Spruce Tuning Fork

Tuning forks are so named because they are used to help tune musical instruments, but they also have a variety of other uses. They are used by audiologists to test for certain kinds of hearing loss, by the police to calibrate the radar guns used to identify speeding vehicles, and by alternative healers as an (unproven) way of improving mental clarity and physical energy. They can also apparently be used by medics to detect a bone fracture in a pinch if no x-ray is available. These applications all rely on the fact that when a tuning fork is struck, that energy is converted into vibrational energy, and its tines oscillate at a particular frequency to produce a specific pitch of note.

This spruce tuning fork is one of a set of 16 made in 2009 by our Director Zoe Laughlin as part of her PhD research. Together these tuning forks investigate the acoustic properties of materials and demonstrate the physics of sound and vibration. Three principle factors influence the production of sound by a tuning fork: the shape of the fork, and the density and elastic modulus (a measure of stiffness) of the material from which the fork is made. Each of the set of tuning forks is the exact same shape, but is made from a different material (an array of metals, woods, plastics and glass). Playing these tuning forks allows us to directly compare how the density and elastic modulus of each material affects the sound the fork produces.

When played, the spruce tuning fork doesn’t produce an audible ring like most of the metal and glass tuning forks. Instead this wooden fork produces a single note that doesn’t last very long at all, and there is a real knack to getting a sound from it. You need to hold the base of the fork firmly in one hand and bring it close to your ear, before pinching the two prongs together and releasing them quickly, so that they vibrate. When played in this fashion, the fleeting sound produced is – surprisingly - higher in pitch than the brass fork, and is closer to that of the steel and gold forks.

Spruce is the overarching name given to a type of softwood produced by about 40 different types of coniferous tree that grow in the cold and temperate parts of the northern hemisphere, like the boreal forests of Alaska (Sitka spruce) and Scandinavia (Norway spruce). Although there are differences between these different spruce species, they commonly all have long fibres, making them very useful in the production of strong paper. They also have a compact, straight and parallel grain, making them very elastic, resistant to splitting and strong relative to their density. That’s why spruce has historically been used to make the masts of ships and the propeller blades of light aircraft and wind turbines.

Its structure also gives this wood its famed acoustic properties: spruce is especially useful as an acoustic dampener, absorbing some frequencies whilst enabling the propagation of others in the manner of an acoustic filter. Spruce does this particularly because of the parallel bands of hard and soft matter that constitute the grain of the material. This could account for the popularity of spruce as a material for making guitars faces and the sounding boards of high-quality pianos, thought its superior strength-to-weight ratio undoubtedly also plays a part.

Read more about our sensoaesthetics research here.