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Need plans for a quad 18 (JBL2245H) bass reflex

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Elliot Thompson View Drop Down
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Elliot Thompson Quote  Post ReplyReply Direct Link To This Post Posted: 15 February 2018 at 11:03am
Originally posted by Roman Roman wrote:

WinISD is for windows. I am on a mac. I will research mac alternatives.

Sounds good. The calculator you are using is too limited for those who are new to loudspeaker building. You need to visually see what is taking place with a graphical chart at the given frequencies. 

Best Regards, 

Elliot Thompson
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DMorison View Drop Down
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Post Options Post Options   Thanks (0) Thanks(0)   Quote DMorison Quote  Post ReplyReply Direct Link To This Post Posted: 15 February 2018 at 1:30pm
Re: Low frequency requirements.

Using the bass guitar as a ballpark reference is, as Elliot points out, only part of the picture.

It’s a pretty good ballpark for getting started, but as with all generalizations, it only takes you so far. There certainly can be content that goes below this in many genres – even from instruments you wouldn’t think go as low.

Herewith the spectrum of track one from Rodrigo Y Gabriela’s self titled album. This is made using only two acoustic guitars which have a normal lowest note of 82Hz, but due to their playing styles you can see that there is a bass peak at around 56Hz, and 40Hz is only a few dB lower in level than that.

At the time I suggested 40Hz, you hadn’t given any example of actual tracks you’d play, but you now have. I listened to a version of that Dawn Penn track on youtube and I think it has strong content at least a bit lower than 40Hz. The analyser I use requires a copy of the file on the hard drive though, so I can’t tell you that for sure.

I’ve already suggested you run some of your own tracks through an analyser – have you done that yet?

If not, please do so – then you’ll actually know something for a fact rather than trusting a couple of random blokes on t’interwebz. ;)

Quote Some previous comments about undesirable outcomes from mismatching a specific driver's characteristics with a cabinet design seem redundant if I am entering the Thiele and Small specifications of my JBL 2245H drivers into the bass reflex cabinet calculator.

Not all calculators are created equal. They don’t all show you the complete picture. The one you linked to at mh-audio.nl for example didn’t seem to want to even show a response graph, much less an excursion curve so there’s no way it can tell you if you’re risking pushing the driver too far.

Elliot’s suggestion of WinISD is a good one – there are programs you can use on a Mac to run windows programs, Parallels is one name I’ve heard of, or if you’re more computer-savvy you can, I believe, set up your Mac to run windows itself – not sure how that works but it may need partitioning the hard drive and setting up a dual boot system.

Re: Power Levels.

The number on the spec sheet is measured without reference to excursion or sound quality – it’s merely the amount of power the driver can withstand without any damage or change to its operating parameters. You use modelling software to determine how much power it’s sensible to apply to a specific driver in a specific cabinet on a case by case basis.

The generalizations (eg 2x RMS) are best applied to whatever power rating you end up with after doing that, not the raw driver rating. To be fair, many people don’t realise that and simply think the nominal rating will be good regardless of what box the driver’s in.

Note – the 2245H is a 300W driver – the 600W you see is already the “program” rating so that’s the one to which you match amp power, not 1200W.

Note 2 – pet peeve of mine:  “RMS” is a mathematical technique that applies to datasets that include both positive and negative values which would cancel out to zero if you did a normal average – this does not apply to power, so anyone you see mentioning Watts RMS is just using it as an (inaccurate) shorthand for “average” power. The best terms are continuous, program and peak – with program being the target for amp size in most common cases. (Again, Elliot will probably point out some exception to this generalization – to be fair he’s trying to get you to think more deeply, but IMO generalizations are a useful starting point as otherwise you can easily get overwhelmed with detail.)

Quote What were the over excursion issues you mentioned? Too much amplifier power making the speakers move too much and causing damage?

If you ask the driver to move further than it can do so (in a linear manner), the first things that happen are 1, the sound quality reduces and 2, it stops getting louder in proportion to the increase in input signal – in other words, distortion increases. If you keep pushing it further then eventually, yes, you can end up damaging it.

Again, this is why you’re best to model in something more capable than that website first.

Quote So would entering the TS specs into a cabinet design calculator guarantee the cabinet matches the driver?

I think you probably know the answer to this already, but just in case: No. As stated above, not all such calculators are equal. Generally the simpler it is, the more assumptions the creator of the calculator has had to make for you, meaning you can’t see the whole picture.

Quote The only information I have about matching driver to cabinet is to enter in the Thiele and Small parameters of the Driver into the speaker box calculator. Anything else I should know.

Lots, if you want a really good outcome.

Even if you can’t run WinISD immediately, download it and read the helpfile – it’s HTML so even a Mac should manage that ;) It contains a lot of good info how to design a project taking account of things like practical box sizes, port lengths, excursion levels etc.

Quote So essentially you are saying the lower I want the lowest frequencies of the box to reproduce the larger the box will need to be. 

Yup, exactly.

Quote Thanks for the Hoffmans Iron Law link. I have read that before but needed a refresher.

It is a spectrum of decisions isn't it. I decide how big a cabinet is too big and work back from there.

That is one sensible way of going about it, yes. There’s no point designing a project that looks amazing on paper, then realizing once you’ve built it you can’t get it out your front door!

Quote Whew! You still with me.

Just thought I would share the specifics of using this online calculator. May help inform any helping me.

My understanding is I can alter the 'box volume' and calculate different results. Maybe I can alter the F-3 frequency as Elliot and DMorison have suggested as the aim. 40Hz or something.

A big question in my mind is why did the calculator recommend: 293.17L

That seems to be the variable that has resulted in a box that is too big and goes down to 17Hz F-3.

Not gonna quote the whole post line by line, I hope what I’ve written above highlights some of the limitations of this particular calculator so just a few points.

First, if you get to using something like WinISD then you input the number of drivers as part of your setting up of the project, so what you see modelled already takes account of that.

I didn’t see any option in that calculator to adjust the tuning frequency, so again, that’s showing how limited this particular one is.

The suggested box size is based on assumptions made by the author of the calculator – usually starting with keeping the response as flat as possible, as deep as possible, hence coming up with a large box size. Not everyone needs that though – again, more complete modelling allows you to take more control of what you get out of the project, rather than relying on assumptions made by others.

Quote Even entering JBL's minimum recommended enclosure volume of 225L times four 900L gives an F-3 of 19Hz?

Half the suggested target of 40Hz.

How accurate is this process and our communications?

I think that calculator might have assumed your 900l was for one driver, hence F3 appearing way deeper than you might have expected. Unless you’d input a revised set of TS parameters with Vas quadrupled first?

Quote Can someone spoon feed me and tell me what they think are some good parameters.

I think Elliot more or less covered this but to put it another way: Speaker designing is too complicated for spoonfeeding, so newcomers do have to either tackle a bit of a learning curve, or get very frustrated. It looks like you’re on the right track though, so please do keep at it J

Unfortunately, the nature of the internet is that a lot of the info you need is spread around in different places, and there’s a lot of limited or even bad info out there. Even trying to answer most of the questions you’ve asked in the last few hours has resulted in this being a long and probably quite hard to follow post.

So, take-away messages: Do some spectral analysis to see how deep you actually need to go, and read the helpfile in WinISD as it is one fairly good starting point for the learning you’ll need to do to get the most out of this.



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DMorison View Drop Down
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Post Options Post Options   Thanks (0) Thanks(0)   Quote DMorison Quote  Post ReplyReply Direct Link To This Post Posted: 15 February 2018 at 4:35pm
OK, got bored and found an mp3 of the Dawn Penn track online, and had a play with it in Audacity.

Download and listen to these 4 clips, and see what (if any) difference you think there is between them.
They're all the same 20-ish seconds of the track.

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