Most headphones function in a very simple manner: sound waves created by the headphones propagate through the air into the ear canal and into the inner ear. However, there are some headphones that have a non-traditional approach to producing sound for listening to music. These headphones can deliver sounds through vibrations to the head bones and tissues, thus enabling users to listen to music without inserting earbuds into the ear canals or using bulky headphones that cover them completely.

Such headphones are called bone-conduction headphones, as they produce vibrations in addition to the conventional sounds created by speakers.

It may appear strange, but the science behind this process is fairly simple. In fact, there is always some kind of bone conduction when a person speaks and hears his own voice. Vibrations generated by the act of speaking pass not only through the air but through the body, which explains why the voice that is recorded differs from the voice we hear when speaking.

In addition, transmission of sounds via the head affects how music is being played. In order to explain why this happens, it is necessary to learn more about how these headphones function and how sound vibration turns into music.

First, Understand How Traditional Headphones Deliver Sound

Before diving into the mechanics of bone conduction, it helps to establish exactly how normal headphones work. Understanding this baseline makes the technical jump to bone conduction much easier to follow.

The Journey of Sound Through Your Ears

Headphones are designed based on the principle of air conduction and transmit sound into the inner ear in the following way. First, sound waves are produced from the headphones as a result of a diaphragm's rapid vibration. These waves move through the air inside the ear canal and eventually encounter the eardrum that starts vibrating.

Then, three small bones in the middle ear, called malleus, incus and stapes, amplify the vibrations and pass them on to the cochlea. It is an incredibly small organ located in the inner ear, which has the shape of a snail and contains fluid. Moreover, it consists of a basilar membrane, which separates the liquid inside the cochlea. As the waves move through the liquid, the sensory hair cells convert mechanical energy to the electrical one. Finally, the signals reach the brain through the auditory nerve, which interprets them as sound. However, the eardrum is not the only place for vibrations to get to the cochlea.

How Bone-Conduction Headphones Actually Work

Whereas miniature speakers have been traditionally used, modern consumer bone-conduction headphones make use of transducers that are electromechanical. This is because the main function of these transducers is to convert electrical audio signals to mechanical vibration. This is different from normal headphones since the transducer does not go inside or on the ears. They usually come into contact with the skin around the ear region.

From Electrical Signals to Mechanical Vibrations

Hearing using a bone conduction device is quite distinct from the one used by earbuds. Once the audio signal sent from the computer to the headphones has been received, it gets converted into mechanical waves through the transducer inside. The transducer moves in sync with the skin of your face and the body tissues beneath it.

This vibration travels through the biological parts of your head, without interacting with the outer and middle ear at all. Mechanical energy turns into motion in the cochlear region. From here, the entire process becomes just like any other form of hearing process, where the sensory hair cells detect the motion in the fluid and send signals to the brain through the auditory nerve.

Does Sound Really Travel Through the Skull Bones?

It is easy to picture bone conduction as one simple process involving the skull, but the reality of it is actually quite complicated. The process of transmission of sounds through the head includes more than one pathway operating at once.

As described in the basic research on bone-conducted sounds and their physiology, there are a number of different ways that this is accomplished. Vibrations lead to movement of the fluid within the cochlea, called cochlear fluid inertia, which is one of the main factors that allow us to hear bone-conducted sounds. Also, the small bones of the middle ear react to the vibration of the skull (ossicle inertia). Moreover, the vibrations cause micro-movement of the walls of the cochlea. Surprisingly, vibrations of the skull also produce secondary sound pressure within the ear canal.

Why Bone-Conduction Headphones Sound Different From Regular Headphones

If you have ever tried a pair of bone-conduction headphones, you immediately notice that they do not reproduce sound in exactly the same way as conventional earbuds. The audio often feels different in texture and weight. This variance comes down to the transducers, the physical vibration pathways and how human tissue filters different frequencies.

Bass Doesn't Feel the Same

Bass sounds that have lower frequencies are critical in the listening experience of music in modern times. In the case of regular earphones, changes in air pressure are used effectively in stimulating the eardrum. In the case of bone conduction headphones that people can buy, the sound waves have to fight their way to the cochlea through a combination of skin, cartilage and bone tissues.

Human tissue has a natural tendency to dampen certain mechanical frequencies, thus, the production of bass tones is noticeably less intense. The transducers have to vibrate more in order for bass tones to be produced, thus, there will be a noticeable buzzing and tickling effect on the cheekbones of the listener.

Sound Leakage and Environmental Noise

Since the transducers are placed totally outside of the ear canal and operate by making vibrations outside of the head, part of the acoustic energy is bound to leak out into the air. As a result, there will be sound leakage, which means that people who happen to be sitting close to you can overhear the low-quality music leaking from the earphones. Additionally, the open-ear technology allows all kinds of background noises to get into your ears automatically.

The Engineering Behind Bone-Conduction Headphones

Designing consumer bone-conduction wearables requires engineers to navigate a highly specific set of physical limitations. Building a traditional speaker is mostly about moving ai, building a bone-conduction transducer is about effectively transferring mechanical energy into the human body.

Why Transducer Placement Matters

The issue of positioning the transducer is significantly more complicated than just finding a place where it will be able to vibrate. The exact position of the transducer, the force of its pressure on the skin and the coupling with the underlying tissue will determine the efficiency o