How do owls fly silently?

08 December 2014

Interview with

Prof Nigel Peake, University of Cambridge

Owls fly nearly noiselessly to avoid alerting prey to their approach. How they do this is the subject of a research project by applied mathematician Nigel Peake, Great Grey Owl by Paul Lewinfrom the Department of Applied Maths and Theoretical Physics at Cambridge University.

Nigel -   I'm a mathematician and I'm interested in trying to predict how much noise is produced by things like aircraft and wind turbines because if you can predict that then maybe you can design them to make them quieter. 

But what I want to talk about tonight is something a little bit different and that's about owls.  It's been known probably for centuries that there are some owls that can fly amazingly quietly.  So, these are big owls like barn owls or great grey owls.  The question is, how do they manage to do that because if you can understand that, then maybe we can do something about aircraft and wind turbines.

Chris -   We heard from Rob at the beginning what sound actually is - its vibrations in the air, but how does the sound vibrations that come from something we do like the sound of or something that's not really very disturbing differ from something which is very distracting or disturbing like an airplane?  What's going on that's different?

Nigel -   Okay, well one of the things is that aircraft noise, annoying noise, has got a very, very broad range of frequencies.  And so, if things aren't in tune, it's producing sound at a whole range of frequencies.  And that's often because the flow is very turbulent, the flow is very complicated and that means that it produces frequencies all over the place.

Chris -   Can you show us through that?

Nigel -   I certainly can.  So, what I have here - a very simple thing - is a canister of compressed air that you have to clean your computer keyboard and it's got a very thin sort of long pipe on the end.  And if I squeeze this, and you can hear there's a hissing noise.  Most of that noise is quite complicated, most of that noise is coming from the turbulent flow that's coming out of the pipe.  This jet is quite low speed so, it's not particularly noisy.  But you can imagine if you've got your aircraft taking off from Heathrow, the jet is far, far faster maybe it's going, close to the speed of sound, 330 meters a second, something like that, and that is really, really noisy.  Now, if I do something else, if I have a - I've got here a ruler which has got a sharp edge and I've got this quiet jet and now, I move the sharp edge into the jet, then what you'll hear is, I get a louder noise.  What's going on there is that although the turbulence by itself isn't very good at producing noise, as soon as you put a sharp edge into it, the sharp edge is very good at then converting the turbulence into the very fast soundwaves.

Chris -   We heard the noise get to a maximum when that ruler edge that you're using was directly in line with the air coming out of the pipe.

Nigel -   That's right, and this is important.  Now, when your aircraft is coming in to land, the engines aren't very noisy.  But what is very noisy are the - what are called the air breaks which are deployed to try and slow the aircraft down, the trailing edge of the wing.  And it's exactly this effect with the ruler.  There's turbulence flowing over the trailing edge and that's amplifying the turbulence, producing a lot of noise.

Chris -   That is at the end of the day, why the airplane is slowing down though because it's shedding energy, it's disturbing lots of air, which is helping it to slow down.  But the downside is that it's making lots of noise.

Nigel -   It's making lots of noise which of course is very bad for the people who live near the airport.  Another thing is that if you have a wind turbine, that's rotating then there's a noise source there which is again, to do with the air flowing over the sharp trailing edge of the turbine blade.

Chris -   But if you stop the wings making that turbulence on an airplane for example, won't it have a problem slowing down?

Nigel -   It can't help the turbulence.  In many ways, that's important because that actually helps the pilots to manoeuvre the aircraft safely.  But what you need to do is to find a way of stopping the turbulence interacting with the sharp edge in such a noisy way.  We believe that's what the owl has done.

Chris -   So, what do owls do then?

Nigel -   Okay, well owls have got two very unusual features about their wings.  So, the first is if you look at the trailing edge then it's got a very flexible brush of small comb.  And then the second thing is, then the feathers of these owls are much, much more complicated than the feathers of any other bird.  In fact, they're much more complicated than the feathers of other owls that don't need to fly silently.

Chris -   And it's because owls need to fly silently to give them the best chance of catching something that they've evolved to have this particular adaptation.

Nigel -   Exactly.  There's two reasons.  One is, if you're hunting a mouse then the mice have rather good hearing.  So, you don't want them to hear you coming.  The other thing is, that if you're trying to hear the mouse, you don't want to be making so much noise yourself.

Chris -   The cost must be to the owl that it's harder to fly though because it's not got the same aerodynamics as an animal that doesn't damp its sound down like this.

Nigel -   Actually, sound is very low energy.  Even very loud sounds are quite low in energy cost.  So for instance, you have the whole of Wembley Stadium shouting throughout a match.  Then the total acoustic energy they would be produced is only about enough to fry one egg.  And that's because the soundwaves have actually very small amplitude.  This proves how remarkable the ear is, detecting these very small amplitudes.

Chris -   And you're trying to copy what the owls do.  Because you worked out what the owls do and now, you're saying, "Can we copy that onto an airplane's wing to make it quieter?"

Nigel -   Yes, and we've managed to do this.  So, we did some mathematics, we did some computing and we design something which we thought would mimic what the owl does.  Now, it doesn't involve sticking a feather on a wing.

Chris -   Because it's a very fluffy airplane.

Nigel -   It would just look silly, wouldn't it?  But we've got this device which we've designed.  We've put it in a wind tunnel and we've tried it and it works.  So, we're really excited about that.  And the next thing is, that we're actually going to put it on a proper wind turbine in a field somewhere and see what happens.  So, that's the next stage.

Chris - Because wind turbines make sounds for exactly the same reasons, that airplanes do.  So, what works for one will work for another?

Nigel -   Yes, and if it works on the wind turbine and maybe, just maybe, it'll work on an aircraft as well.

Chris -   How much disturbance or sound do you think you can save?  How much of a knockdown effect will this achieve?

Nigel -   Well, in the wind tunnel, they went from being quite loud to being almost inaudible.  So, sound is measured in decibels and this was 10 decibels which is an absolutely massive change.

Chris -   Any questions?

Ben -   My name is Ben and I'm from Barrington.  What happens if you put the blunt end to the air cannon instead of the sharp end?

Nigel -   Remember what I said about the aircraft coming in to land?  That's actually another source of noise which is when you put the landing gear sown and that's like a blunt edge.  And you let anybody who's been on an aircraft coming in to land, when the gear goes down, that's very noisy as well.  So, as well as doing something about the sharp end of the ruler, doing something with the bluff end is very important as well because that would solve this problem about the landing gear.

Ginny -   There is a question that could really be put to any of you or all of you.

Chris -   Is it on Twitter, given that we're talking about birds?

Ginny -   It is on Twitter.  Garage Idol asks, "Is there a minimum or maximum limit to frequency or amplitude of sound?"  Who wants to come in on that?

Nigel -   Well, I'll have a go.  So, it's to do with hearing of course.  So, young people's ears are much better than old people's as we heard.  But a young person can hear a sound of zero decibels.  That's the sort of threshold of hearing and then there's a sort of maximum threshold where it starts to cause damage to the ear because the sound is so loud which is about 140 decibels, I think like that. And then you can get much bigger pressures.  So, there are some very loud sounds like volcanoes erupting which can get up towards maybe 190 decibels and 190 decibels is actually 1 atmosphere of pressure which is how much pressure is pressing down on us because of all the air going up into space.

Chris -   Ladies and gentlemen, from the Department of Applied Maths and Theoretical Physics at Cambridge University, Nigel Peake.  Now, this does look very exciting.  Dave and Ginny, two large funnels instead of headphones by the look of it.  What are you up to now?

Ginny -   We've been talking about owls and how good some of them are at flying quietly, but there's another thing that owls are really, really good at that helps them with hunting.  And that's pinpointing where a sound is coming from.  Humans are okay at it, but owls are brilliant.  So, when they're flying, they can work out if that little mouse they can hear is over to the left or over the right that they can swoop down and get it.  We don't have any owls here unfortunately.  We're stuck with humans, so we're going to have a bit of a play with how you work out where a sound is coming from.

Dave -   So, what I've got here is a set of ear defenders and attached to it, I've two tubes.  One from the right hand ear which goes to a funnel on the left hand side and one from the left hand ear which goes to a funnel on the right hand side.  Now, what we need is a volunteer.

Ginny -   What's your name?

Stanley -   Stanley.

Ginny -   Where are you from Stanley?

Stanley -   Trumpington.

Ginny -   Brilliant!  Okay, now Stanley, we are going to put this fantastic thing on your head.  So, Stanley has a pair of big yellow ear defenders on now and from his left ear, there is a tube going up and over his head and a big funnel pointing out to his right.  From his right ear, there's another tube going up over his head and with a big funnel pointing to his left.

Dave -   So now Stanley, I want you to shut your eyes.  What I want you to do is point to the direction which it sounds like I'm coming from.

Ginny -   Stanley has pointed to me and I'm standing on his left.  Dave meanwhile is standing on his right.  Where does it sound like I'm coming from?  So, it sounds like I'm coming from where Dave is.

Dave -   And I'm coming from where Ginny is.

Stanley -   It is like opposite.  I think it sounds like he's on that side.

Ginny -   Is that very confusing?

Stanley -   Yeah.

Ginny -   So, why have we done this to Stanley?

Dave -   So, what we've done is effectively swap the position of his two ears.  Actually, where the sound is coming in to his left is actually on his right hand side now because the tube where the sound is coming into his right ear is actually on the left hand side.

Ginny -   So, when I talk and I'm on his left, the sound goes into the funnel, around the tube and into his right ear.  So, it sounds like I'm over there on your right, yeah?

Stanley -   Yes.

Dave -   You've got a couple of ways of working out what direction things are coming from.  One of them is, which ear it sounds the loudest in and the other more subtle one is, which ear the sound gets to first.  And that's the reason why sometimes it's very, very hard to tell whether something is in front of you or behind you.  Because if something is dead behind you or dead behind you, it gets to both ears at the same time.

Ginny -   So, I'm standing here with nothing on my head and I can tell that you're on my right because your voice gets to my right ear first and you sound louder in my right ear.  We've changed that for Stanley here and that's why he's very confused.

Dave -   Exactly.

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