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27th February 2008, 01:35 PM  #1271  
diyAudio Member
Join Date: Feb 2007
Location: London

Quote:
Just to check  this is the same as a single TH115 in a corner (0.5*pi) ? Cheers Ian 

28th February 2008, 12:26 AM  #1272  
diyAudio Member
Join Date: Feb 2007
Location: London

Quote:
I've done some modelling in Hornresp of what I think is something close to the TH115 (but see comments about simulated vs. measured results) and a dual 18" reflex box with the same external size and cutoff frequency  all results are for a single box in half space (but both boxes gain the same 10dB in a corner or a block of 4 according to Hornresp). I know this is simulation only, but it's the only way I can see of doing a comparison of the two designs with a level playing field  if there is any systematic error in Hornresp's calculations it should at least apply to both designs equally, so the differences should still be correct. Box designs  For the TH115 I'm using the B&C 15TBX100 in a 3m long conical tapped horn (tap points 20cm from each end) with 3:1 CR (best result I could get) and 2000cm2 mouth  this gives just over 300l net volume which I calculate is what is inside the TH115 allowing for all the internal partitions and the bottom rear corner chamfer. For the reflex box I'm using 2 B&C 18PS76 with 320l net volume, (allowing 30l for ports which is a 10" diameter port per driver) tuned to 37Hz. Simulations for box this are with 2V applied to allow for 4ohm nominal impedance (2.82V for tapped horn). Response curves  The tapped horn has an impedance curve which is very close to the TH115, but in spite of lots of tweaking I can't get either a flat response or one that's quite as efficient as the one in the data sheet  from 37Hz to 100Hz it varies from 99dB/W to 102dB/W (3dB ripple), average is about 100.5dB/W, 6dB point from this average is 34Hz. ==> Tom, can you explain why the measured results in the data sheet look better than this? (I have noticed the poor frequency resolution, also the differences between the current data sheet response and the one I have saved from a couple of years ago :) The reflex is not quite maximally flat, from 38Hz to 100Hz it varies from 100dB/W to 101dB/W (1dB ripple), average is 100.5dB/W, 6dB point from this average is 32Hz. Maximum SPL  Maximum cone travel for the tapped horn with 2.82V applied is 0.49mm at 47Hz (also at 33Hz). Mathematical Xmax for the 15TBX100 is 9.5mm, so worstcase power handling is 380W without exceeding Xmax, which is over 4dB less than the 1000W continuous power handling of the driver. This limits peak output to 126dB at 50Hz and 119dB at 33Hz (average would be 130.5dB at 1000W without Xmax limit). Maximum cone travel for the reflex with 2V applied is 0.29mm at 51Hz (also 32Hz). Mathematical Xmax for the 18PS76 is 9mm, so worstcase power handling is 980W without exceeding Xmax, which is 1dB less than the 1200W continuous rating for the drivers. This limits peak output to 130.5dB at 50Hz and 125dB at 32Hz (average inband output would be 131.5dB at 1200W without Xmax limit). Comparison  The 15PZB100 costs 225 euros+VAT, the 18PS75 cost 167 euros each + VAT which is 50% more for a pair than the single 15PZB100, but I would expect the extra cost of building the tapped horn box would compensate for this. According to Hornresp (which might not agree with your SPL measurements for sensitivity, but the cone travel results are similar to those in your white paper) we have the following:  average sensitivity 38100Hz is 101.5dB/W for both boxes (single box on floor, both increase by 10dB in corner or block of 4)  inband ripple is 3dB for tapped horn, 1dB for reflex  6dB point is 34Hz for tapped horn, 32Hz for reflex  Average SPL at rated power is 130.5dB for tapped horn, 131.5dB for reflex  Xmax limited SPL inband is 126dB for tapped horn, 130.5dB for reflex  Xmax limited SPL at 6dB point is 119dB for tapped horn, 125dB for reflex Conclusions  Sensitivity is similar for both boxes, dual reflex has flatter response and higher Xmaxlimited SPL across the operating bandwidth. Even allowing for a 2dB possible sensitivity discrepancy between simulations and measurements in favour of the tapped horn (which I hope Tom can explain), the Xmaxlimited maximum SPL for the tapped horn is still considerably lower. Note that I've allowed a huge 10" diameter port for *each* driver so maximum velocity even at 32Hz is only 11m/s, which should be well below the point where significant port choking or chuffing happens. Before anyone says "it's not fair comparing 2 18" drivers to a single 15"  remember, nobody really cares what's inside the box, only results (size, weight, cost, frequency response, maximum SPL). On this basis it seems that the dual18" reflex does indeed equal or exceed the performance of the single15" tapped horn. I await comments with interest... :) Cheers Ian P.S. If Tom can show otherwise I'll be perfectly happy to be wrong! 

28th February 2008, 02:06 PM  #1273 
diyAudio Member
Join Date: Jul 2005
Location: Goderich

Several members, including myself, have noticed that the calculated amount of excursion is nowhere near the actual excursion in a tapped horn.
For instance, mine full tilt(1400WRMS) with a 40Hz tone, is moving only 5mm( I can't accurately measure it, but its much less than the 22+mm its capable of) each way or so. 
28th February 2008, 02:18 PM  #1274 
diyAudio Member
Join Date: Nov 2006

This brings up an old question of mine and others from a few months ago. Maybe itīs worth investigating actual excursion from built horns and compare it to the simulations. Although I havenīt done measuerements of excursion on my built horns so far, my feeling, too is that cone movement is less than predicted.

28th February 2008, 02:33 PM  #1275 
diyAudio Member
Join Date: Jan 2006

Interesting observation. When i made a TH out of my old br box drivers, my first impression also was, that it could get much louder than before. I have no measurement equipment that could validate this impression, but it impressed me alot

28th February 2008, 02:37 PM  #1276  
diyAudio Member
Join Date: Feb 2007
Location: London

Quote:
It's not what the driver is capable of that matters, it's how much the driver actually moves at a given frequency and drive level compared to how much the simulations say it should be moving. If they're different this would tend to suggest that the calculations in Hornresp are in error rather than the laws of physics being wrong :) David, do you have any comment on this? Ian 

28th February 2008, 02:43 PM  #1277  
diyAudio Member
Join Date: Feb 2007
Location: London

Quote:
I'm sure that if simulations disagree with real life then the simulations will be the source of error, Scotty had it right here... Ian 

28th February 2008, 03:26 PM  #1278 
diyAudio Member
Join Date: Nov 2006

Exactly thatīs my point. If nobody double checked excursion on an actual real TH  how can we be sure that the simulation is 100% correct? After all  simulation is one thing, checking up on reality is another. Without the latter it remains speculation. Not everything is predictable by an simulation. The often mentioned effects of portlosses in a reflexdesign are one example. This has to be measured in real life.
Itīs only fair to raise an eyebrow if a well known driver seems to handle much more input power in a TH than in a reflexdesign. One key to explain this observation is to check up on excursion. I know of several simulation programs which calculate excursion not as peakpeak but as "effective excursion" (donīt know if thats the correct english terms). AjHorn for example does this. So a result of 1mm excursion in the simulation has to be mulitplied by 2.83 in order to get the "real live movement" from peak to peak (or multiplied by 2.83 / 2 to get + results comparable to the numbers in specsheets). I donīt know how hornresp handles this, but I am sure not everybody is aware of this when using ajHorn so many misinterpretations can occur. 
28th February 2008, 03:31 PM  #1279  
diyAudio Member
Join Date: Jan 2006

Quote:
The real problem is, that you want other people to do the work for you. If you really want to achieve something, then DIY. 

28th February 2008, 03:51 PM  #1280  
diyAudio Member
Join Date: Feb 2007
Location: London

Quote:
If there is any error in excursion calculations  like x2 from confusing peak and peakpeak  this would almost certainly apply to all simulations, since the same equations are used everywhere. So any differences in excursion between different boxes are likely to be correct  however if the absolute numbers are wrong then this changes lots of conclusions... In the Hornresp simulations I did for the "TH115" (as close as I could guess) the maximum excursion was +/0.49mm at 47Hz with 2.82V applied. In Tom's white paper (which refers to the TH115) he says that the peak excursion with 63V applied is just over 6mm (I assume this means +/6mm) at 46Hz, which would give 0.28mm with 2.82V applied. This is just over half the number predicted by Hornresp, which is not good :( If Tom's figure is right then this also explains why my calculations showed the TH115 to be Xmax limited at 380W; with his excursion figure this goes up to 1160W which then exceeds the thermal rating of the driver  so my conclusion that the TH115 was Xmax limited is then wrong. I did clearly say that all my figures were based on Hornresp simulations and the assumption that these were correct, and that "If Tom can show otherwise I'll be perfectly happy to be wrong!" (my exact words)  and I stand by that :) Cheers Ian 

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