Introduction

The classical composer Beethoven composed some of his finest music when he was extremely deaf. How did he do this? It is said that he put a wooden stick between his teeth and pushed it against the piano to feel the vibrations. Dame Evelyn Glennie is a famous percussionist who happens to be deaf. When practicing and performing, she can feel the vibrations generated by her instruments.

Many adults who use hearing aids or cochlear implants have difficulty listening to music [1]. For adult musicians and singers who have hearing impairments, deafness can make it difficult to take part in music making [2]. There are 32 million deaf children in the world, so could vibration help them learn to play instruments and perform with other children? Our research investigated the range of musical notes that can be felt safely using vibration, and whether people can tell the difference between notes felt through vibrations.

Can We Hear Through the Skin?

Our ears convert sounds into signals that are sent to the brain so we can identify them. The outer part of the ear—the soft part on the outside of the head—is called the pinna. The pinna helps sound enter the ear canal, which is a short tube that leads to the eardrum in the middle ear. The sound makes the eardrum vibrate. These vibrations are sent to the cochlea in the inner ear by the vibration of very small bones. Inside the inner ear, there are tiny hair cells that are sensitive to vibration, and they convert the vibration into electrical signals to be sent to the brain. Hair cells are called mechanoreceptors because they are receptive to mechanical vibration.

When we feel vibration, signals are sent to the brain by different kinds of mechanoreceptors that are underneath the top layer of the skin. These mechanoreceptors are not as sensitive as those in our ears, but they allow us to feel vibration for some sounds that can be heard with our ears.

Can We Safely Apply Vibration to Hands and Feet?

Very loud sounds can damage the ears, so is vibration safe? When workers dig up roads using a pneumatic drill, they are exposed to high levels of vibration. This can cause a permanent health problem called hand-arm vibration syndrome that makes a person’s fingers feel numb. To avoid this problem, we investigated whether a person could feel musical notes at low levels of vibration that were safe over a long period of time. This is important because musicians and singers spend many hours practicing their instruments. We used experiments to find the lowest levels of vibration that people could feel on their fingertips and the soles of their feet [3]. This was done for different musical notes. These lowest levels are called vibrotactile thresholds. Vibro indicates vibration, tactile indicates the sense of touch, and threshold refers to the lowest level.

Why did we use the hairless skin on the palms of the hands and the soles of the feet? Because this skin is more sensitive than hairy skin, and therefore has a lower vibrotactile threshold. We thought that singers could use their fingertips to feel the vibrations caused by other musicians. However, musicians playing instruments with their hands would need to use their feet to feel the vibrations. To provide the vibration, we used a flat disc that contacts the hand or the foot, connected to a device called an electrodynamic shaker (Figure 1). Shakers are similar to loudspeakers, but instead of producing sound, they only make vibration. A loudspeaker is fragile and is easily damaged so it can not be touched, but a shaker is robust and can support the weight of a hand or foot.

In traditional Western music, a musical scale has seven notes. For example, the scale of C major contains the following notes: C, D, E, F, G, A, B. Each note has a different pitch. You might remember the sound of these seven notes if you have ever sung the “Do-Re-Mi” tune because they are equal to “Do,” “Re,” “Mi,” “Fa,” “Sol,” “La,” “Ti.” In Figure 2 you can see that these are the white notes on a piano keyboard. This sequence of notes is repeated across the entire keyboard, where the next C is an octave higher in pitch. To name the notes in different octaves, a number is added after each note. For example, G5 refers to G in octave 5.

Our experiments showed that all musical notes from C1 (low pitch) up to G5 (high pitch) can be felt clearly on hands and feet. Importantly, these notes could be felt at a safe level that avoided health problems. This range of notes covers more than four octaves, which includes most notes on a piano (Figure 2) and on many other instruments, as well as notes produced by the human voice. The fact that people have more difficulty feeling the high notes from A5 to C8 on the piano reminds us that the sense of touch is less sensitive than the sense of hearing.

For low-pitched notes, we found that the heel and the forefoot had lower vibrotactile thresholds than the fingertip. Most musicians can not use their fingertips while they are playing an instrument, so it is helpful that their feet are more sensitive, and they can use those to detect vibrations. We also found that the vibrotactile threshold for fingertips is similar for people with normal hearing and people who are deaf. This means that vibration could be used by everyone.

Can We Tell the Difference Between Musical Notes Using Vibration?

Identifying relative pitch is a useful skill for musicians. This means they can identify or sing a musical note after they have heard another note. Think about the “Do-Re-Mi” tune again. If you heard “Do” followed by “Mi” and had relative pitch skills, you would be able to say that “Mi” was a higher pitch than “Do.” We carried out experiments with vibration to see if it was possible to identify one note as being higher or lower in pitch than another note when using the fingertip or forefoot [4]. The musicians with normal hearing carried out 16 short training sessions to see if they could improve their skill.

We found that wide pitch intervals were easier to identify than narrow intervals. A wide interval has notes that are far away from each other, such as from C to G. In a narrow interval, the notes are close together, such as from C to D. In terms of “Do-Re-Mi,” it was quite easy to tell “Do” from “Fa,” “Sol,” “La,” and “Ti” but harder to tell “Do” from “Re” or “Mi.” This happened with amateur and professional musicians with normal hearing as well as professional musicians with a hearing loss. The short training sessions improved the relative pitch skill of the musicians with normal hearing.

How Can Vibration be Used to Perform and Learn About Music?

Musicians can use vibration to help them learn an instrument and play music with other people, by sending the sound from each instrument to a mixing desk and then sending it back as vibration to the musician’s body (Figure 3). For example, a guitarist could choose to feel the drums on one foot and their own guitar on the other foot.

Our equipment was installed in the Royal School of the Deaf Derby. Video 1 shows the responses of staff and pupils who used the vibrotactile equipment to understand musical pitch and play music together in their lessons [5]. The music teacher, Matthew Taylor, found that the equipment made a positive change to his teaching and that it created a calmer atmosphere in the classroom. Matthew used the equipment to help his students make a connection between vibration and the pitch of a note. This helped the students decide if one note was higher or lower than another note. Matthew said that his students became more interested, independent, and active in music lessons, and that they really enjoyed playing music together.

Conclusions

Our research shows that vibration can be used at a safe level by people with hearing impairments to help them play music with other musicians. We hope this technique will provide more opportunities for children and adults with hearing impairments to perform, enjoy, and compose music. The research promotes inclusive music education by making music more accessible to children with hearing impairments.

Glossary

Cochlear Implant: ↑ An electronic device inside the inner ear that helps a deaf person to hear.

Mechano-receptors: ↑ Sensors in the body that respond to mechanical pressure or change of shape.

Vibrotactile Threshold: ↑ The lowest level at which a person can perceive vibration through touch.

Musical Scale: ↑ A group of notes arranged in order of their pitch.

Pitch: ↑ The lowness or highness of a note.

Octave: ↑ The interval between two notes where one note has twice or half the frequency of vibration of the other.

Relative Pitch: ↑ The ability to correctly recognize or produce a musical note in terms of its relative position in a musical scale.

Interval: ↑ The distance in pitch between two notes.

Conflict of Interest

The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.

Acknowledgments

This research was supported by the Arts and Humanities Research Council (AHRC) (Grant No. AH/H008926/1, 2010–2013). We would like to say a big thank you to staff and the pupils at Royal School for the Deaf Derby for letting us document their experiences.