How Do Headphones Produce Sub-Bass & Bass Frequencies?
First, let’s repeat a statement I made previously. Low-end frequencies are felt more than they are heard. We can feel the low-end of a kick drum at a live event or the deep bass of a subwoofer on the dancefloor.
Of course, we can sometimes feel the vibrations of bass-heavy music in our headphones but it is faint and localized around our ears.
That is to say that the ways in which headphones produce bass are much less visceral than the ways in which subwoofers produce bass.
Headphones do not have to propel bass frequencies across long distances in order for the listener to hear them. The drivers/speakers of headphones are practically attached to the listeners’ ears.
Proximity
The close proximity of the headphone driver to the eardrum means the driver (headphone speaker) doesn’t have to move as much air to produce a decent bass response when the headphones are worn properly.
The inverse-square law state that the intensity of a sound wave is quartered for every doubling of distance. In other words, the sound pressure level drops by 6 dB for every doubling of distance.
Try moving your earbuds or headphones just an inch from your ears and you’ll hear the distance. Since the low-end requires a lot of energy, more energy will be lost for every doubling of distance. Put another way, a quartering of the intensity of a bass frequency is perceived as a greater drop in loudness.
Closed Space/Tunnel
Many headphone/earbud form factors create a sealed (or at least a semi-sealed) enclosure with the speaker on one end and the eardrum on the other. This coupling of diaphragms allows the bass frequencies to have more of an effect on the eardrum and a greater perceived loudness.
This helps to explain why closed-back headphones often have more perceived bass than open-back headphones and why pushing earbuds further into your ear canal will also boost the perceived bass response.
Bone Conduction
We’ve been discussing the eardrum quite a bit in this section. However, the eardrums themselves aren’t overly reactive to frequencies below 80 Hz. Remember how we feel these low-end frequencies more than we hear them?
Well, our brains still process these low vibrations as sound but we rely more on the resonances of our bodies to sense these sound waves.
With headphones, this is where the ear structure and bone conduction come into play.
Basically, the sound vibrations in the headphones physically vibrate our skull and the tiny bones in our ears, which sends signals to our brain that help us perceive the sound frequencies. This is particularly true of bass frequencies.
Over-the-ear headphones press against our skulls and can so their bass frequencies are more easily perceived.
