[scientizscht (OP)]

Hello

What is the weakest sound that can be detected and from what distance?

Thanks!

[evan_au]

What is the weakest sound that can be detected...?

The intensity of a sound is measured in microPascals.
- The softest audible sound is somewhere around 5 microPascals, although the reference intensity in air is 20 microPascals (which was the traditional "softest audible sound")
- Because human hearing is frequency-sensitive, the softest audible sounds will be between 2 & 4 kHz

Read: https://en.wikipedia.org/wiki/Decibel#Perception

...and from what distance?

Sound intensity follows an inverse-square law as you travel further from the source.
- So the softest sound at the source would only be detectable from 0 distance.
- However, a very loud sound from a far-away source could still be just 5 microPascals when it reaches your ear; so a very loud sound at the source can also (in theory) be the softest perceptible sound at your ear
- In practice, the softest perceptible sound can only be heard in an anechoic chamber; the normal noises outdoors would mask truly quiet sounds. Since practical anechoic chambers are of limited size, the softest sound can only be heard from a few meters away.
See: https://en.wikipedia.org/wiki/Anechoic_chamber

[scientizscht (OP)]

Thanks but I didn't say by human ear.

[chiralSPO]

Thanks but I didn't say by human ear.

Ok, then I think the most sensitive detectors we have for vibrations are those that are looking for gravitational waves (LIGA and VIRGO), which can detect minuscule vibrations. (I can't find the numbers now, and gotta run--but I will say that they are the equivalent of using a seismometer to hear a mouse fart on the other side of the globe.)

Found it: One example is 10⁴⁹ Watts at the source, but the source is 410 Mpc away, or 1.26x10²⁵ meters away. (so not as faint as I thought...)

[evan_au]

The sensitivity of human hearing corresponds to an displacement of the eardrum of about 10% of the width of a hydrogen atom - in the presence of air molecules zipping around at close to 1,000 km/h.
- This sensitivity is possible because a series of levers in the ear matches the impedance of the air to the impedance of the fluid-filled cochlea, maximizing sound transfer efficiency.
https://en.wikibooks.org/wiki/Engineering_Acoustics/The_Human_Ear_and_Sound_Perception

I agree that LIGO is the most sensitive vibration detector we have today.
- It is able to detect vibrations of its mirrors of about 1/10,000 the width of a proton.
- It is sensitive to frequencies between 50Hz and 1000 Hz.
- It operates in a very good vacuum, so it doesn't have to contend with lots of air molecules zipping around
- They plan to implement cryogenic cooling of the mirrors, to further reduce movement of the atoms in the mirrors.
- The energy released by black hole mergers is immense, and the gravitational waves carry a lot of energy, even at the distance of the Earth. But only a very small amount of the energy in the gravitational wave goes into moving the mirrors; the gravitational waves continue on into the cosmos as if they had not encountered the Earth (or the Sun).

See: https://www.ligo.caltech.edu/page/facts

[scientizscht (OP)]

Ok this is an enormous intensity sound from an enormous distance which doesn't have clear relationship to what I am asking.

E.g. this could be the equivalent of 100db at 5m distance which is definitely not the weakest sound.

What is the weakest sound that can be detected from let's say 5m?

[Bored chemist]

It's probably dependent on Brownian motion of the microphone or some such.
It will depend on temperature and how long you are prepared to "listen".
So it's another of your badly worded questions to which there isn't really an answer.

Why do you keep asking them?

[yor_on]

The sound of a photon hitting your eardrum?
=

Not sure how we would detect it though :)

[Bored chemist]

Countless photons hit and are absorbed by the eardrum every second. A roughly equal number are emitted by it.
The momentum transfer is the same in both cases.
And the transfer is twice as big for those photons which are reflected.

But I still think the impact of the air molecules hitting it will be rather bigger.

[syhprum]

This problem worried me as a boy because for hearing they always spoke of decibels but never from a base value.
my teachers could not help but I eventually found a value of 0.00002 dynes per square centimetre what ever that is in SI units.

[Bored chemist]

I gather the current reference is 20µ Pa.