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njw 
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Joined: 26 March 2010 Location: S. Wales Status: Offline Points: 2700 |
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 Posted: 23 April 2025 at 8:41pm |
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That is the most sensible post in the thread!
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Line Array
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Joined: 19 March 2022 Location: New Jersey, USA Status: Offline Points: 204 |
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Posted: 23 April 2025 at 9:17pm |
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the mistake you're making is you're looking at just the cost of the driver, which makes it seem like RCF is the better value because you get 1/2 performance for 1/3 the price. but if you look at the cost of the box as a whole ( the cabinet and the drivers ) and see how much boxes with top drivers cost versus how much boxes with entry level drivers cost then at least here in US it would be like $2,000 for an entry level box and about $7,000 for a top level box, so an upcharge of $5,000 ... but in case of upgrading just the drivers it's only $1,500 more, which then makes the more expensive driver actually the better value ! now you may say that you can build two boxes with RCF for less than one box with 18Sound ... but do you really want to use two boxes to get the output of one ? i mean how do people justify 1RU amps that barely save any space and even less weight and then be willing have an entire separate box of several cubic feet size and over a hundred pounds in weight i will give you this though - two RCF boxes will be a LOT more efficient than one 18Sound box ... that is you will only need HALF the power to get same SPL ... so if it's a permanent install and you don't have to constantly move these boxes then the RCF is the better value ... as for the G-Sub being able to go lower than a full range top ... if that is the case then it is only because dual 18" drivers have 3 times more surface area than a single 15" but if you were to use three 15" woofers from SRX835 in place of two 18" woofers from SRX828 you would basically get the same low end output because they are basically the same driver with just a larger cone. here: the 12" 15" and 18" are basically the same. the SRX basically uses ferrite versions of these. i honestly think good subwoofer drivers are overpriced. i think they're taking advantage of the consumer. it doesn't cost 3 times more to build the driver right. they charge 3 times more because there is 3 times more value in that driver because it allows you to get more output from fewer boxes and people are willing to pay for this. but i can see how if you just want to save money the RCF route allows you to do this by accepting a bit more mid-bass oriented tune, than a true sub-bass tune. it isn't "wrong" per se, you just have to believe that the fundamental doesn't matter, which many people do believe. personally i don't even think it's a question of whether it matters or not but rather if it is worth the $$$ to reproduce it. if you can give up that fundamental then the RCF box will go as loud as the 18Sound box and do so for 1/2 the price and using 1/2 the power. whether it would still be a "sub" is then a philosophical question. Edited by Line Array - 23 April 2025 at 9:27pm |
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ArgustusThe3th
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Joined: 15 January 2025 Location: Brussels Status: Offline Points: 9 |
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Posted: 23 April 2025 at 9:30pm |
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Hey guys,
Well i must tell you that i chose and bought an amp. I am new to this and i loved building these subs. This was my first build and i loved every part of it. I also learned a lot and i know my next build will be much better. At least thats the goal  .The amplifier i chose is the IHOS D3600. The next posts on this feed will probably be about how stupid i am that i chose this amp but i really dont care. I am a student on a budget who's looking for fun projects about the stuff i like and learning a lot about audio and speakers. I know my builds wont win prices but i am ok with that. I could not be happier with the result. They sound great, its warm and powerful and i am ready for my next project. thanks for the tips and recommendations.
Edited by ArgustusThe3th - 23 April 2025 at 9:32pm |
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fudge22
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Joined: 26 July 2022 Location: UK Status: Offline Points: 263 |
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Posted: 24 April 2025 at 1:18pm |
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By half the motor force, I assume that you mean force as in Newtons, (the unit) produced by the magnet and coil. I also assume that BL^2/Re is the expression that you use to calculate the force and by BL^2 you mean the force factor squared. B being the flux density, with the unit Tesla, L the length the coil with unit of metres and Re being a resistance, unit ohms. The unit Tesla, is equivalent to Newtons per Amp times metre – N/(A.m) which can be substituted for the units of B. When you combine all the units in your expression, I don’t see how you end up with just the unit Newtons. If you don’t end up with just the unit Newtons, the result is not a force, and your expression is wrong. I’ll explain using the expression that I learned, because it eliminates squaring anything, as well as being correct. Force = Bli That is force equals flux times length times current, with units of Tesla, metres and Amps. For the pedants, yes I know that the sine of the angle between the magnetic field and the wire should be considered, but in loudspeakers, the angle is usually 90º, therefore is just 1. It is also unit-less, so has no impact here. So, substituting N/A.m for Tesla in the equation, the expression for the units is: Newtons x metres x Amps / Amps x metres The metres and Amps cancel out leaving Newtons, a force. One Newton is the force required to accelerate one Kg by one metre per second squared. We can therefore substitute these units for force. Force = Kg.m/s^2 With your expression I get the result Kg/s, which is mass (Kg) flow rate (per sec)
Next post. You are not stupid. Good engineering is coming up with a satisfactory solution a big part of which is usually within budget. |
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Line Array
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Joined: 19 March 2022 Location: New Jersey, USA Status: Offline Points: 204 |
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Posted: 24 April 2025 at 4:51pm |
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i did not invent BL^2 / Re it is a widely accepted metric for judging subwoofer motor strength strength, not force. units do not matter because it is only used to compare different motors to each other. it is not used to calculate anything. some drivers like this one: advertise BL^2 / Re as one of the specs ( 390 in the case of this 33" woofer ) but generally only Chinese drivers list this parameter because only Chinese can do math these days ... the logic goes like this: if you were to take two motors and put them either in series or in parallel the motor strength should double. ( two arms are twice as strong as one ) let's say the motor is 8 ohm and 32 BL. this would be a fairly typical subwoofer. then BL^2/Re = 128 put them in series it's now 16 ohm and 64 BL. then BL^2/Re = 256 put them in parallel it's now 4 ohm and 32 BL. then BL^2/Re = 256 it doubles no matter how you wire it, so the formula is correct. so this B&C subwoofer for example in 8 ohm: has BL of 26 and Re of 5.3 which comes out to BL^2/Re of 127 or basically the same as our imaginary 8 ohm / 32 BL subwofer. the 33" woofer i linked has 390/127 = 3.1 times the motor strength and also (33/18)^2 = 3.3 times the cone area of that B&C. so it's actually a fairly balanced driver for something that at first appears like a practical joke. of course compared to three B&C drivers this 33" unit has much larger moving mass ... and much longer Xmax ... so it's more of an infra-sub ... but at least they were TRYING to get motor strength in the ballpark of what a 33" cone needs ! it really should have been even higher because a driver with larger moving mass really needs stronger motor, and that driver has like TWO KILOGRAMS of moving mass. but to get the motor strength even higher would make the driver so heavy nobody would be able to lift it. but like i said, at least they tried. i could probably come up with an even moar brillianterer formula that would literally tell you how much $ a subwoofer is worth based on TS parameters alone. BL^2/Re would be part of it but it would have to also take Sd, Xmax, Xmech, MMS and Le/Re MMS and Le/Re being bad while SD, Xmax and Xmech being good. because reality is getting strong motor isn't that hard - just wind a lot of copper on the former - what is hard is doing it without driving up MMS and Le in the process. JBL already came up with a similar formula but for some reason they called it "mass break point frequency" and applied it to compression drivers. their formula is basically BL^2/Re/Mms with some constant factor thrown in. HornResp also uses this formula to model HF rolloff - none of which makes sense. i call BS on them. there is not a single place on the internet that explains what causes mass break point frequency. it is a total mystery. Edited by Line Array - 24 April 2025 at 5:23pm |
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fudge22
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Joined: 26 July 2022 Location: UK Status: Offline Points: 263 |
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Posted: 24 April 2025 at 10:30pm |
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I didn’t think that you did. However in your previous post, you did write motor force, not motor strength.
I think that NASA would beg to differ about the importance of units, and if units do not matter, why does it have units of Kg/s? Why is mass flow rate an indicator of subwoofer motor strength? Plus, what is squaring the force factor and dividing by the resistance, if not calculating? Assuming that you are correct, doing a comparison of the RCF and 18sound drivers gives the following results, using BL^2/R and the manufacturer’s figures. RCF 23.5^2/5 = 110 Kg/s 18sound 25^2/6,8 = 92 Kg/s It gives the RCF driver a slightly higher “motor strength”, despite the slightly lower force factor (Bl).
Will your formula include tariffs? Sorry, I digress; Mms is the combined diaphragm and air load mass. The air load mass is 2.67a^3.po, where a = radius of the diaphram and po = air density. Note that m^3. Kg/m^3 gives Kg. Correct units for the win. The air load mass goes up by the cube of the radius while Sd does up by the square of the radius. Generally, larger diaphragms are heavier than smaller ones because of the extra material. So, the gooder Sd, the badder Mms.
Are you sure that expression attributed to JBL is correct. If so, like you they like to calculate Kg/s A quick poll, should anyone be reading this. All those who agree with JBL and hornresp say yay. Those who agree with Line-Array say nay. Anyway,
try reading a few books the following is from one published in 1954
which mentions a bit about the upper frequency roll off.  The
above text discusses direct radiator loudspeakers. For a compression
driver, while the response is limited principally by the combined mass
of the diaphragm and the voice coil MMD, it can be modified by the front
cavity. If the compliance CM1 of the front cavity were zero, the
response would drop off at the rate of 6 dB/octave. It is possible to
choose CM1 to resonate with MMD at a frequency that extends the response
upward beyond where it would extend if it were limited by MMD alone.
However the response drops off at 12dB/octave. The upper frequency can
be calculated using  |
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Line Array
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Joined: 19 March 2022 Location: New Jersey, USA Status: Offline Points: 204 |
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Posted: 25 April 2025 at 12:32am |
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that was sloppy wording on my part.
Fudge you need to stop fudging numbers. Re for the 18Sound is 3.1 not 6.8 which means its motor strength is double that of the RCF. https://www.eighteensound.it/en/products/lf-driver/18-0/4/18ntlw5000-4 Not surprising considering it has two 4.5" motors versus one 4" motor for the RCF. Of course it isn't so simple - winding material, thickness and number of layers are equally as important as VC diameter. the main benefits of the push-pull motor are: 1 - inherently symmetrical BL and inductance curves 2 - lower weight for given performance level ( because most of magnetic return structure is eliminated ) 3 - lower inductance for given motor strength ( because the two voice coils are wound in opposite directions partially canceling out each other's inductance ) the downsides are: 1 - depth of the driver is increased 2 - potentially getting VC properly centered in both gaps could be more challenging than in a single gap, though this is pure speculation. here is a video of a push-pull driver getting reconed: https://youtu.be/JAipJT_fYms?si=56pJncjNYYQGZCSl
yes i should have said MMD. i said MMS because it is more commonly listed in specs than MMD. airload good. diaphragm mass bad.
appendix 2: https://www.cieri.net/Documenti/JBL/Technical%20Notes/JBL%20Technical%20Note%20-%20Vol.1,%20No.8%20-%20Characteristics%20of%20High-Frequency%20Compression%20Drivers.pdf
you need to stop posting random formulas without any context. either post the entire text from which you got the formula, or explain in your own words. the way i know JBL is wrong is because TAD drivers go beyond what JBL calls the theoretical limit of high frequency roll off, which is actually even seen in the very paper in which JBL makes those claims ( for some inexplicable reason the author decided to sabotage his own thesis as well as JBL by including charts of TAD response - maybe he was getting laid off and decided to give JBL the middle finger ) of course TAD uses beryllium diaphragm and a better phase plug but JBL's formula doesn't say anything about that. it seems JBL's formula only applies to JBL's own drivers of that generation, but it doesn't say that in the paper.
Edited by Line Array - 25 April 2025 at 12:50am |
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FOO
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Joined: 23 December 2018 Location: Denmark Status: Online Points: 954 |
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Posted: 25 April 2025 at 6:56am |
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This Line Array dude is just talking gibberish all day long.
RCF makes great LF drivers. And 18P300 is a great choice. Budget friendly and good sounding. Like Fane Colossus 18XB. Both are good old designs, time proven and battletested for years and years. I had G Subs in the past with 18XB in them. Not the most powerful box but sounded so rich and warm. Just make more boxes if you need higher SPL. And I don't see any issues with +3kw for each box with those RCF drivers. It's better to have headroom and no need for it, rather than needing headroom and not having it. I got LAB PLM10000Q running 3" RCF comp drivers on my current setup. 16ohm/220watts each. Two drivers on each channel, making it 1300watts to run 440watts worth of drivers. Massive headroom, and absolutely no worries as long as you remember some good limiter setups.
Edited by FOO - 25 April 2025 at 6:58am |
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jbl_man
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Joined: 12 January 2005 Location: London. Status: Offline Points: 11308 |
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Posted: 25 April 2025 at 7:50am |
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Yup.
i gave you a warning just a few days ago Line Array, and there you go again, another thread ruined with pointless theoretical ramblings and gibberish. You are obviously an intelligent man, i'm sure there are other forums who would appreciate your posts instead. Enough now.
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Be seeing you.
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APW
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Joined: 13 November 2012 Location: UK Status: Offline Points: 1234 |
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Posted: 25 April 2025 at 9:09am |
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He has run out of people on other forums to p**s off…
Edited by APW - 25 April 2025 at 9:20am |
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The text above was originally written by a human but was 'perfected' by AI. Any remaining grammatical errors are proof of AI's uncanny ability to misunderstand while exuding absolute confidence.
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jbl_man
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Posted: 25 April 2025 at 9:26am |
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He's run out here as well now.
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Be seeing you.
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Robbo
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Posted: 25 April 2025 at 9:33am |
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Thank you Ian---We can all get back to sanity now!!!!!!!!
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KDW32 wrote: