Understanding Frequency Theory in Hearing Psychology
The ear-brain system is a complex instrument, and understanding how we perceive sound has been a topic of extensive research. Currently there are two overlapping theories of how we hear; the place theory of hearing and the temporal theory of hearing. One of these theories, the frequency theory of hearing, offers insights into how we interpret the pitch of sounds. This article delves into the frequency theory, its historical background, and its role in auditory perception.
What is Frequency Theory?
The frequency theory of hearing describes how humans perceive pitch regarding impulses of the auditory nerve. Frequency theory in psychology pertains to the model of how auditory perception interprets the frequency of sound waves as distinct pitches. Its basic premise is that the frequency of the impulse equals the rate of the produced tone, thereby detecting pitch.
This theory of how we hear sounds states that there are pulses that travel up the auditory nerve, carrying the information about sound to the brain for processing, and that the rate of this pulse matched the frequency of whatever tone you are hearing exactly. We thus hear the tone because the pulse traveling up the auditory nerve matches the actual tone. Essentially, we are getting a copy of the real sound.
To put it simply, the frequency theory explains that a sound heard is replicated and matched by the same amount of nerve impulses that are then transmitted to the brain. When an individual hears a frequency of 100Hz, an equivalent of 100 impulses per second are then transmitted via the auditory nerve to the brain. 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.
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.
Historical Context
The historical background of frequency theory can be traced back to the late 19th century in Germany, where scientists began to systematically investigate the relationship between sound frequency and auditory perception. One of the key figures associated with the development of frequency theory is Hermann von Helmholtz, a German physicist and physician. In the mid-19th century, Helmholtz conducted extensive research on the physics of sound and the mechanisms of hearing.
Helmholtz’s research led to the formulation of the frequency theory, which posits that the frequency of a sound wave is directly correlated with the rate of neural impulses traveling along the auditory nerve. Significant events and studies further contributed to the evolution of frequency theory. In the early 20th century, researchers such as Georg von Békésy used techniques like auditory masking to investigate the perception of complex sounds and the role of frequency in auditory processing. Technological advancements, such as the development of more sophisticated tools for measuring and analyzing sound, also played a crucial role in refining the theory and deepening our understanding of auditory function.
Structure and Function of the Ear
To understand how the frequency theory works, it's important to know the structure and function of the ear's components. The human ear consists of 3 units: the outer, middle, and inner ear. Air molecules that vibrate to create sound reaches the ears and are channeled, via the pinna and external auditory canal, past the tympanic membrane, and further processed by the 3 ossicles , in the middle ear, and finally transmitted from the cochlea, in the inner ear. From the cochlea, the vibrations are turned into electrochemical nerve impulses that then travel to the brain via the auditory nerve. Other than hearing, the ear also plays an important role in hand-eye coordination and balance.
Outer Ear
The outer ear is visible to the eye with the pinna, also known as the auricle, protruding out from either side of the head. This area is made up of cartilage that is covered with skin and has a lobule (fleshly lobe with no cartilage) located towards the bottom of the outer ear. Channeling through from the pinna is the short external auditory canal that is closed by the tympanic membrane, better known as the eardrum. The function of the outer part of the ear is to collect the sound waves from a source and guide them through to the eardrum. Once the sound has been absorbed, it becomes an acoustical signal.
Middle 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.
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.
We know from the structure of the cochlea that different parts resonate at different frequencies; the end closest to the stapes resonates at high frequencies and the end furthest from the ossicles resonates at low frequencies. Nerves are connected to hairs located along the cochlea which are stimulated when vibrations are present. A logical conclusion is that each place in the cochlea corresponds to the perception of a given frequency.
Wavelength, Frequency, and Amplitude
Wavelength is measured in frequency and amplitude. Frequency is measured in Hertz (Hz) which determines the pitch of a sound according to the length of the wave. The amplitude of a wave determines the loudness of a sound by looking at the height of a sound wave and is measured in decibels (dB). The so-called normal range of hearing of a healthy young person is between 20 to 20,000Hz. In loudness, this ranges from 0 to 180dB, however, anything over 85dB can cause damage to the human ear.
Limitations and the Volley Principle
The frequency theory believes that sounds heard with frequencies larger than 500Hz cannot be processed by the human ear, as a neuron's action potential is quite short. So, 1 neuron cannot process a sound of a frequency higher than 500Hz.
This theory cannot explain the way sounds of higher than 1,000 hertz are heard since neurons can fire neither more than 1,000 impulses a second, nor more than that, with groups of neurons firing together to provide volleys of nerve impulses. Therefore, the volley principle is used to answer this problem.
The Volley Principle overcomes that of the frequency theory, in that 1 neuron cannot transmit over 500Hz - let alone- 20 000Hz, however, 20 000 neurons with staggering fire rates, can. Thus, when a higher frequency (<500Hz) of sound is heard frequently, instead of 1 neuron completing the transmission of the sound, multiple neurons do. In order to overcome this, the Corti in the cochlea combines the high-pitched sound into a volley (simultaneous firing of neurons) in order to process it. Instead of 1 or multiple neurons transmitting the sound through the cochlea into the auditory nerve. The volley principle explains sounds up to 5,000 hertz by having groups of neurons fire together in unison.
Place Theory
The Place Theory argues that different parts of the cochlea (inner ear) respond to different frequencies. The higher tone one hears, the more excited the oval window is on the cochlea. The lower the tone, the more firing of neurons is happening at the opposite end of the oval window. So. the area of neurons that are firing more rapidly will determine the different frequencies of sound that a person may hear.
The Place theory better explains that different parts of the cochlea, in the inner ear, process sounds of different frequencies. The Frequency TheoryMartin is listening to his favorite song. He loves the way the notes rise and fall in a melodic way.
A problem with the place theory is that the resonance curves turn out to be very broad and they overlap, as shown in the graph below (compare with the resonance graph of amplitude versus frequency in Chapter 4). In other words the sections of the cochlea are low Q-factor resonators. This would seem to make it very difficult for the ear to pick out frequencies which are close together but we know that the just noticeable difference in frequency is about \(1\text{ Hz}\) for frequencies lower than \(1000\text{ Hz}\) for most people.
If the place theory of hearing was correct we would expect that changing the frequency from, say \(250\text{ Hz}\) to \(240\text{ Hz}\) would shift the region of the basilar membrane that vibrates and trigger different nerve cells. But what actually happens is that the region that vibrates at \(250\text{ Hz}\) overlaps with the region that vibrates at \(240\text{ Hz}\) to such an extent that pretty much the same nerves are firing.
We also know from the uncertainty principle, discussed in Chapter 9, that a sharp resonance would mean less information about the duration of the sound. If sharp resonance peaks (high Q-factor for the cochlea) were the mechanism that enabled us to hear frequencies that are close together, we would not be able to hear sudden changes in frequency. A possible explanation to save the place theory might be that neighboring nerves are inhibited by the nerve firing at the center of the excited region.
The Frequency theory of hearing argues that 1 neuron constantly fires in order to process sound vibrations and deliver the impulses to the brain, which only accounts for lower-pitched tones smaller than 500Hz. The Volley principle combats the frequency theory by saying that instead of 1 neuron doing all the work and falling short of its action potential, there are multiple neurons in the Corti of the cochlea, that fire simultaneously in order to process higher-pitched sounds that are above 500Hz. The Place theory takes it a step further to understand that different parts of the cochlea (inner ear) process low and high-pitched sounds.
Real-World Examples
For instance, imagine you’re at a concert and the bass is booming. You can physically feel the vibrations in your chest because low-frequency sounds have a slower rate of vibration. Another example is when you’re trying to have a conversation with someone in a noisy cafe. Despite the background noise, you are able to focus on the person’s voice and understand them.
Additional Concepts
In addition to pitch and timbre, place theory is another closely linked term in auditory perception. According to place theory, our ability to distinguish between high-pitched sounds is less applicable using frequency theory alone. Collectively, these concepts underscore a multifaceted approach to understanding auditory processing.