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Old 12th August 2005, 12:03 PM   #11
Mark Kravchenko --- www.kravchenko-audio.com
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Default THink about it this way.

THe horn length, flare rate, and mouth size dictate the overall response. In a front loaded horn the rear chamber also has alot to do with the response helping to null the reactance that is part of the driver unloading the horn as it reaches Fh ( horn cutoff frequency ). In a backloaded horn you are as you have stated introducing a lowpass filter by creating a volume directly behind the driver inbetween the driver and the horn throat. WHat it does and how it behaves is a function of it's size and it's lining and or stuffing density. The size and the stuffing or lining the walls has to be played with and the results listened to or measured.

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Old 12th August 2005, 02:20 PM   #12
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Default comment on horn against a wall and hornparameters for fostex fe208EZ

I have two points to make within the framework of this thread.

In the last months I am looking for a diy implementation of a 10 cm fostex fullrange. This thread is interesting because it is discussing some fundamentals as a consequence of positioning of some kind of horn against or very near to a wall.

In my view the most commonly choices (when one wants to use a 4 inch full range) narrows to a dedicated fostex cabinet or a buschhorn mkII (may be an updated Cheap Trick 164). And this in combination with a fostex FE103E of a FE108EZ. The last one's are for the 'normal' diy worldwide the easiest available and the most recent incarnations of this sort of fullrangers.

I for myself favours the concept of the buschhorns mkII because of 'looks' and expected performance in combination with my listeningroom (16 square meters, medium damped). I have the possibility to play with speakerpostioning, hotspot and roomboundaries. In my view alike dipole speakers (electrostats f.e) it is principally needed to use horns of this kind with enough space around it, gain by the way of roomboundaries and room for integration of the sound perceived at the listeningposition. Otherwise you end with very uneven sounddispersion and a tiring presentation.

In my opinion and expierence (mostly with very good electrostats, even in small rooms and with small two way bookshelf speakers) a horn implementation will stay difficult with a cabinetposition fully against the wall. In my view a dedicated two way bookshelf, may be helped by a adjustable sub, will give a nicer result.

Only, when the 10 cm unit is comming out of the front of the wall-framework (not so nice to see and for persons with another level of acceptance quite awfull) there is may be a way out. On the net there are some cabinets that have a sort of neck. When you mount the neck horizontal and pace the mouth of the horn as near as possible to the corner walls you can may be accomplish a good result.
This requires quite another form for a horn cabinet than the buschhorn, fostex or CT164. And a good implementation wil require quite some woodworking expierience and motivation to tuning the endresult in a given room.

My advise would be: look again into the possibilities of a rather freestanding little horn so that you can get the most out of this sort of instrument for music retrieval. When you stick to a against the wall implementation go for another way to reach nearly the same, only by another way to load the air in the room for your own enjoyment.


The second point I want to make to the more experienced in maths and computersimulations is the following. In the last threads on diy audio there are several concrete questions on hornparameters such as size and form of the CC, throatsize, hornlength/deployment and mouthsize. I have tried for myself to make some calculations of especially CC size and throatsize. The goal was to get some insight in these parameters in relation to some popular cabinets for the forstex FE108EZ. I came to the conclusion that I am too inexperienced to make thrustfull calculations.

Is it possible to present in this or in a specific thread some theoretical/computersimulated parameters for enclosures around the Fostex FE108EZ? I think that several prospective diy ers have the same question, because they will choose this (relatively new) type according to worlwide availability and their desire to make a one shot approach in line with expected performance.

Not that that theoretical parameters are the end in the discussion or experiments, but they can give some quidance for constructiondetails (fe enough room to make cc and throat somewhat smaller from the start).

In the end we have all to experiment to get the best results out of the chain 'source, amp, horn, room and personal sound preferences'.

Although it helps a lot to know some indicatitions for the parameters for the FE108EZ, because the specs of this type differ quite a lot from those for the older types! And the most discussed horncabinets are at their time develloped specifically for that older types.

I am looking forward to some more information on the critical parameters.

Greetings.
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Old 12th August 2005, 06:32 PM   #13
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Default Re: REad this it gives a good explanation:

Quote:
Originally posted by mwmkravchenko
It attenuates the high frequencies at a rate of about 6db/octave.
The 6db attenuation on top end per octave occurs as result of low end extension. Then the more low end I attempt to extend may make a need for tweeter to fill in the lost high end in frequency response. Guess I shouldn't try to get to much low end out of a 2" driver. Is tuning to 60hz to much? Fs is 153hz

Quote:
Originally posted by Colin
horn decoupling equation, cavity volume for capacitive shunt reactance to equal horn throat resistance at frequency f, where At = throat area

V(cu in) = (2150 At)/f [where At is in sq ins]

V(cu cm) = (5550 At)/f [where At is in sq cm]
Not sure this formula works or I may be using wrong values.
Hornthroat= 94 sq"
Hornmouth= 43.5 sq"
Hornthroat & Hornmouth= 137.5sq"
Fs= 153hz or Ftuning= 60hz
Best I calculate volume as 1932 cu". This is huge, I was using WinISD at 62 cu" (.9 liter) vented enclosure.

Could you maybe show me my error.
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Old 12th August 2005, 06:40 PM   #14
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Default Re: Re: REad this it gives a good explanation:

Quote:
Originally posted by mrlots2do
Not sure this formula works or I may be using wrong values.
Hornthroat= 94 sq"
Hornmouth= 43.5 sq"
Hornthroat & Hornmouth= 137.5 sq"
Fs= 153hz or Ftuning= 60hz
Best I calculate volume as 1932 cu". This is huge, I was using WinISD at 62 cu" (.9 liter) vented enclosure.

Could you maybe show me my error.
Hornmouth in quote is not the opening of the mouth but really a second Hornthroat area.

This is the entire Hornthroat area.
Hornthroat & Hornmouth= 137.5 sq"
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Old 12th August 2005, 06:50 PM   #15
Cortez is offline Cortez  Hungary
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I know the basics about acoustical RLC filters, but i have some uncertainties too.
When we decrease the throat, we must decrease the chamber too, cause when the
throat's impedance is too high, the air will just compressed in the chamber,
instead of going to the horn, like by a BR enclosure, when the port is too long.
But the horn's efficiency depends on the starting throat area, if i'am right.
Therefore have i asked, that why we need a big rear chamber. Theoritacally when the
rear chamber would be 0 size, the air could nothing else to do, like go to the horn,
and therefore should be the efficiency at the maximum, that we can reach with our horn.

My other question: why the horn's length does matter. I understand the need of the
expansion, cause of acoustic transformation, but why we need to create a horn with a
long line ? Whats the different between a shorter and a longer type horn, when the
Mouth/Throat rate is the same ? By a back-loaded horn its maybe logical, cause when the line
is too short, the acoustical shortcut will eliminate the SPL with the front waves.
But why does it matter by a front loaded horn ?

An example:
Click the image to open in full size.
Why is the straight section so long ? Maybe this acts like a TL too ?

(If you want, read my post here too: http://www.diyaudio.com/forums/showt...166#post702166)
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Old 13th August 2005, 03:52 AM   #16
GM is offline GM  United States
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Quote:
Originally posted by Colin
If it helps, I came across this in an article by John Crabbe in HiFi News a few years ago:

horn decoupling equation, cavity volume for capacitive shunt reactance to equal horn throat resistance at frequency f, where At = throat area

V(cu in) = (2150 At)/f [where At is in sq ins]

V(cu cm) = (5550 At)/f [where At is in sq cm]

(JC developed a number of horn designs for the magazine in the 50s and 60s.)


Colin
Greets!

Thanks for sharing! FWIW, I compared the (cu in) formula to a couple of proven BLH designs and one was ~3.7x too large and the other was ~100x!, so without knowing more about his horn designs I recommend these not be used.........

GM
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Old 13th August 2005, 05:01 AM   #17
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Quote:
Originally posted by Cortez
I know that it acts like a low-pass filter, like a reflex enclosure, but what is does with the efficiency ?

And what about using more then one compression chamber in series ?
The first chamber would be right behind the speaker with a small size, and then a reflex-tube-like
narrow section and then an other chamber with bigger volume, and at the end of this chamber
would start our horn with a small cross section.
Greets!

Like any vented design, efficiency varies depending on the alignment. Remember, a typical 'HI-FI' BLH is nothing more than a BR with a huge flared vent. For this reason having multiple gain stages doesn't make any sense to me since this is accomplished by the flare rate ('M', aka 'T', factor). If this doesn't yield enough gain, then use different flare rates in series (aka Klipsch's 'rubber' throat), which would provide better performance overall.

GM
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Old 13th August 2005, 07:13 AM   #18
Cortez is offline Cortez  Hungary
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> Remember, a typical 'HI-FI' BLH is nothing more than a BR with a huge flared vent.
Why ? With a small rear chamber it should act as a horn, should not ?

> For this reason having multiple gain stages doesn't make any
> sense to me since this is accomplished by the flare rate.
What do you mean exactly under "multiple gain stages" ?

> If this doesn't yield enough gain, then use different flare rates
> in series, which would provide better performance overall.
Could you write more about this "rubber throat" ?
Why it helps us to improve the bass radiation ?
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Old 14th August 2005, 12:54 AM   #19
GM is offline GM  United States
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Quote:
Originally posted by Cortez
> Remember, a typical 'HI-FI' BLH is nothing more than a BR with a huge flared vent.
Why ? With a small rear chamber it should act as a horn, should not ?

> For this reason having multiple gain stages doesn't make any
> sense to me since this is accomplished by the flare rate.
What do you mean exactly under "multiple gain stages" ?

> If this doesn't yield enough gain, then use different flare rates
> in series, which would provide better performance overall.
Could you write more about this "rubber throat" ?
Why it helps us to improve the bass radiation ?

Greets!

Why 'what'? It's the physics of the situation and I don't know how to further simplify it, so if you can't grasp this concept then I don't know how you're going to ever understand BLH design. Anyway, read my response here, maybe it will help: http://www.diyaudio.com/forums/showt...815#post702815

Each one of your multiple compression chambers would be a 'gainstage', i.e an amplification chamber, if properly designed of course!

Not too much to write about. I mean it's my understanding that PWK coined the term around 1940 to describe the multiple flare rate of his corner horn. He may also have been the first to do it since I've never seen one before Altec Lansing developed the 811 sectoral lens in the late '40s. Both these used fast initial expansions to control their HF response, then slow down to the Fc flare rate.

This yields less gain near Fc though, so for more gain you'll need a much slower initial flare rate, then speed it up to keep it a semi-reasonable length/BW. Or not, as the case may be as is done with the B*** AWC, which is a long straight pipe, relying on loading the back side of the driver with another one. It yields a rather underdamped response, but used as a true sub this may not be a 'big deal' depending on your room design/performance goals and you can't beat it for simplicity: Acoustic wave canon

GM
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Old 14th August 2005, 10:51 AM   #20
Cortez is offline Cortez  Hungary
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Default adapt a horn to the environment...

> Why 'what'?
You said, that a classic hifi horn is just a BR with a huge port.
And I said to this, that when we have a very small (0 size)
rear chamber, it cant be act as a BR, cause the air cannot be
compressed and therefore it can just go to the horn.

> Each one of your multiple compression chambers would be a 'gainstage'
How ? imho a chamber only shifts the phase end increase the speed of air in the output port.
A horn would gain, cause of acoustic transform, but a simple chamber how ?

I'am still curious, that how we should size a horn, when we want
to include our room's size to the calculations. I think when a horn
operates indoor, the acoustical conditons are quite different, cause
in a smaller, closed air-space we need a little more power to compress
the rooms' air related to an open air environment, where we
do not have this problem, and can concentrate only to the transform.
So in a little room maybe a smaller mouth would be better.
The flare could be the same, but the throat, and the mouth size
should maybe decreased, shouldnt ?
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