PHYSIOLOGICAL ACOUSTICS 
125(2009); http://dx.doi.org/10.1121/1.3097446View Description Hide Description
The sound field in the external ear can be subdivided into a distinctly three-dimensional part in front of pinna and concha, a fairly regular part in the core region of ear canals, and a less regular part in the drum coupling region near the tympanic membrane. The different parts of the sound field and their interaction have been studied using finite elements. A “pinna box” enclosing the pinna provides both a realistic coupling of the external space to the ear canal and the generation of sound. The sound field in the core region turns out to be not that regular as mostly assumed: near pressure minima and maxima “one-sided” isosurfaces (surfaces of equal pressure magnitude) occur, which are inconsistent with the notion of a middle axis, in principle. Nevertheless such isosurfaces can be seen as part of a “fundamental sound field,” which is governed by the principle of minimum energy. Actually, the sound transformation through narrow ducts is little affected by one-sided isosurfaces in between. As expected, the beginning of the core region depends on frequency. If the full audio range up to 20 kHz is to be covered, a location in the first bend of the ear canal is found.
125(2009); http://dx.doi.org/10.1121/1.3097464View Description Hide Description
The aim of this research is to extend previous studies of the time-frequency features of otoacoustic emissions(OAEs) using information about the properties of the signals at low frequencies. Responses to 0.5 kHz tone bursts were compared to OAEs that were evoked by click stimuli and by 1, 2, and 4 kHz tone burst stimuli. The OAEs were measured using 20 and 30 ms intervals between stimuli. The analysis revealed no differences in the time-frequency properties of 1, 2, and 4 kHz bursts measured using these two different acquisition windows. However, at 0.5 kHz the latency of the response was affected significantly if a shorter time window was used. This was caused by the fact that the response reached a maximum after an average time of 15.4 ms, and lasted a few milliseconds longer. Therefore, for this particular stimulus, the use of a 30 ms time window seems more appropriate. In addition, as an example of the possible application of low-frequency OAEs, signals were measured in patients suffering from partial deafness, characterized by steep audiograms with normal thresholds up to 0.5 kHz and almost total deafness above this frequency. Although no response to clicks was observed in these subjects, the use of 0.5 kHz tone bursts did produce OAEs.
125(2009); http://dx.doi.org/10.1121/1.3097768View Description Hide Description
Spontaneous otoacoustic emissions (SOAEs) were measured longitudinally for durations up to 19.5 years. Initial ages of the subjects ranged from 6 to 41 years. The most compelling finding was a decrease in frequency of all emissions in all subjects, which was approximately linear in %/year and averaged 0.25%/year. SOAE levels also tended to decrease with age, a trend that was significant, but not consistent across emissions, either within or across subjects. Levels of individual SOAEs might decrease, increase, or remain relatively constant with age. Several types of frequency/level instabilities were noted in which some SOAEs within an ear interacted such that their levels were negatively correlated. These instabilities often persisted for many years. SOAEs were also measured in two females over the course of their pregnancies. No changes in SOAE levels or frequencies were seen, that were larger than have been reported in females over a menstrual cycle, suggesting that levels of female gonadal hormones do not have a significant direct effect on SOAE frequencies or levels.
125(2009); http://dx.doi.org/10.1121/1.3097471View Description Hide Description
Aperiodicity of speech alters voice quality. The current study investigated the relationship between vowel aperiodicity and human auditory cortical N1m and sustained field (SF) responses with magnetoencephalography. Behavioral estimates of vocal roughness perception were also collected. Stimulus aperiodicity was experimentally varied by increasing vocal jitter with techniques that model the mechanisms of natural speech production. N1m and SF responses for vowels with high vocal jitter were reduced in amplitude as compared to those elicited by vowels of normal vocal periodicity. Behavioral results indicated that the ratings of vocal roughness increased up to the highest jitter values. Based on these findings, the representation of vocal jitter in the auditory cortex is suggested to be formed on the basis of reduced activity in periodicity-sensitive neural populations.