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27th May 2008, 12:56 PM  #71  
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Join Date: Jun 2007

Quote:
Many thanks for the above information. Hornresp uses the same isophase wavefront model for all horn flare types (including the OS waveguide). Not having any measured results to compare the OS W/G predictions against, there is probably little that I can do at this stage to improve the generalised model. It is interesting to note that it appears to be only with OS waveguides that the predicted versus actual results are significantly different (reportedly). I see that you have assumed corner loading in your calculations. Perhaps the results might have been closer if 2 Pi had been used? Kind regards, David 

28th May 2008, 10:02 AM  #72  
R.I.P.
Join Date: Oct 2005

Hello David,
Thanks for your reply which confirms my thoughts. You'll find as attached file 2 simulations performed under Hornresp using the 2 Pi solid angle as you recommanded. Please note that I choose the same dimensions for the horn (Fc = 160Hz as in the Azura 160 horn and the waveguide. Here is a picture of the Azura 160 horn http://www.azurahorn.com/images/Yama...B_on_AH160.jpg and its measured response is here: http://www.azurahorn.com/6681_on_160.pdf The simulations performed with Hornresp confirm what I know since my reading of H. Olson (e.g. fig 5.5 p.105 in Acoustical Engineering, Ed. Prof. Audio. Journ. 1991): throat impedance for a conical horn (and an OS waveguide behaves very much the same regarding throat impedance) is very low in the low frequency domain compared to the throat impedance of an exponential horn or an hypex horn. From this lesser loading at LF of the diaphragm there is with the conical horn or the waveguide a lost in efficiency in the conversion by the driver from electrical energy to acoustical energy (at frequencies at which exponential or hypex horns still have a high efficiency). I was puzzled the first time when Earl Geddes wrote on AudioAsylum that the LF part of the reponse of different horns and waveguide (... of comparable dimensions) is not related to the profile between the throat horns and waveguide : see: http://www.diyaudio.com/forums/showt...56#post1521956 "Any device placed on a compression driver loads the same above some LF point. They will all approach the characteristic impedance of air divided by the throat area. So everything "loads" the same. Thats why I ignore "loading" as irrelavent. Loading and cutoff are both concepts that are not relavent factors in a device whose primary intent is to control directivity." As he possess a recognized authority even I thought that I was minsinterpreting his thought and pass over. What do you think of Earl Geddes formulation? (I'll undesrtand if you don't reply...) Best regards from Paris, France JeanMichel Le Cléac'h Quote:
Earl Geddes: Any device placed on a compression driver loads the same above some LF point. They will all approach the characteristic impedance of air divided by the throat area. So everything "loads" the same. Thats why I ignore "loading" as irrelavent. Loading and cutoff are both concepts that are not relavent factors in a device whose primary intent is to control directivity. 

29th May 2008, 09:07 AM  #73 
diyAudio Member
Join Date: Jun 2007

Hi JeanMichel,
Many thanks for the additional information. The comparison of predicted vs measured results for the Azura 160 horn is very interesting. The low frequency rolloff point seems to have been reasonably accurately predicted. Are your simulation results showing the constant directivity power response, or the onaxis pressure response? "Any device placed on a compression driver loads the same above some LF point." The above statement can be readily verified using Hornresp, by checking the acoustical impedance charts for different horn types having identical throat areas. At 20000 hertz the throat acoustical impedances are effectively the same for all types (normalised resistance ~ 1, normalised reactance ~0). With regard to the OS waveguide itself, strictly speaking the device is actually a horn and not a waveguide (by definition, a waveguide is a nonradiating transmission line with the propagated energy being "guided" from one point to another). Also, from a physics and geometry viewpoint there is nothing really special or unique about the 'OS waveguide'. It is simply yet another horn flare, asymptotic to a conical horn, but with a transition at the throat which facilitates the wavefront shape changing from plane to curved as the sound propagates down the horn. Kind regards, David 
29th May 2008, 10:27 AM  #74  
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Join Date: Oct 2005

Hello David,
In both examples, the curves are related to constant directivity response. Thanks for your remark about Earl's sentence. As in the discussion it came when we disussed comparison of the efficiency at low LF between horns and waveguides, I was thinking (probably wrongly) that this not well defined "some LF point" was situated by example in the frequency domain for which the resistive part of the acoustical impedance was a fraction ( e.g. "half") of the resistive part of the acoustical impedance at highest frequency. Also I did a parallel with what Earl Geddes wrote in his reply to the AudioXpress editor" http://www.gedlee.com/downloads/Horn...ry%20reply.pdf B. Kolbrek wrote: "Well at least the horn equation is good for predicting the shapes that one needs in order to achieve good impedance loading – at least at low frequencies!” E. Geddes: "Sure, except that it turns out that virtually any shape connecting a given throat and mouth area will yield approximately the same impedance – within a dB or so. In the long wavelength region where the Horn Equation is valid, shape is simply not a significant factor, the waves don’t see shape. Basically they only see the inlet and the outlet areas and the distance between them and everything else is of secondary importance." I interpreted this, and I am not the only one (it was even translated by someone on a French audio forum), as if Earl Gedddes was saying that whatever is the flare shape the throat impedance was the same at low frequency for horns and waveguides having comparable dimensions... But this is probably due to my poor knowledge of the arcanes of the English language. Best regards from Paris, France JeanMichel Le CLéac'h Quote:


30th May 2008, 02:48 PM  #75  
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Join Date: Jun 2007

Quote:
Thanks for the extra background information  I can now see why there could be some confusion out there :). The above statement is certainly true for very short horns and for long slowflaring horns where the mouth area is not much larger than the throat area. Following are the measured throat impedances for an exponential horn and a tractrix horn having similar throat areas, mouth areas and axial lengths. Based on this data, I leave it to you to decide whether the comment can be applied generally to all horns :). It would be really interesting to be able to compare the measured exponential and tractrix throat impedance results against those for either a parabolic or conical horn having identical dimensions. To complete the picture, I have also included Hornresppredicted results for the above exponential and tractrix horns (Ang = 2.0 x Pi, S1 = 20.27, S2 = 2307.22 and L12 (Exp and Tra) = 55.90). The Hornresp data was exported to Excel and converted to show values of ka (2 x Pi x f / 34400 x 27.1) rather than frequency along the xaxis, to make it easier to directly compare the predicted and measured results. You can see that the predicted resistance and reactance curves for the exponential horn have the same overall characteristic shapes as the measured results for the exponential horn. Also, the predictions for the tractrix horn compare reasonably well to their respective measured results. The comparisons are certainly close enough for the purposes of calculating theoretical power responses (to an acceptable degree of accuracy). If the Hornresp predictions for an equivalentlydimensioned conical horn are anything to go by, then the above statement could probably be justifiably questioned :). Kind regards, David 

30th May 2008, 02:50 PM  #76 
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Exponential  Measured

30th May 2008, 02:51 PM  #77 
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Tractrix  Measured

30th May 2008, 02:53 PM  #78 
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Exponential  Predicted

30th May 2008, 02:54 PM  #79 
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Tractrix  Predicted

30th May 2008, 04:46 PM  #80  
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Join Date: Oct 2005

Hello David,
Thanks for the reply. I can see that we agree on a possible questionning about Earl's sentence ;) Thanks also for the impedance measurements (is it possible for you to indicate where did the measured curves of the acoustical impedance were published). Too bad I gave my conference about "axisymetrical horns" last saturday, my introduction to Hornresp would have been improved with those data. Best regards from Paris, France JeanMichel Le Cléac'h Quote:


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