Technically, a tuning fork is an acoustic resonator. When struck it produces several tones – a fundamental and at least one harmonic – but the fork’s shape tends to minimize the harmonics and within a few seconds only the fundamental can be heard. The tone a fork makes is determined primarily by the length of its “tines” (or prongs). Longer tines vibrate more slowly and thus produce a lower tone. Shortening the length of the tines allows them to vibrate faster and thus produce a higher sound. The invention of the tuning fork is generally credited to the British musician, John Shore, in 1711. Strong used his fork as a pitch standard to tune musical instruments, a task for which they are still used today. In the 19th century, advances in manufacturing made it possible to create extremely precise tuning forks, which were made in sets and used as tone generators to identify and measure other sounds. By the last decades of the 19th century, tuning forks were among the most precise of all scientific instruments. Specialized techniques were developed to use them for measuring different kinds of vibrations, and they were frequently used as high-precision timing standards. Albert Michelson, for example, used light reflected from the vibrating tines of a tuning fork to make his historic measurements of the speed of light. In the 20th century, the development of electronic technologies for measurement and precision timing quickly replaced technologies that employed mechanical tuning forks. One notable exception has been the introduction, around 1960, of tiny quartz tuning forks in high-precision watches. Maintained in motion by batteries, the resonating forks far exceed the accuracy of conventional mechanical watches.
Tuning Forks on Resonators
This set of 3 tuning forks, each mounted on its’ own resonator box, was made in Paris between about 1870 and 1900. The tuning forks were each milled from a single blank of fine steel and were then precisely tuned to produce a single, specific, tone. The resonator boxes that they are bolted to are wood, made from the same spruce often used in stringed musical instruments. Spruce wood is naturally responsive to sound vibrations and is the ideal material for this application. Its’ use in these acoustic instruments reminds us that Rudolph Koenig, the great 19th century acoustic instrument maker who manufactured them, was originally trained as a musical instrument maker.
Beats Apparatus
This instrument, made in the 1950s, was designed to demonstrate the acoustic phenomena of “beats”. Beats are produced when two similar sounds interact. If the sound’s frequencies differ by only a few wavelengths, their combined tone will contain rapid changes in loudness. These are the beats, and they are caused by the physical interaction of sound waves as they alternately combine creatively, to increase the sound, and destructively, to reduce it. Beats provided a convenient way to measure sound in the 19th century, because if the frequency of one of the tones was known, counting the number of beats per second would allow a person to precisely calculate the frequency of the second tone. This was the method that Scheibler developed to produce his tonometer (below) and this procedure can be demonstrated with this apparatus.
Tonometer
A “Tonometer” is a carefully constructed set of tuning forks which were used, by comparison, to determine the pitch of other sounds. It was the German silk manufacturer (and acoustic researcher) Johann Scheibler who first suggested this instrument, in 1834, and it was he who built the first one.
Scheibler constructed many tonometers during his life, and different sets would have different numbers of tuning forks. His most advanced design consisted of 56 forks, which together covered the range of a single octave (from A220 to A440) at 4 wavelength intervals.
In 1876 the acoustic instrument maker Rudolph Koenig expanded the idea of a tonometer from a single octave to the entire range of human hearing. He constructed a tonometer of 670 tuning forks which ranged in pitch from 16 to 4,096 hertz. Because of the difficulty in producing tiny tuning forks, sounds above that frequency were produced by rubbing precisely made steel rods with a rosined cloth. Koenig’s great tonometer was exhibited at the Philadelphia Exposition of 1876 and was widely regarded by American scientists as the most scientifically important instrument at the event.
Adjustable Tuning Fork
Adjustable forks were used primarily as teaching instruments, although they may have had medical applications as well. These instruments seem to have been introduced in the 1890s and were common in 20th century high school laboratories. Adjustable forks could take the place of several individual forks and had the added advantages of being relatively inexpensive and quite durable. Moving the adjustable weight on each tine changes the tone by effectively changing the tines' length. Thus moving the weight down the tine raises the tone, and moving the weight up the tine lowers it. To produce a pure tone, it was important that the weights on both tines be precisely opposite each other.