increasing upper rolloff corner |
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Ibex
Young Croc Joined: 27 May 2009 Status: Offline Points: 1013 |
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Posted: 10 August 2010 at 6:56am |
Cheers!
Referring to the formulas of Keele's low-frequenzy horn design using thiele/small driver parameters the upper rolloff corner is given due to the effects of driver moving mass: fhm=2*fs/Qts Is this the same for mid-range horns? My thought was to increase the drivers fs in order of a small as possible rear chamber to get an extendet operating range du to higher upper roll off corner. (ie: 18Sound 8NM610, RCF MR8N301/MR10N301) Does this work with every driver? gzg |
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_djk_
Old Croc Joined: 23 November 2004 Location: United States Status: Offline Points: 6002 |
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http://www.xlrtechs.com/dbkeele.com/PDF/Keele%20%281977-05%20AES%20Preprint%29%20-%20LF%20Horn%20Design%20Using%20TS%20Paras.pdf
Yes, it is the same for midrange horns. A small rear chamber can improve response at Flc (can be modeled in Hornresp) Reducing Vfc (for instance, with a phase plug) can extend the Fhc |
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djk
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Ibex
Young Croc Joined: 27 May 2009 Status: Offline Points: 1013 |
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thx _djk_, already know these papers....
Am I right, that only the free air resonance frequency of the driver and not the resonance frequency of the whole system (driver+rear chamber) has influence on the flh upper rolloff frequency? what about the above mentioned drivers including a sealed rear chamber? which thiele/small parameter do I have to use to simulate them in a flh with hornresponse and what value would be correct in case of the rear chamber? Anyone experienced with these drivers? |
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Ibex
Young Croc Joined: 27 May 2009 Status: Offline Points: 1013 |
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Out of the Keele papers:
What about the case if fc is higher than flc (that would be in the case of a midrange horn)...? |
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_djk_
Old Croc Joined: 23 November 2004 Location: United States Status: Offline Points: 6002 |
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In the case of the drivers listed above that have chambers, it is the system resonance and the Qtc that dictate the mass corner. It should give the same result as the free air Fs and Qts.
You would have to contact the manufacturer of the driver to either get the enclosed rear volume, or the free air Fs and Qts of the driver. You are probably not going to reactance annul a mid horn design anyway, so just stick in a number and see what happens. |
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djk
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Ibex
Young Croc Joined: 27 May 2009 Status: Offline Points: 1013 |
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so, the ratio of Fs/Qts and Fc/Qtc is always the same?
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Meat
Old Croc Joined: 05 June 2009 Location: Manc Status: Offline Points: 1514 |
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Could a Linkwitz transform be used to reduce qts and make the upper corner move higher up? I've previously asked about using these in bandpass horns to make them go lower in singles by increasing it but the answer was that the circuit wouldn't handle enough power and would waste too much energy, surely in a midrange horn where the power levels are lower this could work.
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Ibex
Young Croc Joined: 27 May 2009 Status: Offline Points: 1013 |
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Fhm of the Ciare 6.38MR3 is around 1.6khz, but the recommended upper x-over frequency of the Limmer 042 is around 4.5khz incl. a phase plug.
Does a phase plug extend the response above Fhm or is it only by the use of such a phase plug possible to reach the upper rolloff corner of a driver? Res5 must be awesome with an operating range for the 8" from 450hz up to 6khz! I wish a driver like this would be available for DIY! Even the void air motion 8" midrange only goes up to 1.7khz... |
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doober
Young Croc Joined: 03 January 2006 Location: Cornwall UK Status: Offline Points: 1118 |
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Fhm is the beginning of a 6db/octave rolloff. This can be balanced (to some extent) by the horn's narrowing dispersion with rising frequency to make the on axis response more flat. The power response still falls off, but this isn't a problem, it's the sensitivity which matters.
Fhvc (12db/oct) and fhc (18db/oct) are more serious extension killers. The 6.38mr3 has fhvc above 7khz. A 65cc front chamber will get you up to 4.5khz, this is where a good phase plug comes into it. Kieran |
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_djk_
Old Croc Joined: 23 November 2004 Location: United States Status: Offline Points: 6002 |
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"Could a Linkwitz transform be used to reduce qts and make the upper corner move higher up? "
A Linkwitz transform doesn't change the actual Q of the system, it's just EQ. So from that standpoint, yes, you can EQ the system to extend the bandwidth.
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djk
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Ibex
Young Croc Joined: 27 May 2009 Status: Offline Points: 1013 |
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Thx Kieran! Now a lot of things seem to be clearer! So, does it make sense to choose a driver with a low mass corner frequency and a high voice coil inductance corner frequency and squeeze the usable passband between these two in combination with a 6db/oct rising midhorn response to get it flat? (low horn cutoff = mass corner, high horn cutoff = inductance corner) gzg |
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doober
Young Croc Joined: 03 January 2006 Location: Cornwall UK Status: Offline Points: 1118 |
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You still need the highest mass corner you can find. There is no problem using the driver below it's mass corner.
The effect of the horn on the sensitivity depends on the horn's shape and size, but generally the lower frequencies will spread more than the higher, hence you will need a larger efficiency (power response) at the low end to get a flat response. Kieran |
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