I If I clap once to a wall, and then walk forward, will I hear an echo?

[phantomvommand]

This is a diagram:
------------ reflective wall
X --> Y

I stand at position X and clap towards a reflective wall. Theoretically, could I walk to some point Y, where there is constructive interference between my clap and the reflected clap, resulting in my hearing a much louder sound. Is this possible?

 

[tech99]

The clap is just a pulse of sound, so the returning echo has nothing to interfere with when it gets back to you. The echo will not be enhanced by changing position.

 

[phantomvommand]

The clap is just a pulse of sound, so the returning echo has nothing to interfere with when it gets back to you. The echo will not be enhanced by changing position.

If the pulse were sent in all directions, can’t I say there is a horizontal direction and another towards the wall, which can interfere to form an echo?
Or do you mean that pulses fundamentally cannot interfere? May I know why not? Thanks so much

 

[tech99]

In order to distinguish an echo, the pulse must be short. So the original pulse has left you when the echo reaches you. So no interference.

 

[Dale]

If you want interference like this then you need to broadcast a continuous tone, not a brief pulse of sound.

 

[hutchphd]

And if you listen to modulation of the two tones (outgoing vs. reflected) it will tell you how fast you are walking at the wall!

 

[berkeman]

If the pulse were sent in all directions, can’t I say there is a horizontal direction and another towards the wall, which can interfere to form an echo?
Or do you mean that pulses fundamentally cannot interfere? May I know why not? Thanks so much

The problem with multipath in this situation is as others have said, it is an impulse. With just one wall, the path length to the wall and back will form two sides of a triangle, and the distance between your clap position and your "listening" position will be the 3rd side of the triangle. And we know that 2 sides of a triangle have to be a longer total distance than the 3rd, right?

Instead, ask what happens if you are between two parallel walls, and set up a microphone a distance away from you along a path that is also parallel to the two walls. When you clap you will hear a first clap from the microphone when your clap sound makes it there on the direct path. Then a little later you will hear the result of the two reflected claps arriving at the microphone. What do you think that 2nd event will sound like? Do you think you can fine-tune some positions to get different interference effects? It might be fun to try.

Also look at the sound pressure plots from claps to understand what the propagating waveform looks like...

https://www.acoustics.asn.au/journal/2013/2013_41_2_Fletcher_paper.pdf

Also, for a very fun experiment, try to find a long tube about 50 meters long that is closed at the other end and is about a meter in diameter. When you clap into the open tube end, you get a very surprising echo a short time later. I first encountered this at the Exploratorium in San Francisco a number of years ago.

Edit/Update -- Here is the Exploratorium exhibit I was referring to. They recently moved the Exploratorium so now the exhibit looks like it's outdoors:

https://www.exploratorium.edu/exhibits/echo-tubes

 

[davenn]

And if you listen to modulation of the two tones (outgoing vs. reflected) it will tell you how fast you are walking at the wall!

but he isn't walking "at the wall" he's walking parallel to the wall X ----> Y

Theoretically, could I walk to some point Y,

Depending on distance between you and the wall and the distance between X and Y, the reflected, the reflected sound is likely to have propagated out past Y before you get there ... speed of sound is fast ! ~ 340 metres/sec (@15deg C)

so how about redefining your diagram with some real numbers

 

[PeroK]

I first encountered this at the Exploratorium in San Francisco a number of years ago.

I always associate San Francisco with free love, drugs and rock and roll. It seems you can discover physics there too!

 

[A.T.]

This is a diagram:
------------ reflective wall
X --> Y

I stand at position X and clap towards a reflective wall. Theoretically, could I walk to some point Y, where there is constructive interference between my clap and the reflected clap, resulting in my hearing a much louder sound. Is this possible?

It's possible with a curved wall.

 

[phantomvommand]

but he isn't walking "at the wall" he's walking parallel to the wall X ----> Y
Depending on distance between you and the wall and the distance between X and Y, the reflected, the reflected sound is likely to have propagated out past Y before you get there ... speed of sound is fast ! ~ 340 metres/sec (@15deg C)

so how about redefining your diagram with some real numbers

Even if I got to point Y somehow, the 2 pulses will reach at different times, so I don't suppose I'll get interference...

 

[phantomvommand]

It's possible with a curved wall.

How so? The 2 pulses will never interfere since they will never reach at the same time due to different path lengths

 

[tech99]

If we use a pulse we create a traveling wave and there is a flow of energy and information and the distance can be found. But with a continuous tone, the outward and return signals create a standing wave and then there is no flow of energy or information. That is why this acoustic radar cannot ascertain distance if we use a continuous wave and cannot exploit standing waves if we employ a pulse.

 

[hutchphd]

but he isn't walking "at the wall" he's walking parallel to the wall X ----> Y

This is a diagram:
------------ reflective wall
X --> Y

I stand at position X and clap towards a reflective wall. Theoretically, could I walk to some point Y, where there is constructive interference between my clap and the reflected clap, resulting in my hearing a much louder sound. Is this possible?

Wow, that's interesting.
I look at that diagram and assume that the dotted line represents his path towards the wall from position X to position Y. You may well be correct but only the OP knows for sure!

 

[hutchphd]

Also the whispering gallery should be mentioned. The Mormon Temple in Salt Lake is one example of this use of an ellipsoid. The architects at Cornell used to host a Spring Festival on t he quad...somebody built a big semi-ellipse positive pressure inflatable to demonstrate...it was quite remarkable and apparently unforgettable.

 

[berkeman]

Also the whispering gallery should be mentioned. The Mormon Temple in Salt Lake is one example of this use of an ellipsoid. The architects at Cornell used to host a Spring Festival on t he quad...somebody built a big semi-ellipse positive pressure inflatable to demonstrate...it was quite remarkable and apparently unforgettable.

Yeah, there was another exhibit at the Exploratorium like that. Two large reflecting bowls (each about 3 meters tall) placed about 30 meters apart. If you sat in the well-placed chairs at the foci, you could talk to your partner at the other station with just whispers. Pretty cool.

Edit -- Looks like they've moved that exhibit outdoors (they moved the Exploratorium a year or two ago to one of the piers by the Bay):

https://www.exploratorium.edu/exhibits/archimedes

 

[A.T.]

How so? The 2 pulses will never interfere since they will never reach at the same time due to different path lengths

 

[phantomvommand]

View attachment 288137

Yes, thanks. I expect that for the ellipse one would hear 1 pulse first due to the direct transmission from focus 1 to 2 along a horizontal straight line without any reflection, and then a second echo that is much louder due to the constructive interference from all the pulses reflecting off the ellipse?

 

[anorlunda]

It sounds like you think of these pulses as packets, like projectiles. If you think of the source and the echo in terms of spreading and interfering wave fronts, does that make it clearer?

 

[A.T.]

Yes, thanks. I expect that for the ellipse one would hear 1 pulse first due to the direct transmission from focus 1 to 2 along a horizontal straight line without any reflection, and then a second echo that is much louder due to the constructive interference from all the pulses reflecting off the ellipse?

Yes.

 

[A.T.]

Just for fun. The reflections in an ellipse starting midway between the focus points:

 

[A.T.]

I was once in the center of a radome, like the one below. It's a bizarre experience when you talk, and hear a slightly delayed very loud echo, that also repeats. You should be able to reproduce this with a mic, loudspeakers and some delay in between.