Search

Ambient noise is “the new secondhand smoke.” Like secondhand smoke, excessive ambient noise is not just a nuisance but is a health and public health problem for millions of Americans. Excessive ambient noise causes hearing loss; disrupts sleep, function, and communication; and causes non-auditory health effects including cardiovascular disease and death. The sounds that matter to people are the ones reaching their tympanic membranes, or perhaps the cochlear hair cells or auditory processing cortex. Specific evidence-based noise exposure levels affect human health. 30 dBA (LAeq(8)) disrupts sleep. 45 dB (Ldn) disturbs concentration. 55 dB daily average (Lden) has non-auditory heath effects, including cardiovascular disease and death, due to stress responses including activation of the autonomic nervous system and neuroendocrine axis. Approximately 60 dBA (LAmax) interferes with speech comprehension for the hearing impaired. 70 dBA daily average (LAeq(24)) causes hearing loss, and 70 dBA (LAmax) interferes with speech comprehension for those with normal hearing. 85 dBA (LAeq(8)) is the occupational recommended exposure level in the United States, and 85 dBA (LAeq(1)) is the recommended exposure limit to prevent hearing loss. Acoustic scientists and engineers must be aware of these specific, evidence-based noise exposure levels and must work to reduce excessive ambient noise.

Many avian species possess an intracranial air-filled passage, directly connecting the medial surfaces of the tympanic membranes, called the interaural canal. It is known to greatly improve directional hearing by passive acoustics in small animals where the external interaural delay is too minute to allow temporal neural coding. For long, the avian interaural canal was assumed to be a simple cylindrical cavity. Contrary to this, we discovered through CT scans and other techniques that many birds (e.g., zebra finches and pigeons) do in fact have a rather elaborate system of interconnected air-filled cavities throughout the entire skull. The cavities communicate directly or indirectly with the tympanic membranes. How does this network affect the directional hearing in birds? On one hand, it may simply be an adaptation to flight and play little or no role in hearing. On the other hand, theoretical considerations suggest that the directional response may be optimized through frequency dependent “tuning” of at...

In his Ph.D. thesis, Albert described his elegant behavioral study demonstrating that two ears are necessary for frogs to localize sound. Next, he elucidated a series of mechanisms in the frog CNS that is responsible for encoding sound source direction. Inspired by this work, we examined some of the peripheral mechanisms involved in sound processing in the frog. For example, frogs and toads are capable of producing calls at potentially damaging levels that exceed 110 dB SPL at 50 cm. In most frog species, the tympanic membranes (TMs) communicate directly via the large, permanently open Eustachian tubes, resulting in an inherently directional asymmetrical pressure-gradient receiver. One active mechanism for auditory sensitivity reduction involves the pressure increase during vocalization that distends the TM, reducing its airborne sound sensitivity. Another states that if sounds generated by the vocal folds arrive at both surfaces of the TM with nearly equal amplitudes and phases, the net motion of the ear...

Hearing‐impaired listeners experience increased difficulties recognizing speech and localizing sounds in adverse environments. This study investigated the benefits of signal processing in binaural hearing aids designed to preserve cues that accompany spatial location. The ability of listeners to localize click‐trains in noise was tested, along with their ability both to localize and to identify words in noise (a dual task). Listeners experienced two types of bilateral hearing aid fittings: (1) a custom fitting that provided appropriate gain while also matching the phase measured near the tympanic membranes without the hearing aids, and (2) a conventional fitting (using the same hearing aid device) that provided the same gain with noncustom, linear phase. Testing occurred for each fitting immediately and following 3‐months listening experience using a within‐listener, within‐device, randomized, single blind, crossover design. A rating‐scale questionnaire was administered to assess perceived speech‐ hearing and spatial abilities. In the dual task, the listeners exhibited superior localization ability for the phase‐preserving fitting initially and after 3‐months experience. This advantage did not occur for the click‐train localization task. Listeners rated their spatial abilities higher with the phase‐preserving fitting, although little improvement was observed or reported for speech hearing.

The sound pressure at the eardrum of anesthetized chinchillas was measured with a probe‐tube microphone implanted into the external auditory meatus. The ratio of this pressure to the free‐field pressure measured at a point corresponding to the center of the animal's head was determined as a function of frequency (sound pressure transformation function, SPTF). Two distinctly different types of SPTF's were found. They differed by as much as 15 dB in the frequency range 1–8 kHz. Several of the ears that exhibited one type of SPTF showed slight perforations in the tympanic membranes. It appeared that a static pressure difference across the eardrum was responsible for the other type of naturally occurring SPTF's. Both types of SPTF's could be artificially produced by either creating or by equalizing static middle‐ear pressure. [This work was carried out at the Massachusetts Institute of Technology and supported in part by the National Institute of Health (Grant), the Joint Services Electronics Program, and NAS...

Four young adult squirrel monkeys were trained in restraining chairs to track their auditory detection thresholds by means of a single‐lever shock‐avoidance procedure. Pure tones from 250 Hz through 12 kHz were presented free field, and measurements were obtained before and for at least 15 weeks following laser irradiation. Control audiograms were taken after each animal was surgically operated by reflecting the tympanic membranes and fenestrating both cochleas with a small burr. About one month later, each animal was reoperated and irradiated bilaterally through the burr holes with a neodymium laser. One, two, four, or eight light pulses were delivered into the inner ear through a flexible fiber optic. Subsequent to obtaining the 10‐week postirradiation audiograms, the monkeys were trained on earphones, and each ear was assessed independently. [Work supported in part by National Institute of Neurological Diseases and Blindness.]

It is usually contended that so‐called subjective tones are due to vibrations introduced either by asymmetrical action of the tympanic membrane or by nonlinear functioning of the ossicles. The experiments here reported were designed to determine whether the tympanic membrane and the ossicles play any part in the generation of these phenomena. Individuals who had neither tympanic membranes nor ossicles served as observers. The harmonic constitution of each of the complex tones used as stimuli was such as to make the fundamental tone audible to the normal ear, when it was absent and only two or more overtones were present. It was easy to discover whether or not an observer heard this fundamental tone by having him match the pitch of the complex tone with a pure tone produced by a beat‐frequency oscillator. The harmonics present in each of the complex tones was determined by means of harmonic analysis. The results of the investigation indicate that the tympanic membrane and the ossicles are probably not invo...

By means of a beat‐frequency audio‐oscillator, amplifier and a calibrated sound reproducer capable of high acoustic output levels it was found that 27 observers with complete bilateral hearing loss, due either to congenital defect or meningitic infection, but with normal tympanic membranes, possessed average value thresholds of feeling. This finding is interpreted to signify that the threshold of feeling is not causally related to stimulation of the acoustic portion of the eighth cranial nerve. The threshold of feeling in 33 ears lacking tympanic membranes because of the radical mastoid operation or prolonged otitis media was raised 20 db or more above the average values for normal ears, which suggests that the end organs responsible for the threshold of feeling are located within the tympanic membrane and epithelium lining the combined middle ear cavity and external auditory meatus. Individuals with pronounced hearing loss and intact tympanic membranes cannot tolerate high levels of sound amplification i...

Vibrational holography was performed on tympanic membranes (TM) in human temporal bones obtained immediately after death. Displacement patterns at the first mode (and also at higher modes) were essentially the same as those found in cats (Khanna et al., 1969). Findings in both species can best be reconciled with the catenary concept of Helmholtz (1868). Owing to the curvature of the TM's surface, a force transformation is brought about between the membrane (larger displacement/lesser force) and the malleus (lesser displacement/larger force). Point‐by‐point comparisons of displacement amplitudes along the manubrium with those in adjacent regions of the posterior quadrant suggested that forces exerted upon the manubrium decrease with distance from the ossicular rotational axis so that FORCE×FORCE ARM≅CONSTANT. The amplitude differences between TM quadrants (Khanna et al., 1969) are explained by the fact that the manubrium and the ossicular rotational axis form an acute angle, open anteriorly (Barany, 1938). Thus, the manubrium appear to rotate around two axes. With frequency, the efficiency of the system varied in a manner resembling the middle‐ear impedance curve. [Supported by NIH grants.]