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Hybrid compression ratio

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Young Croc
Young Croc


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    Posted: 10 June 2018 at 2:11pm
I did a few sims of cubo-like long flared port reflexes and the efficiency seems really quite amazing. The outer dimensions are also around 150-160l which once built would still probably be below 200 so this looks very promising. The design is a small chamber about 70l with around 400cm^2 'port' area or throat depending how you think about it.

In a hybrid the rear horn can be thought of as a port so something like 400cm^2 would be a generous port area (same rear volume modelled in winISD for normal reflex gives below 20m/s air speed). For a horn this would be around a 3:1 compression ratio which for my quite reflexey drivers sounds like quite a lot. Is there some sort of accepted rule regarding this?


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Old Croc
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Post Options Post Options   Thanks (0) Thanks(0)   Quote mini-mad Quote  Post ReplyReply Direct Link To This Post Posted: 10 June 2018 at 5:23pm
Have you tried asking Mr Cubo himself what rule of thumb he likes to use? That could be a good starting point.. 


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Young Croc
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Post Options Post Options   Thanks (0) Thanks(0)   Quote mobiele eenheid Quote  Post ReplyReply Direct Link To This Post Posted: 10 June 2018 at 9:01pm
For an 18" hybrid I would start at an 1.5 - 2.5 (:1) compression ratio. 1.5 for 30 - 35 Hz content, 2.5 for 40 - 45 Hz and up. However for something like an 18NLW9600 I would look up to about a 3.5 compression ratio. A 70 litre chamber for an 18" sounds pretty all-round.

400 cm^2 for a regular reflex sounds generous enough, for a hybrid you can squeeze the start of the horn/ port a bit but only if it becomes so much larger then 400 cm^2 near the end. If the last part of the horn forms the correct (special) end termination (of the 400 cm^2 port, you can get away with much smaller (then for reflex ports desired) throat area and average cross sectional area as well.

The more you tell me, the more specific I can be (driver Sd, cabinet f3, outer dimensions, internal cabinet volume, stuff like that.

Cubo/ Johan
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Young Croc
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Meat Quote  Post ReplyReply Direct Link To This Post Posted: 10 June 2018 at 9:42pm
Thanks Johan,
I'm going to have to redo the hornresping as I managed to not save it - I think I overwrote it with data for a cubo sub trying to work out where my design was in comparison to something tested and working. Once I get back to where I was with the sims I'll put some data up. The drivers I have are BMS 18N850v2.

When you say special termination what do you mean? Something like a cosine horn?
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Young Croc
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Post Options Post Options   Thanks (0) Thanks(0)   Quote mobiele eenheid Quote  Post ReplyReply Direct Link To This Post Posted: 17 June 2018 at 11:07pm
I'd say under normal reflex circumstances 400 cm^2 is as low as I would go for the 18N850v2, for a hybrid or horn (particular for this driver) that would be indeed quite small. In this case that's mostly acceptable because of the large compression chamber. The larger the compression chamber, the less the compression ratio matters. Another way would be use that special termination.

With special termination I was aiming at a large flare in front of the port, somewhat like a hyperbolic horn with T: 0.00 (approximated by 2 conical segments). The port and flare length combined regulate the lower tuning and a large horn area gives high output at the upper part of the frequency range. If the first part (the port) is so small that the turbulence becomes so high, that you can hear it (at the end of the port), the flare can also help to ease down the turbulence, so that it becomes in-audible again by the time it reaches the horn mouth.

To be honest I wouldn't use the BMS in a hybrid design smaller than Cubo Sub, because it will take a hybrid design of that size to align the full excursion capabilities, with it's rated power handling. Unless you're playing sine waves all the time, in which case you design is spot on. For most music styles however even a plain reflex, in combination with a suitable high pass, should be nearer 200 - 250 L imo to prevent it from burning way before it's tearing.

Johan 
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