If you've ever seen what looks to be a pair of odd-looking headphones that do not go over or even in the listener's ear, you've likely seen a pair of bone conduction headphones. These strange devices serve the same purpose as regular headphones but work on different principles.
What are bone conduction headphones? Bone conduction headphones are transducers that convert electrical energy (audio signals) into mechanical wave energy (physical vibrations). They do so with piezoelectric drivers coupled to the listener's jaw and/or cheekbones. The vibrations extend to the inner ear and are interpreted as sound.
Of course, there's more to it than that, and in this article, we'll discuss these strange headphones in great detail to improve your understanding of bone conduction headphones and headphones in general.
A Primer On Headphone Drivers
Before we begin our full discussion on bone conduction headphones, let's discuss the most critical component of any headphone: the driver.
The headphone driver is the transducer component that makes converting audio into sound a reality. Without a pair of drivers, a pair of headphones would be capable of producing sound when connected to an audio source. This would render them practically useless.
Most headphones utilize moving-coil dynamic drivers that work on principles of electromagnetism.
In these models, the audio signal (AC voltage) is sent through a conductive coil attached to a diaphragm. This conductor/diaphragm combo is housed within a permanent magnetic field.
As the AC audio signal varies the coil's magnetic field (via electromagnetic induction), the coil/diaphragm oscillates back and forth, producing sound waves as it does so.
The less popular planar magnetic, electrostatic and balanced armature driver designs also work with an oscillating diaphragm to produce sound waves that mimic the audio signals. Planar magnetic and balanced armature designs are also electromagnetic, while electrostatic drivers work on electrostatic principles.
For more information on headphone drivers and each driver type mentioned above, check out the following My New Microphone articles:
• What Is A Headphone Driver? (How All 5 Driver Types Work)
• What Are Dynamic Headphones And How Do They Work?
• Complete Guide To Planar Magnetic Headphones (With Examples)
• Complete Guide To Electrostatic Headphones (With Examples)
• The Complete Guide To Balanced Armature IEMs/Earphones
Bone conduction headphones technically have drivers, too. However, they are unlike the previously-mentioned headphone driver types and deserve their own category.
Unlike the other types, bone conduction drivers are not concerned with creating sound waves that interact with the wearers' eardrums. They still convert electrical audio signals into physical vibrations, but these vibrations are designed to travel through solids (our bones) rather than through air.
For this reason, bone conduction headphones are worn around the ear but not on or in the ear. The vibrations are sent through our heads to our inner ears rather than through the air to our eardrums.
So how do the bone conduction drivers turn audio signals into mechanical waves without producing much sound? The short answer is piezoelectricity.
What Is Piezoelectricity?
The term “piezo” is derived from the Greek word “piezein,” which means to squeeze or press.
It makes sense, then, that piezoelectricity allows us to squeeze piezoelectric crystalline structures (such as quartz) and convert mechanical energy (the squeezing) into electricity and vice-versa. If we pass electricity through the same crystals, their molecules would “squeeze” themselves” by vibrating back and forth.
The piezoelectric effect produces an electrical potential (voltage) across the sides of a mechanically stressed crystal structure.
What is a crystal?
A crystal is a solid material made of highly ordered molecules, atoms, and ions arranged in a lattice that extends in all directions. A crystal of infinite size would be made endless repetitions of the same basic atomic building block known as the unit cell.
In most crystals, the unit cell is symmetrical. Piezoelectric crystals are special since they are not symmetrical in their atomic arrangement.
Rather, the electrical charges of piezo crystals are perfectly balanced. The charges still exist, but a positive charge in one place is effectively cancelled out by a nearby negative charge.
Squeezing and/or stretching a piezo crystal causes a slight deformation in the structure. This deformation pushes some atoms closer together and pulls other atoms further apart.
Deforming the crystal, therefore, changes the naturally balanced electrical charge within the crystal. This causes net electrical charges to develop, leading to a positive charge on one side of the crystal and a negative charge on the opposite side of the crystal.
The reverse is also true of a piezoelectric crystal, which defines the working principle of bone conduction headphones.
By applying a voltage across the crystal and passing an electrical current through it, we produce the need for the atoms to rebalance themselves to try and find equilibrium which deforms the crystal. If the current is alternating (AC), like an audio signal, the crystal will actually vibrate according to the direction and amplitude of the current.
How Do Bone Conduction Headphone Driver Work As Transducers?
As with any headphone or speaker, the main purpose of bone conduction headphones is to convert electrical audio signals into something we can hear.
Note as well that, like all other headphone and speaker drivers, the bone conduction “driver” must receive analog audio signals (as opposed to digital audio signals) to work properly.
This is because analog signals are continuous alternating currents, while digital audio signals are only digital representations of analog signals with binary code, samples, and bit-depths.
This point about analog and digital is important to know, especially considering that most bone conduction headphones have wireless Bluetooth technology.
Bluetooth transmits digital audio wirelessly, so bone conduction headphones must have built-in wireless receivers with DACs (digital-to-analog converters) to convert the digital audio into analog audio that will cause the headphone vibrations.
For in-depth information on wireless Bluetooth headphones, check out my article How Do Wireless Headphones Work? + Bluetooth & True Wireless.
The DAC output makes a separate circuit with the left and right headphone drivers. The left audio signal (AC voltage) is passed through the left driver while the right audio signal (AC voltage) is passed through the right channel.
Each piezoelectric driver has two electrical lead wires: one at one side and the other at the opposite side. At any given point, while the headphones are passing audio, the piezo crystal has equal but opposite voltage at either side.
As we've learned in the previous section, applying a potential difference (voltage) across a piezoelectric crystal causes it to deform.
When the audio signal produces a current flow in one direction, the crystal will be squeezed. The crystal will be stretched when the audio signal inevitably produces a current flow in the opposite direction.
Since audio signals mimic sound in the typical range of 20 Hz – 20,000 Hz, the crystal with squeeze/stretch or, in other words, vibrates in the range of audible sound.
These vibrations are passed through the bones of the skull to the cochlea, where they are then transduced back into electrical impulses for our brains (sense of hearing) to perceive as sound!
How Do We Hear Bone Conduction Headphones?
Regular headphones are worn with their drivers to the exterior of the ear. Earphones and most hearing aids are worn within the ear canal.
Bone conduction headphones are worn very differently, with their “drivers” pressed against the listener's cheek and/or jawbone.
Bone conduction headphones are relatively new to the market, and, therefore, many of these headphones are designed with Bluetooth technology.
The headphones are often designed with a band that wraps around the back of the listener's head; and has the drivers sit just in front of the ears, pressed against the cheek or jaw bone.
The shape of a typical pair of bone conduction headphones can be seen in the following Aftershokz Aeropex:
Human hearing is made possible by the cochlea. The cochlea is a spiralled, hollow, fluid-filled conical chamber of bone in the inner ear.
The key component of the cochlea is the organ of Corti, which effectively acts as a transducer that converts mechanical wave energy (vibrations) into electrical energy (nerve impulses for the brain to observe as sound). The organ of Corti is distributed along the partition separating fluid chambers in the coiled tapered tube of the cochlea.
Note that each ear has its own cochlea.
There are two main ways to vibrate our cochleae and, therefore, there are two main ways that we sense the sound of our environment (and our headphones).
Before we get into each method, let's have a look at a diagram describing the anatomy of the human ear:
In this article, I'll be oversimplifying the working principles of our sense of hearing. To get into the anatomy and mechanics of the ear and brain would require many more articles than a relatively simple script explaining bone conduction headphones.
With that being said, let's get into it.
The first method of stimulating the cochlea (and, therefore, our sense of hearing) is via the eardrum.
Sound waves are channelled through the ear canal and interact with the eardrum. The eardrum is a flexible membrane that vibrates according to the sound pressure variations at its surface.
Eardrum vibrations are transmitted through the ossicles (three tiny bones) in the air-filled middle ear to the cochlea in the fluid-filled inner ear.
The middle ear bones essentially provide impedance matching between the sound waves in the air at the eardrum and the waves in the fluid at the cochlea. Further protection is provided by muscles in the middle ear that offer a stiffening reflex.
A second flexible membrane called the round window passes the vibrations of the middle ear to the inner ear, allowing for the smooth displacement of the inner ear fluid caused, ultimately, by the entering sound waves.
As the fluid inside the inner vibrates, the cochlea sends sound information via the auditory nerve to the cochlear nucleus in the brainstem. The complex system works as a transducer, converting mechanical wave energy into electrical energy (microphones do the same).
To learn more about how microphones work, check out my article How Do Microphones Work? (A Helpful Illustrated Guide).
The electrical energy is made of nerve impulses that are perceived by (heard by) the brain.
This first method is the primary method by which most headphones are heard.
Headphone drivers produce sound waves in or near our ears which are picked up by our eardrums and heard by our brains.
However, we also hear headphones via a secondary method of hearing, which turns out to be the main method of bone conduction headphones.
The second method involves stimulating the cochlea via the skull rather than through the ear.
Vibrations in the audible range of 20 Hz – 20,000 Hz do not only vibrate the eardrum. They vibrate our bodies.
These vibrations can be transmitted through the soft and hard tissues of our bodies. Therefore, vibrating headphones cause, to some extent, vibrations through our bones that are transmitted to our inner ear.
These vibrations, like the ones travelling through our outer and middle ears, cause an auditory response in our brains.
Bone conduction headphones work on this method, completely bypassing the outer and middle ears.
By vibrating against our skulls, bone conduction headphones transmit the information of the audio signal to our cochlea, which is then perceived by our brains.
Pros & Cons Of Bone Conduction Headphones
Knowing the advantages and disadvantages of bone conductions will help us to understand the technology better. The pros and cons are summarized in the table below:
|Better choice for listeners with middle ear and/or eardrum damage
|Relatively poor sound quality
|Does not cover the listener's ears
|May be uncomfortable for some head shapes
|Allows for easy environmental listening
|Does not offer any noise-cancellation
|Less likely to damage hearing
Pros Of Bone Conduction Headphones
Better Choice For Listeners With Middle Ear And/Or Eardrum Damage
If a listener has conductive hearing loss or an otherwise damaged eardrum or middle, bone conduction headphones can significantly improve listening enjoyment.
There is a reason bone conduction technology is used in hearing aids.
Does Not Cover The Listener's Ears
Because bone conduction headphones work by transmitting vibrations through our skulls, they do not have to be placed over the ears.
This may improve comfort for those who do not like wearing headphones over their ears or holding earphones in their ear canals.
Allows For Easy Environmental Listening
The fact that bone conduction headphones do not cover our ears also allows us to hear our environment.
Being aware of your environment is important in many situations, such as walking down the street or bicycling on the road. Bone conduction headphones allow us to listen to our audio without closing us off from hearing our environment.
Less Likely To Damage Hearing
Regular air conduction headphones can easily be turned up too loud and be listened to for too long. On top of that, our ear canals act naturally as acoustic amplifiers that further increase the sound pressure levels at our eardrums. This can lead to hearing damage.
Though bone conduction headphones can certainly cause damage to the inner ear if turned up too loud, it is very unlikely.
Increasing the volume of a pair of bone conduction headphones causes the drivers to vibrate with greater intensity, but much of this energy is lost to the environment.
Cons Of Bone Conduction Headphones
Relatively Poor Sound Quality
Because bone conduction headphones only act on one part of our hearing, their audio quality is relatively poor.
Some would argue that the sound quality of bone conduction headphones matches that of low-end earbuds. However, no bone conduction headphones come close to sounding as clear and powerful as high-end headphones.
If superb audiophile quality is what you're after, bone conduction headphones are not for you.
May Be Uncomfortable For Some Head Shapes
To effectively vibrate our skull bones, the bone conduction headphones must be pressed against our heads. This, of course, causes pressure on our cheek and/or jawbones which can be rather uncomfortable for some people.
Compare this to the soft cushions of a nice pair of circumaural (over-ear) headphones, and the comfort differences will be apparent.
Does Not Offer Any Noise-Cancellation
This is the flip side of not closing our ears off to the environment.
Though hearing our environment is good in some situations, other situations call for noise-cancellation for a more immersive listening experience.
Bone conduction headphones offer no noise cancellation.
For more information on headphone noise-cancellation, check out my articles How Do Noise-Cancelling Headphones Work? (PNC & ANC), Passive Vs. Active Noise-Cancelling Headphones and Do Noise-Cancelling Headphones Work With Or Without Music?
Bone Conduction Headphone Examples
To better understand bone conduction headphones, let's look at a few examples.
Note that bone conduction headphones are relatively new and are not produced by major headphone manufacturers (yet). Aftershokz is the first company to specialize in higher-end bone conduction headphones, and we'll start our list of examples with their Titanium model.
The Aftershokz Titanium is a Bluetooth wireless bone conduction headphone with excellent wearability, 6-Hour battery life, IP55 dust/water resistance, and dual noise-cancelling mics for telephony.
- Frequency response: 20 Hz ~ 20,000 Hz
- Sensitivity: 100 dB/mW ± 3 dB
- Weight: 36 g
- Particle resistance: IP55
- Bluetooth: 4.1
The Akaso G101 is another Bluetooth wireless bone conduction headphone with excellent wearability, 6-Hour battery life, IP55 dust/water resistance, and a microphone for telephony.
- Frequency response: Specification not given
- Sensitivity: Specification not given
- Weight: 33 g
- Particle resistance: IP55
- Bluetooth: 5.0
Kasono B2 is yet another Bluetooth wireless bone conduction headphone. This lightweight model has a 5-Hour battery life, IP55 dust/water resistance, and a microphone for telephony.
- Frequency response: Specification not given
- Sensitivity: Specification not given
- Weight: 1.6 oz (45 g)
- Particle resistance: IP55
- Bluetooth: 4.2
The IP code (Ingress Protection code) is a measurement and classification of the degree of protection provided by mechanical casings and electrical enclosures against intrusion, dust, accidental contact, and water.
The first digit refers to solid particle protection on a scale from 0 to 6, while the second digit refers to liquid ingress protection on a scale from 0 to 9.
The IP55 classifications given to each of the bone conduction headphone examples above means they are protected against dust and water jets.
More specifically, ingress of dust is not entirely prevented, but it must enter in sufficient quantity to interfere with the operation of the headphones. The headphones are also protected from water based on a test that has water projected by a nozzle (6.3 mm (0.25 in)) against the headphones from all directions.
What is the difference between air and bone conduction? Air conduction hearing occurs through the air near the ear. It involves sound waves in the air that interact with the ear canal and eardrum. Bone conduction hearing occurs through vibrations in the body that are picked up by the inner ear’s specialized nervous system.
What is better, air conduction or bone conduction? For optimal hearing, we need to sense audible vibrations through air conduction and bone conduction. If we have hearing damage in our eardrum and middle ear, bone conduction will work better. If we have damage in our inner ear, we will likely hear via air conduction with more clarity.
Choosing the right headphones or earphones for your applications and budget can be a challenging task. For this reason, I've created My New Microphone's Comprehensive Headphones/Earphones Buyer's Guide. Check it out for help in determining your next headphones/earphones purchase.