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 impair sound quality. Hearing aids and sound reinforcement systems use adaptive algorithms to detect and suppress feedback in real time. Mechanical measures such as tight-fitting earmolds or directional microphones also minimize the risk of feedback. 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 tone audiometry, the patient holds a control when they hear the tone and releases it when it disappears. This produces an accurate 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 petrous bone can lead to hearing loss, tinnitus, and vertigo. Imaging (CT, MRI) provides a detailed view of the petrous bone in order to detect pathological changes. Surgical procedures in this area require the utmost precision in order to preserve nerve structures.

An acoustic filter selects certain frequency ranges and suppresses others in order to shape sound spectra in a targeted manner. Hearing aids use multiband compression filters that 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, with the transition bandwidth and slope being decisive factors. In hearing aid technology, the filter characteristic determines which speech frequencies are emphasized and which interfering frequencies are suppressed. Adaptive filters dynamically adjust their characteristics to changing listening situations. 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 allows for broader transitions. In hearing aids, the quality is selected so that speech bands are clearly separated and background noise is minimized. However, excessive quality can cause resonance effects and sound coloration. Fine-tuning the Q factors is part of the hearing aid fitting process performed by the audiologist.

The slope refers to how quickly a filter attenuates outside its passband, measured in dB/octave. Steep slopes (e.g., 24 dB/octave) suppress unwanted frequencies more effectively, but can lead to phase distortion. In hearing systems, a compromise is made between attenuation and natural sound. Slope also influences crosstalk between adjacent filter bands. Adaptive systems vary the slope depending on the situation to achieve optimal speech intelligibility.

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

Formants are resonant frequency bands in speech that are created by vocal tract shaping and characterize vowels. The first two formants (F1, F2) are crucial for distinguishing between 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 switching between speech sounds, such as from consonants to vowels. They are important acoustic cues for speech perception and phoneme recognition. Distorted or weakened transitions lead to comprehension problems, especially in the presence of background noise. Audiological tests evaluate formant transitions in real time. Speech training can improve the perception and production of these transitions.

A free field is an acoustically unlimited space without reflective surfaces in which sound propagates spherically. 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, anechoic chambers or open-field setups are used.

In free-field measurements, sound pressure is determined in an open, low-reflection environment in order to obtain accurate 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 floor reflections are minimized by shielding. 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 audible range, it typically ranges from 20 Hz to 20 kHz, with speech predominantly occurring 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 in 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 tones. 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 multiband hearing aids, the audio spectrum is often divided into 4–16 bands in order to specifically process speech and interference frequencies. Each band can be compressed, amplified, or attenuated separately. The band limits and bandwidths are adjusted 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 dependence. Hearing aids typically cover 100 Hz to 8 kHz to optimally amplify speech. Frequency ranges are shown in audiograms and technical specifications. Limitations in the frequency range directly affect sound quality and intelligibility.

The frequency response shows the amplification or attenuation of a system across the frequency spectrum. In hearing aid technology, it documents how different frequencies are adjusted in the output signal. A linear frequency response reproduces the input signal without distortion, while compressed response curves improve speech components. Measurements in a free field or with an artificial ear provide accurate curves. Clinical fitting software visualizes the 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 aids 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 replicate selectivity by using narrow filter bands. Loss of selectivity leads to broader masking and poorer speech comprehension. Psychoacoustic tests measure selectivity using masking paradigms.

Frequency distortion occurs when a system amplifies or attenuates certain frequencies unevenly, thereby altering the sound spectrum. This can be caused by nonlinear filters, overdrive, or diaphragm damage. In hearing aids, distortion is minimized by linear amplification stages and feedback suppression. Measurements using sine sweeps and spectral analysis quantify distortion. High distortion levels impair naturalness and speech intelligibility.

A crossover divides an audio signal into several bands so that they can be amplified or filtered separately. In multi-channel hearing aids, it enables differentiated compression and noise reduction per band. Passive crossovers work with coils and capacitors, active ones with electronic filters. The slope and filter quality determine the selectivity between bands. Precise crossovers prevent crosstalk and phase errors.

Functional hearing disorders occur when no organic damage can be detected, 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 results and everyday performance. Diagnosis combines objective measurements (OAE, AEP) with behavioral audiometry. Treatment includes psychological support and habitual hearing training.

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