Inverse Square Law

[RayDunzl]

Imagine a speaker radiating omnidirectionally.

You probably don't have to get too far away from it before you can barely hear it.

It obviously dims out as you move away.

Everyone agree so far?

Why can you still see it well?

Why doesn't it's image dim like its sound? Aren't both following the Inverse Square Law?

What is the difference between its radiated sound and image in this sense?

It does get smaller... But I don't seem to observe the same range of dimming.

What's the deal?

 

[amirm]

The Square Distance Rule only applies to the direct sound. Reflected sounds after certain distance dominate and you can keep moving further and no drop occurs in sound level in a closed space. This is called Critical Distance or Dc for short.

You can see this effect in a set of measurements I made in a rather large space:

This is a log scale so you expect the drop off from left to right be a straight line as is the case for part of the distance in blue. Once we hit the critical distance (noted with "1.0" on that graph), the drop off slows down and the curve flattens more or less.

Light follows the same rule. If you have highly diffused lighting, it also won't drop off as rapidly after a while. Of course what reflects light and sound are different things.

This is how much I can pretend to know about this topic .

 

[DonH56]

Because we can see better than we can hear?

The hand-waving explanation is that the speaker is not the light source but rather illuminated by a light source, then light is reflected back for you to see. The absorption of light energy through the atmosphere is drastically different than the absorption of acoustic energy. You can see a candle flickering from a long, long way aways on a dark night because light attenuation in air is relatively low compared to sound attenuation. (The absorption mechanisms are also somewhat different.) You could also calculate relative energy levels from the speaker and the light source, then factor in the sensitivities of our eyes vs. our ears, but I'll leave that to a scientist...

This did make me think of an old quote from Dr. Joyce Brothers (IIRC): "The reason most women would rather be beautiful than smart is because most men can see better than they can think!"

 

[fas42]

Sound energy does carry a long way, but it has to be 'authentic' in the first place. A few days ago we had Anzac day, a local memorial day to the fallen in war, and bagpipes are everywhere. I went outside in my small township, and someone, probably quarter of a mile away, was playing this instrument. Clear as a bell, no dimming, subjectively, at all! Of course, the source sound has to be intense - which is where audio systems usually fall flat on their faces - raucous, brazen, beating you into a pulp are the phrases usually thought of when the volume goes up ... we still have some way to go, ...

 

[March Audio]

Sound energy does carry a long way, but it has to be 'authentic' in the first place. A few days ago we had Anzac day, a local memorial day to the fallen in war, and bagpipes are everywhere. I went outside in my small township, and someone, probably quarter of a mile away, was playing this instrument. Clear as a bell, no dimming, subjectively, at all! Of course, the source sound has to be intense - which is where audio systems usually fall flat on their faces - raucous, brazen, beating you into a pulp are the phrases usually thought of when the volume goes up ... we still have some way to go, ...

This is TOTALLY incorrect. The instrument at 1/4 mile will sound very different. a very quick search will provide lots of info on this. I found this.

http://www.sfu.ca/sonic-studio/handbook/Sound_Propagation.html

 

[fas42]

Of course it will sound different, but the sound signature will strike one as being correct! This was in response to the OP, where it was said

You probably don't have to get too far away from it before you can barely hear it.

We'll have a competition, a set of bag pipes, and a recording of the same: play the recording at nominally the same volume as the real instrument; and have a listen 1/4 mile away - which will be the winner?

 

[iridium]

Because we can see better than we can hear?

The hand-waving explanation is that the speaker is not the light source but rather illuminated by a light source, then light is reflected back for you to see. The absorption of light energy through the atmosphere is drastically different than the absorption of acoustic energy. You can see a candle flickering from a long, long way aways on a dark night because light attenuation in air is relatively low compared to sound attenuation. (The absorption mechanisms are also somewhat different.) You could also calculate relative energy levels from the speaker and the light source, then factor in the sensitivities of our eyes vs. our ears, but I'll leave that to a scientist...

This did make me think of an old quote from Dr. Joyce Brothers (IIRC): "The reason most women would rather be beautiful than smart is because most men can see better than they can think!"

DonH56 wrote: "The reason most women would rather be beautiful than smart is because most men can see better than they can think!"
So TRUE; it must be a universal law!
iridium.