What are Binaural Beats?

1. what sound is

Let’s start at the very beginning and try to understand what sound is in the first place.

In physics terms, sound is a vibration that travels (more precisely, propagates) through a physical medium (a substance with mass) as alternating increases and decreases in its density. 

To be even more precise, in physics sound is a mechanical wave that manifests as these pressure variations in our medium of interest.

In psychology, sound is understood as the perception of this wave by a listener (human or animal). In other words, sound is how our bodies detect these vibrations and how our brains interpret them as meaningful experiences.

2. visualising sound waves

Let’s use a simplified animation of such a sound wave, to illustrate the key points that I made above.

Credit for this animation goes to Dr Daniel Russell at Penn State University, who has an excellent page on acoustics.

Imagine that all the dots in the animation above are the particles in the air around you (the medium that we described earlier). A disturbance of some kind (perhaps the movement of the speaker cone of your stereo, represented above by the grey line on the left) is responsible for the wave you see. As the speaker cone moves into the air that is directly in front of it, it pushes into the air particles and compresses them, resulting in a temporary increase in air density and pressure. These particles, in turn, push on the particles ahead of them and so on. As the speaker cone returns to its starting position (or even further back), it creates an open space that the air particles rush to occupy, resulting in a decrease in air density and pressure.

This most basic form of periodic wave is what we call a sine wave. In the waveform below, you can see the pressure variations over time. The taller the peaks of these variations are (the higher their amplitude) the louder you’ll experience this sound. In the case below, the peaks are of constant amplitude, so you’ll hear the sound as having stable loudness (volume, if you like).

3. beats

Alright, we’re almost ready to talk about Binaural Beats, I promise. First, though, I want to talk about beats in general.

Let’s picture what would happen if we added a second sine wave to the one above, more specifically one that is just a few Hz different to it, let’s say, 430Hz:

If we look at the two waves in the same plot (with the 430Hz tone in red), we see that because they’re ever so slightly different in frequency, their peaks start to drift away from each other over time:

In time of course, their peaks would start coming back together again and this oscillation in how their peaks coincide with one another results in what we call a beat in acoustics. If we played these two sounds simultaneously, the two wave forms will add to one another and the resulting wave would look like this:

Here, you can clearly see the beat as an overall increase and decrease in the pressure changes. The frequency of this beat is equal to the difference in the frequencies between the two tones, in this case 10Hz, which is to say that this periodic change in pressure happens 10 times per second.

If you listen to the sum of these two tones (below), you’ll notice is that you perceive this beat as a periodic change in the volume of the sound you hear. From the wave form above, you can already see these changes in the amplitude of the sound; that’s what you perceive as a beat.

4. binaural beats

Finally we can talk about binaural beats! So far, the beats we’ve talked about represent a physical interference between two sounds (the 430Hz and 440Hz tones in the example above). You can hear this beat over speakers or over headphones and the result is pretty much the same. You don’t need two ears to perceive this beat, so it’s sometimes referred to as a monaural beat (monaural meaning involving one ear).

What if you were to present each of these two tones to a different ear? We can easily do that with headphones these days and you’ll likely have a pair nearby, so bring them now and connect them to the device you’re using to read this page, because without them, you can’t perceive the binaural beat.

The sound below is a stereo file with the 430Hz tone presented in the left ear and the 440Hz tone presented in the right ear. Listen to it now with your headphones on.

What do you hear when you listen to this sound? Most people hear the same beating pattern as in the previous example, though often the sound also seems to move left and right at the same time.

Try these experiments:

1. With your headphones on, compare the monaural beat with the binaural beat. Do you notice any differences?
2. Unplug your headphones and compare the monaural and binaural beat examples through your speakers. What differences do you notice now?
3. Finally compare the experience of listening to either of these two examples through your headphones and through your speakers. What do you make of these comparisons?

5. different beat frequencies

As we saw, in both monaural and binaural examples, the frequency of the beat is determined by the frequency difference of the two tones. What this means is that we can create different beat frequencies, simply by changing the frequencies of these two tones. Try these examples:

6. what’s so special about binaural beats?

Without venturing into conversations about whether binaural beats have any psychological effects, the curious thing about them is that they are a sort of auditory illusion. Since the two tones are presented separately to the two ears, they can’t physically interact with one another. The beat that you hear when you listen to them through your headphones is created by your auditory system, as it compares the inputs from your two ears.