The octave illusion is an auditory illusion where the listener hears tones differently than they are presented. The use of the octave in this illusion is due to the fact the higher note of the octave has twice the frequency of the lower note in the octave. If you are not a musical person, an octave is two notes being eight notes apart. The musical scale goes from A to G and one octave is eight notes above or eight notes below a note. An example of this would be playing an A note and going up the scale to the next A would be one octave.
Diana Deutsch first discovered the Octave Illusion in 1973. She was showing the differences of individual perception in hearing. This illusion was created using a pattern of two notes spaced an octave apart being presented alternatively. The sequence was presented to an individual through headphones, where the right ear will hear the high tone while the left ear hears the low tone and switches to allow the left ear to hear the high tone while the right ear hears the low tone.
This pattern produced a multiple illusions for the listeners. Many people heard a tone, which was switching from ear to ear, with the pitch switching from high to low. For these people the phenomenon was even present when the headphones are reversed. Other people heard a single tone switching from ear to ear while the pitch remains the same, as the tone seems to switch location. Another perception the listener heard a low tone switching from ear to ear whose pitch shifts by a semitone, combined with an intermittent high tone in the other ear.
The octave illusion also produced an unexpected discovery. There was a difference in the perception of the illusion by people who were right-handed, compared to people who were left-handed Right-handers tend to hear the pattern as the high tone on the right side and the low tone on the left side, regardless of the position of the headphones. However, the results of people who are left-handed show many multiple results as to where the high tone and low tone appear.
The octave is the same note but the higher note has twice the frequency of the lower note. Hoping to find out if the octave is a somewhat magical combination of tones to play, a modified illusion will be presented to listeners to determine if the same effects occur. Where the notes in the Octave Illusion were eight notes apart, in the modified illusion the notes will be just four notes apart. If the results are the same it will show how the octave is not the magical piece of this auditory illusion. If this illusion provides different information it will open up chances to determine what happened through the results presented. This will also provide an opportunity to compare the results between the handedness of people and the relation to sound perception compared to the octave illusion.
The participants of this experiment range from eighteen to twenty-one years old. Because this experiment is an auditory illusion the participants will need to explain any hearing problem they previously or currently have. Also because of the anticipation of results depending on handedness the participants will also be required to let the person running the experiment know.
This illusion was created using a computer program called Garage Band. Using the piano option I was able to create two sets of tones four notes apart. One of these sets of tones was created to only enter the right headphone and the other to only enter the left headphone. After this illusion was created I exported it to a media player on the computer to make it an audio file. The next thing you need is a set of headphones; it is best to use larger ones to eliminate outside noises.
To run this experiment an original version of the Octave Illusion needed to be created. Instead of the two notes being eight notes apart they will need to be four notes apart. This is where the program, Garage Band, is used. First I recreated the Octave Illusion using the piano option. I had to make two copies of the notes to be played and changed the options so one set plays in the right ear and the other set plays in the left ear. The next step is to create my illusion using the notes being four notes apart. To do this I just took the Octave illusion and moved every other note down to create my illusion. This also requires two separate sets of notes to get one playing in the right ear and the other in the left ear. Each of these illusions needs to be at least twenty to thirty seconds long to get the desired effect.
The participants will put the headphones on making sure to match the right headphone to the right ear and left headphone to the left ear. The Octave Illusion will be played for the participant two or three times, they will then communicate to whoever is running the experiment and they will record how the participant perceives the notes being played into their ear. The participant will then switch the headphones around to see if the illusion has changed. If there is any perceived change, any changes will be recorded.
Next the participants will listen to the new illusion. Same as before with the headphones on correctly the listener will listen to the illusion for up to thirty seconds. They will then communicate what they have heard to the recorder. Just as before the participant will be asked to turn the headphones around to see if that alters the results. The person running the experiment will discuss with the participants to discover if the second illusion had the same results as the first experiment.
At the end of the experiments the results from the people who are right-handed will be compared to the results of the people who are left-handed to determine if there are in fact any noticeable difference in the results. The results of the new illusion will be compared to the results of the Octave Illusion to discover if the octave is a special pair of notes or if the illusion is noticeable with any two notes that are different.
The results of this experiment show commonalities with the original octave illusion, as well as results unique to the experiment. The expected results of this experiment where that participants would not be able to hear the correct notes which are being played but will hear an alternative form. There was not one person who was able to hear the illusion correctly, but the results were almost as expected.
The results from the modified illusion came just as expected. There were different perceptions, but they were just as expected. The pattern seemed to stay the same when the participants switched the headphones. The most common result of this illusion was hearing the higher note in the right ear and the low note in the left ear.
There were several different results obtained from the modified illusion. Seven of the participants reported hearing a constant high note playing in the right ear while the low note stays in the left ear. When switching the headphones around these people observed the same pattern of sounds. This confused the participants, thinking they would hear the opposite when the headphones are switched. The second highest response reported by five of the listeners was hearing the high tone alternating from ear to ear with a constant low tone being played in the right ear. This pattern also appeared when the listener switched the headphones.
 Figure 1: Shows the actual pattern and the percieved patterns presented in this illusion. The red notes show the note being heard in the right ear and the blue note shows the note being heard in the left ear. The line labeled 1 shows the listener hearing the high note in the right ear and the low note in the left ear. The line labeled 2 shows the listener hearing the high tone alternating from ear to ear and a constant low tone in the right ear. |
There also were results unique to the modified illusion. Some of the listeners reported what seemed to be an after tone of the high tone in their right ear. It sounded as if the tone was reverberating in their ear. Also another unique result occurred when the listener focused on hearing from their opposite ear to try and hear the other tone. When the listener focused on their other ear they in fact did hear the constant tone they were hearing before but they could not hear it in the original ear. This only occurred when the participant focused on the other ear but they could never hear it in both ears at the same time.
Through this experiment I was hoping to determine if the octave illusion occurs because the octave may be a Magical Number. From the results of the illusion I created by just using notes four notes apart rather than eight notes apart these illusions appear to have similar results. The results show that there were two main perceptions of the presented illusion. One had the participant hearing a constant high note in the right ear with an alternating low note from ear to ear. The other perception was hearing the high note alternating ears and the low note constantly in one ear.
There was one noticeable difference when comparing the results from the octave illusion to the modified illusion. There were cases where during the modified illusion the listener perceived an after tone that seemed to resonate from the higher tone. This tone actually was the low note being played in their ear. I figure this occurred because of the closeness of the two notes being played. This was not evident in the octave illusion because the high note and low note are spaced twice as far away than in the modified illusion.
Another perception unique to the modified illusion was when concentrating more on one ear than the other the listener was able to hear the notes alternating in their ear. This was only possible by the listener concentrating on hearing from the opposite ear, but when they did this they lost the ability to hear the notes alternating in the other ear. I feel this was happening due to lateralization. The listener will focus their attention through one of their ears, only to lose the sound they were focusing on through the other ear.
The results taken from this illusion are very similar to those of the original Octave Illusion. The two main perceptions taken from the octave illusion were also noticeable within the modified illusion. The only results I could not compare was the differentially of right-handed and left-handedness. This was due to the low amount of volunteers who performed in the experiement.
Through the modified illusion I was hoping to show that the octave was not the only distance to get an observable illusion. I feel I proved my theory to an extent because of the nearness of the results of these two illusions. Using two notes being half as far apart on the musical scale gives the same type of illusion as two notes being an octave apart. This illusion did not give me all of the information I was looking for, but it leads to other interesting questions. My theory about the modified illusion allowed for knowledge about the hearing perception of alternating notes, but other modifications would allow for further knowledge and understanding. There are numerous experiments you can attempt, including; creating one illusion with the notes being more than one octave apart, and another illusion with the notes being one note apart. This would help determine if the illusion is stronger the further apart the notes are and weaker the closer the notes are to each other.
I feel my modified illusion gives some further knowledge about perceived hearing while further understanding can be gained. I hope this experiment serves as a stepping-stone for other experiments in the future to determine the difference in strength of the illusion depending on the distance between the two tones. The results themselves show a similarity between the original octave illusion and the modified illusion. However, there were also different perceptions than the original illusion which helps discover new ideas on testing the illusion strength. I was not expecting to get the same results but I am glad there are similarities, but the most important part was the differences.
Click on the following links to access the original octave illusion and the modified version. To get the full illusion make sure to have a set of stereo headphones.
Click here to hear The Octave Illusion To get the desired illusion you will need stereo headphones.
Click here to hear The Modified Illusion This illusion also requires the use of stereo headphones.
References:
Deutsch, D. An Auditory Illusion. Journal of the Acoustical Society of America 1974
Deutsch, D. The Octave Illusion Revisited. Journal of Experimental Psychology: Human Perception and Performance 2004, 30, 355-364