In this video from Casey Connor, we look at common and fascinating audio illusions and phenomena. An audio illusion is a false perception of a real sound. Confirmation bias is mainly responsible for the illusions due to the brain’s preference towards using our prior knowledge and experience. In essence, we fill in the gaps of what we think is most likely to occur.
Psychoacoustics is the study of the perception of sound. These videos attempt to gather all of the various interesting phenomena that fall in to this category in one condensed series, including many neat illusions. We will also cover a few fascinating geeky topics relating to hearing. I’m also going to release a few of these illusions as short standalone videos for easier sharing. If this series misses any that you think should be covered, do let me know in the comments. Check the full playlist here, and check the description for corrections, clarifications, and an index to the different parts of the video. Most of these phenomena will not make any sense if you’re listening on a cell phone speaker, and even if you have million-dollar speakers in a fancy studio, you will need to use headphones for some of these effects to work at all. Even cheap headphones on an old smartphone should suffice. You might want to watch this series with your hand on the metaphorical volume knob — I’ve made every effort to normalize the levels to an appropriate loudness, but I’ll be using a lot of pure tones, and they reproduce pretty differently on different playback systems, so the sounds may be too loud or too quiet on your setup. You have been warned.
Headphones will be required for this illusion to work. When two tones combine, the wavefronts of the sound can mix in a constructive or destructive fashion. Meaning, sometimes they cancel each other out, and sometimes they reinforce each other. Listen as the pitch of these two tones gets closer to each other. These are both just simple, pure tones. The pulsing effect you are hearing is called “beating”, and it happens as the two tones with close frequencies interact with each other. You can see the visual analog of this in one version of a Moiré pattern, which demonstrates nicely what happens when two visual patterns with similar but not-identical spatial frequencies overlap. So this is nothing surprising. Simple math shows us that we should expect this change in amplitude to occur when we add two waves together. It’s just basic physics, and it results from the tones combining in air as we described. So: let’s play both of these tones. Right now, they are both playing in both ears, so they combine to form the beating before they enter your ears. But listen to what happens as we slowly spread them apart, so that 500 Hz is moving to one ear only, and 504 Hz is moving to the other. Now they are totally panned to either side. One ear hears this, and the other hears this. But when they both play you still hear a beating, even though there is only one tone in each ear. There is no physical mixing of the sounds at all, so we shouldn’t hear the beats any more, right? This is a purely psychoacoustic effect produced by your brain’s interpretation of the sound. But it doesn’t happen for frequencies above about 1500 Hz. Here’s a 2000 Hz tone in one channel, and a 2005 Hz tone in the other. These frequencies are well above 1500, and you probably don’t hear a beat between them. Now let’s play those two high frequencies together, but this time both of them are present in both ears. Notice that there is a very audible beat frequency present. When the sound mixes before entering your ear, the beating is perceptible. Otherwise, these two tones are too high for the binaural beat effect to happen.
This class of illusions involves tricking the ear’s perception of pitch change. These are sometimes referred to as “barber-pole” illusions, because they echo the visual illusion of a rotating barber-pole. The lines on this pole are forever moving down, but never really go anywhere. The audio you are hearing, called a “Shepard-Risset tone”, sounds as if it is descending in pitch, and, it is, really, but it somehow continues forever, where a normal descending pitch would eventually get so low that you can no longer hear it. How is this magic accomplished? Recall that most sounds, including this one, are actually composites of frequencies. This tone is built of many frequencies one octave apart. Some are very low, some are very high. Your ear picks all this up and sort of vaguely places the “real” pitch somewhere in the middle. Perhaps you perceive this tone as being rooted here. At any rate, as all of these frequencies start to descend, the lowest frequencies are subtly faded out, and new frequencies are subtly faded in at the top. We thus create an auditory barber-pole. You can imagine that there are lots of tricky illusions you can create using overtone manipulation. Legendary researcher Diana Deutsch demonstrated a method of altering the odd harmonics in a tone as the pitch descended. Check this walk down a 12 tone scale. You can see how the harmonics subtly fade away as the tone dscends, until the mutated tone becomes equivalent to the tone we started at, thus creating another form of circularity.
The Tritone Paradox
Let’s say you take one Shepard tone, that is, a stack of sine waves an octave apart from each other. Here’s is a freeze-frame of the spectral graph of this tone. Now we’re going to play another Shepard-Risset tone that is half an octave away from the first tone. (Half an octave is also known as a “tritone”.) That spectrum is shown overlaying the first spectrum. If you look at the two graphs, you can see that there is ambiguity as to which tone is higher than the other. You may have heard a clear difference, and to be honest your playback system may be biasing these results, but it would seem reasonable to guess that if we played these two tones for many different listeners, one of two outcomes would be likely: either the listeners would randomly distribute themselves into two camps: one that thought the first tone was higher, and one that thought the second tone was higher, or that a majority of listeners would find one of the tones to be higher. What Deutsch and her colleagues found was stranger: there was a clear preference as to which tone was higher, but the particular preference depended on where in the absolute pitch scale the tone occurred. Meaning, if the first tone was a C, there would be one bias. If the first tone was an E, or F, or G♯, that preference might change. You can see the preference shift on this graph. What was really surprising about this was that these are absolute pitch positions. It is generally accepted that most people do not have perfect pitch perception, yet here was a result demonstrating that at least on some level an average person had some kind of absolute pitch detection going on. Furthermore, the results varied dramatically depending on the culture the person came from, whether their language was tonally-based, and so on. Clearly our brains are being programmed in some ways that predispose us to certain perceptions.
Tempo Circularity: The Risset Beat
As with pitch circularity, we can play tricks with rhythm to give the impression that tempo is slowing down or speeding up indefinitely. In this example, the tempo is indeed slowing, yet somehow the music never stops moving along. It’s the same sort of barber-pole effect. As the tempo slows, imperceptible fast elements are nested into the beat. As they start to slow down and get louder, your ear can pick them out. The slowest elements get slower and slower, and gradually quieter, until they fade out completely. Note that there is a demonstration Risset beat you can find all over the internet with a quick search — it’s a great example, but it changes tempo gradually, and to really experience the illusion properly you have to listen to it for a while — at least 45 seconds. Unfortunately a lot of the places that clip is used only play it for a short time, and imply that the beat isn’t actually speeding up. It is of course, and especially if you only listen for 20 seconds, the beat is certainly just accelerating in a conventional fashion. So, don’t get confused: listen to it for a minute or two so you can actually perceive the circularity.
The combination tone is a psychoacoustic phenomena where extra tones are perceived when tones are combined. It also helps if they are loud, so you might bump the volume up a little. Don’t damage your hearing, though. Here is a tone with a 220 Hz fundamental, and here is one with a 330 Hz fundamental. If you listen carefully to their combination, you may be able to perceive a phantom tone at 110 Hz, and another at 550 Hz. These are the sum and difference frequencies of our two original tones. You can see in the graph that there is in fact no signal down at 110 Hz, but it sounds like there is. Part of the explanation is the “missing fundamental” effect that we’ll talk about next.
Listen to this little run of notes. Most people perceive the following pure tones as being present in that run. Watch them on the graph as they descend. Now let’s go back to the original audio and see how it registers on the graph. Strange — the pure tones are down here, and that’s where we heard the original notes as well, but in fact there were no such frequencies present in those original notes. Many natural sounds, including the sounds of most instruments, produce frequencies that are integer multiples of the fundamental frequency. If we pause our graph of this plucked guitar string, we see a fundamental at 110, but we also see peaks at 220, 330, and so forth. These are the so-called “harmonics” of the sound. We can isolate different components of the spectrum to hear more clearly what each is adding to the overall sound. Our ears are quite used to this sort of pattern. So used to it, in fact, that it will sort of “fill in” missing parts when it expects them to be present. Listen to what happens if we remove the lowest peak, also known as the “fundamental”, from the sound. It sounds thinner, not as rich and full, but it still sounds like the same note, despite the missing fundamental. This phenomenon is exploited by mix engineers to make music more listenable on cell phones. If you’re listening on a cell phone without headphones, this bass line will be almost totally inaudible. This bass line, however, will come through. The difference is that the second bass line has overtones present in the tone of the instrument. Even though the cell phone can’t reproduce the lowest frequencies, the ear is provided with clues from the higher frequencies and is able to “fill in” the missing fundamental. If you’re listening on a cell phone with headphones, try rewinding and listening again through the built-in speaker. And plug those headphones back in before we move on.
Did you know that your ear can itself make sound? Weird, huh? In roughly half of all people with normal hearing, at least one of their ears actually generates very quiet tones that can actually be detected. In very rare cases, these sounds can be loud enough for another person to hear. Usually, though, they require specialized tiny microphones to be inserted in to the ear and only a computer analysis of the resulting signal will reveal the tones. This is part of normal hearing in most cases, and doesn’t cause any problems. In a small percentage of people these tones can be percevied as tinnitus, or a persistent ringing in the ear, which can be annoying. (Ask me how I know.) I searched for a recording of these sounds and I’m sad to say I found none. Maybe it’s not possible to make a listenable recording. Let me know if you have any leads! There are other forms of otoacoustic emission as well, which result from different kinds of stimulation of the ear with external sounds. One form of these emissions is triggered by the combination tones we talked about earlier. These “distortion product otoacoustic emissions” are what Adam Neely covered in a recent video on the subject. Thanks for watching. Part 2 will be coming out pretty soon, so stay tuned. A big thanks to my Pateron patrons that are helping to keep this channel going. You’d of course be more than welcome to join them. There’s a link down below. If you’d like to make a one-time to donation to help support the channel there’s also a link for that. And head over to my second channel if you’d like to see a variety of different sorts of videos over there.