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 to the auditory nerve with high precision of temporal and amplitudinal information. Radial fibres have large, myelinated axons that enable fast conduction speeds and are crucial for speech intelligibility. Damage to these fibers, for example through 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 area of the cochlea that stabilize nerve fibres 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 and inflammation can contribute to degeneration of Raphael's bands. 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 quiet environmental noise would otherwise 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 permanent hum. Technical measures such as noise suppression 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 are generated 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. If hearing loss occurs or these signals are masked, speech modulation suffers, resulting in a loud or quiet voice. In cochlear implant recipients, reafferentation is partially restored by direct electrical stimulation. Research is investigating how increased reafferent feedback can improve speech therapy outcomes.

Recruitment refers to a pathologically altered perception of loudness in which loud sounds are suddenly perceived 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 assessed using loudness scaling tests and Bekesy audiometry. In hearing aids, recruitment is compensated for by adapted compression algorithms to improve comfort and intelligibility. Without compensation, those affected find loud noises 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 (sound, broadband noise) triggers an impedance change, which is recorded using tympanometry. The reflex threshold and burst provide information on sound conduction, nerve integrity and central processing. Asymmetrical or absent reflexes indicate otosclerosis, nerve lesions or central disorders. Reflex audiometry supplements sound and speech audiometry with objective diagnostic data.

The ear control loop describes feedback loops between the auditory system, the brain and feedback mechanisms such as the stapedius reflex. It regulates amplification, protective reflexes and phonatory adjustments in order to maintain homeostasis in the auditory system. Disturbances in the control circuit lead to hyperacusis, tinnitus or a lack of 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 reaction. 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. Stimulation thresholds are basic data for treatment 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 into cochlear fluids, where hair cells generate electrochemical signals. Afferent nerve fibers relay action potentials to the cortex via brainstem stations. Each relay extracts specific features such as time or level differences. Disruptions along the chain lead to various 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 of the ear canal and the cavum conchae emphasize certain speech frequencies around 2-4 kHz. 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 usable hearing in the event 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 need to be supplemented by hearing aids. Greater residual hearing improves speech comprehension and facilitates hearing aid acceptance. Measurements of residual hearing are used in the selection of compression parameters and amplification limits. Changes in residual hearing over time indicate progression or therapy success.

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

The retrolabyrinthine space, also known as the vestibular space, lies behind the bony labyrinth and comprises cranial nerves, vessels and connective tissue between the labyrinth and the cerebellopontine angle. It is clinically relevant for tumors (e.g. acoustic neuroma) and inflammatory processes that cause dizziness and hearing loss. Operations in this area require gentle intraoperative monitoring of the auditory brainstem outlets. Anatomical knowledge of the retrolabyrinth space is essential for approaches in otoneurosurgery. Postoperative imaging monitors resection completion 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 the acoustic information, while outer hair cells realize the cochlear amplifier through active feedback. Damage to these receptors, for example through noise or ototoxins, leads to sensorineural hearing loss and reduced frequency resolution. Research is aimed at regenerating receptors through gene therapy or stem cells.

Reciprocal inhibition in the stapedius reflex describes the neurological counter-circuit in which activation of the stapedius muscle inhibits the contraction of the tensor tympani. This reciprocal inhibition optimizes middle ear mechanics by avoiding over-attenuation and enabling reflex adaptation to different types of noise. An intact reciprocal mechanism ensures balanced protective reflexes for impulsive and continuous sound. Pathological disorders of reciprocal inhibition can lead to reduced reflex amplitude and increased noise sensitivity. The examination is carried out using combined reflex audiometry and EMG measurements.

In the superior olive nucleus complex in the brain stem, there are reciprocal connections between the left and right nucleus 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 amplification. Reciprocal connections are fundamental for functions such as the binaural masking advantage. Lesions of this network lead to central auditory processing disorders and poorer directional perception.

A directional microphone is a type of microphone that preferentially picks up sound from a specific direction - usually the front - and attenuates noise coming from the side or rear. In modern hearing aids, it improves the signal-to-noise ratio by reducing noise from other directions. Different directional characteristics (cardioid, supercardioid, omnidirectional) allow adaptation to specific hearing situations. Adaptive systems automatically switch between directional and omni mode depending on the ambient noise. Directional microphones increase speech intelligibility, 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 carried out quickly and serves as an initial differentiation between conductive and sensorineural hearing loss. The Weber test is also used to test lateralization.

The tube sound, also known as the tympanic sound, is a hollow or tube-like sound pattern that the patient hears when the ear is flushed or 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 an effusion or eardrum perforation. Specific audiometric tones can reproduce the tube sound in audiometry. Therapeutically, any tympanic sound symptomatology is addressed with targeted otitis treatment or tympanostomy tube treatment.

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

The round window is a flexible membrane opening at the end of the scala tympani that enables pressure relief when the oval window is mechanically stimulated by the stapes. It ensures that the fluid volume in the cochlea remains constant and allows 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 perilymphatic 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 into the perilymph and acts as a passive relief valve. Its elasticity and thickness vary along the membrane and influence the impedance matching. Damage leads to loss of perilymph, dizziness and hearing loss. In microsurgical interventions, the membrane is reconstructed in perilymph fistulas using filling materials to restore tightness.