Unraveling Auditory Illusions: Examples with the Piano
Just as visual illusions reveal the inner workings of our visual system, auditory illusions expose the complex processes our brain uses to interpret sound. While we are familiar with visual illusions, other types of sensory illusions can be more elusive. For instance, the 'phantom limb' syndrome in amputees or the 'burning' sensation from chili are examples of touch illusions. In the realm of sound, auditory illusions demonstrate how our perception can be tricked, leading us to hear things that aren't quite what they seem.
In music, several elements are universally recognized by the brain, including rhythm, tempo, melody, timbre, volume, and tone. These are fundamental aspects of human musical perception. The brain groups these perceived sound elements together to create coherence. This article explores musical perceptions and cognitive interpretations through the neural connections and brain areas involved in sound processing, particularly with the piano as an example.
Understanding Harmonics and the Missing Fundamental
When we hear a sound, we perceive its basic tone (fundamental) and several other notes (frequencies) simultaneously. These are the harmonics of the sound signal. If you remember the beep your phone made on the home line (A 435 Hz), that sound didn’t actually exist. The brain is so well attuned to harmonics that, given a sound signal, it quickly recognizes all the harmonics, allowing it to infer information about the fundamental note. This effect can be useful for a wide range of sound equipment to produce notes lower than they are actually capable of.
This phenomenon is present when we hear the lowest notes of a piano or a double bass: we don’t actually hear 27.5 Hz or 35 Hz because these instruments don’t produce enough energy at these frequencies.
Perceptual Filling-In: Creating Melodies from Fast Notes
In visual perception, our brain can fill in missing information to perceive a complete figure, even when part of it is hidden. This visual bias also works in the same way with hearing, creating musical elements that don’t really exist. Listening to Christian Sinding’s “Rustle of Springs” or the beginning of Chopin’s “Fantaisie Impromptu” (Op. 66), the notes go so fast that a melody seems to emerge.
Gestalt Principles in Auditory Perception
The auditory system applies gestalt principles of similarity and proximity to connect similar sounds together, so as to organize and interpret sound stimuli in a coherent and meaningful way. In the intro to The Eagles’ “One of These Nights”, we hear the bass and guitar as one instrument. This same kind of effect is found in the song “Lady Madonna” by The Beatles. In an instrumental section, the members sing into the microphone while covering their mouths with their hands, and what we hear resembles a saxophone tone. The notes played, together with the experience and expectation that this instrument is typical of this kind of music, cause our brain to infer that it is indeed a saxophone.
The Shepard Tone: An Infinite Ascending Melody
On the visual side, Shepard tables demonstrate how our brain’s rules alter our perception. Shepard tables are actually a pair of identical parallelograms representing the tops of two tables, but the illusion makes the tables look completely different as the rules of three-dimensional objects take over. These deceptions of the brain were uncovered by Roger Shepard, an American scientist specializing in cognitive psychology. The scale allows us to compose melodies that never end: the Shepard tone. We perceive an infinite ascending or descending melody, although the tones don’t really change pitch. Penrose’s staircase is a good visual representation of this. The application of this technique adds suspense and tension, and can be used to give the impression of acceleration in the soundtrack. Hans Zimmer often uses this technique in his compositions.
The effect can also be used in reverse.
The Octave Illusion: Conflicting Pitch Perception
The octave illusion is a fascinating auditory illusion that defies our usual perception of pitch. In this illusion, a pair of tones separated by an octave interval is presented. We should hear a lower and a higher note with both our ears. This is due to the brain’s selection of information and our body’s lateralization. As there are more right-handed people, there are more people who use their right ear to hear. Higher-pitched sounds are perceived by the right ear, and lower-pitched sounds by the left. The opposite is true for left-handers.
The brain, relying more on high-pitched sounds to perceive the origin of sound accurately, interprets that the high-pitched sound comes from one side and the lower-pitched sound from the other.
It’s fascinating that even after becoming aware of auditory illusions, we often continue to perceive them as such.
Examples of Auditory Illusions
- Falling Bells: This is a recording of a paradox where bells sound as if they are falling through space. As they fall their pitch seems to be getting lower, but in fact the pitch gets higher. If you loop this sample you will clearly see the pitch jump back down when the sample repeats.
- Virtual Barbershop: Number 5 - ‘Virtual Barbershop’ features a session at the barber’s and is stupendous when listened to on headphones. The final seconds are great! Hear (!) you perceive something in space, when it’s just in your head. These are examples of ‘dummy head’ recording.
| Illusion | Description | Musical Example |
|---|---|---|
| Missing Fundamental | Brain infers the fundamental note from its harmonics. | Lowest notes of a piano or double bass |
| Perceptual Filling-In | Fast notes create the perception of a melody. | Christian Sinding’s “Rustle of Springs”, Chopin’s “Fantaisie Impromptu” (Op. 66) |
| Gestalt Grouping | Similar sounds are grouped together to form a coherent perception. | The Eagles’ “One of These Nights”, The Beatles’ “Lady Madonna” |
| Shepard Tone | Creates the illusion of an infinitely ascending or descending melody. | Hans Zimmer's soundtracks |
| Octave Illusion | Different ears perceive different pitches in a pair of tones separated by an octave. | N/A |