The occlusion effect occurs when an object fills the outer portion of a person's ear canal, causing that person to perceive echo-like "hollow" or "booming" sounds generated from their own voice.
The bone-conducted sound travels to the cochlea through different pathways. The outer ear pathway corresponds to the sound pressure generated in the ear canal cavity due to the vibration of the ear canal wall, which constitutes the source of the occlusion effect. At low frequencies, the outer ear pathway is negligible when the ear canal is open but dominates when it is occluded. The occlusion effect is thus objectively characterized by an acoustic pressure increase in the occluded ear canal at low frequencies and which can be measured with a probe-tube microphone.[1]
Considering that the vibrating ear canal wall acts as an ideal source of volume velocity (also known as volumetric flow rate), the occlusion device increases the “opposition” of the ear canal cavity to the volume velocity imposed by its wall and thus increases the amplitude of the acoustic pressure that is generated in reaction, leading to the occlusion effect.[2]
The acoustic impedance of the ear canal cavity represents its “opposition” to the volume velocity transfer and governs its reaction in terms of acoustic pressure. In other words, the occlusion effect is mainly due to the increase of the acoustic impedance of the ear canal cavity when it is occluded.[2][3][4][5]
A person with normal hearing can experience this by sticking their fingers into their ears and talking. Otherwise, this effect is often experienced by hearing aid users who only have a mild to moderate high-frequency hearing loss, but use hearing aids which block the entire ear canal. The occlusion effect is also deemed to be a notable source of discomfort to workers wearing shallowly inserted passive occlusion devices such as earplugs.[6][7]
Active occlusion algorithms are needed to help people with severe hearing loss adequately. If a person suffers from "near-normal low-frequency hearing and mild to moderate hearing loss of up to 70 dB at mid and high frequencies," hearing aids with increased vent size or hollow ear-molds/domes are more suitable for them in lessening the extent of the occlusion effect.[8] In the latter case, the open-fitting decreases the ear canal acoustic impedance and thus the occlusion effect.
For earplug users, an incomplete seal has a similar effect at frequencies lower than the Helmholtz resonance formed by the system (the neck of the resonator corresponding to the incomplete seal at the earplug/ear canal wall interface and the resonator cavity being the partially occluded ear canal). In the general case, the deep-fitting reduces the occlusion effect because the volume velocity imposed by the ear canal wall to the occluded ear canal cavity decreases since the surface as well as the vibration amplitude of the remaining ear canal wall diminish with the insertion depth.[citation needed]
↑ Stenfelt, Stefan; Reinfeldt, Sabine (January 2007). "A model of the occlusion effect with bone-conducted stimulation". International Journal of Audiology. 46 (10): 595–608. doi:10.1080/14992020701545880. PMID17922349. S2CID39146020.
↑ Occlusion effects, Part II: A study of the occlusion effect mechanism and the influence of the earmould properties, Report by M. Ø. Hansen, T. Poulsen, and P. Lundh, Technical University Denmark, 1998.
↑ Zurbrügg, T.; Stirnemannn, A.; Kuster, M.; Lissek, H. (1 May 2014). "Investigations on the physical factors influencing the ear canal occlusion effect caused by hearing aids". Acta Acustica United with Acustica. 100 (3): 527–536. doi:10.3813/AAA.918732.
↑ Rating and ranking methods for hearing protector wearability. J.G. Casali, S.T. Lam and B.W. Epps - Sound & Vibration, 1987.
The middle ear is the portion of the ear medial to the eardrum, and distal to the oval window of the cochlea.
The cochlea is the part of the inner ear involved in hearing. It is a spiral-shaped cavity in the bony labyrinth, in humans making 2.75 turns around its axis, the modiolus. A core component of the cochlea is the Organ of Corti, the sensory organ of hearing, which is distributed along the partition separating the fluid chambers in the coiled tapered tube of the cochlea.
A hearing aid is a device designed to improve hearing by making sound audible to a person with hearing loss. Hearing aids are classified as medical devices in most countries, and regulated by the respective regulations. Small audio amplifiers such as personal sound amplification products (PSAPs) or other plain sound reinforcing systems cannot be sold as "hearing aids".
A resonator is a device or system that exhibits resonance or resonant behavior. That is, it naturally oscillates with greater amplitude at some frequencies, called resonant frequencies, than at other frequencies. The oscillations in a resonator can be either electromagnetic or mechanical. Resonators are used to either generate waves of specific frequencies or to select specific frequencies from a signal. Musical instruments use acoustic resonators that produce sound waves of specific tones. Another example is quartz crystals used in electronic devices such as radio transmitters and quartz watches to produce oscillations of very precise frequency.
A hearing test provides an evaluation of the sensitivity of a person's sense of hearing and is most often performed by an audiologist using an audiometer. An audiometer is used to determine a person's hearing sensitivity at different frequencies. There are other hearing tests as well, e.g., Weber test and Rinne test.
Bone conduction is the conduction of sound to the inner ear primarily through the bones of the skull, allowing the hearer to perceive audio content even if the ear canal is blocked. Bone conduction transmission occurs constantly as sound waves vibrate bone, specifically the bones in the skull, although it is hard for the average individual to distinguish sound being conveyed through the bone as opposed to the sound being conveyed through the air via the ear canal. Intentional transmission of sound through bone can be used with individuals with normal hearing — as with bone-conduction headphones — or as a treatment option for certain types of hearing impairment. Bones are generally more effective at transmitting lower-frequency sounds compared to higher-frequency sounds.
The auricle or auricula is the visible part of the ear that is outside the head. It is also called the pinna, a term that is used more in zoology.
An earplug is a device that is inserted in the ear canal to protect the user's ears from loud noises, intrusion of water, foreign bodies, dust or excessive wind. Since they reduce the sound volume, earplugs may prevent hearing loss and tinnitus, in some causes.
The acoustic reflex is an involuntary muscle contraction that occurs in the middle ear in response to loud sound stimuli or when the person starts to vocalize.
Earmuffs are clothing accessories or personal protective equipment designed to cover a person's ears for hearing protection or warmth. They consist of a thermoplastic or metal head-band that fits over the top or back of the head, and a cushion or cup at each end to cover the ears.
Conductive hearing loss (CHL) occurs when there is a problem transferring sound waves anywhere along the pathway through the outer ear, tympanic membrane (eardrum), or middle ear (ossicles). If a conductive hearing loss occurs in conjunction with a sensorineural hearing loss, it is referred to as a mixed hearing loss. Depending upon the severity and nature of the conductive loss, this type of hearing impairment can often be treated with surgical intervention or pharmaceuticals to partially or, in some cases, fully restore hearing acuity to within normal range. However, cases of permanent or chronic conductive hearing loss may require other treatment modalities such as hearing aid devices to improve detection of sound and speech perception.
The ear canal is a pathway running from the outer ear to the middle ear. The adult human ear canal extends from the pinna to the eardrum and is about 2.5 centimetres (1 in) in length and 0.7 centimetres (0.3 in) in diameter.
Tympanometry is an acoustic evaluation of the condition of the middle ear eardrum and the conduction bones by creating variations of air pressure in the ear canal.
Noise-induced hearing loss (NIHL) is a hearing impairment resulting from exposure to loud sound. People may have a loss of perception of a narrow range of frequencies or impaired perception of sound including sensitivity to sound or ringing in the ears. When exposure to hazards such as noise occur at work and is associated with hearing loss, it is referred to as occupational hearing loss.
Seashell resonance refers to a popular folk myth that the sound of the ocean may be heard through seashells, particularly conch shells. This effect is similarly observed in any resonant cavity, such as an empty cup or a hand clasped to the ear.
Hearing, or auditory perception, is the ability to perceive sounds through an organ, such as an ear, by detecting vibrations as periodic changes in the pressure of a surrounding medium. The academic field concerned with hearing is auditory science.
Real ear measurement is the measurement of sound pressure level in a patient's ear canal developed when a hearing aid is worn. It is measured with the use of a silicone probe tube inserted in the canal connected to a microphone outside the ear and is done to verify that the hearing aid is providing suitable amplification for a patient's hearing loss. The American Speech–Language–Hearing Association (ASHA) and American Academy of Audiology (AAA) recommend real ear measures as the preferred method of verifying the performance of hearing aids. Used by audiologists and other hearing healthcare practitioners in the process of hearing aid fitting, real ear measures are the most reliable and efficient method for assessing the benefit provided by the amplification. Measurement of the sound level in the ear canal allows the clinician to make informed judgements on audibility of sound in the ear and the effectiveness of hearing aid treatment.
Cartilage conduction is a pathway by which sound signals are transmitted to the inner ear. In 2004, Hiroshi Hosoi discovered this pathway and named “cartilage conduction”. Hearing by cartilage conduction is distinct from conventional sound-conduction pathways, such as air or bone, because it is realized by touching a transducer on the aural cartilage and does not involve the vibration of the skull bone. Therefore, cartilage conduction is referred to as the “third auditory pathway”.
A hearing protection device, also known as a HPD, is an ear protection device worn in or over the ears while exposed to hazardous noise and provide hearing protection to help prevent noise-induced hearing loss. HPDs reduce the level of the noise entering the ear. HPDs can also protect against other effects of noise exposure such as tinnitus and hyperacusis. There are many different types of HPDs available for use, including earmuffs, earplugs, electronic hearing protection devices, and semi-insert devices.
Hearing protector fit-testing, also known as field attenuation estimation system (FAES), measures the degree of noise attenuation when using the hearing protection device by a particular worker in the enterprise. Such measurements are necessary because noise attenuation is very variable among workers, and it can be close to zero for a significant proportion of them. Individual anatomical differences and varying ability to correctly apply HPD do not allow predicting noise attenuation in workers based on laboratory measurements with satisfactory accuracy. This is typically carried out using one of the available fit-testing hardware and software systems. The effectiveness is typically measured as a personal attenuation rating (PAR) which is subtracted from the known noise exposure to estimate the total noise exposure a single person has when wearing the tested hearing protection device (HPD). The Occupational Safety and Health Administration, National Hearing Conservation Association and National Institute for Occupational Safety and Health recommend it for all workers used HPD as a best practice, and describes existing testing methods and how to incorporate them in hearing conservation programs.
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