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Sound source wave interference effect simulations

Started by David Pinnegar, April 27, 2011, 01:23:48 PM

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David Pinnegar

Hi!

I hope these links might be helpful for anyone doing church PA etc.

http://www.burton-manor.co.uk/Audio/LAthoughts.htm
http://www.burton-manor.co.uk/Audio/Audio.htm
http://www.gedlee.com/downloads/directivity.pdf
http://www.eurasip.org/Proceedings/Eusipco/Eusipco2010/Contents/papers/1569292745.pdf
http://audioroundtable.com/misc/nflawp.pdf

I have been looking at horn physics and am intrigued by the shape of the wavefront emerging from a horn of different shapes. The one rule of horns is that the wavefront is normal to the surface of the horn - so whether conical, exponential or tractrix, all produce a spherical wavefront of a different radius of apparent source . . .

What is interesting in the original Edison acoustic grammophone recording concept is that a horn, on the basis of a shperically emerging wave seems excellent at emitting sound, but I'm not at all sure of the function of a horn in the process of receiving sound, which will be either planar wavefronts or inverse spheres. What is happening? Is the function of a horn better analysed in terms of angular reflecting surfaces?

Best wishes

David P

dragonser

Hi,
I always thought that the reason a horn was used for recordings was that it was needed to amplify the sound [ in an acoustic manner ] as the recording process was not that sensitive.
the introduction of electric recording techniques improved the quality of recordings no end.
[ I mean sensitivity and frequency response ].
I think I saw an Aes article where someone has analysed the early recording equipment ?

regards Peter B

Quote from: David Pinnegar on April 29, 2011, 12:36:43 AM
Hi!


I have been looking at horn physics and am intrigued by the shape of the wavefront emerging from a horn of different shapes. The one rule of horns is that the wavefront is normal to the surface of the horn - so whether conical, exponential or tractrix, all produce a spherical wavefront of a different radius of apparent source . . .

What is interesting in the original Edison acoustic gramophone recording concept is that a horn, on the basis of a spherically emerging wave seems excellent at emitting sound, but I'm not at all sure of the function of a horn in the process of receiving sound, which will be either planar wavefronts or inverse spheres. What is happening? Is the function of a horn better analysed in terms of angular reflecting surfaces?

Best wishes

David P

David Pinnegar

Quote from: dragonser on April 29, 2011, 01:53:36 AMI always thought that the reason a horn was used for recordings was that it was needed to amplify the sound [ in an acoustic manner ] as the recording process was not that sensitive.

Hi!

Yes - that's the effect but I'm not at all satisfied that the operation of a horn is a reversible function, unlike a parabolic reflector.

. . . Does anyone have any ideas?

Best wishes

David P

KB7DQH

QuoteYes - that's the effect but I'm not at all satisfied that the operation of a horn is a reversible function, unlike a parabolic reflector.

Well it works at microwave frequencies with electromagnetic waves... ;D ;) 8)

For this I have direct experience... Horns are also used to "feed" parabolic reflectors...

I can only imagine the math gets to be different when dealing with sound pressure waves in air ;)

(note to self: Should try "enclosureless" mounting of loudspeaker at the focal point of (one of) my leftover 6 foot microwave antennae and see if it is as effective as the "big wood horn" ;D ;)

Note to forum: Abandoned railway tunnels present some unique acoustic situations, I imagine because of the different propagation modes and frequencies which may be used to excite this as a "waveguide"... More effective at some than others...

Eric
KB7DQH



The objective is to reach human immortality—that is, to create things which are necessary to mankind, necessary to the purpose of the existence of mankind, and which have become the fruit that drives the creation of a higher state of mankind than ever existed before."

Colin Pykett

Acoustics at the level David has introduced is horrendous if one wanders into the mathematics.  But a horn is a transformer not an amplifier, which means it is passive (no energy is introduced as it is with an amplifier).  It is also linear otherwise the sound it emits would suffer harmonic and intermodulation distortion.  For these reasons it is therefore also reversible, just as a transformer is.  Consequently, horns were also used as receivers in the earliest days of acoustic recording directly onto a wax disc.

Textbooks such as Beranek's 'Acoustics' cover all the issues, but mathematically.  Before electrical reproduction appeared, the finest acoustic gramophones were based on the research of Maxfield and Harrison, who even worked out little details such as the optimum design of a sound trap joint for the lid so that the chatter of the needle was completely suppressed.  (Hope-Jones also used such traps for his swell shutters, but I don't know how carefully they were designed.  He would not have had the necessary mathematical background for this sort of thing himself).  These gramophones also had miniature Venetian-blind type volume controls at the mouth of the horn controlled by a knob.

It only became apparent how good they were when used to play the 'ultimate' 78 rpm discs produced towards the end of this technology, i.e. the late 1940's/early 50's, before they were replaced by vinyl LP's and EP's.  These recordings were intrinsically often very good when new and pristine, and when played on an M&H acoustic reproducer they sounded very fine indeed, much better than the typical electric Dansette of the day with its Collaro autochanger!  As a child, I had a then-new 78 rpm recording of Isobel Baillie singing Hear My Prayer etc, and it was almost as though she was in the room when played on this type of acoustic machine (with a thorn needle!).

Sorry for the ramble, but it's all good stuff from a bygone age.

Colin Pykett

David Pinnegar

Dear Colin

I was hoping you'd pick this up.

Is the speed of the wave the same at the edge as in the open space of the horn? I'm simply intrigued at the frequency implications of the horn as a waveguide in terms of spherical and inverse spherical wavefronts, because logic suggests that the waveshape coming in is going to have a different effect to a waveshape going out.

The theory of conical horns is interesting in so far as they are simply angular truncators and arguably the purist of transformers . . . but with the problem of the impedance mismatch at the mouth causing resonances . . .

Best wishes

David P

Colin Pykett

Hi David,

I'm not sure one can look at different parts of column in the way you suggest, because there is but one wave travelling within it and one can't really chop it up into pieces.  As to wavefront shape, a spherical wavefront (in the Fresnel or near-field region) rapidly approximates to plane (in the Fraunhofer or far-field region) as the wave propagates away from the source.  Therefore, whether the horn is emitting or receiving energy I suspect it's the same thing in terms of wavefront, because a plane wave approximation can probably be used in both cases at most frequencies of interest.  However I see the point you are making, and a proper analysis would have to consider Fresnel number and how it varies with frequency and horn aperture before developing a deeper understanding.

Conical horns do indeed have a frequency-dependent mismatch at the end which causes resonances.  This is in fact how the resonator of a conical organ reed pipe (such as a trumpet) works, and it imparts the characteristic tone of the pipe in question.  The frequency dependence arises from the end correction, which varies with frequency (harmonic number) and with the degree of flare (i.e. how wide the pipe is at the open end).  It's not a subject we can take very far here, apart from boring everybody else to death, but it is dealt with in books such as 'The Physics of Musical Instruments' by Fletcher and Rossing.  There's nothing which hasn't been worked out in any degree of detail you wish for, but it can be rather hard going.

These are just instant musings, so best wishes in your endeavours

Colin Pykett

KB7DQH

Some very intriguing "horn audio concentrators" were employed in  pre-radar World War Two for
receiving the noise of incoming aircraft and no doubt they were optimized dimensionally for this service.

War museums in the South of England???

Eric
KB7DQH
The objective is to reach human immortality—that is, to create things which are necessary to mankind, necessary to the purpose of the existence of mankind, and which have become the fruit that drives the creation of a higher state of mankind than ever existed before."

David Pinnegar

Quote from: Colin Pykett on April 30, 2011, 12:31:04 AM
I'm not sure one can look at different parts of column in the way you suggest, because there is but one wave travelling within it and one can't really chop it up into pieces. 

Hi!

In view of simiularities with organ pipes, I make no excuses for starting off trains of thought in the horn direction.

There are two things that are worrying me conceptually at the moment -
1. If we imagine a pressure wave entering the front of the horn, there will be a slice of air carrying a wave of a volume of air at higher pressure wanting to release its pressure as soon as it can. As the wave travels down the horn, does the speed increase? Its a long time since I have analysed waves arriving on the shore of a beach - certainly in the shallow area the vertical element of the wave increases, as there is nowhere else for the volume of water to go, and this is the amplification that we would expect as the wave travels down the constricting passage of a horn, and from memory, the speed increases. So the wavelength must increase. Can we understand the opposite happening in reverse?

2. If we look at the sum of what happens at the throat of a receiving horn, it must be an integral of what is happening at the mouth over the whole area, and this assumes that the wave can be analysed as a dissection of its angular parts . . .

Best wishes

David P

Colin Pykett

No need to apologise David!  But I think I can detect a common misconception about wave propagation here.   And don't think I'm preaching in any way either, but it's important not to confuse the motion of air molecules as they vibrate (longitudinally) about a mean position when propagating a sound wave, and the motion of the bulk medium (the air itself) as it would flow down a hose when inflating a car tyre for example.  The two are completely different.  They can co-exist simultaneously but there's no need for them to.

Whether I am correct or not, I also think you will have to take a deep breath and wade into the theory as in the books I've suggested (or others - I only mentioned two which I happen to have and have used often).  Again, please don't think I'm being offhand, but I can't see an easy way to assist much further here. 

All the issues which you have raised are dealt with and can be understood, but the trouble is that I know of no way in which it can be done without invoking the mathematics.  That might be a limitation of my understanding of course and maybe someone else could explain it all non-mathematically.  But I have found in physics generally, as I'm sure you have also, that one can only go so far without calling on the maths.

Colin Pykett

David Pinnegar

Quote from: Colin Pykett on April 30, 2011, 01:59:21 PMBut I think I can detect a common misconception about wave propagation here.   And don't think I'm preaching in any way either, but it's important not to confuse the motion of air molecules as they vibrate (longitudinally) about a mean position when propagating a sound wave, and the motion of the bulk medium (the air itself) as it would flow down a hose when inflating a car tyre for example.  The two are completely different.  They can co-exist simultaneously but there's no need for them to.

Dear Colin

Yes - appreciated - but the blurring will be dependent on volume and frequency. Obvoiously at high frequency one is looking at pure wave movement, but at low frequency and at high enough amplitude, the air molecules actually have to move somewhere in order to transmit the wave. It's possibly a concept akin to the question of whether a photon is a particle or a wave . . . where the behaviour is certainly in reverse in terms of frequency. One sees it demonstrably on YouTube Yob videos demonstrating ear splitting car subwoofer systems where gullible sycophantic females get their hair blown around, or in one example from memory her eye sucked out by the physical movement of the air at the car window . . .

The maths can be useful, but sometimes a conceptual approach can lead to useful insights, particularly as to whether assumptions behind the maths hold good.

It's for the reason of the movement of the air and the requirement for the molecules to shift physically that horn comression ratios are generally recommended not to exceed 4:1 and were the mathematics of horns to be perfectly understood, whilst they might behave perfectly according to the maths, people will always say that they sound "horny". Whether one views them as lenses / magnifying glasses, complex continuous wavefront mirrors producing wavefronts of constructive interference, or merely as pressure transformers, they are fascinating and not entirely predictable objects. . . .

Best wishes

David P