Why do headphone speakers sound 'tinny' when far from the ear?

Question

For me it is counter-intuitive: I should be hearing more low frequency sounds (bass) at a greater distance from a headphone speaker (like I hear only the bass when standing outside a club), because the bass travels further and high frequency sounds get absorbed easier on the way to the ear.

But of course it sounds like the bass is completely absent when headphones are located far from the ear, and it sounds like only the very high frequency sounds survive.

What is the physics behind this - why do headphones sound more 'tinny' when distance from the ear is increased?

Answer 1

The speakers in a headphone couple poorly to 3-dimensional space full of air- that is, there is a severe impedance mismatch between the itsy bitsy speaker cone in the earcup and the air that is more than one cone diameter away from it, for low frequencies. This means if you hold the headphones at arm's length, all your ears hear is the fizzy treble portion of the audio output.

To hear the bass requires you to be within the near field of the itsy bitsy speaker which in practical terms means that the distance from the speaker cone to your ear is less than one cone diameter. In this regime, the cone movement is well-coupled to your eardrum and the bass comes through loud and clear.

To obtain good coupling and therefore good radiation of sound power at low frequencies in a big room requires replacing the itsy bitsy speaker with one of greater diameter- the bigger, the better. This is why large sound reinforcement systems use special speakers with diameters of 15" or even 18" to get good bass response, even if your ear is 50 feet from the speaker cone.

Answer 2

The answer given above that emphasizes the importance of the near field is incorrect. The reasoning there is a blind application of acoustic theory without basic physical understanding of the particular case at hand.

That answer can be disproven simply by first mounting the headphones and listening to music with bass tones, and then slightly cracking the seal between the headphones and the side of your head. The bass tones immediately reduce in volume, even though you haven't much altered the distance between your ear and the speaker (near field). This result illustrates that a correct explanation must include the fact of the sealed volume of air contained between the headphones and your eardrum.

A better answer is that the low frequencies are able to periodically increase and decrease the air pressure within the volume much more than the higher frequencies can, simply because the amplitude of vibration of the speaker is larger for the low frequencies than the high frequencies. The higher the frequency, the less the speaker moves, causing less air pressure in the volume. The lower the frequency, the more compression of the air volume. In fact, if the frequencies are below about 20 Hz, you cannot hear the tone, but you can feel it physically, and that could enhance the overall enjoyment of the music. Though not many headphones have such low frequency responses.

We can thus say that the sealed air volume acoustically couples the headphone speaker to your eardrum, and the amount of coupling is frequency dependent.

The same thing happens with ear buds. Have you ever wondered why ear buds can produce much of the same low frequencies that huge bass woofer speakers can? This is the reason. It's the coupling between the speaker in the bud and your eardrum, via the closed volume of air in the ear canal. The buds contain a coupling mechanism that the woofers don't have.

Answer 3

A potential partial reason why only high frequencies are audible could be due to human hearing capability.

The equal loudness contours on this wiki link clearly show that at lower frequencies, our ears need a higher sound intensity to perceive the same level of loudness as compared to higher frequencies. Moreover, as the intensity decreases, the lower frequencies are the ones that are the most affected.