Horn Loudspeaker Project.

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Ray P
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#16

Post by Ray P »

I've attached below the 2D vertical section of the horn profile.

Overall height is just shy of 2metres. Mouth diameter is around 67cms prior to the final flare. Front to back depth is around 90cms. These are large dimensions but the intention is that the units will be pushed into room corners, this and the vertical tapering will mean they won't dominate as much as you might think.

Ray
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Audio_Works
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#17

Post by Audio_Works »

Ray P wrote: Ah, that's what you meant; yes, have used HornResp but will check out the other one. That's what I meant by horn theory around the throat, profile etc. and I thought you were into something much deeper around modelling the theoretical behaviour of different materials etc. That said, as I'm using the profile data from an established and well-considered design I'mnot spending a lot of time on this aspect but a lot more on the realisation of the less-compromised profile.

Ray
oh, i wish i knew some software tools that could do that kind of modeling.

anyways, i remembered i have read about concrete and plater as material for horns and looking in Beranek's Acoustics i found the paragraph:
The material from which a horn is constructed is very important. If the side walls of the horn resonate mechanically at one or more frequencies in the range of operation, "dips" in the power-output curve will occur. Undamped thin metal is the least desirable material because the horn from which it is made will resonate violently at fairly low frequencies. Heavy metals, covered on the outside with thick mastic material so that mechanical resonances are damped, are much better. A concrete or plaster horn 1 or 2 inch in thickness is best because of its weight and internal damping.
Plywood is commonly used in the construction of large horns. Although it is not as satisfactory as concrete, it gives satisfactory results if its thickness exceeds 3/4 in. and if it is braced with wooden pieces glued at frequent, irregular intervals.
So your thoughts were correct, plaster is a good material.

About the BLH, you can also change the properties of the compression chamber by adding or subtracting dampening material. Basically the air from that chamber after a certain frequency acts as a radiation resistance so playing around with stuffing material you can also be a little flexible on the dimensions of the chamber. Maybe I'm wrong tho.
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Ray P
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#18

Post by Ray P »

Audio_Works wrote: oh, i wish i knew some software tools that could do that kind of modeling.

anyways, i remembered i have read about concrete and plater as material for horns and looking in Beranek's Acoustics i found the paragraph:
The material from which a horn is constructed is very important. If the side walls of the horn resonate mechanically at one or more frequencies in the range of operation, "dips" in the power-output curve will occur. Undamped thin metal is the least desirable material because the horn from which it is made will resonate violently at fairly low frequencies. Heavy metals, covered on the outside with thick mastic material so that mechanical resonances are damped, are much better. A concrete or plaster horn 1 or 2 inch in thickness is best because of its weight and internal damping.
Plywood is commonly used in the construction of large horns. Although it is not as satisfactory as concrete, it gives satisfactory results if its thickness exceeds 3/4 in. and if it is braced with wooden pieces glued at frequent, irregular intervals.
So your thoughts were correct, plaster is a good material.

About the BLH, you can also change the properties of the compression chamber by adding or subtracting dampening material. Basically the air from that chamber after a certain frequency acts as a radiation resistance so playing around with stuffing material you can also be a little flexible on the dimensions of the chamber. Maybe I'm wrong tho.
Thanks for the reference about plaster, it confirmed my thinking that it is rigid and well damped. Still a devil to work with though and I dread the thought of building one only to find that it gets cracked moving it... That said, I do recall a guy building concrete box loudspeakers a few years ago (may still be?) that were well regardsd for their 'absence' of cabinet colourations.

Thinking out loud, maybe a plaster horn with an outer lamination of grp to protect it......

And then there's the question of the type of plaster; I would be inclined towards plaster of paris as I know you can buy it easily, including scrim impregnated with PofP (used for making hills etc. in model railway layouts) and it sets quickly. Not sure if you have to do it all in one go though. More research needed.

I know some people experiment with compression chamber stuffing/damping and vary the volume with little bags of lead shot so it should be possible to tune it effectively.

Ray
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Ray P
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#19

Post by Ray P »

A quick search shows that there are a lot of different grades of plaster available, including some that are very dense/strong/chip resistant.

http://www.maragon.com/plaster-of-paris.html#Init

Ray
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#20 Plaster

Post by IslandPink »

Plaster should be good if it's sufficiently dense, and thick enough ( > 1" ) . Marc Henry makes plaster mid/HF horns for 'Musique Concrete' . I saw some of his early ones a few years ago at ETF2004, and they were very nicely made and seemed hard and 'dead' acoustically .

http://www.musique-concrete.com/MC/Technical.html
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#21

Post by Audio_Works »

I've never worked with plaster myself but i know you should stay away from those sound proofing plasters because it is like a very very dense swiss cheese.
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Ray P
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#22

Post by Ray P »

Whilst plaster looks like a good material the weight looks a bit scary; a very rough calculation, assuming a wall thickness of 4cms, suggests a finished weight well in exces of 250Kg per speaker. I'll double check and try to refine that a bit later.

Ray
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#23

Post by planet10 »

Ray P wrote:... why should a FLH design be different to a BLH? Surely they are both 'horns', for which the optimal profile cross section is round, that just differ in length etc.
In a FLH you are trying to get as extended and as smooth a high frequency as possible.

In a BLH you DO NOT want high frequency leakage out of the horn. Having a circular smooth expansion is counter to this requirement.

dave
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#24

Post by Andrew »

Yes, spot on in my humble Dave, that's what makes a BLH so difficult to get right.

Andrew
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#25

Post by Audio_Works »

Hello Dave,

I don't know what you meant by smooth expansion but having a low expansion rate and a long path will kill most of the high freq energy.

I agree tho that FLH and BLH are completely different things that work in different ways and do different things, i don't even see a point in comparing them.
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#26

Post by chrisby »

is it too late to correct the usage of term "compression" chamber when referencing BLH?

FLH or compression drivers have compression chambers, BLH have the room on the other side of the driver.
I agree tho that FLH and BLH are completely different things that work in different ways and do different things, i don't even see a point in comparing them.
indeed, but the point is that many folks consider all "horns" to follow the same acoustic principles, even or perhaps particularly those that are highly compromised by minimal expansion or undersized mouth unless coupled to room boundaries, and are really operating more like under-damped TLs with a flared terminus than anything else - not that they can't deliver satisfying sound
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Ray P
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#27

Post by Ray P »

planet10 wrote:
Ray P wrote:... why should a FLH design be different to a BLH? Surely they are both 'horns', for which the optimal profile cross section is round, that just differ in length etc.
In a FLH you are trying to get as extended and as smooth a high frequency as possible.

In a BLH you DO NOT want high frequency leakage out of the horn. Having a circular smooth expansion is counter to this requirement.

dave
Yes, I know. Clearly they are quite different beasts but the point I was making was simply that there is no reason why a BLH should not have a round section profile moulded as so many FLHs do. To confuse a round cross section BLH with the design of a FLH on the basis of that cross section is simply wrong.

Ray
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#28

Post by SimonC »

Ray P wrote:
Audio_Works wrote:Since you are a CAD user, a 5 axis CNC and lots of plywood could save you a lot of trouble but it's pricey.
Hence trying to work out 3D in AutoCad - it aint easy!

Ray
Hi Ray,

I drive a 3D cad system for a living, if it would help out I'm happy to kick this around as a lunchtime 'project' to model it in 3D.
Drop me a PM with your email address if you're interested :-)

Cheers
Simon C
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Ray P
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#29

Post by Ray P »

SimonC wrote:Hi Ray,

I drive a 3D cad system for a living, if it would help out I'm happy to kick this around as a lunchtime 'project' to model it in 3D.
Drop me a PM with your email address if you're interested :-)

Cheers
Simon C
Simon, that's very kind of you, I'll be in touch shortly....

Ray
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Ray P
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#30

Post by Ray P »

I've just been exploring the use of plaster further, in particular the weight.

I've refined the calculation for the weight of the horn profile and it comes out quite a bit less than my initial estimate but still a pretty hefty 175Kg,

If anyone cares to check my estimate I used the following steps;

1. Measure the vertical cross-sectional area of the horn profile in sq cms.

2. As that area is based on the diameter of the round profile, multiply the cross-sectional area by 3 (circumference of a circle is approximately *3 the diameter). This gives an approximate surface area for the horn profile.

3. MUltiply the surface area of the profile by the proposed wall thickness for the horn (I assumed 4cms but I would probably vary it, thinner near the throat, thicker near the mouth)) to give you the volume of the plaster that would make the walls of the profile. Divide the result by 1000 to concert cubic centimetres to litres.

4. Based on the stated yield of the preferred plaster (volume per Kg of plaster = about 0.6l = about 1.6Kg per litre) multiply the volume calculated in step 3 by 1.6 to convert the volume to Kgs of plaster.

More thought required.

Ray
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