Understanding Frequency Theory in Psychology: How We Perceive Sound
The frequency theory in psychology pertains to the model of how auditory perception interprets the frequency of sound waves as distinct pitches. It suggests that the firing rate of auditory neurons conveys specific frequencies to the brain, where they are decoded as various sounds.
To fully understand this theory, it's essential to explore the structure and function of the ear, as well as related concepts like the volley principle and place theory.
The Structure of the Ear
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.
Let's take a closer look at each part:
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.
Frequency Theory Explained
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.
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. Essentially, we are getting a copy of the real sound. 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.
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.
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.
Humans can detect sound waves with frequencies that vary from approximately 20 to 20,000 Hz. Probably of greatest interest to psychologists are the frequencies around 500-2,000 Hz, the range in which sounds important to speech typically occur. Humans are most responsive to sounds between 1,000 and 5,000 Hz, and are not likely to hear very low or very high frequencies unless they are fairly intense. For example, the average person is approximately 100 times more sensitive to a sound at 3,000 Hz than to one at 100 Hz.
The relationship between frequency and pitch is predictable but not always simple. That is, as frequency increases, pitch becomes higher. At the same time, if the frequency is doubled, the resulting sound does not have a pitch twice as high. In fact, if one listens to a sound at a given frequency, then a second sound at twice the frequency, the pitch would have increased by one octave in pitch. Each doubling of frequencies involves a one-octave change, for example, the Middle C note on a piano has a frequency of 261.2; the C note one octave higher is 522.4, a change of 261.2 Hz.
When an individual hears a complex sound consisting of many different wavelengths, such as a human voice, music, and most sounds in nature, the ear separates the sound into its different frequencies. This separation begins in the inner ear, specifically the basilar membrane within the cochlea. The basilar membrane is a strip of tissue that is wide at one end and narrow at the other. When the ear responds to a low frequency sound, the entire length of the basilar membrane vibrates; for a high frequency sound, the movement of the membrane is more restricted to locations nearer the narrow end.
The ability to hear declines with age, although the loss is greatest for high frequency sounds. At age 70, for example, sensitivity to sounds at 1,000 Hz is maintained, whereas sensitivity to sounds at 8,000 Hz is markedly diminished.
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.
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.
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.
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.
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.
Key Concepts in Auditory Perception
Here's a summary of key concepts:
| Concept | Description |
|---|---|
| Frequency Theory | Pitch perception based on the firing rate of auditory neurons. |
| Volley Principle | Multiple neurons firing simultaneously to process high-frequency sounds. |
| Place Theory | Different parts of the cochlea respond to different frequencies. |
Collectively, these concepts underscore a multifaceted approach to understanding auditory processing.