Shubha Nadadur:1999
Introduction
The inner ear (cochlea) is a coiled passageway in the temporal bone of the head. Whenever sound is present in the inner ear, small hairs (cilia) on the basilar membrane respond, converting the sound energy to neural signals.
The varying stiffness of the basilar membrane is very important in determining what sounds are heard. At the end of the cochlea the membrane is relatively elastic. In the middle of the cochlea, towards the base the stiffness increases. The stiff membrane shakes immediately in response to a sound. The net effect of the variations in response of different parts of the membrane can lead to nonlinear effects.
Methods
Two tuning forks were struck to set them vibrating. One was placed near the ear, while the other was held farther away. The positions were then reversed. Attention was focused on each one of them in turn. Finally the two forks were placed near one ear together. Attention was shifted from the individual sounds to the combination of sounds.
Data
Observation
When both forks were close to one ear a third frequency was distinctly heard. The two tuning forks with frequencies A440 and C523 produced a single third sound of different pitch.
The following questions arise ...
1 What is the frequency of this third sound?
2 Is this frequency a linear or non linear combination
of the primary frequencies?
3 If non linear, what is the order of this non linearity?
4 Is this combination occurring in the ear itself, or in
the brain?
Analysis
The two tuning forks were struck at the same time. Absence of a third sound wave indicates that the sound is not a simple combination of the two vibrations in the air.
The SoundVision program did not detect the third sound that I distinctly heard. I needed a method to find out the frequency of this third sound. First, I made sure that others heard the third sound, then I focused on the third sound and tried to match its frequency with the note produced by a piano. Trial and error showed that this frequency was being produced by the F note with frequency 349 Hz.The simple addition of the two oscillations....
.... is not F!
Location of the effect
The two possibilities within the body are the brain and the ear. First, I tried out whether this phenomenon was occurring in the brain. That is the brain likes to hear this third sound and combines it. I put one fork close to one ear and the other fork close to the other ear. In this case I did not hear the third sound. I heard the third sound only when the two tuning forks were brought near one ear. Hence, this phenomena is occurring within the ear.
The ear causes the phenomenon. The human ear is divided into three main parts: the external ear, the middle ear and the inner year. To explain this phenomenon, let us look at the external ear. Is this non linearity in the ear drum? I believe that it is not in the ear drum for the following reasons: when I move the two forks away from my ear, so that the perceived intensity from each one decreases, I still hear the third sound. If it is due to non linear response in my ear drum, it should fall off in loudness much faster with distance.
To confirm these results, I tried the same method on another major triad - CEG. Tuning forks with frequency C and G were struck together to produce a third sound, namely E.
Editor's comments
The account above has been edited without changing the sense or intent. The phenomenon is a well known curious effect. With the recent purchase of SoundScope two frequencies can more easily be generated and adjusted for loudness, in either one of two external speakers. The effect could be further investigated. A person with perfect pitch could be consulted. As an easy reference the table at right has been added.
It might be possible to build a vibrating structure with a nonlinear response ..... if this were possible, more confidence might be generated in the explanation presented here.