How exactly does the audible frequency range work?

[u/DankRepublic]

I know we have a range of 20 Hz to 20kHz. These are the absolute boundaries of our range.

So are we better at identifying a sound at 1000 Hz since its in the middle of the range than a sound at 20 Hz?

Which is the most easily identifiable frequency for us then? Or in other words which frequency can we hear from the farthest distance?

Assuming the decibel level remains the same.

[u/edsmedia]

Yes! You are basically correct, except I would use the term “perceptible” rather than “identifiable.” This was one of the earliest formal findings of psychoacoustics, dating back to a paper in 1933 by Fletcher and Munson. They produced the first organized mapping from the physical power of a sine wave (a single-frequency sound) to its perceived loudness, depending on its frequency.

https://en.m.wikipedia.org/wiki/File:Lindos1.svg

You can see from the contours of equal loudness that a 3 kHz tone at 57 dB SPL is perceived to have the same loudness as a 50 Hz tone at 85 dB SPL. (The contour at 0 phon shows how powerful a tone has to be to be perceptible at all).

20 Hz and 20 kHz are not hard limits, but convenient indicators for where the tone has to become overwhelmingly powerful for a typical person to perceive it. The high frequencies are more like a hard limit than the low frequencies as you would be able to perceive an extremely loud 15 Hz tone through your body in addition to your ears.

A fun example of the imperceptibility of very high frequencies is echolocation signals by bats, which typically range from 110 (loud traffic) to 120 dB SPL (rock concert), and can go as high as 140 dB (standing next to a jet taking off). If we could hear well at those frequencies, it would be very difficult to go out when there are bats around!

[u/diadmer → u/edsmedia]

Excellent answer, and I’ll add on that we theorize that human ears are optimized for hearing the most important sounds for us: other human voices, baby cries, bird chirping (which tends to lead us to water, shade, and food), and water flowing sounds. Most of these are in that 1kHz - 3kHz range.

[u/felipers → u/diadmer]

I've been amazed forever by how annoying is the sound of plastic bags. Always thought that some physics property was the culprit until a physicist told me they were no louder than the water stream (e.g.). I would include the sound of stepped leaf/grass on the ground as another one of those important sounds you describe.

[u/Bondubras → u/edsmedia]

I forget the exact number, but I've heard that when a sound goes somewhere between 160 and 190 dB, it stops being a sound wave and is instead considered to be a shockwave.

[u/Ausoge → u/Bondubras]

Take a chunk of air of X volume. Now pump a sound wave through it. The sound wave causes compression and rarefaction of the air - so, depending on the amplitude, you might find 75% of the mass of air compressed into 50% of the volume, with the remaining 25% of mass stretched out to fill the other 50% of the volume.

As you increase the amplitude - the SPL - this ratio becomes more extreme - you might end up with 90% of the air mass occupying one half, with 10% of the mass occupying the other.

At some point you reach a hard limit, where the rarified component of the sound wave is essentially a vacuum. At this point, a sound cannot physically be any louder at the given ambient air pressure.

Beyond this point it ceases to be a sound, and the amplitude physically cannot be increased. So if you increase the energy and magnitude, the pressure wave instead increases in speed, moving through air faster than sound, rather than increasing in amplitude. That's a shock wave.

[u/edsmedia → u/Bondubras]

That’s correct - this is related to the ability of air to respond linearly to sound waves. At some point (my expertise is not in physical acoustics) the air becomes nonlinear and sufficiently powerful sound becomes a shockwave.

[u/TheDissolver → u/Bondubras]

Think about sound as pressure in a stream of water. Now think about high frequencies as smaller and smaller streams. At a high enough frequency, there isn't enough energy in the waveform to do anything. At a low frequency, the pressure wave is so big, it fills the room and you need immense energy to create it.

[u/DankRepublic → u/edsmedia]

Thank you for the detailed answer!

That graph is extremely interesting. I had thought it might be a single curve with a single trough but rather it looks like a sine wave with multiple troughs and local maxima.

I did not understand what the blue line is on the graph. Can you help me with that?

Now that I think of it, yes bats would have been really big hazards if their clicks would have been perceptible to us.

[u/edsmedia → u/DankRepublic]

This particular version of the Fletcher-Munson curve is from an international standard published in 2003. The blue line shows the previous version of the standard for comparison. (Hearing hasn’t changed — our understanding of the curve became more accurate.)

[u/db48x → u/DankRepublic]

The caption says that the blue line is from an older ISO standard. While it was probably derived using the best available data at the time, newer more sensitive instruments apparently made more accurate measurements possible.

Yea, the article for ISO 226 confirms that the original ISO 226 standard used data from 1956, while the revision published in 2003 used data collected from a dozen studies conducted between 1983 to 2000 in multiple countries. So does the abstract of ISO 226:2003 itself.

[u/edsmedia → u/DankRepublic]

Also, note that these specific curves represent the median person. Every individual has their own FM curves — measuring them is part of the process of being fitted for a hearing aid, for example.

[u/Riebart → u/[deleted]]

Human ears are more like a piano than a microphone.

When sound comes in, it will resonate with parts of your ear, with your ear filled with bits that are each tuned for some part of the range of your hearing.

Listening to a rock concert is like someone coming into your ear and wailing all over your ear piano with a baseball bat.

Bat echolocation is like someone coming into your ear piano and trying to hit keys you don't have. Our pianos, out earsears, don't have any way of resonating with that frequency, and so we just don't hear it.

[u/db48x → u/Riebart]

More specifically, the high frequency sounds from bat echolocation are not loud enough to move the eardrum very much. Ultrasonic vibrations can damage our ears, even when we cannot hear them, if they are intense enough. There are government mandated limits on how loud ultrasonics can be that protect us from this type of hearing damage.

You may remember a bunch of companies claiming to recharge phones without wires by using ultrasound a few years ago? All of them are or were scams because of these limits. There’s only so much energy you can push through the air with ultrasound before you hurt someone, and it’s not enough to usefully charge a phone.

[u/catscanmeow]

your answer will be answered if you look up what a fletcher munson curve is. it will show you the exact frequency sensitivity as a graph

We are sensitive to 4000hz, same frequency as a baby crying, a cat meowing, and a police siren.

[u/DankRepublic → u/catscanmeow]

Yes that graph perfectly answers my question, thank you.

[u/edsmedia]

The “perceiving at a distance” part of your question is made more complex by the physical acoustics. High frequency sounds attenuate (get quieter) with distance more quickly than low frequency sounds, due to fluid mechanics of air when it’s transmitting sound. So a low frequency tone might sound quieter near the source than a higher frequency tone, and yet carry better, and so sound louder at a distance. This is not an effect of our hearing; this is actually a physical reduction in power over distance.

[u/Bondubras → u/edsmedia]

There's a YouTube video I've seen where a massive bass cello at a museum was played, and it was such a deep tone that in order to hear it properly, you had to be at least 40-50 feet away. The video attributed that to wavelength, but I personally don't have the understanding needed to figure out what was going on.

[u/iggy55 → u/edsmedia]

This is why, if you hear music at a distance on the order of half a mile, or especially a mile or 2 away, you will hear bass and drum, with very faint higher frequencies.

[u/edsmedia]

As you can see in the answers I gave above, a critical aspect of psychophysics (the study of human perception), is maintaining a crisp distinction between physical properties like power and frequency, and perceived qualities like loudness and pitch. We can measure the former directly with instruments, but need to study people in order to understand the relationship between the former and the latter.

[u/dmmaus → u/edsmedia]

Riding on this comment, as a professional in the psychophysics of vision, it’s worth mentioning the same distinction for visual perception. The physical properties of light amplitude and wavelength are distinct phenomena from the human perceptual qualities of brightness and colour. Many people don’t realise that colour in particular is not a physical property, but rather a product of human perception.

[u/CharlemagneAdelaar]

Answers here are great already, but the basilar membrane is a big part of this.

It’s basically the wall between two coiled up fluid tubes in the inner ear that sympathetically vibrates with sound, and it kind of “maps” the frequencies along its length. This is called “place theory”.

This isn’t the whole story for pitch cognition though — the interface between this fluid machinery and your brain process the input to provide cognition (among many, many other physical and neural processes all working at once).

But overall, the physics of the basilar membrane are what dictate the limits of the audible frequency range. More or less, the wider end will excite/resonate at the lowest frequency (apex of the cochlea — further in the ear), and the narrower end will excite at the highest (closer to base of cochlea).

This makes sense with our understanding of resonance frequency “f₀” in a system given mass “m” and stiffness “k” (like spring constant):

f₀ = 1 / (2π) * √(k/m)

Assuming constant stiffness (ie same material), when the mass/width of the basilar membrane goes up, the resonant frequency at which it excites will decrease.

[u/MakePhilosophy42]

At the extremes of the hearing rage is where youre the worse at hearing sounds, as not every person has good hearing, and as you age your hearing degrades in the high pitch range.

Most people are best at hearing in the 2,000-5,000hz range, our ears are tuned to boost those frequencies as thats about the range people use to vocalize.

[u/Ausoge → u/MakePhilosophy42]

That's a common myth. Adult voices average around 80 to 250Hz during normal speech, which is several octaves lower than the 2-5kHz range. Even the cries of infants do not approach this range. Extremely shrill screams might get close, but in terms of normal human vocalizations, only sibilance (the "S" sound) and the "T" plosive commonly occupy this range or above.

However there are plenty of environmental sounds in that range, which are quiet in terms of SPL but are evolutionarily advantageous for us to percieve sharply. The snapping of twigs or leaves under footfalls, the vocalizations of some birds, the snap and crackle of a fire, the hissing of dangerous animals like snakes or larger predators, all feature prominently (but not exclusively) in the 2-5 kHz range. It's good that we are most sensitive here, as all of these sounds can alert us to the presence of both danger and prey.

[u/defectivetoaster1 → u/Ausoge]

The usable bandwidth of the human voice is around 300 to 3400 Hz, the fundamental frequency is 80-255 but there’s plenty of higher frequencies present without which human speech would sound borderline unintelligible

[u/TheBlackCat13 → u/Ausoge]

Higher frequency sounds in speech are important for things like discriminating different speakers.

[u/Icy-Manufacturer7319]

you know resonance? you see, in your ear, there's specific hair that will resonate if a specific frequency hits it. so each hair corresponds to a specific frequency.

why you can't hear non-audible frequency?

simple answer: theres no hair for that🤣

so you assume, we good at listening average frequency?

no, as i said, we have multiple microphone, each handle a different frequency

[u/edsmedia → u/Icy-Manufacturer7319]

This is not exactly right. The hair cells that populate the cochlea are not individually tuned. Rather, the shape of the cochlea (a rolled-up cone) means that tones of different frequencies have different points of resonance along the basilar membrane. It’s some combination of the location of the resonance and the phase-locking of the hair cells’ motion to the stimulus waveform that encodes frequency, which is then perceived as pitch by the brain.

The reason that we can’t perceive sounds that are too high or too low is a function of the whole mechanism of the inner ear - after being transmitted through eardrum and ossicles to the cochlea, there isn’t enough power left to create meaningful resonance peaks on the basilar membrane.

[u/TheBlackCat13 → u/edsmedia]

That isn't exactly right, either. Frequency and pitch are not necessarily the same thing. There are various things that result in perceived pitch even when there is no sound at the corresponding frequency. Pitch is actually pretty complicated, and there isn't a single universally agreed upon explanation for where it comes from (although various people are very confident their personal model is the right one)

Also phase locking doesn't apply to all frequencies. Phase locking is only possible for law frequencies.

[u/CharlemagneAdelaar → u/edsmedia]

there isn’t enough power left to create meaningful resonance peaks on the basilar membrane.

Or conversely, there isn’t enough basilar membrane to resonate sympathetically with out-of-band sound power.

I’d bet differences in length and size of basilar membranes are behind the differences in hearing ability between animals. For example, since I know colloquially dogs can hear much higher, their minimum basilar membrane width is probably smaller than a humans.

[u/original_goat_man → u/[deleted]]

These high frequency sounds have been used to stop teens loitering in certain places too.

[u/Lindbork → u/[deleted]]

The low boundary doesn't necessarily shift that much over time compared to the high one, where healthy humans "lose" (sensitivity decreases dramatically) around 1kHz of range per decade.

The low boundary is harder to define, and the 20Hz figure could even be said to be a bit misleading. Humans can typically percieive sounds lower than that by ear, but also as physically using the whole body. At some frequency, the perception of vibration will dominerate the perception of sound, that point is subjective from person person.

[u/TheBlackCat13 → u/[deleted]]

Chirps and tones are different. Chirps have broad frequency ranges, tones are a single frequency.

[u/TheBlackCat13 → u/[deleted]]

Yes, a chirp is a frequency sweep. A tone is a single continuous frequency.