Martin Poulin
Direct radiator or horn?
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If using a horn mid-range and horn tweeter set up, doesn't it make sense to use a horn loaded bass and low-mid range too?
What better way to integrate subjectively with a horn… than with another horn?
Make sense? Hold on…not so quick.
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I'm talking about the 50-500 Hz range here folks, lower than 50 Hz, where horns aren't practical. A multiple subwoofer combination is by far the better option. Who wants to build 4 huge horns to get 20-50 Hz duty and then try and "hide" them in a room? Impractical.
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Direct Radiator or Horn mid-bass then?
I think matching efficiency is primordial. The average compression driver and horn combination achieves 106-108 dB in 1w/m. This is true for big 4" diaphragm compression drivers, most bullet tweeters… and everything in between.
Some ring type compression driver even reach 110 dB and 111 dB efficiency!
Horn loaded 8" drivers, like the B&C 8PE32, will also get ~110 dB 1w/m and cover 150Hz to 1 kHz!
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Direct radiator options:
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A - Single 12", 15" or 18" pro woofer, they won't give much more than 95-98 dB 1w/m. The ones that can reach 100 dB and above won't reach low enough for our intended range. So, "A" won't match efficiency of the midrange!
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B - Double driver combination does get 6 dB efficiency gain, but result in harder to drive 4 ohms load, which results in an average of 101-103 dB 1w/m. We’re getting closer to a match but we aren't there yet.
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C - A quad driver gets another 6 dB gain, but the impedance will drop to 2 ohms. Keeping the original 8 ohm impedance by using a parallel/series connection will result in only in a 3 dB gain vs the option "B" (2 driver in connected in parallel).
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Option C will result in 104-106 dB 1w/m efficiency which nearly matches the average compression efficiency of the driver. Therefore, I recommend using a quad driver.
Of course, 6 or 8 drivers could be used for even more efficiency gain, but this start to get real expensive.
Option C being the best: Buying 8 quality woofers with appropriate cabinet size and weight, quickly becomes out of reach for average user. Most professional companies instead compromise and choose option B. Others audio companies offer option C as their ultimate solution.
push pull arangement in bass to reduce distortion
A7 specs
push pull arangement in bass to reduce distortion
Besides dual or quad woofer choice, look how similar the professional offering is...
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If something work fine, why try to re-invent the weel?
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TAD, QSC, JBL, LW, Klipsch, pro audio technology
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Only LW differs from traditional horn set-up and offer a interesting mid-range line array option.
Besides the Vintage Altec stuff shown above (way too small bass front horn to be effective anyway), only Klipsch offers an option with mid bass horn. Its's called: The MCM Grand
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Pictured on the left is a Klipsch stacked 15" MWM style horn bass bin. (2x 15" woofer inside)
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Why Klipsch are they the only ones offering bass horn?
Yea, Klipsch are all about horns and their reputation is solid I guess, but is that a valid reason, heritage?
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They also offer the same MCM system but with a quad 15" bass reflex woofer instead of the horn. (picture below)
I wonder which version is the better seller?
I do think that horns are great as front load. However, to reproduce bass, they need to be hilariously big.
A 1:4 wavelength is the minimum (most aim for 5:16 ratio and this is still small), so 50 Hz horns need be at least 1.8 meters long... (~5.9 feet)
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This brings us to compromise #1. The bass horn must be folded to get to an acceptable footprint. By doing so, this limits the high frequency, and introduces all kinds of resonance and artifacts that are the result of horn folding.
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The above Klipsch Grande MCM "MWM horn" style has one big 180° bend, with a rear facing driver which reflects sounds onto the back wall. As sounds hits a wall, it makes the speaker enclosure resonate and this vibration creates distortion.
Using horns as compression devices, results in more pressure inside the chamber than traditional bass reflex speakers. As a result, vibration and resonance inside the enclosure worsen, which then requires proper bracing that is VERY important in horn construction.
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Below is frequency response of various Klipsch bass horns (all folded design)
Disregard anything above ~500 Hz, as they’ll be taken over by midranges and tweeters.
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None of them is exactly smooth. All have big dips between 150 Hz - 300 Hz, in our 50-500 Hz target range.
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The Big MWM horn of the MCM Grand Cinema line shows dips at both the 148 Hz and 320 Hz. Further, it’s dead below 70 Hz. Not reaching low frequency enough.
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Whilst the Jubilee horn isn’t too bad, its corner loaded to help with low frequencies, but shows issues at ~280 Hz. In the real world, speaker placement should not be corner loaded for proper imaging and soundstage rendering. That being the case, the Jubilee bass response would not be as good as the graph suggests.
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I know what I'm NOT building...
Finally, here’s an interesting read from the then head engineer at Electrovoice.
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The article is dated 1976, which was likely the pinnacle of horn design. However, take a look at the DR Keele conclusion.
That was back in a time when the average amplifier rarely exceeded 70 watts and efficiency was considered very important.
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Times have changed. Today, pro amplifiers routinely exceed 1000 watts, rendering ultimate high efficiency goals obsolete.
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Direct Radiator Woofer Development.
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The average 15" woofer can now take up to 5X the power of a 1970 driver. Thus being able to play much louder as a direct radiator.
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The average pro 15" woofer comes in up to 15mm Xmax vs. the older design 4mm to 5mm Xmax. (average 3x more displacement)
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Voice coils are bigger than ever now, this limit power compression. A 4" voice coil size is my minimum size for 15" woofers. Some 18" woofers actually have huge 6" voice coils!
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Voice coils have clever forced air cooling methods, resulting is reduced power compression than historical "non-vented coils".
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Physics:
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Horn oriented woofers are limited mainly by 2 Law of Physics. Magnet strength and moving mass.
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The magnetic strength of woofer motors (unity of such measure is called: BL), BL is still about the same, as magnet technology remained the same. A BL of 16-25 range is still the norm for pro-style woofers, and a BL> 30 is almost impossible as you reach point of magnetic saturation.
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Horn woofer must have high BL and no improvement in that regard have occurred in 30+ years.
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Woofer cone mass is also about the same today as it was in the 1970's.
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This moving mass does limit driver ability to reach high frequencies, as 1970’s paper cone weighed about the same as today's paper cones.
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Bass-horn high frequency limits will roll-off as a result of such mass, often in the 300-500 Hz range for a 12" driver and lower for an average 15".
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No real progress in cone mass reduction either.
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If Dr Keele was to conduct his tests today, I think the average 4 x 15" bass reflex arrangement would be much stronger choice no matter what horn it's compared against. Newer direct radiator driver progressed tremendously while horn oriented woofers didn't.
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To put one more nail in the coffin of bass-horn, read Wayne Parham’s post about horns:
https://audioroundtable.com/forum/index.php?t=msg&goto=74408&
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My Personal Journey: Nov 2018
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When I moved from a single sealed 15" to dual sealed 15" woofer, both subjective improvement and dynamic slam were not subtle. Within 5 minutes, there was no turning back. Ever!
I now use a quartets of 15" woofer per side, subjective gains are even better.
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I can afford to have a sealed cabinet resulting in limited low bass, as I have 18" subwoofer kicking in below 50 Hz. If I was more limited however, I’d go bass reflex in order to maximize low frequency extension, per commercial offering shown above.
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I'm using the Fane 15XS Prime. (Direct radiator oriented woofer)
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Please, have a look at my "quarter pie horn" page, I've built a prototype of a decent design midbass horn
I've put sound clip
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