Digit Geek
Digit Geek > Recent Articles > Science > These audio tricks have always fooled you

These audio tricks have always fooled you

Just like any of the senses, our ears are also prone to trickery. And that fact is utilised by music producers and movie-makers to fool you all the time!

Our ears play a major part in how we perceive our reality. While Marvel’s Daredevil might have taken the listening superpower to a whole new level, an average person also depends on what he hears to complete their understanding of their environment. For instance, an approaching siren always makes you step aside from the road and a high pitched shriek will heighten your senses. These instinctive reactions, coupled with how our brain perceives the different type of sounds, leads to a lot of sound illusions. These illusions are particularly taken advantage of by sound engineers, be it for movies, music or games. Let’s have a look at some interesting examples of how they do it.

P.S Familiarity with some audio jargon will help you a lot with this one

Human-Ear
A Human-Ear diagram

 

Boom!

When the ear experiences loud noises, the middle ear muscles contract to keep the delicate cochlea safe from the increased vibrational energy being transmitted. As a result, the loudness reduces immediately after the initial burst. However, the brain is used to sensing this sudden change and still perceiving the sound as loud and sustained.

This particular trick is used in cinematic sound design to create the illusion of sustained loud noises, like explosions and gunfire, without actually giving a theatre full of moviegoers hearing damage. It’s also used by music producers, especially to punctuate the drops in a club or electronic track.

 

The Phantom of the Melodies

Sometimes music consists of patterns repeating at very high speed, with little or no difference between subsequent notes. When played fast enough, the brain identifies the subtly different notes and strings them together to form a melody. This same melody disappears when the sound is played slowly.

Shepherd’s Paradox

This particular illusion is very interesting. A pair of chords being played sound as if they are continuously rising up in scales, but in fact, they get nowhere. In fact, if you loop a sample like this, it will be impossible to detect where it starts and where it ends.

Shepard Tones
Shepard Tones down-pitch, spectrum, linear scaled

If you’re wondering where you might have heard a tone like this, you need to look no further than the sound of the Bat-pod in Dark Knight and Dark Knight Rises. Since the filmmakers didn’t want the bat-pod to change gears and continuously accelerate, a Shepard tone was used to create the special sound of Batman’s motorcycle-esque vehicle. You should also check out how it was used to create an increasing sense of danger more recently in Dunkirk, another Christopher Nolan movie, here.

The Haas Effect

Helmut Haas described this effect in 1949. He was studying the relation between original sound and its initial reflections with respect to human hearing. According to his research, as long as the initial reflections are no more than 35ms delayed and 10dB louder than the original sound, the discrete sounds are interpreted as one single sound. This effect is put to use to create the illusion of a stereo width starting from just a mono source.

A layout for Haas effect.
A layout for Haas effect. Source

 

For instance, if you have a mono source like guitars, it’s a helpful trick to duplicate the original piece, pan one to the extreme right and the other to the left. Then delay either sample by about 10ms to 35ms (results may vary depending on the application being used and particular sample). This makes the listener’s mind perceive a lot of space and depth and leaves the centre of the soundstage open for other instruments. This is also useful to make sure that a mono signal is not masked by other instruments at the centre.

Fletcher-Munson Effect

Okay, this one might be a bit complicated to wrap your heads around if you have no clue what an equaliser does. Our ear is more sensitive to midrange sound, and as a result, relative levels of different instruments at different frequencies changes depending on the volume you’re listening it to. This phenomenon is also referred to as the nonlinear response of the human ear. And it can be used to create the illusion of high volume even at low volume levels. This is the reason why professional sound mixers do so at low volumes (also to avoid ear fatigue).

By reducing the midrange or by boosting the high and low frequencies in a track, an impression of loudness at lower volumes can be created. For instance, just listen to Nirvana’s Nevermind for a classic, thundering example of scooped mids dynamics. The same effect, when reversed, is useful in highlighting mid-range sounds – e.g when you want to highlight the lead vocals versus backing vocals.

The snare drum effect that you can hear in Johnny Cash’s Walk the Line was created by inserting a piece of paper between the strings in his guitar.

Tritone Paradox

The tritone paradox, or the scale illusion, is a special case of Shepard’s Paradox. In this, a pair of Shepard tones is separated by an octave, which might then be heard as ascending or descending.

In fact, there’s a very interesting way to experience this paradox. Go to this link and before you listen to the audio, make sure you’ve adjusted your headphones for equal loudness on each ear. Once you’re done, play the audio and try to understand what you’re experiencing. Generally, you’ll need to identify on which ear you’re experiencing the higher and lower tone respectively. Now, turn the headphone around and listen again. Chances are that you’ll be hearing the high and low tones in the same ear as before. This can be described as the brain assigning the higher and lower tones to different ears, with results differing between right handed and left handed people. Additionally, with both the tones being Shepard tones, they are heard to be continuously rising or falling without actually getting anywhere.

McGurk effect

The phrase ‘seeing is believing’ is one of the best descriptions of the McGurk effect. This effect refers to the combination of the audio of one phonetic syllable with the visual pronunciation of another leading to the listener hearing a third syllable. This is another example of the brain’s conditioning affecting our experience of sound from beyond the auditory senses.

Lips
McGurk effect is nothing but your mind lip reading for you

 

For instance, of the audio of someone saying ‘Ba’ is combined with the visual of lips pronouncing ‘ga’ you’re very likely to hear the syllable ‘da’. While there are no practical benefits, hence applications, of this effect, it is pretty interesting to observe it for improving our understanding of how the brain processes inputs from different senses.

And these are not the only tricks out there! If you find some of your own, or experience something entirely new, do let us know at editor@digit.in.

Arnab Mukherjee

Arnab Mukherjee

A former tech-support desk jockey, you can find this individual delving deep into all things tech, fiction and food. Calling his sense of humour merely terrible would be a much better joke than what he usually makes.