# The Naked Scientists Forum

### Author Topic: Can you hear the flies?  (Read 4731 times)

#### blakestyger

• Guest
##### Can you hear the flies?
« on: 23/06/2008 17:05:18 »
There is an ancient woodland in Cambridgeshire called Hayley Wood and a visit to it is memorable because some distance from it is a maggot farm (the buildings can be made out across the fields, but there is a pervasive smell of decay.
Last time I went, about 5 years ago, I remember the smell but can't recall whether I could hear the flies buzzing or not. You certainly couldn't hear one fly at that distance but if there were millions of them would they be audible? In other words, are decibels at a low level detectable when multiplied at the same level?

#### RD

• Neilep Level Member
• Posts: 8089
• Thanked: 51 times
##### Can you hear the flies?
« Reply #1 on: 23/06/2008 23:29:41 »
The sound waves would add together...
Quote
Superposition of Waves
The principle of superposition may be applied to waves whenever two (or more) waves travelling through the same medium at the same time... The net displacement of the medium at any point in space or time, is simply the sum of the individual wave displacements. This is true of waves which are finite in length (wave pulses) or which are continuous sine waves.
http://www.gmi.edu/~drussell/Demos/superposition/superposition.html

But the sound intensity would fall off rapidly from as you moved from the maggot farm, according to the inverse square law.

http://hyperphysics.phy-astr.gsu.edu/hbase/Acoustic/invsqs.html
« Last Edit: 23/06/2008 23:38:29 by RD »

#### lyner

• Guest
##### Can you hear the flies?
« Reply #2 on: 24/06/2008 09:14:42 »
You need to remember that the Inverse Square Law only applies to a Point Source. If the source is extended - and a Fly Farm would need to be bigger and bigger to support a large number of flies - the loudness would not necessarily drop off at a rate following than ISL.

#### Bored chemist

• Neilep Level Member
• Posts: 8599
• Thanked: 41 times
##### Can you hear the flies?
« Reply #3 on: 24/06/2008 19:19:06 »
If the swarm looks small it probably acts like a point source of sound.
As a  rough guess I can hear a fly 3 metres away. The inverse square law tells me I could hear 4 flys 6 metres away and a million flies 3000 metres away (provided there's no walls or whatever in the way.
For 5 miles - about 8Km you would need about (8000/3)squared flies- something like 6 million flies.
The next question is how many flies were there and how far away were you?

#### lyner

• Guest
##### Can you hear the flies?
« Reply #4 on: 24/06/2008 22:53:31 »
Quote
If the swarm looks small it probably acts like a point source of sound.
Only a spherical swarm, I think. And flies don't actually swarm, not being social animals like bees; they will probably spread out into a cloud.
In any case, the Inverse Square Law is often quoted where it doesn't apply. I was just being pedantic.

#### Bored chemist

• Neilep Level Member
• Posts: 8599
• Thanked: 41 times
##### Can you hear the flies?
« Reply #5 on: 25/06/2008 07:07:34 »
From a distance it will act like a point source located at the COG of the swarm, apart from "self absorbtion" effects.

#### lyner

• Guest
##### Can you hear the flies?
« Reply #6 on: 25/06/2008 11:30:41 »
From a distance it will act like a point source located at the COG of the swarm, apart from "self absorbtion" effects.

That is too simplistic. Take an extreme case to show why.
Imagine the swarm was, in fact, two swarms, separated by several metres. If you were much nearer one swarm than the other, the sound level would be much higher than if all the flies were at the CM of the two swarms and would seem to come from the nearer swarm.

A similar argument applies to gravitational fields / potential of non-spherical objects. If you are near the Moon, you will fall 'down' towards the Moon and not to the CM of Earth and Moon, combined, which is actually below the Earth's surface.

The only issue is what you can describe as 'at a distance'. In the case of the fly farm, I suggest that, to hear them at all, you would need to be relatively near, so you would be in 'near-field' conditions because they are, necessarily, spread out in order to buzz at all.

Having read the original question again, the point about dB adding up is correct apart from BC's point about flies on one side absorbing some of the sound from flies on the remote side.
Ten times as many flies - 10dB increase in sound energy - less a bit for absorption.

#### LeeE

• Neilep Level Member
• Posts: 3382
##### Can you hear the flies?
« Reply #7 on: 25/06/2008 14:12:24 »
Lots of maggots don't necessarily mean lots of flies - it seems that house flies typically lay several batches of 75-150 eggs over the course of several days, totalling up to 500 eggs per fly.

So roughly speaking, and with a fudge factor, the number of flies is only likely to be around 0.3-0.4% of the number of maggots.

Still quite a lot of flies though, I guess.

#### Bored chemist

• Neilep Level Member
• Posts: 8599
• Thanked: 41 times
##### Can you hear the flies?
« Reply #8 on: 25/06/2008 18:05:44 »
This clause "If you were much nearer one swarm than the other" is inconsistent with this one "From a distance "
I wasn't kidding when I put that requirement in.
The original question says "the buildings can be made out across the fields,"
I can't see how that can be anything other than "From a distance".

This is a bit academic unless someone has a handle on the number of flies. (And due credit to LeeE for at least considering it.)
The walls of the building will make a difference too but I think typical attenuation figures for walls can probably be found on the web.
Another point that no one has looked at is the attenuation of sound as it travels through air. The effect isn't big but it will reduce the sound level and also the timbre of the sound because the attenuation is frequency dependent. The details of the sound attenuation also depend on temperature, pressure and relative humidity. There's every chance that, even on a still day, the audibility of the swarm would depend on the weather.
Of course, if the wind blows the sound towards you it will increase the audibility and if there's a reflective wall behind the swarm it will increase the apparent noisiness of the swarm. Other geographical features might make more difference still (though the original question implies line-of -site transmission will dominate).

It might be easier to go there and listen. :-)

#### blakestyger

• Guest
##### Can you hear the flies?
« Reply #9 on: 25/06/2008 19:18:11 »
Thanks - a good discussion. I tried to find the 'phone number of the maggot farm, so I could ask how many flies they had, but couldn't.
A fishing tackle shop owner told me today that the farms get a lot of hassle because of the smell so they keep their heads down; He had once been to one in Essex but said he hadn't heard any flies.
So it's just another false memory I guess.

#### Alan McDougall

• Neilep Level Member
• Posts: 1285
• Thanked: 14 times
##### Can you hear the flies?
« Reply #10 on: 25/06/2008 20:37:19 »
Yes more flies more noise

#### lyner

• Guest
##### Can you hear the flies?
« Reply #11 on: 25/06/2008 21:59:28 »
Quote
Of course, if the wind blows the sound towards you
Oh BC! How could you?
The wind doesn't 'blow' sound - the incremental velocity is tiny. It's a refraction effect due to the velocity gradient from zero on the ground to full speed at some height. It tends to bend sound downwards when you are downwind and up when you are upwind.
Also, the buildings can be distinguished as buildings - not points on the horizon.

We are nit - picking, tho', I think!
Good sport.

#### The Naked Scientists Forum

##### Can you hear the flies?
« Reply #11 on: 25/06/2008 21:59:28 »