Tartini tones, also known as combination tones, are fascinating auditory illusions. They occur when two pure tones are played simultaneously, creating the perception of a third tone, lower in frequency than the original two. This third tone, the Tartini tone, has a frequency equal to the difference between the two original frequencies (f2 – f1). Remarkably, this perceived tone has no corresponding physical vibration in the external world. Nothing outside your head vibrates at that specific frequency. It’s a phantom sound, created within the listener’s auditory system.
The Science Behind the Illusion
How are these phantom tones generated? While the exact mechanism isn’t fully understood, it’s believed to arise from non-linear processes within the ear itself. The ear, unlike a simple microphone, doesn’t respond linearly to incoming sound waves. Complex interactions within the cochlea, the inner ear’s hearing organ, are thought to produce these difference tones.
Tartini Tones vs. Heterodyne Tones
It’s important to distinguish Tartini tones from heterodyne tones, which can also result from the interaction of two frequencies. Heterodyne tones arise when two signals are processed by a non-linear system, leading to the creation of new frequencies, including the difference frequency (f2 – f1), as well as sum frequencies (f2 + f1) and others. Unlike Tartini tones, heterodyne tones do have a physical presence. They are actual vibrations in the air, measurable with a microphone.
Several musical instruments rely on non-linear systems to produce sound. Woodwind reeds, brass player’s lips, and bowed strings are all examples. When two notes are played simultaneously on these instruments, heterodyne tones are produced alongside the original notes. For example, playing a note on a didjeridu and simultaneously singing a fifth above it creates a strong heterodyne tone an octave below the played note.
This technique is especially important for the didjeridu, allowing players to extend the instrument’s range downwards. Similarly, double stops on a violin create heterodyne components due to the non-linear interaction of the bow with the two vibrating strings. These heterodyne tones are not illusions in the same way Tartini tones are; they have a real physical presence in the sound produced.
Experiencing Tartini Tones
Because Tartini tones are relatively weak, they are best perceived with higher-pitched notes, where the difference tone falls within a frequency range of optimal human hearing sensitivity. Heterodyne tones, on the other hand, are much stronger and can be used to create impressive bass notes on instruments like the didjeridu, saxophone, or brass instruments.
The exploration of Tartini tones offers a glimpse into the complex processing that occurs within our auditory system, highlighting the difference between the physical reality of sound waves and our subjective perception of them. While these phantom tones may be illusions, they offer a valuable insight into the fascinating interplay between physics and perception in the world of sound.