Odd acoustic phenomena

[MarkOne]

You may have seen my thread in the lounge about the live event that happened just upon the road from me.

There seemed to be something really odd about the acoustics and I'm at a loss to explain it.

So the venue is a repurposed early 20th century lido (The pool area filled in and now used for events like ice skating in the winter, a big funfair and music festivals through the summer.) The whole area (about 100m square) is surrounded by a big stone wall, around 4m high. The south side - the one we face - has a building down its length and the steep sided roof is probably another 3m high.

From what I could see of the images posted online the festival stage was, your average truss built affair topped with an arched awning with flown arrays left and right and some centre fills, and what looked like subs under the stage.

From our living room, around 250m away the mid frequencies were clear, but the bass was pretty non existent.

But here's the thing. A couple of people I know could clearly hear the bass, from hillside locations one and two miles away.

I'd have thought the bass being more or less omnidirectional shouldn't have been blocked by the structure for me. So I don't really understand what was going on.

Thoughts

[Hugh Robjohns]

They could have been running a cardioid configuration for the subs, or the wall and building might have been large enough to provide some shadowing effect?

[James Perrett]

I've experienced a somewhat similar phenomenon on Castle Field in Southsea a few years ago. This is a field surrounded on three sides by (at a guess) 8 metre high banks. The other side is the sea.

I could hear the midrange clearly from about a mile away and continued to hear it as we approached. When we got inside the field there was almost no midrange to be heard - just bass and high frequencies. It was almost as if the midrange was being beamed over the top of the banks.

This was a regular event and I don't remember this phenomenon being quite so apparent at other times so maybe the sound people had done something different for that gig.

[Mike Stranks]

... and don't discount meteorological effects...

... Glasto had to wind levels way back a few years ago one evening (Saturday?) because of distant volumes being way above what is permitted. That was put down to the atmospheric conditions...

[James Perrett]

That was put down to the atmospheric conditions...

I wonder if geology matters too? I used to live a few miles from where Boomtown is held. The sound from the festival was much louder inside the house than outside - particularly the low bass from one particular morning soundcheck .

[RichardT]

Could it be a diffraction effect around the barriers - both sites had high walls?

[jb1t]

Hi, the most likely explanation for this is Refraction, a bending of a wave due to changes in a) density of a medium through which soundwaves travel or b) changes in temperature.

Sound is drawn towards cool air so out-doors during the day it generally rises upwards towards the cooler air as the ground is warmer.
At night this usually inverts with the ground colder and the air warmer sound may initially be headed at 45 degrees or more upward but it is soon pulled downward towards the colder ground. This can 'pull' sound over to the opposite side of hills covering many miles.
E.g. T in the Park at Balado heard mid-range clearly in Dunning over the Ochil Hills for 8 miles/ 12km