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An Introduction to the Knowledge of Bone Conduction

The snake living in the ocean gets the vibration of the sound in the water through the mandible and transmits it to the ears; after Beethoven's hearing loss, he bit the baton with his teeth, and put the other end on the piano, and the sound made by the piano through the baton Into the ear, so as to continue to create.

In fact, we often experience another mysterious transmission channel of sound unconciously, that is bone conduction. Cover your ears with your hands and talk to yourself. No matter how small the sound is, we can hear what we say. This is the result of bone conduction.

Ⅰ. What is bone conduction?

Air conduction and bone conduction have a very different sound effect. This is because the sound transmitted through the air is affected by the environment, and its energy will be greatly attenuated, resulting in a great change in timbre, but the bone conduction sound passes through the skull and reaches the inner ear directly. This way the energy and tone of the sound attenuate and change relatively little. Therefore, the hearing is not the same.

There are two ways for sound to enter the inner ear: one way is through the external auditory canal→tympanic membrane→ossicular chain, called air conduction; the other way is directly through the skull vibration conduction into the inner ear, called bone conduction. Under physiological conditions, bone conduction is far less effective than air conduction, but bone conduction will play an important role when the middle ear pressurization effect is destroyed. This is also the basic acoustic principle for some patients with external auditory canal atresia to fit bone anchored hearing aids.

When sound waves are conducted from the skull to the cochlea, the vibration of the skull causes the inner ear lymph to vibrate accordingly, which further causes the basement membrane to vibrate, thereby generating hair cell excitement.

 To put it simply, air conduction is a way of spreading through the air, so that everyone around you can hear the sound; while bone conduction is spreading through the skull, and only yourself can hear the sound.

Ⅱ. Bone conduction: a neglected transmission channel

There are three ways of bone conduction: including mobile bone conduction, compression bone conduction and through bone drum. In the first two ways, sound waves are directly transmitted into the inner ear through the skull, which is the main way of bone conduction. In the latter way, sound waves are first transmitted to the tympanum through the skull and then into the inner ear through the tympanum, which is the secondary way of bone conduction.

1. Mobile bone conduction

In mobile bone conduction, when sound waves act on the skull, the entire skull including the cochlea vibrates repeatedly. Due to the inertia of the inner ear lymph fluid, the vibration of the lymph fluid slightly lags behind the cochlear bone wall in each vibration cycle. When the cochlear bone wall shifts upward during the vibration period, the displacement of the lymph fluid cannot keep up with the displacement of the bone wall temporarily, causing the cochlear window membrane to protrude; when the cochlear bone wall shifts downward, the inertia of the lymph fluid displaces the soleplate of the stirrup outwards. During the vibration period, the two windows are alternately convex, causing the basement membrane to move back and forth and vibrate.

In mobile bone conduction, in addition to the inertia of the lymph fluid that makes the basement membrane vibrate, the inertia of the ossicular chain also plays the same role. Since the ossicular chain is suspended in the tympanum, the connection with the skull is not strong. When the skull moves, its inertia makes the displacement of the ossicular chain slightly lag behind the cochlear wall. The result of this process is the displacement of the stirrup floor in the vestibular window, which is equivalent to the vibration of the stirrup floor caused by air conduction. When the sound frequency is below 800Hz, the mobile bone conduction plays a major role.

2. Compression bone conduction

In compression bone conduction, when the vibration of sound waves is conducted to the cochlear bone wall through the skull, the skull, including the cochlear bone wall, periodically expands and compresses with the density of the sound waves. In the dense phase, the cochlear bone wall is compressed, but the cochlear lymph fluid has very little compressibility and can only move to the cochlear window and vestibular window. Because the amount of lymph fluid in the vestibular scala is larger than that in the tympani, the ratio of the two is 5:3. At the same time, the activity of the cochlear window membrane is much greater than that of the pedestal floor. Therefore, when the sound waves are dense, the compressed The bone wall encourages the perilymph fluid in the semicircular canals to be squeezed into the vestibular scala with a larger volume, and then into the scala tympani with a smaller volume, and the activity of the cochlear window membrane is greater than the bottom plate of the peduncle, which causes the basement membrane to shift to the scala tympani. During the sonic sparse phase, the labyrinth bone wall swells, the lymph fluid returns to its original position, and the basement membrane shifts upward. The repeated alternating effects of sparse and dense sound waves make the cochlear basement membrane vibrate up and down, forming an effective stimulation to the cochlear hair cells. According to the above-mentioned mechanism, the greater the difference in the mobility of the two windows, the greater the displacement of the basement membrane, and the greater the effective stimulus produced. Therefore, when the resistance of the ossicular chain increases, the compression bone conduction increases. The bone conduction of sound waves with frequencies above 800 Hz is mainly compression bone conduction.

3. Through bone drum

Through bone drum means that when the skull is stimulated by sound waves and vibrates, the sound waves are transmitted to the external auditory canal, tympanum and the surrounding air, causing the tympanic membrane to vibrate, and then the sound waves are transmitted to the inner ear through normal air conduction. This approach may have special meaning in listening to one's own voice.

Based on our core technology-bone conduction, Thunder Blast plans to launch consumer food and electronic products, business machines and medical equipment. The world's first Lavoli music lollipop that we just launched is also based on bone conduction technology, allowing people to listen to music while eating lollipops, and they can only hear it by themselves. Relying on the ergonomic design concept, Lavoli music lollipops has good experience, and the product design is both beautiful and practical. There are many types of lollipop shapes to choose from, so you can enjoy delicious candies when listening to music, which is a dual wonderful experience. If necessary, please consult Lavoli.

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