If you have ever seen a pair of strange-looking headphones that neither enter the listener’s ears nor the listener’s ears, then you may have seen a pair of bone-conduction headphones. These strange devices have the same functions as ordinary headphones, but the principles are different.
1. What are bone conduction headphones?
Bone conduction headphones are transducers that convert electrical energy (audio signals) into mechanical wave energy (physical vibration). They connect piezoelectric actuators to the listener’s jaw and/or cheekbones. The vibration extends to the inner ear and is interpreted as sound.
Of course, there are more things. In this article, we will discuss these strange earphones in detail to improve your understanding of bone conduction earphones and general earphones.
Getting started with headphone drivers
Before we begin to discuss bone conduction headphones in full, let’s discuss the most critical component of any headset: the driver. The headphone driver is a converter component that converts audio into sound reality. Without a pair of drivers, a pair of headphones can produce sound when connected to an audio source. This will make them practically useless.
Most earphones use moving coil dynamic drivers based on electromagnetic principles. In these models, the audio signal (AC voltage) is sent through a conductive coil connected to the diaphragm. This conductor/diaphragm combination is installed in a permanent magnetic field.
When the AC audio signal changes the coil’s magnetic field (through electromagnetic induction), the coil/diaphragm oscillates back and forth, and when it does so, it produces sound waves. The less popular planar magnetic, electrostatic, and balanced armature drive designs also work with the oscillating diaphragm to generate sound waves that mimic audio signals. Planar magnetic and balanced armature designs are also electromagnetic, while the working principle of electrostatic actuators is electrostatic.
Bone conduction headsets also have drivers technically. However, they are not like the aforementioned headphone driver types and should have their own category. Unlike other types, bone conduction drivers do not care about generating sound waves to interact with the wearer’s eardrum. They still convert electronic audio signals into physical vibrations, but these vibrations are designed to pass through solids (our bones), not through the air.
Therefore, bone conduction headphones are worn around the ears, not on or in the ears. Vibrations are transmitted to our inner ears through our heads, not to our eardrums through the air. So, how does a bone conduction driver convert an audio signal into a mechanical wave without producing too much sound? In short, it is piezoelectric.
2. What is Piezoelectric?
The word “piezo” is derived from the Greek word “piezein”, which means squeeze or squeeze.
Therefore, piezoelectricity allows us to squeeze piezoelectric crystal structures (such as quartz) and convert mechanical energy (squeezing) into electrical energy, and vice versa. The molecules of the crystal will vibrate back and forth if we pass current through it.
The piezoelectric effect generates electrical potential (voltage) on both sides of the mechanically stressed crystal structure.
3. What is a crystal?
A crystal is a solid material composed of highly ordered molecules, atoms, and ions, arranged in a lattice extending in all directions. An infinite crystal will continue to repeat basic atomic building blocks called units.
In most crystals, the single unit cell is symmetrical. Piezoelectric crystals are special because their atomic arrangement is asymmetric. On the contrary, the charge of the piezoelectric crystal is completely balanced. The charge still exists, but the positive charge in one place is effectively cancelled out by the negative charge nearby.
Squeezing and/or stretching the piezoelectric crystal will slightly deform its structure. This deformation pushes some atoms closer and pulls others further away.
Therefore, the deformation of the crystal changes the natural balance of charges in the crystal. This results in a net charge, which results in a positive charge on one side of the crystal and a negative charge on the other side.
The situation with piezoelectric crystals is just the opposite, which defines the working principle of bone conduction headphones. By applying a voltage to the crystal and passing an electric current, we create the need for atomic rebalance, trying to find a balance that deforms the crystal. If the current is alternating current (AC), like an audio signal, the crystal will actually vibrate according to the direction and amplitude of the current.
4. How does a bone conduction headphone driver work as a transducer?
Like any earphone or speaker, the main purpose of bone conduction earphones is to convert electronic audio signals into something we can hear. Also note that like all other headphone and speaker drivers, the bone conduction “driver” must receive an analog audio signal (not a digital audio signal) to work properly.
This is because the analog signal is a continuous AC signal, and the digital audio signal is just a digital representation of the binary code, samples, and bit depth of the analog signal.
Particularly since most bone conduction headsets have wireless Bluetooth technology, this point about analog and digital is crucial. Bluetooth transmits digital audio wirelessly, so bone conduction headphones must have a built-in wireless receiver with a digital-to-analogue converter (DAC) to convert digital audio to analog audio, causing the headset to vibrate.
The DAC output forms a separate circuit with the left and right headphone drivers. The left audio signal (AC voltage) passes through the left driver, and the right audio signal (AC voltage) passes through the right channel. Each piezoelectric actuator has two wires: one on one side and the other on the other side. At any given point, when the earphone passes audio, the piezoelectric crystal has equal but opposite voltages on both sides.
As we learned in the previous section, applying a potential difference (voltage) to a piezoelectric crystal will cause it to deform. When the audio signal generates a current in one direction, the crystal will be squeezed. When the audio signal inevitably generates a current in the opposite direction, the crystal will be stretched.
During the simulation of an audio signal, the crystal vibrates in the range of 20 Hz-20000 Hz, which can be heard as squeezing or stretching. These vibrations are transmitted through the skull to the cochlea, where they are transduced back into electrical impulses, allowing our brain (hearing) to perceive sound!
5. How do we hear bone conduction headphones?
Ordinary earphones are worn outside the ears of the driver. Earphones and most hearing aids are worn in the ear canal. Bone conduction headphones are worn in very different ways, with their “drive” pressed against the listener’s cheek and/or jawbone.
Due to their relative newness on the market, most bone conduction earphones use Bluetooth technology for their control. The headset is usually designed with a strap that wraps around the back of the listener’s head; the driver sits in front of the ear, close to the cheek or jaw. The cochlea makes human hearing possible. The cochlea is a spiral, hollow, fluid-filled conical bone cavity in the inner ear.
The key component of the cochlea is the Corti organ, which effectively acts as a transducer, converting mechanical wave energy (vibration) into electrical energy (the brain treats nerve impulses as sound). The Corti organs of the cochlea are distributed on the partitions that separate the fluid chambers in the spiral conical tube of the cochlea.
There is a cochlea in each ear. There are two main ways to vibrate the cochlea, so we have two main ways to perceive the sound of our surroundings (and our headphones). Before we discuss each method, let’s take a look at the diagrams describing the anatomy of the human ear:
Anatomy of the human ear
In this article, I will oversimplify the working principle of our hearing. To gain insight into the anatomy and mechanics of the ear and brain, more articles are needed, rather than a relatively simple script explaining bone conduction headphones.
Having said that, let’s get started.
The first way to stimulate the cochlea (that is, our hearing) is through the eardrum. Sound waves travel through the ear canal and interact with the eardrum. The tympanic membrane is a flexible membrane that vibrates in response to changes in surface sound pressure. The tympanic membrane vibration is transmitted through the small bones (three small bones) in the air-filled middle ear to the cochlea in the fluid-filled inner ear.
The middle ear bone basically provides impedance matching between the sound waves in the air of the eardrum and the sound waves in the cochlear fluid. Further protection is the stiff reflex provided by the middle ear muscles.
Another type of flexible membrane called a round window transmits the vibration of the middle ear to the inner ear, so that the inner ear fluid caused by sound waves entering the inner ear can flow smoothly.
When the internal fluid vibrates, the cochlea sends sound information to the cochlear nucleus in the brain stem through the auditory nerve. This complex system is like a transducer that converts mechanical wave energy into electrical energy (this is also done by microphones).
Electricity is made up of nerve impulses that the brain perceives (hears). The first method is the main method by which most headphones are heard. Headphone drivers generate sound waves in or near our ears. These sound waves are received by our eardrums and then heard by our brains.
However, we also listen to headphones through the second hearing method, which proved to be the main method of bone conduction headphones. The second method is to stimulate the cochlea through the skull instead of the ears. The vibration of 20 Hz-20,000 Hz in the audible range not only shakes the eardrum. They shake our bodies.
These vibrations can be transmitted through the soft and hard tissues of our body. Therefore, vibrating earphones cause vibration to some extent, and the vibration is transmitted to our inner ears through our bones. These vibrations, like the vibrations passing through our outer and middle ears, cause auditory responses in our brains.
This is how bone conduction headphones work, completely bypassing the outer and middle ears. Through the vibration of the skull, bone conduction headphones transmit audio information to the cochlea, which is then sensed by the brain.
6. Pros and cons of bone conduction headphones
Understanding the advantages and disadvantages of bone conduction will help us better understand this technology. The pros and cons are shown in the following table:
Advantages of bone conduction headphones
Here are the main benefits of using bone conduction headphones.
- A better choice for listeners with middle ear and/or eardrum damage
If the listener has conductive hearing loss or the eardrum or middle ear is damaged for other reasons, bone conduction earphones can significantly increase the enjoyment of listening. Bone conduction technology is used in hearing aids for a reason.
- Can’t cover the listener’s ears
Because bone conduction headphones work by transmitting vibrations through the skull, they do not need to be worn on the ears.
For those who don’t like wearing headphones or putting them in the ear canal, this may improve their comfort.
- Make environmental listening easy
Bone conduction headphones do not cover our ears, which also allows us to hear surrounding sounds.
In many cases, it is important to be aware of your environment, such as walking on the street or riding a bicycle on the road. Bone conduction headphones allow us to listen to our own voice without turning off our hearing.
- Less likely to damage hearing
Ordinary air conduction headphones can easily be turned on too loudly and listened to for too long. Most importantly, our ear canal acts as a sound amplifier, further increasing the sound pressure level of our eardrum. This may cause hearing loss.
Although bone conduction headphones will definitely cause damage to the inner ear, if the volume is too high, it is very unlikely. When bone conduction headphones are turned up to their maximum volume, the driver will produce increased vibrations, but the majority of energy is lost to the environment.
Disadvantages of bone conduction headphones
Besides, there are some disadvantages when using bone conduction headphones.
- Poor sound quality
Since bone conduction earphones only act on a part of our hearing, the sound quality is relatively poor.
Some people might say that the sound quality of bone conduction headphones is comparable to that of low-end earplugs. However, no bone conduction headset can be as clear and powerful as a high-end headset. If you are after first-class sound quality, bone conduction headphones are not for you.
- Some head shapes may be uncomfortable
In order to vibrate our skull effectively, bone conduction headphones must be pressed against our head. This of course puts pressure on our cheeks and/or mandible, which can be quite uncomfortable for some people.
Comparing it with the cushion of a beautiful surround ear (over-ear) headset, the difference in comfort will be obvious.
- No noise cancellation service
This is the other side of not being outside of the environment.
Although in some cases it is good to hear the environment around us, in other cases, in order to get a more immersive listening experience, noise needs to be eliminated. Bone conduction headphones do not provide noise cancellation.