How is it that we can listen to music and hear each individual note? This has to do with the frequency theory of hearing and the structure of the ear. Learn more about the frequency theory and then take a short quiz.
The Frequency Theory
Martin is listening to his favorite song. He loves the way the notes rise and fall in a melodic way. Martin doesn’t stop to think about how his ears allow him to identify the different pitches of the notes.
He just knows he loves this song.So, why do we hear the difference between notes in a song instead of just monotone notes? This is attributed to the frequency theory of hearing. The frequency theory of hearing states that the frequency of the auditory nerve’s impulses corresponds to the frequency of a tone, which allows us to detect its pitch.The way it works is that sound waves cause the entire basilar membrane to vibrate at different rates, which, in turn, causes the neural impulses to be transmitted at different rates.
Basically, when we hear a musical note, it causes specific vibrations in our ears that lets us hear that specific pitch. Lower notes vibrate at slower speeds, while higher notes vibrate at higher speeds. As pitch increases, nerve impulses of the same frequency are sent to the auditory nerve.
This means that a tone with a frequency of 700 hertz produces 700 nerve impulses per second. It is the speed in which the neural signals move along the brain that determine the pitch.
Structure of the Ear
So, how does Martin perceive musical sound? He absorbs sound through the outer ear, which consists of the external auditory canal and the pinna, also known as the auricle. Once the sound has been absorbed, it becomes an acoustical signal. The tympanic membrane, or the eardrum, separates the middle ear and outer ear.After the acoustical signal makes its way to the middle ear, the movement of the ossicular chain causes the acoustical signal to become mechanical.
The ossicular chain consists of the malleus, incus, and stapes and carries the signal to the inner ear. This is where the sound enters the cochlea. It is the cochlea that transforms the signal into nerve impulses that are carried to the brain via the auditory nerve. The brain perceives these nerve impulses as music.
Volley Principle & Place Theory
The major flaw in frequency theory is that the neurons fire at a maximum of about 1,000 impulses per second, so frequency theory would not account for sounds above 1,000 hertz. This means that Martin would not be able to hear the high notes of his favorite song! This problem led to the development of the volley principle, which states that groups of neurons in the auditory nerve fire in unison, as opposed to each neuron firing at the same time, thereby creating volleys of nerve impulses.
Auditory nerves can create volleys up to 5,000 impulses a second. This means that the frequency theory of hearing can account for sounds up to 5,000 hertz.But what about tones that are higher than 5,000 hertz? These sounds are dependent upon place coding.
According to the place theory of hearing, we are able to hear different pitches due to sound waves of different frequencies activating different parts on the basilar membrane of the cochlea. In other words, different parts of the cochlea are activated by different frequencies. This explains how we can hear sounds at and above 5,000 hertz.
According to the frequency theory of hearing, the frequency of the auditory nerve’s impulses corresponds to the frequency of a tone, which allows us to detect its pitch. Sounds come into the ear as acoustical signals and are later transformed into nerve impulses by the cochlea. The auditory nerve transfers these nerve impulses to the brain. Frequency theory of hearing can only account for sounds up to 5,000 hertz. The place theory of hearing accounts for sounds at or above 5,000 hertz.
When you are finished, you should be able to:
- Describe the frequency theory of hearing
- Explain the functions of the ear’s components
- Recall what the volley principle is
- Summarize how the place theory of hearing accounts for sounds at or higher than 5,000 hertz