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Sound Reinforcement - Forums for Live Sound Professionals - Your Displayed Name Must Be Your Real Full Name To Post In The Live Sound Forums => SR Forum Archives => LAB Subwoofer FUD Forum Archive => Topic started by: Antone Atmarama Bajor on July 29, 2005, 12:05:56 am

Title: Flare Rate Transitions
Post by: Antone Atmarama Bajor on July 29, 2005, 12:05:56 am
     Hello All I've been working with a guy what does some work on horns, and he's been teaching me about how expansion transitions in flare rates as well as end of flare can be seen in the impedance chart.  And that they are all significant events that cause coloration's and ringing etc..

    I've been noticing that on a lot of the horn Resp models that people have been posting including the Lab Sub,  Have some very High Q looking peaks in the impedance charts.  I imagine that these have a lot to do with the transitions in the cabinet and probably significantly color the sound?  I'm not sure if the lab12 is a Hyperbolic flare rate.  But I was wondering if the lab12 is a hyperbolic is there no way to keep the cab its current dimensions and lessen the extreme transitions of the horn flare.  In order to make the impedance and frequency response more linear?  Or would One have to just build a non folded version of the Lab 12 to eliminate the Transition Resonances?

Sum-
Title: Re: Flare Rate Transitions
Post by: Wayne Parham on July 29, 2005, 03:59:30 pm

Even if the horn walls are perfectly smooth along the flare, the horn will still have impedance spikes.  This is the horn acting as a quarter-wave resonator down near the flare frequency.  It is a result of the discontinuity at the mouth, causing reflections back to the throat.  That is what sets up the tuned pipe resonance.  As the horn mouth is extended, the larger it becomes, the more these ripples are smoothed.

Title: Re: Flare Rate Transitions
Post by: Antone Atmarama Bajor on July 30, 2005, 01:44:27 pm
     I know that the mouth wavelength  I suppose width and length should cause a reflection.  As well as the throat.  

    I thought that if you had a hyperbolic flare that was perfectly smooth you wouldn't see any reflections until the mouth.  Except that we arent working with Perfect flair rates.  Or is it Tractrix Horns that only perform without any reflection until the mouth.

    I'm just wondering if any of the Reflections (impedance spikes) we see in say the lab Graph are related to the Folding/Flare Rate Transitions.  I would imagine that some of them would have to be dirrectly related.

    I was also wondering if we made our transitions at Tempered Wavelenth intervals if we could increase the musicality of Bass Horns.  Say make a transition every Half OR 1/4 Step?  Yikes thats a lot of transitions if you're trying to make a 3 Octave Bass Horn.

    Does anybody do this currently?
    If not don't forget to give me credit if you use it.

    Though we may ruin Atonal music if forever.

Sum-
Title: Re: Flare Rate Transitions
Post by: Wayne Parham on July 31, 2005, 04:38:47 am

A tractrix horn has a discontinuity at the mouth, just like every other finite horn.  Regardless of the shape, it will still have those impedance spikes even if the flare is uniform.  Non-uniformities may just make more of them.

Title: Re: Flare Rate Transitions
Post by: Peter Morris on August 01, 2005, 01:33:54 am
Any discontinuity in the horn is a potential point that will cause a reflection, but its related to the wavelength, the discontinuity has to be large in comparison to the wave length to cause a problem.

There are some java animations some where (??) on to the web that demonstrates how waves travel around corners at different frequencies and the related reflections.

The biggest discontinuity is the mouth exit, the usual way on minimising these problems is to use a mouth where the exit angle is quite large, approaching 180 degrees, or tractrix like in profile.  The trick is to minimize reflections and maintain the desired pattern control for a HF horn.

I think that most of the discontinuities in the LAB will not cause problems in the intended operating range. ---  distortion is not consider a problem with the LAB ---  

The only problem with the LAB is that you need so many of them, less than 4 to 6 of them in half space does not have enough mouth area and there will be a 1 / 4 wave length resonance, which is probably a good thing despite the distortion – you trade a little LF resonance for LF gain when you don’t have enough boxes.

Peter
Title: Re: Flare Rate Transitions
Post by: Antone Atmarama Bajor on August 02, 2005, 01:17:57 pm
     I'm not sure if the energy reflected back into the horn at transitions will be measured as distortion, but it certainly can cause phase cancellations and ringing And other such things.  

    I guess some of that can be minimized if your amplifier has a good dampening factor.

    I'm sure that the wave length relationships of flare expansion transitions would have some effect on the musicality of the horn.  Although making a horn had a flare transition every 1/2 step in relation to a tempered scale would make it incredibly difficult to build.  It would probably sound better than a horn that had transitions that fell somewhere in between the tempered intervals (At least for western Tempered music) since the reflections would accentuate Non Harmonious notes.

    I suppose you could make a horn that was "Just" tuned to one key but it would have limited application.

    I know a lot of you insist the wave forms are so big that the volume of the horn expansion is more critical than the flare.  
But I would venture to guess that a Sub Horn that can reproduce 3+Octaves Thats 36+ discreet 1/2step tempered intervals in music.  
The flare will not couple to all of those intervals equally if the flare is not fabricated to accommodate all of those intervals.

    Not that I'm going to rush out and build one that does now I'm not ready for something like that.  I think I'm going to start with my little vented box first.

    Anyhow just something to chew on.
"Tonal Harmony Is the Bastardisation of Western Music"- I forget

-Sum
   
Title: Re: Flare Rate Transitions
Post by: Antone Atmarama Bajor on August 04, 2005, 12:47:58 pm
27.5*2^(x/12)
Title: Re: Flare Rate Transitions
Post by: Michael 'Bink' Knowles on August 05, 2005, 04:20:58 pm
The only real way I see you getting the answer you are looking for is for you to construct a straight LAB horn and compare it with the standard design.

-Bink (free advice that make you work hard)
Title: Re: Flare Rate Transitions
Post by: Antone Atmarama Bajor on August 06, 2005, 06:11:06 pm
Hmm Looking at the impedance spikes in the lab sub they do look like they are Harmonically related, Fundamental, 2nd, 3rd, 4th but the impedance peaks don't look like the fall dead on to those intervals like I would expect.  Since they are all related to the Mouth, where does the frequency shift on the impedance peaks come from?

    So a hyperbolic horn will typically have impedance peaks that are Harmonic Related to the Low Cut-off of the mouth?

    Thats no fun.  How do we do away with them?

Sum-    

   
Title: Re: Flare Rate Transitions
Post by: Peter Morris on August 07, 2005, 07:31:37 pm
What you need to do is make a model in Hornresp.  Model it with perfect sides, a smooth exponential horn.

Just for the sake of the argument make it 250 cm long, a throat area of 500 and a mouth of 6000.  Put about 65 litres in the driver enclosure and drive it with a good 18 (BL 25, Mmms 200)

Now have a look at the impedance curve in half, quarter and one eight space and then look at it as an infinite horn.  Surprise surprise – as the mouth size approaches that of an infinite horn the impedance goes flat, no harmonically related spikes.  

Now try it as a collection of straight-sided conical sections – provided they approximate the horn curve there will be no additional spikes in the impedance curve, it will look exactly the same as the first “smooth” horn over the intended operating range.

You should now know where those spikes in the impedance curve come from and that for any practical horn there is not much you can do about it.

Bottom line is that the discontinuities in the thoat (excluding the mouth) will not cause a problem until the wave lengths become short in comparison to the size of the discontinuities.  There is not much you can do about the mouth other than use a large horn or stack of horns …. and the LAB and many other horns do sound very good!

If you want to go higher then the discontinuities are a problem (short wave lengths).  Have a look at Bill Fitzmaurices folded mid horn designs which use smooth curves to solve the problem. www.billfitzmaurice.com
Title: Re: Flare Rate Transitions
Post by: Michael_Elliston¶ on August 07, 2005, 07:42:19 pm
Peter wrote on Mon, 01 August 2005 17:33

Any discontinuity in the horn is a potential point that will cause a reflection, but its related to the wavelength, the discontinuity has to be large in comparison to the wave length to cause a problem.

The biggest discontinuity is the mouth exit, the usual way on minimising these problems is to use a mouth where the exit angle is quite large, approaching 180 degrees, or tractrix like in profile.  The trick is to minimize reflections and maintain the desired pattern control for a HF horn.

I think that most of the discontinuities in the LAB will not cause problems in the intended operating range. ---  distortion is not consider a problem with the LAB ---  

The only problem with the LAB is that you need so many of them, less than 4 to 6 of them in half space does not have enough mouth area and there will be a 1 / 4 wave length resonance, which is probably a good thing despite the distortion – you trade a little LF resonance for LF gain when you don’t have enough boxes.

Peter


There are some java animations some where (??) on to the web that demonstrates how waves travel around corners at different frequencies and the related reflections.



Do you prefer one large horn of 3200Litres volume? Wink
Remember that High Q resonances are less audible than low Q ones -from the articles Ive read.

http://www.diyaudio.com/wiki/index.php?page=basshorn

Two Davids


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Written by W.H.Geiger at 17 Jul 2003 17:33:17:

As an answer to: Re: Horn Flare vs. Segmentation written by mike.e at 16 Jul 2003 22:00:08:

ME,

David McBean has produced an excellent piece of work.
And, you get it for free! This phenomenon seldom occurs in life.

For some horn simulations in real time, see the work of David Burners

Thesis Defense
http://www-ccrma.stanford.edu/~dpberner/Defense/pres.html

Also his other publications may be of interest.
http://www-ccrma.stanford.edu/~dpberner/downloads.html

Note, that in this setting, resonances rule supreme.

Regards,

WHG

Title: Re: Flare Rate Transitions
Post by: Peter Morris on August 08, 2005, 12:54:17 am
Thanks Mike....

Bigger is always better, is it ???

The resonance issue - its not so much the Q but where it is in this case.  With the LAB its around 30Hz so its not that critical, at those frequencies its more about energy than defining the timbre of an instrument, but if it was at 80Hz then it would be an issue.

FWIW I did actually model (I had to pay for it) my 18 inch version of the LAB with a program that I guess you could say is like a finite element acoustic model – is capable of predicting a lot of things including directivity at various frequencies and will take into account anomalies in the horn  - The poor computer has to think for about an hour or so per run, anyway it agreed with what McBean simpler model had predicted.

BTW that nice power point presentation with all that math assumes plane waves propagating down the horn, which is what you would expect from the second order differential equation that describes the wave propagation in a horn – the problem is they’re not quite ... hence the problems with models.
Title: Re: Flare Rate Transitions
Post by: Wayne Parham on August 09, 2005, 01:11:47 am

I agree with the direction you're going here.  As the horn approaches an infinite horn, it becomes less resonant.  But as it is made smaller and smaller, it acts more like a tuned pipe.  There was a discussion similar to this in a thread on AudioRoundTable.com called "Basshorn or Transmission Line."  You might check it out and throw in your 2
Title: Re: Flare Rate Transitions
Post by: Michael_Elliston¶ on September 15, 2005, 10:44:30 pm
Peter wrote on Mon, 08 August 2005 16:54

Thanks Mike....

Bigger is always better, is it ???

The resonance issue - its not so much the Q but where it is in this case.  With the LAB its around 30Hz so its not that critical, at those frequencies its more about energy than defining the timbre of an instrument, but if it was at 80Hz then it would be an issue.

FWIW I did actually model (I had to pay for it) my 18 inch version of the LAB with a program that I guess you could say is like a finite element acoustic model – is capable of predicting a lot of things including directivity at various frequencies and will take into account anomalies in the horn  - The poor computer has to think for about an hour or so per run, anyway it agreed with what McBean simpler model had predicted.

BTW that nice power point presentation with all that math assumes plane waves propagating down the horn, which is what you would expect from the second order differential equation that describes the wave propagation in a horn – the problem is they’re not quite ... hence the problems with models.



Yes at LF its different WRT Q and audibility. The investigations by HARWOOD etc revealed Q=1 resonances in the midrange,and panel colourations needed much improvement.


Ive heard about FEA/FEMM having high price tags for the moment. As it becomes more common place it should reduce.

On a related note, cheap parallel computing is easier too through linux and cheap PCs.

Your saying that the waves arent quite plane waves? Are they generally distorted? Does it make an audible difference? Geddes has done some AES work on this too.