28
$\begingroup$

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?

$\endgroup$
3
  • 1
    $\begingroup$ Related: physics.stackexchange.com/q/697399/2451 $\endgroup$
    – Qmechanic
    Commented Aug 30, 2023 at 8:02
  • $\begingroup$ Isn't it a simple as this: Assuming white noise coming out from the headphones (i.e. all frequencies have same amplitude), isn't it simply that lower frequencies have less energy than higher ones? Maybe that's why bass speaker have quite some size, while "tweeters" don't. $\endgroup$
    – U. Windl
    Commented Aug 30, 2023 at 8:03
  • $\begingroup$ like I hear only the bass when standing outside a club I think it's a bad comparison, because club building walls may have low resonating frequencies which may partially match with music bass sound frequency, and so walls/ground/ceiling "dances" (aka reverberates) with music together re-emitting these sounds to the outside, while high frequency sounds are absorbed pretty well by walls. Headphone has no such situation and hence comparison with club may be irrelevant here. $\endgroup$ Commented Sep 10, 2023 at 12:49

3 Answers 3

22
$\begingroup$

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.

$\endgroup$
6
  • 5
    $\begingroup$ I feel like the focus of the number of "cone diameters" away from the speaker driver doesnt paint a clear picture at all. For a 8 mm driver, and an average 371 mm long arm, you're ~46 diameters away. For a 15" speaker, 46 diameters away is ~57 ft, not much further than the 50 ft you mention for a "good" bass response. Or alternately, being less than 15" away from a large speaker obviously isnt it's intended use either. It's not clear why your cone diameter rule only applies for small speakers, so it's probably not worth focusing on. $\endgroup$
    – JMac
    Commented Aug 29, 2023 at 18:42
  • 5
    $\begingroup$ One more important factor here is the "acoustic short-circuit" concept. When the headphone is worn, there is less bleed of sound from one side to the other and thus the headphone does not act as a dipole (which is a very bad low frequency radiator) but becomes more like monopole (which is a less bad - still bad - low frequency radiator). $\endgroup$
    – ZaellixA
    Commented Aug 29, 2023 at 19:13
  • 3
    $\begingroup$ @JMac I guess it's not just about the ratio between speaker size and distance, but also about the ratio between speaker size and sound wavelength. Otherwise it would be just as bad for high frequencies. So increasing speaker size should help in two different ways. $\endgroup$
    – jkej
    Commented Aug 29, 2023 at 22:29
  • 1
    $\begingroup$ I have ear buds with the best (consumer) bass I've heard aside from a dedicated subwoofer. I'm pretty sure direct contact with the ear/head is the only explanation and that it is relevant to this question. $\endgroup$
    – JimmyJames
    Commented Aug 30, 2023 at 21:38
  • $\begingroup$ @JimmyJames There is more to it than simply 'distance from the ear' since the question is asking about the attenuation of low frequency sounds vs high frequency sounds, and why low frequency sounds do not appear to travel as far as high frequency sounds, in the specific instance of small speakers (headphone size), counter to what one might intuitively assume (that low freq should survive and high freq attenuate - not the other way around) $\endgroup$ Commented Sep 1, 2023 at 0:07
4
$\begingroup$

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.

$\endgroup$
2
  • 1
    $\begingroup$ This is exactly why sources all over the web emphasize how any gaps in sealed headphones will cause the bass to leak. For a demonstration of how even a tiny leak kills the bass response, compare Figures 4, 6, & 8 in "B&O Tech: Airtight excuses". $\endgroup$ Commented Sep 2, 2023 at 14:26
  • 1
    $\begingroup$ David Bailey, Thanks for the link. My guess on the nature of the "resonant" peaks shown at the low frequencies is that the combined volume and the opening form a Helmholtz resonator, and it's the resonant frequency of this resonator that we see in the graphs. It's a shame that the author doesn't define by what the "magnitude" of the Ordinate is. $\endgroup$
    – ttonon
    Commented Sep 2, 2023 at 17:40
1
$\begingroup$

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.

enter image description here

$\endgroup$

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge you have read our privacy policy.

Not the answer you're looking for? Browse other questions tagged or ask your own question.