[bigshot]

I was reading a book of short stories by Ambrose Bierce today and it included a ghost story about a phantom Civil War regiment. It mentioned the phenomenon called "acoustic shadow"- a battle that was audible in the distance 8 miles away in one direction, was inaudible in another direction a half mile away. It's an interesting idea, and I wondered if there are places in the world that are known for this.

I know that sound can be focused on a specific spot. When I visited Washington DC once, I took a tour of the Capitol building and under the dome, there was a spot where you could hear whispers from many yards away at the other side of the dome. And I remember a demonstration of a pin dropping at the Mormon Tabernacle auditorium. But I've never heard of acoustic shadows. Does anyone know of any places like this?

 

[gregorio]

But I've never heard of acoustic shadows. Does anyone know of any places like this?

Yes, almost any place on the planet!

Sound is pressure waves travelling through the air, so anything that affects the movement of air, such as wind or other meteorological effects or anything that can impede the transmission of pressure waves, such as buildings, walls or topographical features, can all create acoustic shadows.

G

 

[megabigeye]

Where I grew up in West Virginia there was a similar (the same?) phenomenon: we could hear a train coming several miles up the valley at our home, but it could not be heard much closer / lower in the valley. The hills created a sort of natural amphitheater that projected the sounds in certain directions but not others. As I recall, how loud it was depended on the wind and weather and the time of year.

 

[Ghoostknight]

isnt it the same with speakers to some degree? on my nearfield setup volume subjectively doubles if i sit in the sweetspot vs to the side, pretty sure this can be classified as accoustic shadowing too (i guess unless you have a reverb heavy room)

the reason is of course "waveguiding" and i guess this can also be replicated on a much greater scale with basses etc

 

[gregorio]

isnt it the same with speakers to some degree?

Depends, how many hills, valleys and how much wind and weather is there between you and your speakers?

on my nearfield setup volume subjectively doubles if i sit in the sweetspot vs to the side

Then you’ve got very seriously screwed up acoustics in your room!

G

 

[gregorio]

As I recall, how loud it was depended on the wind and weather and the time of year.

You can get acoustic shadowing, even in the same direction and without any topographical features. Certain meteorological conditions (temperature inversions for example) can allow sound to propagate long distances through fairly high altitude “refraction channels”, return to earth and be audible, whist the sound is inaudible at locations closer to the sound source in the same direction. This phenomena has been observed on numerous occasions, going back nearly 400 years, although it’s not particularly common.

G

 

[megabigeye]

You can get acoustic shadowing, even in the same direction and without any topographical features. Certain meteorological conditions (temperature inversions for example) can allow sound to propagate long distances through fairly high altitude “refraction channels”, return to earth and be audible, whist the sound is inaudible at locations closer to the sound source in the same direction. This phenomena has been observed on numerous occasions, going back nearly 400 years, although it’s not particularly common.

G

Cool! So, if I understand, sound travels more easily through cooler air; so during a temperature inversion sound can follow the cool air, traveling from one location on the ground, up to a higher altitude, and then back down to the ground in another location? Bypassing some of the intermediate places on the ground? Mother Nature can do some wild and wonderful stuff!!

 

[gregorio]

So, if I understand, sound travels more easily through cooler air; so during a temperature inversion sound can follow the cool air, traveling from one location on the ground, up to a higher altitude, and then back down to the ground in another location?

From what I know, it’s a bit more complex than that. You need a specific set of air temperature, density and humidity conditions forming a band (refraction channel) at some, presumably not too high, altitude. A sound source occurs, maybe an artillery battle for example and the sound waves propagate spherically (mostly upwards and outwards though). Those which travel horizontally are eventually absorbed by air (the inverse square law) but some of the sound which travels upwards gets trapped in that band/“refraction channel” (a bit similar to sound travelling down a pipe) and can propagate far further than those at ground level, until getting refracted back to the ground again at some other more distant location. I believe a similar effect can occur with light btw (mirages).

Independent verified accounts exist of sound travelling around 40 miles further via this process than across the ground. Although again, you need an exceptionally loud sound source (like a cannon battle) and very rare meteorological conditions.

That’s about as far as my knowledge of this phenomena extends I’m afraid and I’m happy to be corrected if anyone knows the details more accurately.

G

 

[bigshot]

Where I grew up in West Virginia there was a similar (the same?) phenomenon: we could hear a train coming several miles up the valley at our home, but it could not be heard much closer / lower in the valley. The hills created a sort of natural amphitheater that projected the sounds in certain directions but not others. As I recall, how loud it was depended on the wind and weather and the time of year.

That’s neat. It would be interesting if there were acoustic tourist attractions… places you could go that would have unexpected echoes or transmission of sound over remarkable distances. I bet there are conditions where sound directionality bends like a funhouse mirror too… or perhaps ones that shift pitch.

It seems that acoustic shadows were common in the civil war. Battles were lost because the generals didn’t hear them coming.

 

[megabigeye]

That’s neat. It would be interesting if there were acoustic tourist attractions… places you could go that would have unexpected echoes or transmission of sound over remarkable distances. I bet there are conditions where sound directionality bends like a funhouse mirror too… or perhaps ones that shift pitch.

It seems that acoustic shadows were common in the civil war. Battles were lost because the generals didn’t hear them coming.

There's the Ear of Dionysius, but I think there are other caves that also have whispering galleries. I vaguely recall going to one once upon a time. I really can't remember where, though. Maybe Wieliczka Salt Mine in Poland? There's a chapel in Wieliczka, so maybe that's what I'm thinking of. The acoustics certainly aren't the main attraction, though!

 

[gregorio]

It would be interesting if there were acoustic tourist attractions… places you could go that would have unexpected echoes or transmission of sound over remarkable distances.

There are, quite a few in fact!

There's the Ear of Dionysius, but I think there are other caves that also have whispering galleries.

There are quite a number of whispering galleries around the world. Arguably the most famous is the Whispering Gallery in St. Paul’s Cathedral (London) where the effect was studied (and explained) in the late 1800’s by Lord Rayleigh, one of the founders of modern acoustics and psychoacoustics. But there are dozens around the world and not an uncommon attraction in some science museums.

The basic theory is based on “Whispering Gallery Waves (or Modes)” - Named after Rayleigh’s discovery in St. Paul’s; A type of wave created by a curving reflective surface where the sound intensity near that surface decays by the inverse of distance rather than by the inverse square of distance (in a free field). So, someone whispering near that surface can be heard 30 meters away by someone else near that same surface, while someone at half that distance (but substantially away from the wall) would hear nothing, as the sound intensity would have decayed below audibility. This phenomena can also apply to light and certain other EM waves.

G

 

[Ghoostknight]

Then you’ve got very seriously screwed up acoustics in your room!

yea its called "treated" and "nearfield monitors"

 

[gregorio]

yea its called "treated" and "nearfield monitors"

Or to be more precise, “very badly treated”, to the point of completely wasting your time/money and very poor/inappropriate near field monitors.

Obviously, there should not be half the perceived sound level if you’re just sitting to the side if your sweet spot, unless by sitting “to the side” you mean roughly three times the distance from your speakers. In which case, that obviously isn’t the same effect as discussed, it’s just the natural reduction of level roughly in line with the inverse square law.

G

 

[Ghoostknight]

Or to be more precise, “very badly treated”, to the point of completely wasting your time/money and very poor near field monitors.

of course you know it, lol

 

[gregorio]

of course you know it, lol

What’s shocking is that you’ve presumably spent time/money on that treatment and don’t know it, lol!!

G