Acoustic feedback occurs when a microphone picks up sound from the speaker and amplifies it again, creating a feedback loop. This typically manifests as whistling or humming and can severely degrade sound quality. Adaptive algorithms are used in hearing aids and sound reinforcement systems to detect and suppress feedback in real time. Mechanical measures such as tight earmoulds or directional microphones also minimize feedback risks. An optimally tuned system prevents audible artifacts for the user.

In field tone audiometry, continuous tones at defined frequencies and levels are presented via headphones or loudspeakers to determine hearing sensitivity. Unlike with impulse probes, the patient holds a control as soon as they hear the sound and releases it when it disappears. This produces a precise threshold curve that documents adaptation behavior and hearing range. The procedure is particularly suitable for research and differential diagnosis of rare hearing disorders. Modern devices automate the process and store the results digitally.

The petrous bone (os petrosum) is part of the temporal bone and surrounds the inner ear as well as the auditory and vestibular nerves. Its dense bone structure protects the sensitive sensory organs from mechanical influences. Inflammation or tumors in the temporal bone can lead to hearing loss, tinnitus and dizziness. Imaging (CT, MRI) shows the petrous temporal bone in detail in order to detect pathological changes. Surgical interventions in this area require the utmost precision in order to protect nerve structures.

An acoustic filter selects certain frequency ranges and suppresses others in order to shape sound spectra in a targeted manner. Multi-band compression filters are used in hearing aids to emphasize speech and attenuate background noise. Filter types such as high-pass, low-pass, band-pass and notch filters allow specific interventions in the frequency spectrum. The characteristics of a filter are described by parameters such as slope and quality (Q factor). Precise filtering improves speech intelligibility and sound quality.

The filter characteristic defines how strongly and in which frequency range a filter attenuates or amplifies. It is represented graphically in the frequency response, whereby the transition bandwidth and slope are decisive. In hearing aid technology, the filter characteristic determines which speech frequencies are emphasized and which noise frequencies are suppressed. Adaptive filters dynamically adjust their characteristics to changing listening situations. A precise design prevents sound distortion and reduces listening effort.

The filter quality (Q factor) describes the sharpness of a resonance peak in a bandpass or notch filter. A high Q value means a narrow bandwidth with steep edges, while a low Q value enables wider transitions. In hearing aids, the Q value is selected so that speech bands are clearly separated and noise is minimized. However, too high a Q factor can cause resonance effects and sound coloration. Fine-tuning the Q-factors is part of the hearing aid fitting by the acoustician.

The slope describes how quickly a filter attenuates outside its passband, measured in dB/octave. Steep slopes (e.g. 24 dB/octave) suppress unwanted frequencies more strongly, but can lead to phase distortion. In hearing systems, a compromise is chosen between attenuation effect and natural sound. Edge steepness also influences the crosstalk of neighboring filter bands. Adaptive systems vary the slope according to the situation in order to achieve optimum speech intelligibility.

An FM system transmits speech signals wirelessly via FM radio from a transmitter unit (teacher microphone) directly to the receiver in the hearing aid. This improves speech intelligibility in noisy environments or large rooms, as ambient noise is blocked out. FM receivers are often integrated in hearing aids or available as accessories. Range and sound quality depend on transmitter power and antenna concept. Regular calibration ensures reliable transmission without interference.

Formants are resonant frequency bands in speech that are created by vowel tract formation and characterize vowels. The first two formants (F1, F2) are crucial for distinguishing vowels. In speech audiometry, formants are analyzed to diagnose articulation disorders. Hearing aids and speech processors emphasize formants to improve intelligibility. Spectral analysis visualizes formant position and width.

Formant transitions describe the dynamic shift of formants when changing between speech sounds, for example from consonant to vowel. They are important acoustic cues for speech perception and phoneme recognition. Distorted or attenuated transitions lead to comprehension problems, especially in background noise. Audiological tests evaluate formant transitions in real time. Speech training can improve perception and production of these transitions.

A free field is an acoustically unlimited space without reflective surfaces in which sound propagates in a spherical shape. In audiometry, free-field conditions are simulated in order to test hearing aids and loudspeakers objectively. Measuring microphones record the sound pressure at various distances from the sound source. Free-field measurements provide data for sound reinforcement planning and room acoustics optimization. In practice, low-reflection chambers or open-field setups are used.

In free-field measurements, the sound pressure is determined in an open, anechoic environment in order to obtain precise level and frequency response data. The loudspeaker and microphone are positioned at standardized distances, usually 1 m. The results are used to calibrate audiometer headphones and loudspeaker systems. Sources of error such as ground reflections are minimized by shadowing. Free-field measurements are the reference for room and product acoustics.

Frequency refers to the number of oscillation cycles per second and is measured in Hertz (Hz). In the hearing range, it typically ranges from 20 Hz to 20 kHz, with speech being predominantly between 250 Hz and 4 kHz. Frequency analysis is central to audiometry, otoacoustic emissions and hearing aid filter design. The cochlea and auditory cortex are organized tonotopically, meaning that different frequencies are processed at different locations. Changes in frequency perception can indicate cochlear or central disorders.

Frequency resolution describes the ability to perceive two closely spaced frequencies as separate sounds. It depends on the filter bandwidth and the membrane capacity of the cochlea. High resolution is essential for music and speech recognition, as many overtones are close together. Narrow filter bands are used in hearing aids to maximize frequency resolution. Psychoacoustic tests determine individual resolution limits.

A frequency band is a defined range of frequencies that is processed by a filter or amplifier. In multi-band hearing aids, the audio spectrum is often divided into 4-16 bands in order to process specific speech and interference frequencies. Each band can be compressed, amplified or attenuated separately. The band limits and bandwidths are adapted to the hearing loss profile. Fine tuning of the bands optimizes speech intelligibility and sound fidelity.

The frequency range indicates the entire spectrum in which a system (ear, microphone, loudspeaker) operates. For the human ear, this range is approximately between 20 Hz and 20 kHz, with individual variability and age dependency. Hearing aids typically cover 100 Hz to 8 kHz in order to optimally amplify speech. Frequency ranges are indicated in audiograms and technical specifications. Limitations in the frequency range have a direct impact on sound quality and intelligibility.

The frequency response shows the amplification or attenuation of a system over the frequency spectrum. In hearing aid technology, it documents how different frequencies in the output signal are adjusted. A linear frequency response reproduces the input signal without distortion, compressed response curves improve speech components. Measurements in the free field or with an artificial ear provide exact curves. Clinical fitting software visualizes frequency response and allows fine tuning.

Frequency modulation (FM) changes the carrier frequency of a signal depending on a modulation signal, such as speech. FM systems in hearing acoustics transmit audio signals wirelessly with high interference immunity. FM receivers in hearing systems decode the modulated signal and improve speech intelligibility in noisy environments. Technical parameters such as modulation index and bandwidth determine transmission quality. FM is standard in schools and conference systems for the hearing impaired.

Frequency selectivity describes the ability of the ear or filter to process individual frequencies separately. In the cochlea, it is created by tonotopic tuning of the basilar membrane. Hearing aids attempt to reproduce selectivity by using narrow filter bands. Loss of selectivity leads to wider masking and poorer speech intelligibility. Psychoacoustic tests measure selectivity via masking paradigms.

Frequency distortion occurs when a system amplifies or attenuates certain frequencies unevenly, which changes the sound spectrum. This can be caused by non-linear filters, overdrive or diaphragm damage. In hearing aids, distortion is minimized by linear amplification stages and feedback suppression. Measurements with sine sweeps and spectral analysis quantify distortion. High distortion impairs naturalness and speech intelligibility.

A crossover splits an audio signal into several bands in order to amplify or filter them separately. In multi-channel hearing aids, it enables differentiated compression and noise suppression per band. Passive crossovers work with coils and capacitors, active ones with electronic filters. The edge steepness and filter quality determine the selectivity between bands. Precise crossovers prevent crosstalk and phase errors.

Functional hearing impairment occurs when there is no evidence of organic damage, but hearing behavior is impaired. The causes are often psychological, such as stress or attention disorders. Symptoms include fluctuations in the hearing threshold and discrepancies between test and everyday performance. Diagnostics combines objective measurements (OAE, AEP) with behavioral audiometry. Therapy includes psychological support and habitual hearing training.

Functional tests examine specific aspects of hearing and balance function, such as the stapedius reflex, tube function or vestibular stimuli. They supplement audiograms with information on mechanical and central processing. Standard tests are tympanometry, VEMP and caloric testing. Results are incorporated into differentiated diagnoses and treatment plans. Modern test systems automate processes and provide reproducible data.