Physiology III - Bone Conduction PDF

CMSD5280 Audition II Physiology #3: Bone Conduction Dr. Olivier Valentin, Ph.D. OUTLINE Air conduction path to the inner ear Bone conduction path to the inner ear Candidacy case studies Occlusion effect Air conduction path to the inner ear...

CMSD5280 Audition II Physiology #3: Bone Conduction Dr. Olivier Valentin, Ph.D. OUTLINE Air conduction path to the inner ear Bone conduction path to the inner ear Candidacy case studies Occlusion effect Air conduction path to the inner ear 1- Pinna 2- Ear canal Outer Ear  When sound reaches the ear, it first encounters the outer ear, which consists of the pinna and the ear canal. The pinna’s unique shape serves two key purposes: it aids in sound localization in the vertical plane and provides natural sound amplification, particularly enhancing mid-range frequencies between 2000 and 7000 Hz. After being shaped and slightly amplified by the pinna, the sound waves travel down the ear canal, another source of sound amplification composed of an external cartilaginous portion and an inner bony portion. Air conduction path to the inner ear 1- Pinna 2- Ear canal 3- Tympanic membrane (eardrum) 4- Malleus 5- Incus 6- Stapes Outer Ear Middle Ear  At the end of the ear canal, sound waves reach the tympanic membrane, also known as the eardrum, a thin, translucent structure marking the beginning of the middle ear. The tympanic membrane consists of three layers and is highly sensitive to sound vibrations. As sound waves cause compressions and rarefactions, the tympanic membrane vibrates, setting the ossicular chain—comprising the malleus, incus, and stapes—into motion. This movement converts acoustic energy into mechanical energy. The three ossicles are interconnected and suspended within the middle ear cavity by ligaments, allowing them to function as a lever system to efficiently transmit sound energy. The final ossicle, the stapes, passes these vibrations to the oval window of the cochlea, a flexible membrane that acts as the gateway to the inner ear. Air conduction path to the inner ear 1- Pinna 2- Ear canal 3- Tympanic membrane (eardrum) 4- Malleus 5- Incus 6- Stapes 7- Eustachian tube 8- Tympanic cavity (middle ear) 9- Cochlea (inner ear) 10- Cochlear nerve Outer Ear Middle Ear Inner Ear  The inner ear, or cochlea, is a spiral-shaped structure containing two distinct fluids: perilymph and endolymph. These fluids flow through three parallel canals within the cochlea's two and a half turns: endolymph moves within the cochlear duct (also called the scala media), while perilymph circulates in the scala vestibuli and scala tympani. When the stapes moves at the oval window, it creates waves in the perilymph. These waves cause the basilar membrane to undulate from the base to the apex of the cochlea. This motion sets off a chain of events, triggering sensory transduction. As the basilar membrane vibrates, it interacts with the overlying tectorial membrane, bending the stereocilia of hair cells located on the basilar membrane. This bending leads to the depolarization of hair cells, which release neurotransmitters to stimulate the auditory nerve endings. The auditory nerve then transmits these signals to the next stages of the afferent auditory system. Air conduction path to the inner ear Most of the acoustical energy is reflected when traveling from low impedance (e.g., air) to high 1- AC Anatomical Structures impedance (e.g., water) 2- Physics of AC Sound Transmission  When sound travels from a low-impedance medium like air (such as the external ear) to a high-impedance medium like water (such as the inner ear), almost all of the acoustical energy is reflected. Air conduction path to the inner ear Most of the acoustical energy is reflected when traveling from low impedance (e.g., air) to high 1- AC Anatomical Structures impedance (e.g., water) 2- Physics of AC Sound Transmission This impedance mismatch is overcome by the inner ear using two mechanisms  The middle ear addresses this mismatch using two key mechanical mechanisms. Air conduction path to the inner ear Most of the acoustical energy is reflected when traveling from low impedance (e.g., air) to high 1- AC Anatomical Structures impedance (e.g., water) 2- Physics of AC Sound Transmission This impedance mismatch is overcome by the inner ear using two mechanisms 1. Tympanic membrane focuses sound onto smaller oval window, amplifying pressure  First, the large-diameter tympanic membrane focuses sound energy onto the much smaller oval window (17 times smaller). This difference in area amplifies the sound pressure. Air conduction path to the inner ear Most of the acoustical energy is reflected when traveling from low impedance (e.g., air) to high 1- AC Anatomical Structures impedance (e.g., water) 2- Physics of AC Sound Transmission This impedance mismatch is overcome by the inner ear using two mechanisms 1. Tympanic membrane focuses sound onto smaller oval window, amplifying pressure 2. Ossicles increase mechanical force between tympanic membrane and oval window  Second, the three ossicles of the middle ear act as a lever system. Their arrangement increases the mechanical force transmitted from the tympanic membrane to the oval window. Air conduction path to the inner ear Most of the acoustical energy is reflected when traveling from low impedance (e.g., air) to high 1- AC Anatomical Structures impedance (e.g., water) 2- Physics of AC Sound Transmission This impedance mismatch is overcome by the inner ear using two mechanisms 1. Tympanic membrane focuses sound onto smaller oval window, amplifying pressure 2. Ossicles increase mechanical force between tympanic membrane and oval window Taken together, these mechanisms provide a gain of up to 25 dB at 1000 Hz  Together, these mechanisms provide a sound pressure gain of approximately 20 dB between 250 Hz and 500 Hz, with a maximum of 25 dB around 1000 Hz. This gain diminishes by about 6 dB per octave at higher frequencies. Air conduction path to the inner ear 1- AC Anatomical Structures 2- Physics of AC Sound Transmission 3- AC and Hearing Loss  When hearing loss is not due to a middle ear problem, hearing aids are the most common intervention to compensate for sensorineural hearing loss, which typically affects the inner ear or auditory nerve. Hearing aids do more than just amplify sound: the microphone captures sounds, which are then decomposed into several frequency channels. Each channel is processed and amplified according to its specific frequency range. This customization allows hearing aids to be tailored to an individual's unique hearing loss and enables additional functions, such as noise reduction and feedback cancellation. hearing aids come in two main families. The first type is behind-the-ear (BTE) hearing aids, which sit behind the ear and are connected to an earmold or earpiece that fits inside the ear canal. Air conduction path to the inner ear 1- AC Anatomical Structures 2- Physics of AC Sound Transmission 3- AC and Hearing Loss  The second type is in-the-ear (ITE) hearing aids, which are custom- molded to fit directly into the ear canal or the ear's outer portion. Both types have their advantages and drawbacks. For example, ITE hearing aids are more discreet than BTE, but may cause a greater occlusion effect, which will be discussed further later in the lecture. Bone conduction path to the inner ear 1- Historical overview of BC 2- Physics of BC Sound Transmission  BC hearing aids placed directly on the head stimulate the cochlea through vibrations that are transmitted via the human skull and surrounding soft tissues. When the skull is stimulated by vibrations, it exhibits complex responses that vary with the frequency. Bone conduction path to the inner ear 1- Historical overview of BC 2- Physics of BC Sound Transmission  At the lowest frequencies, below the resonance frequency of the skull mechanical point impedance (150 to 400 Hz), the skull behavior can be approximated with rigid body motion, meaning all parts of the skull vibrate together in the same direction without significant deformation. Bone conduction path to the inner ear 1- Historical overview of BC 2- Physics of BC Sound Transmission  For frequencies between approximately 400 and 1000 Hz (where the first global skull resonance appears), the skull behaves like a mass- spring system, where large parts of the skull move in-phase, similar to how a spring oscillates under tension Bone conduction path to the inner ear 1- Historical overview of BC 2- Physics of BC Sound Transmission  Between 1 and 2 kHz, the skull transitions from a mass-spring like behavior to being dominated by wave transmission. The type of wave transmission differ between the cranial vault and the skull base. In the skull base, wave speed appears to remain relatively constant at around 400 m/s, while in the cranial vault, wave speed is frequency- dependent, starting at approximately 250 m/s at 2 kHz and increasing to around 300 m/s at 10 kHz. Bone conduction path to the inner ear 1- Historical overview of BC 2- Physics of BC Sound Transmission 3- Mechanisms of BC  Several mechanisms contribute to bone conduction. The first mechanism is called compressional bone conduction. When the temporal bone vibrates, it causes alternate compression and expansion of the cochlear capsule. Since the cochlear fluids are incompressible, this leads to bulging at compliant points, such as the oval and round windows. If both windows were equally compliant, the cochlear partition would not displace (as seen in a). However, because the round window is more compliant than the oval window, compression of the cochlear capsule pushes the fluid in the scala vestibuli downward, displacing the basilar membrane (as seen in b). This effect is further reinforced by the larger surface area of the vestibule and scala vestibuli compared to the scala tympani (as seen in c). Bone conduction path to the inner ear 1- Historical overview of BC 2- Physics of BC Sound Transmission 3- Mechanisms of BC  The first mechanism is called distortional bone conduction. Distortional bone conduction occurs because the volume of the scala vestibuli exceeds that of the scala tympani. As a result, distortions of the cochlear capsule lead to compensatory displacements of the cochlear partition, even in the absence of compliant windows. Bone conduction path to the inner ear 1- Historical overview of BC 2- Physics of BC Sound Transmission 3- Mechanisms of BC  Inertial or ossicular-lag bone conduction refers to the contribution of the middle ear to bone conduction. The ossicles in the middle ear move side to side rather than front to back. Barany discovered that bone conduction was most effective when a vibrator was placed on the side of the head, where it vibrated the skull in the direction of ossicular movement, and least effective when placed on the forehead, where the skull vibrated perpendicular to ossicular movement. This mechanism occurs because the ossicles, suspended like pendulums, move relative to the skull when it vibrates, causing a rocking motion of the stapes at the oval window and stimulating the cochlea. Inertial bone conduction is particularly relevant in otosclerosis, a condition in which fixation of the stapes results in hearing loss. Interestingly, although bone conduction would be expected to be impaired at low frequencies, bone conduction thresholds are elevated around 2000 Hz, known as Carhart's notch, due to the resonant frequency of the ossicular chain. Bone conduction path to the inner ear 1- Historical overview of BC Osseotympanic 2- Physics of BC Sound Transmission 3- Mechanisms of BC bone conduction IS THAT ALL?  In addition to the contributions from the inner and middle ear, the outer ear also plays a role in bone conduction, referred to as osseotympanic bone conduction. Vibrations from the skull are transmitted through the outer ear canal to the tympanic membrane. When the external auditory canal is closed or blocked (e.g., by an earphone or hearing aid), the vibrations are trapped more effectively within the ear canal. This results in the amplification of low-frequency bone-conducted sounds, as the closed ear canal prevents the escape of sound waves. This phenomenon, known as the occlusion effect, leads to an increase in the loudness of low-frequency sounds and will be discussed further later in the lecture. Bone conduction path to the inner ear 1- Historical overview of BC 2- Physics of BC Sound Transmission 3- Mechanisms of BC 4- BC consumer devices  Bone conduction has applications that extend beyond audiology, with one popular consumer device being bone conduction earphones. These earphones transmit sound vibrations directly through the skull, bypassing the outer and middle ear. They are commonly used by athletes, cyclists, and runners, as they allow users to enjoy music or take calls while keeping their ears open to important environmental sounds, like traffic. This feature enhances safety, enabling users to stay aware of their surroundings while still enjoying their audio content. While bone conduction earphones offer the advantage of situational awareness, their sound quality may not be as rich or clear as traditional air-conduction earphones, particularly at lower frequencies. However, for many users, the ability to remain aware of their environment outweighs the trade-off in sound quality. Bone conduction path to the inner ear 1- Historical overview of BC 2- Physics of BC Sound Transmission 3- Mechanisms of BC 4- BC consumer devices 5- Cartilage Conduction (CC)  Air conduction (AC) and bone conduction (BC) are the two primary mechanisms by which sound reaches the inner ear. In 2004, Hosoi discovered that vibration of the aural cartilage, when a transducer is placed on it, can generate audible sound with clarity comparable to that of AC and BC. Based on this finding, he coined the term "cartilage conduction (CC)" to describe this phenomenon. The key difference between BC and CC lies in the medium through which vibrations are transmitted—while BC relies on cranial bone vibrations, CC involves vibration of the cartilage in the external ear. Bone conduction path to the inner ear 1- Historical overview of BC 2- Physics of BC Sound Transmission 3- Mechanisms of BC 4- BC consumer devices 5- Cartilage Conduction (CC)  Several applications have since been developed based on Hosoi's work. For instance, CC hearing aids are particularly beneficial for patients who are unable to use conventional AC hearing aids due to conditions such as atresia of the external auditory canal or severe otorrhea. Bone conduction path to the inner ear 1- Historical overview of BC 2- Physics of BC Sound Transmission 3- Mechanisms of BC 4- BC consumer devices 5- Cartilage Conduction (CC)  Additionally, cartilage conduction headphones have been designed to transmit vibrations directly to the skin, bypassing the ear canal, and delivering sound to the inner ear, much like bone conduction headphones. Bone conduction path to the inner ear 1- Historical overview of BC 2- Physics of BC Sound Transmission 3- Mechanisms of BC 4- BC consumer devices 5- Cartilage Conduction (CC) 6- BC applications in audiology 6.1- BC hearing test  Because BC thresholds are less affected by a conductive hearing loss than AC thresholds, BC threshold measurements accompanied by AC threshold measurements have been used to distinguish between conductive and sensorineural losses. Bone conduction path to the inner ear 1- Historical overview of BC 2- Physics of BC Sound Transmission Type of vibrator not a significant influence if 3- Mechanisms of BC static force > 4N 4- BC consumer devices 5- Cartilage Conduction (CC) 6- BC applications in audiology 6.1- BC hearing test  Several factors can influence BC thresholds; however, as long as the static force applied exceeds 4 Newtons, the type of vibrator used does not significantly affect the measurements. Bone conduction path to the inner ear 1- Historical overview of BC 2- Physics of BC Sound Transmission Type of vibrator not a significant influence if 3- Mechanisms of BC static force > 4N 4- BC consumer devices Forehead = overall lower sensitivity compare to 5- Cartilage Conduction (CC) mastoid 6- BC applications in audiology 6.1- BC hearing test  Traditionally, BC threshold measurements have been conducted with the transducer placed on the mastoid portion of the ipsilateral temporal bone. However, some argue that the forehead could be a better location for BC testing because it is less sensitive to variations in stimulation position and is less affected by the middle ear status. In addition, BC testing on the forehead is less affected by airborne sound radiation from the BC transducer compared to the mastoid. Despite this, the forehead generally exhibits lower sensitivity than the mastoid - by more than 11 dB in the typical test frequency range, This difference can be important, especially for patients with low-frequency sensorineural impairments or profound hearing loss, where the BC vibrator's output may be limited by nonlinear distortion at high levels. Bone conduction path to the inner ear 1- Historical overview of BC 2- Physics of BC Sound Transmission Type of vibrator not a significant influence if 3- Mechanisms of BC static force > 4N 4- BC consumer devices Forehead = overall lower sensitivity compare to 5- Cartilage Conduction (CC) mastoid 6- BC applications in audiology 6.1- BC hearing test Masking of the nontest ear is required  When using the mastoid as a stimulation site, the transcranial attenuation between the ears is 0 to 15 dB. In contrast, using the midforehead as a stimulation site results in no transcranial attenuation. Masking of the nontest ear is therefore a requirement to achieve testing of one ear, regardless of BC site used. It I important to remember that masking can affect the thresholds obtained. First, it is harder to detect a tone if masking noise is used. Second, an earphone in or on the ear may introduce an occlusion effect that improves the low frequency BC thresholds up to 20 dB. Third, the level and frequency of the masking noise is important. Inadequate masking may allow the nontest ear to participate, whereas excessive masking will falsely depress the BC threshold. The optimal masking is achieved with narrowband noise at the test frequency. Bone conduction path to the inner ear 1- Historical overview of BC 2- Physics of BC Sound Transmission Type of vibrator not a significant influence if 3- Mechanisms of BC static force > 4N 4- BC consumer devices Forehead = overall lower sensitivity compare to 5- Cartilage Conduction (CC) mastoid 6- BC applications in audiology 6.1- BC hearing test Masking of the nontest ear is required Middle ear diseases impact BC Hearing  Many middle ear abnormalities can affect BC hearing: for example, otosclerosis of the stapes will induce a loss of approximately 20 dB at 2 kHz, with lesser losses at frequencies below and above this frequency Bone conduction path to the inner ear 1- Historical overview of BC 2- Physics of BC Sound Transmission 3- Mechanisms of BC 4- BC consumer devices 5- Cartilage Conduction (CC) 6- BC applications in audiology 6.1- BC hearing test  Similar to air conduction hearing tests, bone conduction hearing tests require proper calibration of the bone vibrator to ensure accurate measurements. According to standard calibration procedures, the bone vibrator must be calibrated using an artificial mastoid, a device designed to simulate the mechanical properties of the human skull. The artificial mastoid is coupled to the BC transducer, which is calibrated to produce a known force. Bone conduction path to the inner ear 1- Historical overview of BC 2- Physics of BC Sound Transmission 3- Mechanisms of BC 4- BC consumer devices 5- Cartilage Conduction (CC) 6- BC applications in audiology 6.1- BC hearing test 6.2- Percutaneous BC device 1- Bone 2- Skin 3- Titanium anchor 4- Abutment  Bone conduction hearing devices are another example of BC application in audiology. These specialized hearing aids do not transmit sound through air conduction; instead, they vibrate the skull to send sound directly to the cochlea via bone conduction. These devices are particularly beneficial for individuals who cannot use conventional hearing aids. For instance, patients with recurrent ear infections or malformations of the external auditory canal can benefit from bone conduction devices, as they allow sound amplification to bypass the outer ear. Bone conduction implants can be percutaneous , meaning the transducer is directly coupled to the bone by means of a permanent skin penetration. Bone conduction path to the inner ear 1- Historical overview of BC 2- Physics of BC Sound Transmission 3- Mechanisms of BC 4- BC consumer devices 5- Cartilage Conduction (CC) 6- BC applications in audiology 6.1- BC hearing test 6.2- Percutaneous BC device Processor Abutment Titanium anchor  The BAHA system from Cochlear and the Ponto system from Oticon Medical are two examples of percutaneous bone conduction devices. They consist of a sound processor attached to an abutment, which is connected to a small titanium anchor implanted in the bone. Bone conduction path to the inner ear 1- Historical overview of BC 2- Physics of BC Sound Transmission 3- Mechanisms of BC 4- BC consumer devices 5- Cartilage Conduction (CC) 6- BC applications in audiology 6.1- BC hearing test 6.2- Percutaneous BC device 6.3- Transcutaneous BC device  The other type of bone conduction implant us transcutaneous, meaning one part of the transducer is implanted and the other part is kept outside the intact skin and soft tissue. Compared to percutaneous implants, transcutaneous leaves the skin intact and help avoiding skin complication and accidental fixture loss as the transducer is permanently implanted in the temporal bone. This option is particularly well-suited for patients where poor hygiene may be a concern, although such devices are more invasive. Bone conduction path to the inner ear 1- Historical overview of BC 2- Physics of BC Sound Transmission 3- Mechanisms of BC 4- BC consumer devices 5- Cartilage Conduction (CC) 6- BC applications in audiology 6.1- BC hearing test 6.2- Percutaneous BC device 6.3- Transcutaneous BC device  The most common transcutaneous bone conduction implants are the Bonebridge from MED-EL, the Sentio from Oticon Medical, and the Osia from Cochlear. These devices transmit the sound signals to a transducer that is permanently implanted on the temporal bone, Bone conduction path to the inner ear 1- Historical overview of BC 2- Physics of BC Sound Transmission 3- Mechanisms of BC 4- BC consumer devices 5- Cartilage Conduction (CC) 6- BC applications in audiology 6.1- BC hearing test 6.2- Percutaneous BC device 6.3- Transcutaneous BC device  In addition to surgically implanted devices, MED-EL offers a non- surgical bone conduction solution with the ADHEAR. Compared to transcutaneous and percutaneous devices, the ADHEAR is less invasive and uses a unique no-pressure adhesive adapter, ensuring all-day comfort. However, its power is more limited than that of surgical devices. Bone conduction path to the inner ear Device choice based on pure threshold average (PTA) BC threshold 1- Historical overview of BC 2- Physics of BC Sound Transmission 3- Mechanisms of BC 4- BC consumer devices 5- Cartilage Conduction (CC) 6- BC applications in audiology 6.1- BC hearing test 6.2- Percutaneous BC device BC PTA 0.5-4 kHz ≤ 25 dB 6.3- Transcutaneous BC device 6.4- Candidacy criteria  Indeed, in term of candidacy, the ADHEAR from MED-EL is suitable for patients with BC thresholds below 25 dB in the range of frequencies 500 Hz and 4000 Hz Bone conduction path to the inner ear Device choice based on pure threshold average (PTA) BC threshold 1- Historical overview of BC 2- Physics of BC Sound Transmission 3- Mechanisms of BC 4- BC consumer devices 5- Cartilage Conduction (CC) 6- BC applications in audiology 6.1- BC hearing test 6.2- Percutaneous BC device BC PTA 0.5-4 kHz ≤ 45 dB 6.3- Transcutaneous BC device 6.4- Candidacy criteria  The Bonebridge from MED-EL and the Sentio from Oticon Medical are suitable for patients with BC thresholds below 45 dB in the 500 Hz to 4000 Hz frequency range Bone conduction path to the inner ear Device choice based on pure threshold average (PTA) BC threshold 1- Historical overview of BC 2- Physics of BC Sound Transmission 3- Mechanisms of BC 4- BC consumer devices 5- Cartilage Conduction (CC) 6- BC applications in audiology 6.1- BC hearing test 6.2- Percutaneous BC device BC PTA 0.5-3 kHz ≤ 55 dB 6.3- Transcutaneous BC device 6.4- Candidacy criteria The Osia from Cochlear is suiable for patients with BC thresholds below 55 dB in the 500 Hz to 3000 Hz range Bone conduction path to the inner ear Device choice based on pure threshold average (PTA) BC threshold and the size of the air bone gap (ABG) 1- Historical overview of BC 2- Physics of BC Sound Transmission 3- Mechanisms of BC 4- BC consumer devices 5- Cartilage Conduction (CC) 6- BC applications in audiology 6.1- BC hearing test 6.2- Percutaneous BC device BC PTA 0.5-3 kHz ≤ 65 dB 6.3- Transcutaneous BC device 6.4- Candidacy criteria + ABG 0.5-4 kHz > 30 dB  And finally, the Ponto from Oticon Medical and the BAHA from cochlear are suitable for patients with an air-bone gap exceeding 30 dB in the range of frequencies 500 Hz and 3000 Hz, and BC thresholds below 65 dB in the range of frequencies 500 Hz and 4000 Hz Bone conduction path to the inner ear 1- Historical overview of BC 2- Physics of BC Sound Transmission PTA BC threshold 3- Mechanisms of BC computation 4- BC consumer devices 5- Cartilage Conduction (CC) 6- BC applications in audiology 6.1- BC hearing test 6.2- Percutaneous BC device 6.3- Transcutaneous BC device 20 + 25 + 30 + 35 PTA 0.5−3kHz = = 28 dB HL 4 6.4- Candidacy criteria  To calculate the Pure Tone Average (PTA) for Bone Conduction (BC) thresholds between 500 and 3000 Hz, you simply take the average of the BC thresholds at the frequencies of 500 Hz, 1000 Hz, 2000 Hz, and 3000 Hz. For example, if the thresholds are 20 dB at 500 Hz, 25 dB at 1000 Hz, 30 dB at 2000 Hz, and 35 dB at 3000 Hz, you would add these values together: 20 + 25 + 30 + 35, which equals 110 dB. Then, divide the sum by 4 (since there are four frequencies in the range) to get the average. So, 110 divided by 4 equals 27.5, rounded up to 28 dB. Bone conduction path to the inner ear 1- Historical overview of BC 2- Physics of BC Sound Transmission Air bone gap (ABG) 3- Mechanisms of BC computation 4- BC consumer devices 5- Cartilage Conduction (CC) 6- BC applications in audiology 6.1- BC hearing test 6.2- Percutaneous BC device 50 − 20 + 60 − 25 + 75 − 30 + (80 − 40) ABG 0.5−4kHz = 6.3- Transcutaneous BC device 4 6.4- Candidacy criteria 30 + 35 + 45 + 40 ABG 0.5−4kHz = = 38 dB 4  To calculate the Air-Bone Gap (ABG), between 500 and 4000 Hz, we need to look at the difference between the air conduction thresholds and the bone conduction thresholds at each frequency. The ABG is calculated by subtracting the bone conduction threshold from the air conduction threshold at each frequency, and then averaging those differences. Let’s use the example where the air conduction thresholds are 50 dB at 500 Hz, 60 dB at 1000 Hz, 75 dB at 2000 Hz, and 80 dB at 4000 Hz. And the bone conduction thresholds are 20 dB at 500 Hz, 25 dB at 1000 Hz, 30 dB at 2000 Hz, and 40 dB at 4000 Hz. Now, for each frequency, we subtract the bone conduction threshold from the air conduction threshold: At 500 Hz, 50 minus 20 gives us 30 dB. At 1000 Hz, 60 minus 25 gives us 35 dB. At 2000 Hz, 75 minus 30 gives us 45 dB. At 4000 Hz, 80 minus 40 gives us 40 dB. Now, we add all these differences together: 30 + 35 + 45 + 40, which equals 150 dB. Finally, we divide this sum by 4 to get the average gap: 150 divided by 4 equals 37.5, round up to 38 dB Candidacy case studies Right BC Left BC CASE #1 Right AC Left AC 55-Year-Old Female Teacher Works in a classroom setting with significant background noise Difficulty understanding speech in noisy environments BTE OR ITE HEARING AIDS  This case is typically a sensorineural hearing loss. The recommendation would be either BTE or ITE hearing aids, as the air- bone gap is the objective methodology don’t take into account the direct bone conduction to the cochlea and is therefore not reliable Occlusion effect 1- Definition and mechanisms 2- Model pathways for BC and AC 3- OE impact on hearing protection  According to the World Health Organization, worldwide hearing loss estimates increased from 120 million people in 1995 to 250 million in 2004. In the U.S., The National Institute for Occupational Safety and Health (NIOSH) evaluates that 22 million workers are affected by occupational noise-induced hearing loss. Occlusion effect 1- Definition and mechanisms 2- Model pathways for BC and AC 3- OE impact on hearing protection  Since it is often difficult, for technological or economical reasons, to reduce noise at its source, the most commonly used solution to protect workers from noise exposure consists in using hearing protection devices Occlusion effect 1- Definition and mechanisms 2- Model pathways for BC and AC 3- OE impact on hearing protection !!! ?  However, this solution also presents several well-known limitations, such as difficulty in communicating which raises safety concerns. Occlusion effect 1- Definition and mechanisms 2- Model pathways for BC and AC 3- OE impact on hearing protection I CAN'T HEAR WHILE WEARING MINE... NO HPD?! HUH?!  Additionally, many people report not wearing their hearing protectors because the occlusion of the ear canal induced a modification of their own voice perception and creates a discomfort that bring them to remove their protectors, or to wear them intermittently. Reducing the OE induced by wearing hearing protectors has the potential to increase the auditory comfort of hearing protectors and could help preventing occupational hearing loss in the future. Occlusion effect 1- Definition and mechanisms 2- OE impact on hearing protection 3- Model pathways for BC and AC 4- OE impact on in-ear hearing aids  Hearing aids are also affected by the occlusion effect: hearing voices with a hollow or booming echo-like sound, decreased total sound volume, and in some instances, distorted sound perception, are reported by hearing aids users. Such side effects might lead to people not wear their HA Occlusion effect 1- Definition and mechanisms 2- OE impact on hearing protection 3- Model pathways for BC and AC 4- OE impact on in-ear hearing aids There are some solutions to avoid these problems: by changing the style of the dome (open style dome), shortening the length of the vent, or increasing the vent diameter. Recap of Today’s Learnings Recap of Today's Learnings (1/3) Four mechanisms contribute to bone conduction: Compressional bone conduction Distortional bone conduction Inertial or ossicular-lag bone conduction Osseotympanic bone conduction Bone conduction has application that extend beyond audiology (BC earphones) BC relies on cranial bone vibrations, CC involves vibration of the cartilage in the external ear BC threshold are less affected by a conductive hearing loss than AC thresholds => help to distinguish between conductive and sensorineural losses Recap of Today’s Learnings Recap of Today's Learnings (2/3) Several factor can influence BC thresholds: Static force Placement (forehead = overall lower sensitivity compared to mastoid) Masking of the nontest ear is required Percutaneous BC hearing aids are more powerful than transcutaenous Transcuraneous = suitable for patients with poor hygiene, but more invasive Device candidacy is mainly based on PTA BC threshold and ABG While SSD patients can benefit from BC hearing aids, there are other solutions (CROS hearing aids, CI) Recap of Today’s Learnings Recap of Today's Learnings (3/3) OE mostly impacts the perception mostly in the low frequency range (below 1000 Hz) OE can lead people to not wear hearing protectors, and hearing aids as well due to the "acoustical" incomfort the induced The OE induced by hearing aids can be reduced by: Changing the style of the dome (open style dome) Shortening the length of the vent Increasing the vent diameter