Teacher background information

This elaboration provides students with opportunities to consolidate their knowledge and understanding of how sound is generated and how sound energy is transferred through different mediums, such as air and water. As in all cultures, Aboriginal and Torres Strait Islander peoples have many diverse technologies, knowledges and processes that involve the transfer of sound energy. Students will have opportunities to develop an appreciation of how Aboriginal and Torres Strait Islander peoples’ knowledge of sound propagation through different mediums influences the design of technologies including sound instruments, herding and signalling devices.

When people consider the transfer of sound energy, they commonly think of sound transfer through air. However, there are less well-known examples of Aboriginal and Torres Strait Islander understandings of how sound travels through the medium of water. This understanding was exploited to develop fish herding technologies and practices. For example, the traditional practice of clapping stones together under water to frighten fish closer to the surface for easier hunting has been documented throughout Australia. An interesting practice that provides evidence of this understanding is the driving of spears into sand under water by Quandamooka peoples on Minjerribah (North Stradbroke Island) to generate a specific signal for dolphins to assist in the mutual herding and hunting of mullet. These practices could be investigated to demonstrate to students the transfer of sound through liquid.

Musical instruments developed by Australia’s First Nations peoples can be used to illustrate the propagation of sound waves through solids and air. First Nations Australians constructed and played a range of different musical instruments; their selection and use being influenced by available resources, cultural practices and spiritual beliefs. The most iconic and widely recognised of these instruments is, arguably, the didjeridu. Its name is not a traditional language word but is believed to have been coined as an onomatopoeic description of the sound it makes, introduced after colonisation.

Traditionally, the didjeridu was played only in the northern parts of Australia, and was an object afforded the utmost cultural and spiritual significance. Laws and protocols governed who could play it, when and where, and specified the ceremonies at which it could be played. The didjeridu is depicted in rock paintings dated at more than 1500 years old, making the didjeridu at least this age but possibly much older. It is therefore a likely contender for the title of the world's oldest wind instrument. Today however, the didjeridu is regularly played across Australia, and also in other parts of the world.

A didjeridu is basically a hollow tube approximately 1 to 3 metres in length. The tube can be made from bamboo, tree trunks or branches that have been hollowed out by termites. It may be cylindrical or flared at one end to create a more conical shape. Once a suitable trunk or branch has been identified, it is further manufactured by: stripping its bark, usually with a stone axe; cleaning out any termite residue; and applying bees wax to one end to form a better fitting mouth piece, helping to create a seal for the player.

As the basic shape and hollowing of each didjeridu are principally the result of natural processes, each didjeridu is unique, and its acoustic behaviour is determined by the length and shape of its bore. The diameter of the bore also affects the amplitude, or loudness of the sound that is generated. Other factors that contribute to the complex nature of the didjeridu’s unique sound are the frequency of the vibration created by the player’s lips, the shape of the player’s vocal tract and the force of the air flow travelling through the instrument.

Two general rules apply to the sound created by woodwind instruments. First, the greater the length of the instrument, the lower is its pitch. Longer didjeridus have lower fundamental frequencies than shorter instruments. Second, flared instruments play a slightly higher pitch than unflared instruments of the same length.

The fundamental frequency is the lowest natural frequency for which a standing wave is produced, and thus at which an instrument resonates. That is, the standing wave produced has a node at the closed, mouth end and an antinode at, or near, the open end. The wavelength of this fundamental standing wave is four times the length of the instrument. Therefore, its frequency is given by the equation:

\(Frequency\;(f_1)\;=\;\frac{speed\;of\;sound\;in\;air\;(v)}{4\;x\;instrument\;length\;(L)};\;that\;is,\;f_1=\;v/4L\)

Taking the speed of sound in air as 340 m.s^-1 means a didjeridu that has a length of one metre has a fundamental frequency of approximately 85 Hz.

The other natural frequencies for a pipe closed at one end are the odd harmonics; that is, the third, fifth, seventh, etc. harmonics. Their frequencies are given by:

Frequency of 3rd harmonic \(f_3\;=\;3v/4L\)

Frequency of 5th harmonic \(f_5\;=\;5v/4L\)

Frequency of 7th harmonic \(f_7\;=\;7v/4L\)

In playing the didjeridu, the initial sound wave is created by the player’s lips. The wave travels in every available direction, both forwards into the instrument but also backwards into the vocal tract of the player. The vocal tract acts as a resonator amplifying certain frequencies while suppressing others. Thus, skilled players manipulate not only the force of the air from the lungs and the vibrations of their lips, but also the shapes of their mouths and their tongue articulations. In so doing they alter the frequency, wavelength and amplitude of the sound wave as it reflects through the lips, pushed by the movement of air from the lungs, and through into the bore of the instrument. This produces the distinct and unique sounds for which the didjeridu is renowned.

By using the didjeridu as an example, students have an opportunity to investigate some of the fundamental principles of acoustics and develop a deeper understanding of how sounds are generated and propagated through different mediums.