Radial fibers are afferent nerve fibers that originate from the inner hair cells and run radially to the modiolar ganglion. They transmit the primary auditory signals with high precision in terms of temporal and amplitude information to the auditory nerve. Radial fibers have large, myelinated axons that enable fast conduction speeds and are crucial for speech intelligibility. Damage to these fibers, for example due to noise or ototoxins, leads to hidden hearing loss despite normal hearing thresholds. Research approaches aim to protect or regenerate radial fibers in order to compensate for synaptic wear and tear.

Raphael's ligaments (ligamenta spiralia interni) are fine connective tissue structures in the modiolar region of the cochlea that stabilize nerve fibers and blood vessels. They run radially between the modiolus and the basilar membrane and support the spatial arrangement of the afferent and efferent fibers. Their integrity is important for undistorted signal transmission and nutrient supply to the hair cells. Histological studies show that aging processes and inflammation can contribute to degeneration of Raphael's ligaments. A better understanding of this structure could open up new therapeutic approaches for sensorineural hearing loss.

The noise floor is the lowest constant background noise of an electronic or acoustic system in the absence of an input signal. In hearing aids, it defines the lower limit of amplification, as otherwise quiet ambient noises would be masked by the inherent noise. A low noise floor is desirable in order to make weak signals clearly audible without the user perceiving a constant hum. Technical measures such as noise reduction algorithms and high-quality components reduce the noise floor. Audiological measurements document the noise floor during calibration and quality control of hearing systems.

Reafferent signals are sensory feedback signals that arise during speech when sound reaches the ear via air and bone conduction. They enable self-monitoring of volume, pitch, and articulation and control the phonatory reflex. Hearing loss or masking of these signals impairs speech modulation, resulting in a voice that is too loud or too soft. In cochlear implant users, reafferentation is partially restored through direct electrical stimulation. Research is investigating how enhanced reafferent feedback can improve speech therapy outcomes.

Recruitment refers to a pathologically altered perception of loudness in which loud sounds are suddenly perceived as much louder, while soft sounds are not heard. This effect occurs in sensorineural hearing loss when the compression properties of the cochlea are disturbed. Clinically, recruitment is measured using loudness scaling tests and Bekesy audiometry. In hearing aids, recruitment is compensated for by customized compression algorithms to improve comfort and intelligibility. Without compensation, those affected perceive loud noises as unpleasant or painful.

Reflex audiometry measures acoustically evoked muscle reflexes in the middle ear (stapedius reflex) and facial nerve area to test the function of the middle ear and brainstem pathways. A test stimulus (tone, broadband noise) triggers an impedance change, which is recorded using tympanometry. Reflex threshold and latency provide information about sound conduction, nerve integrity, and central processing. Asymmetric or absent reflexes indicate otosclerosis, nerve lesion, or central disorders. Reflex audiometry supplements tone and speech audiometry with objective diagnostic data.

The ear control loop describes feedback loops between the hearing system, brain, and feedback mechanisms such as the stapedius reflex. It regulates amplification, protective reflexes, and phonatory adjustments to maintain homeostasis in the auditory system. Disruptions in the control loop lead to hyperacusis, tinnitus, or poor volume control. Computer models of the control loop support the development of adaptive hearing aid technologies. Understanding the dynamics of the control loop is crucial for targeted therapies and rehabilitation strategies.

The stimulus threshold is the minimum stimulus level (sound pressure, voltage) that triggers a measurable physiological or psychological response. In audiology, it corresponds to the hearing threshold, defined for each frequency in the audiogram. In evoked potential measurements (ABR, ECochG), the stimulus threshold is also referred to as the lowest level that still generates a signal. Stimulus thresholds are basic data for fitting decisions and fitting algorithms in hearing aids. Changes over time document progression or therapy effects.

Auditory stimulus transmission involves mechanical, chemical, and electrical processes from the outer ear to the auditory cortex. Sound waves are transmitted via the eardrum and ossicular chain to the cochlear fluids, where hair cells generate electrochemical signals. Afferent nerve fibers transmit action potentials via brainstem stations to the cortex. Each relay extracts specific features such as time or level differences. Disruptions along the chain lead to different forms of hearing loss and processing deficits.

Resonance is the amplification of vibrations when the excitation frequency matches the natural frequency of a system. In the ear, resonances in the ear canal and cavum conchae cause certain speech frequencies around 2–4 kHz to be emphasized. Technical resonators such as Helmholtz filters in hearing aids use the same principle to shape sound spectra. Excessive resonance can lead to sound coloration and feedback. Room acoustic resonances (room modes) are controlled by damping and diffusers.

Residual hearing refers to the remaining, still usable hearing in cases of hearing loss and is defined in the audiogram as the difference between the hearing threshold and the comfort threshold. It determines which signal components can be perceived without amplification and which must be supplemented by a hearing aid. Greater residual hearing improves speech comprehension and facilitates hearing aid acceptance. Measurements of residual hearing are taken into account when selecting compression parameters and amplification limits. Changes in residual hearing over time indicate progression or therapeutic success.

A retrocochlear lesion affects the auditory system beyond the cochlea, usually in the area of the vestibulocochlear nerve or higher up in the brainstem. It leads to central auditory pathway disorders, which can manifest themselves in combination tests (e.g., ABR latency prolongation) and in speech comprehension tests. Those affected often have discordant findings, such as a normal otoacoustic emission pattern but disturbed evoked potentials. Causes include acoustic neuromas, multiple sclerosis, or vascular infarcts. Diagnosis requires imaging techniques such as MRI for localization and follow-up.

The retrolabyrinthine space, also known as the vestibular space, lies behind the bony labyrinth and encompasses cranial nerves, vessels, and connective tissue between the labyrinth and the cerebellopontine angle. It is clinically relevant in cases of tumors (e.g., acoustic neuroma) and inflammatory processes that cause vertigo and hearing loss. Surgery in this area requires careful intraoperative monitoring of the auditory brainstem outputs. Anatomical knowledge of the retrolabyrinthine space is essential for access in otoneurosurgery. Postoperative imaging checks the completeness of resection and complications.

The receptors in the inner ear are the inner and outer hair cells on the basilar membrane of the organ of Corti. They convert mechanical vibrations into electrochemical signals by stereocilia opening mechanosensitive ion channels. Inner hair cells primarily encode acoustic information, while outer hair cells implement the cochlear amplifier through active feedback. Damage to these receptors, for example due to noise or ototoxins, leads to sensorineural hearing loss and reduced frequency resolution. Research aims to regenerate receptors through gene therapy or stem cells.

Reciprocal inhibition in the stapedius reflex describes the neurological counteraction whereby activation of the stapedius muscle inhibits contraction of the tensor tympani muscle. This reciprocal inhibition optimizes middle ear mechanics by preventing excessive damping and enabling reflex adaptation to different types of noise. An intact reciprocal mechanism ensures balanced protective reflexes in response to impulsive and continuous sound. Pathological disturbances of reciprocal inhibition can lead to reduced reflex amplitude and increased noise sensitivity. The examination is performed using combined reflex audiometry and EMG measurements.

In the superior olive complex in the brainstem, there are reciprocal connections between the left and right nuclear areas, which exchange interaural time and level information contralaterally. This networking enables binaural processing and precise localization of sound sources. Each half of the nucleus inhibits the opposite side depending on the level difference in order to achieve contrast enhancement. Reciprocal connections are fundamental for functions such as the binaural masking advantage. Lesions in this network lead to central auditory processing disorders and poorer directional perception.

A directional microphone is a type of microphone that primarily picks up sound from a specific direction—usually from the front—and attenuates noise coming from the sides or rear. In modern hearing aids, it improves the signal-to-noise ratio by reducing background noise from other directions. Different directional characteristics (cardioid, supercardioid, omnidirectional) allow adaptation to specific listening situations. Adaptive systems automatically switch between directional and omnidirectional modes depending on the ambient noise. Directional microphones improve speech comprehension, especially in noisy environments.

The Rinne test is a clinical hearing test in which a tuning fork is held alternately on the mastoid (bone conduction) and in front of the ear (air conduction). A positive Rinne result (air conduction better than bone conduction) indicates normal or sensorineural hearing. A negative result indicates conductive hearing loss in the tested ear. The test can be performed quickly and is used to make an initial differentiation between conductive and sensorineural hearing loss. The Weber test is also used to check for lateralization.

The tube sound, also known as tympanic sound, is a hollow or tube-like sound pattern that the patient perceives during ear irrigation or when the eardrum is thin. It occurs when sound waves in the air-filled middle ear are modulated by fluid particles. Clinically, hearing this phenomenon helps to diagnose effusion or eardrum perforation. Specific audiometry tones can reproduce the tube sound in audiometry. Therapeutically, any tympanic sound symptoms are addressed with targeted otitis treatment or tympanic tube insertion.

Acoustic feedback occurs when the signal emitted by the loudspeaker is picked up again by the microphone and amplified once more, resulting in a feedback loop with whistling or humming. In hearing aids and public address systems, feedback is suppressed by adaptive algorithms, tight-fitting earmolds, or directional microphones. Mechanical measures such as sealing and microphone placement minimize the risk of feedback. Uncontrolled feedback can severely impair listening comfort and speech comprehension. Modern systems detect feedback early and adjust filters in real time.

The round window is a flexible membrane opening at the end of the scala tympani that allows pressure relief when the oval window is mechanically stimulated by the stapes. It ensures that the fluid volume in the cochlea remains constant and enables traveling waves on the basilar membrane. Injuries or stiffening of the round window, for example due to surgery or trauma, lead to sound conduction problems and can trigger a perilymph fistula. Clinically, the round window is used as an access point for cochlear implants. Pathological changes can be detected in CT scans and by tympanogram analysis.

The round window membrane is a thin, gelatinous membrane that closes the round window and provides mechanical flexibility. It transmits pressure fluctuations from the scala tympani to the perilymph and acts as a passive relief valve. Its elasticity and thickness vary along the membrane and influence impedance adaptation. Damage leads to perilymph loss, vertigo, and hearing loss. In microsurgical procedures, the membrane is reconstructed with filling materials in cases of perilymph fistula to restore tightness.