44
$\begingroup$

We can see buildings, doors, cars etc. as light falls on it gets reflected to us. but why doesn't the same thing happen with sound? I mean why don't we hear sound reflecting that much?

$\endgroup$
  • $\begingroup$ I've deleted some off-topic comments. $\endgroup$ – David Z Apr 7 '16 at 23:33
  • 7
    $\begingroup$ There are actually blind people that can use echo location just like bats that use this reflection of sound $\endgroup$ – Ivo Beckers Apr 8 '16 at 9:49
  • 5
    $\begingroup$ Did it ever happen to you that the source of a prominent sound (e.g., a siren, music, construction noise) in a city was somewhere entirely different than where you thought it was? If yes, this was very likely due to sound reflecting from buildings. $\endgroup$ – Wrzlprmft Apr 9 '16 at 5:05

12 Answers 12

178
$\begingroup$

We do. Normally the reflections are too quick to hear distinctly, and in an environment like a room they rapidly become diffused into a mush which a sound engineer would call reverberation. In larger spaces you can often hear distinct echoes as well or instead: a good way to play with this is to clap your hands (once) in a quiet hall: you will hear the first echo and then hear the subsequent echoes mix into reverb.

The reflective and absorbent properties of rooms and halls are absolutely critical to how pleasant they are to be in and how usable they are for music and so on: people spend a lot of time worrying about this, and if they get it wrong you know.

One reason people are not very aware of this is that it happens all the time, wherever you are. You can build spaces which do not reflect sound -- anechoic chambers -- and it is very odd indeed being in one.

If you record music electronically (so, from an electronic source with no microphone) as is now common, then it is critical to add simulated reverberation to the sound: reverb units (often now done in software of course) are probably the most common effect in recording studios.

So reflected sound is absolutely pervasive.

$\endgroup$
  • $\begingroup$ I've deleted an off-topic comment discussion. $\endgroup$ – David Z Apr 7 '16 at 23:33
  • 1
    $\begingroup$ Helmut Haas described this in his Precedence effect. If the time between two reflections is to short the separate impulses (wave fronts) merge. This is a binaural psychoacoustic effect and therefore only applies to humans. Read on here: en.wikipedia.org/wiki/Precedence_effect $\endgroup$ – Max Apr 9 '16 at 10:40
  • 1
    $\begingroup$ Another way to hear it is to go to a college when the marching band is playing. Sometimes you can find places where a building obscures the band itself, but you can hear the echos off of the wall on the other side. When I find such a place, I get the distinct impression that the snare drum is being played behind the building that I'm hearing echos from. The effect happens everywhere, but colleges happen to be a great place to find snare drums and nice tall buildings. $\endgroup$ – Cort Ammon Jun 24 '18 at 0:52
  • $\begingroup$ @CortAmmon: that's a really good example, it's happened to me as well. $\endgroup$ – tfb Jun 24 '18 at 9:59
  • $\begingroup$ There's a particularly clean example here (just south of the central library at Imperial College London), where the library blocks the light & sound path to the Queen's Tower, but both can reflect on the glass-fronted building on the other side of Imperial College Road. When the tower's bells ring, there's a remarkable impression that the sound is coming from the tower's reflection behind the glass windows. $\endgroup$ – Emilio Pisanty May 6 at 14:34
47
$\begingroup$

Our eyes have excellent spatial resolution. We can tell the difference between objects only a fraction of a degree apart. This is possible due to both the construction of the eye and the fact that visible light has wavelengths that are tiny on our scale. Signals that arrive simultaneously can be independently detected.

Our ears do not have this level of precision. While we can generally determine the direction of a single sound through the combination of several clues, frequencies that we are sensitive to can't be "imaged" in a similar manner. A mid-range tone of 500Hz would have a wavelength of over 60cm. That makes for poor resolution possibilities with human-scale sensors.

Reflections happen all the time and we hear them, but we can't easily distinguish between the original source and the reflection unless the time difference between them is larger than normal.

$\endgroup$
  • 3
    $\begingroup$ You make it sound as if this were a bad thing. The "design" is very much necessary exactly because there's so much reflected sound - if short echoes weren't filtered out, you'd have tons of acoustic noise that would make hearing much harder. And it's not like a short echo adds any information you didn't hear already :) $\endgroup$ – Luaan Apr 5 '16 at 21:43
  • 4
    $\begingroup$ @Luaan: I would say exactly the opposite -- we have so little sensitivity to reflected sound because there was hardly any of it in our ancestral habitat of trees and grassland, so it wouldn't have been useful. $\endgroup$ – TonyK Apr 5 '16 at 22:46
  • 6
    $\begingroup$ @Luaan it actually adds a ton of information about the geometry of the surrounding area; that's how echolocation works. If you have the right neural circuitry to process the echoes then they're not "noise" at all. But humans have pretty poor abilities in that regard. $\endgroup$ – hobbs Apr 6 '16 at 5:10
  • 5
    $\begingroup$ This answer uses physical differences between light and sound waves to point out why seeing sound reflections like we do light reflections is impractical for humans. It could do with a bit more about how our eye is mostly a lens+pin hole camera, while our ear is mostly a resonance cavity, but really that difference is due to the scale difference in the waves they are monitoring. $\endgroup$ – Yakk Apr 6 '16 at 13:56
  • 3
    $\begingroup$ The comparably low spatial resolution of audible frequencies is mainly not caused by the "sensor's" capabilities but simple, plain wave physics. Objects and locations much smaller than one wavelength cannot be detected or located even theoretically. And even probing an object of 1m with 0.5m waves will only give a very unsharp representation of its position. That's why e.g. bats use ultrasonic frequencies. $\endgroup$ – JimmyB Apr 8 '16 at 13:07
25
$\begingroup$

As someone who's done substantial amounts of live sound for bands, often in rooms in pubs which are acoustically "interesting", it certainly does happen with sound, and you and everyone else hear it all the time. My only possible conclusion is that you haven't listened carefully enough to what you hear.

The reason people like singing in the bathroom is that they generally have lots of reflective surfaces, and that gives a lot of natural reverb. We can't distinguish pitch as accurately when there's a lot of reverb, so this makes our singing sound "better". For the same reason, singers often use reverb when playing live. Before electronic reverb was available, recording engineers used various reverberent spaces to make this happen, either recording the artist in that room, or playing back that recording in the space and recording the resulting echo. It's still widely used for acoustic music, especially classical and folk.

With live sound, a major issue is preventing feedback howls. The main source of these is the monitor speakers (pointing at the performers) being heard by the microphones. This is mainly countered by microphones being designed to be "deaf" to the rear. However if the wall behind the performer has no acoustic damping, it can (and will!) reflect that sound back into the microphone. To add to the amusement, a reflective end wall can (and will!) also reflect the sound from the front-of-house speakers back to the stage, which gives yet another source of feedback. The longer the distance, the lower the pitch of the howl. The next time you're at a gig with an incompetent soundman, use this to guess the cause of the feedback. :)

Electronic recreation of echoes turns out to be easy in principle but hard to do well. You have early reflections (echoes directly off the surface which travel the shortest distance), then after some time you have a more general "mush" of reflections bouncing in various directions which reach you in a varying time, and you have a tail-off effect as the various echoes stop bouncing around the space. The simplest way to recreate the echo/reverb of a particular space is to generate an "impulse response" - you create a loud click (hand-clap would do) and record how that impulse decays. You then get a sample-by-sample recreation of what happens to sound, and you can apply it to your sounds. However this only recreates one space, in one position. More sophisticated reverb algorithms attempt to model how sound bounces off surfaces and combines together. Some (particularly for recording and live sound) have abandoned sounding "like" anything at all, and instead have just focussed on producing reverb characteristics which sound pleasing to the ear.

$\endgroup$
  • $\begingroup$ Worth noting that people still do use acoustically-nice spaces even for electric music. It's less common than it was because digital reverbs are very good now and good live rooms in studios are often big, but it still is done by those who can afford it. (Comment for general readers, obviously already known by person who wrote answer!) $\endgroup$ – tfb Apr 5 '16 at 15:03
18
$\begingroup$

The main reason why we don't hear reflections of sound is related to how our hearing works. The psychoacoustic explanation to this is called the precedence effect. It states that when two or more sounds arrive to the listener within a short enough time(roughly under 50ms) this is perceived as a single sound event. The localization of the sound is dominated by the sound that arrives first.

Second thing that has an effect is the attenuation of sound. When traveling in the air the sound pressure level at a certain distance is inversely proportional to the distance. Sound is also attenuated when it meets a surface and reflects. Depending on the properties of the surface, part of the sound is absorbed and part reflected back.

Imagine that you are in a room talking with somebody. In this situation you there will be direct sound from the speaker and then delayed copies of the sound reflected to you from the walls. The reflected copies will have a certain delay that depends on the room size and certain attenuation that depends on the room size and the acoustic properties of the walls. If the reflected sound arrives within the first 50ms or is attenuated too much then you will hear just a single sound event and no echoes.

So how big would the room need to be that you would hear a reflection from a wall ? Knowing that the speed of sound in room temperature is approximately 343 m/s it can be calculated that sound travels ~17m in 50ms. This means that you would need to be 8.5m away from the closest wall in order to hear the reflected sound. In addition to this the walls would have to be reflective enough that the reflection won't be too quiet.

2nd or further order reflections often would arrive late enough to be perceived as separate sounds but they are usually not loud enough. In an empty room they would be but usually furniture, carpets and such absorb the sound enough to stop the echoes.

$\endgroup$
  • $\begingroup$ This is by far the best answer. $\endgroup$ – StrongBad Apr 8 '16 at 0:08
  • $\begingroup$ I think this is the best answer as it clearly explains why we often don't hear any echoes when we might expect to, just as light is reflected. The reason we don't is a function of our physiology and not physics. The physics behaves as expected. $\endgroup$ – Nathan K Apr 8 '16 at 14:06
13
$\begingroup$

As the above answers have stated, we do hear such reflected sound but normally do not notice. However, if you ever get the opportunity stand inside a closed anechoic chamber. You will then "hear" the total absence of all reflected sound. To say it is weird is an understatement - it feels like your ears are being sucked out by silence.

$\endgroup$
  • 1
    $\begingroup$ Imagine it would be something like standing in an open field with freshly fallen snow. $\endgroup$ – Neil Apr 5 '16 at 8:32
  • 2
    $\begingroup$ @Neil: it's much weirder than that, although in the same direction of weirdness. Snow reflects quite a lot of sound, I guess. $\endgroup$ – tfb Apr 5 '16 at 8:57
  • $\begingroup$ Stand between two mattresses, it is close to that. Using ear protectors tends to make you hear sounds from inside your head and neck via the Eustachian tube (breathing, etc). $\endgroup$ – user95006 Apr 5 '16 at 18:03
11
$\begingroup$

Whispering rooms/galleries are another good example. In the simplest case, an elliptical room, sound echoing off the walls allows one person standing at one focus of the ellipse to clearly hear everything at the other focus. Step away from the focus, though, and the effect disappears.

$\endgroup$
8
$\begingroup$

Some blind people actually use sonar-like techniques to "see". This is in the press from time to time, e.g. http://www.sciencemag.org/news/2014/11/how-blind-people-use-batlike-sonar.

$\endgroup$
  • $\begingroup$ Note that it's not limited to blind people and that it can be taught. You can learn to walk with your eyes closed. There's an organisation called the "World Access for the Blind" that offers classes on echolocation. $\endgroup$ – slebetman Apr 11 '16 at 7:41
4
$\begingroup$

Some good answers here but one area that has not been covered is a sort of threshold effect. An ear senses sound pressure, not "rays" of sound. So it responds mainly to the strongest signals, and the brain tends to tune out noise. We attend to one signal in sound, generally speaking. But for vision, all that "noise" of light bouncing around off of objects IS what we see. We see ALL the distinct reflections simultaneously with the many pixels of the retina. An analogy for light would be to cover your eyes with pieces of Styrofoam cup: you could only see how bight it was, not which direction the light was coming from.

Hearing is more like receiving radio waves, where the strongest signal "captures" the receiver. This is the only way it can work when the receiving element has only one "pixel" (whether wide or narrow).

So, the answer is that eyes are Two Dimensional receivers, and ears receive a one-dimensional stream, at least as far as spatial resolution is concerned. (Ears have resolution for pitch and phase, but that is not relevant to echoes.)

$\endgroup$
  • $\begingroup$ Blind people who've trained themselves echolocation describes having 360 degree "vision" and can see individual objects in a room. So it's not exactly one pixel. But it is very-very low resolution compared to vision. $\endgroup$ – slebetman Apr 11 '16 at 7:45
2
$\begingroup$

It is because of our reaction time ,speed of sound and disturbance in atmosphere. As our reaction time is 1/10th of a second and speed of sound is about 343.2 metres per second. So sound travels 34.32 metres in 1/10th of a second. To distinguish between 2 sounds the distance between them must be 34.32 metres and if we want to notice reflection of sound(echo) then we must count the distance back and forth i.e 34.32metres and half of it would be 17.16 metres. So to distinguish between original sound and its reflection,the distance between person and reflecting surface must be at least 17.16 metres ,but again sound loses its energy on travelling and in an urban area where so many noises are disturbing the medium of travel..it becomes very difficult to notice such phenomenon. Though we notice it everyday in form of reverbs created in day to day places,but they are hardly distinguishable.

$\endgroup$
  • 4
    $\begingroup$ Reaction time and resolution of hearing perception are two different things despite being measured in units of time. E.g: you can easily "feel" milliseconds of discrepancy between sound and vision when watching a movie. I would assume that left-right channel would be close to this as well. $\endgroup$ – luk32 Apr 5 '16 at 15:47
  • $\begingroup$ Phase matters too. The ears are about a half wave apart at 1000 Hz. That is part of the reason our sound sensitivity is so good there: we can use phase to determine direction in space quite well. A half a millisecond differential in two ear "signals" is definitely informative for nerve cell response. $\endgroup$ – user95006 Apr 5 '16 at 18:38
2
$\begingroup$

Why don't we hear sound reflecting that much?

Mainly, it has to do with energy and wavelength.

When light hits an object with rough surfaces such as a building, it is scattered in every direction. Because the light illuminating the building has such a high energy, your eye is able to pick up on just a tiny fraction of that scattered light, in spite of the inverse square law for intensity. Moreover, your eye focuses that light into a clear image, which your brain recognizes as a building.

Now sound is different. Firstly, there is no great source of sound in the sky illuminating everything with noise. The ambient sounds you hear are actually fairly low energy. If a building scattered sound in every direction the way it scatters light, you would simply not hear echoes at a distance.

However, here is the thing: sound has much longer wavelengths than visible light. When the wall of a building reflects sound, even though that wall may appear rough to the naked eye (concrete or brick or whatever), from the perspective of sound, that building is a specular surface which reflects sound in a mirror-like fashion.

The reason you can hear an echo bouncing from a building wall is precisely because it does not scatter the sound, but reflects it like a mirror. Consider that the wavelength of sound at 10 kHz in air is about 3.4 cm To scatter 10 kHz sound, a fairly high frequency, you need bumps which are in that ballpark: say, at least about half that size. Buildings are usually smoother than this. And scattering lower frequencies needs even larger bumps: 34 cm for 1 kHz, 340 cm for 100 Hz.

Now imagine that the building were actually constructed of mirrors: perfectly smooth mirrors with no visible seams or joints. You would then not see it very well! You would see images reflected in the building, but the building itself would be hard to make out, especially if its walls were perfectly vertical, so that the sky and horizon reflection appeared continuous with the real horizon.

So that's basically the primary reason you don't hear a building well: it reflects sound in a specular way, and so you hear sound sources which are being reflected, and not the building itself. Moreover, since the sound isn't scattered, for you to hear these reflected sounds, you have to be standing at the correct angle of reflection, relative to the angle of incidence. You simply don't hear everything that is hitting the building: only those sound sources which hit the wall at the opposite angle to your vantage point, the same way that you don't see every light source in a mirror.

Secondly, your ears are not very selective for direction, and so the reflections you do hear are mixed with other reflections and the direct sound. The more complex this mixture is, the harder it is to determine where everything is coming from. Complex, numerous echoes just create an ambient sense which we call reverberation. Usually, when echoes are isolated, we can guess what is reflecting them: "Aha, a second clap came from the direction of that building; so it must be that it's reflecting from its wall".

Thirdly, related to the second point, your ears do not focus sound into an image. If you had an optical-like focusing mechanism for sound (say, an acoustic dish), forming an image of a building based on ambient sound striking that building would require low-wavelength (high frequency) sound of great intensity, and the building would have to have a very rough surface, whose features are larger than the wavelength, so that it scatters that sound. Recall that at 1 kHz, the wavelength is 34 cm, so surface irregularities much smaller than 34 cm basically look like a smooth surface to 1 kHz sound.

$\endgroup$
2
$\begingroup$

This is purely a matter of human perception.

Our eyes contain literally millions of separate photoreceptors, which enables us to build up a clear picture of our surroundings. On the other hand we have only two ears!

Admittedly each ear is able to differentiate a wide range of frequencies, and in that way has a superiority over the eye, which can only detect three separate frequency bands (which we perceive as the three primary colours) but that really doesn't compensate for the amazing imaging power of our eyes.

Other animals, for example bats and certain whales, hunt by echolocation. They make noises and and listen for the echoes that come back. you too, if blindfolded, can clap your hands and if you hear an echo, you would deduce that you are not in an open space, but must be near a wall or in a cave. It's just that the sense isn't nearly as well developed in humans.

So, sound does reflect, all the time. But there is one important caveat. Sound travels at about 330m/s, so a sound of 1kHz (about the optimum range for our hearing) has a wavelength of about a foot (30cm.) To get a good, clean image the wavelength of whatever medium must be several times smaller than the object it is reflecting off.

Humans have developed imaging technologies based on sound. But they don't tend to use frequencies of 1kHz. For small objects, they use much higher frequencies with shorter wavelengths. This makes it possible to see, for example, a clear picture of a baby in its mother's womb: https://en.wikipedia.org/wiki/Ultrasound. Engineers also use ultrasound to detect cracks in metal.

There are also SONAR techniques used in water, often with lower frequencies, for mapping much larger areas. Low frequencies are better in this application, as they are less attenuated by water, and less confused by random reflections off small objects. They can be used by trawlers to find fish, or by warships to find enemy submarines. They can even be used by oil prospectors to find deposits of oil deep under the ocean floor. This image of a shipwreck shows what can be achieved with the right equipment, and time: http://phys.org/news/2010-09-sonar-historic-shipwreck-poses-oil.html

$\endgroup$
0
$\begingroup$

As Ville explains, this is because of the way we perceive sound - how our brains work. If you stand at one end of a room having bare hard walls and clap your hands, you may be able to notice the echo. Under normal "room temperature" conditions, sound travels at about 1,100 ft/sec; if you try this at one end of a 40ft room, the echo will return in about 70ms, taking just a little longer than the threshold at which your brain will perceive it as a distinct event.

The reasons we don't routinely notice the echos which are always present are:

  • The surface is too close; we perceive the echo as part of the same event as the original sound (Ville's answer)
  • The surface is too far away - sound waves propagate radially outward, so intensity diminishes with the square of the distance; the reflected sound intensity will diminish at the fourth power of distance between source and reflector. It doesn't take much distance for an echo to fall below either the threshold of perception or the level of ambient noise.
  • The echo is perceptible but we subconsciously filter it out because it conveys no useful information to us.
$\endgroup$

protected by Qmechanic Apr 5 '16 at 19:56

Thank you for your interest in this question. Because it has attracted low-quality or spam answers that had to be removed, posting an answer now requires 10 reputation on this site (the association bonus does not count).

Would you like to answer one of these unanswered questions instead?

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