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Old 5th Mar 2018, 10:39 am   #1
Diabolical Artificer
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Default Speaker cabinet query.

Not sure if this is the right section....

I'm after building a speaker cab for a sub-woofer. Modelled in WinISD the dimensions are
91.4cm high, 58.5cm wide and 36.5cm deep. I want to reduce the height, knock off 10cm say, and add some on the depth to recuperate space lost by doing so.

For the life of me I can't remember how to do this, what to times by and divide and all that. Google is a waste of time, it points me to software like solid works which I don't have. I often have this problem; how to phrase your question to get the right result.

Anyhoo, how do i figure this out on good old paper?

I then have to figure out the port; WinISD says 10cm port, I presume by that's 10cm diameter pipe with a vent mach of 0.27. I'd prefer to leave an open slot at the bottom of the cab face rather than muck about with bits of pipe. Any ideas?

Andy.
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Old 5th Mar 2018, 11:59 am   #2
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Default Re: Speaker cabinet query.

I may have solved it but if someone could check me figures..

First I converted dimensions to inches by dividing by 2.5, so dia are H 36 1/2" by W 23 1/2" by D 14 1/2"
I fudged the figures a bit, IE made 23.4 = 23 1/2 and 14.6 = 14 1/2. that gives us 3.7 ft cubed.

To get the port you need 12 - 16 inch per square per ft cubed, so 16 x 3.7 = 59.2" sq. the width is fixed in stone so 59.2/23.5 = 2.5 or 2 1/2". You add this space onto your cab.

Therefore H becomes 39". We want to knock off 10cm or 4" + our 2 1/2" for port. 3.7ft cubed is 12469.86" sq + our port (851.8, call it 852" sq) total volume = 13,321.86 call it 13,322 sq.

To get depth 13,322/ (23.5x32.5) = 17.44 etc call it 17 1/2" . Therefore we need a cab with internal dimensions of H 32 1/2" by W 23 1/2" by D 17 1/2" ..... I think.

Just have to add on internal bracing to get final dimensions.

A.
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Old 5th Mar 2018, 11:59 am   #3
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Default Re: Speaker cabinet query.

Just multiple them all together. That gives you the volume.

Then, reduce the dimension you need, and increase one of the others. Then multiply them all together again to see how the volume has changed. Or re-arrange the formula:

If volume = width x height x depth, then depth = volume / (width x height)

BTW, there's a fair bit of lattitude with these sorts of designs, and WinISD is good, but it only a model. I've found that you get better results if you measure the TS parameters of the drivers you have, rather than relying on the datasheets or what might be in the WinISD driver database already. Generally, I've had better results by tuning the port down in frequency from what it suggests. What WinISD says - and any of them, frankly - must be regarded as the starting point, not a finished design.

Regarding the slot port, WinISD can calculate that for you. But the trouble with slot ports is that it's hard to tweak them once the cabinet is built (see previous paragraph )- you need to mill some slots in the sides to allow the slot to slide in and out until you get the desired frequency, and it's hard to do that without getting air leaks, etc.
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Old 5th Mar 2018, 1:22 pm   #4
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Default Re: Speaker cabinet query.

Thanks Mark, school algebra is resurficing from the depth's. I was going off the slot idea, pipe it is. Does the volume given by WinISd include port volume?

I got the TS parameters from Eminence not from WinISD, so hopefully all will be well. I'm aware there's more to this speaker building lark and that the WinISD model may be off. Trouble is 18mm MDF ain't cheap, hate to have to start again.

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Old 5th Mar 2018, 1:42 pm   #5
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Default Re: Speaker cabinet query.

One of the cool things about WinISD is the interactive modelling that allows you to see the effect of altering the volume of the box and the port tuning just by dragging your mouse. If you spend a bit of time doing this, you'll soon get a feel for what's critical and what's not. Altering the box volume by 10%, for example, rarely makes a massive difference - but the port tuning does. That's why you *will* need some way to alter the tuning once the box has been made. Luckily, plastic pipe is much cheaper than MDF

The safest way to ensure you've actually tuned the port to the frequency you want is to look at the impedance curve. You don't need fancy equipment to plot this, and you don't even need to measure the values of impedance - you're just looking for the minimum that indicates the port frequency, so a simple oscillator and DMM set to AC might be all you need (I prefer to use a 'scope), plus some means to measure frequency.

Be careful with data from manufacturers. Audax were not exactly brilliant for their AP100Z0 drivers I used years back: http://www.markhennessy.co.uk/audax/background.htm - my results were a lot closer than most other folk on DIYAudio at the time (for example: http://vikash.info/audio/audax/). If your drivers are new, then like these Audax units, they might well need some running in before the parameters stabilise.

What is the drive unit?
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Old 5th Mar 2018, 2:05 pm   #6
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Default Re: Speaker cabinet query.

Unfortunately my driver is a bit cr*p so changing the box, IE making it bigger results in a very small increase in low frequency response. Am also limited to what enclosure I can use, sealed is out so that leaves ported. X order etc is out too but I wouldn't tackle a complicated design as it's my first build.

If it goes pear shaped I can always use the box as a coffin though my head will have to poke out the top. : )

A.

PS I'll probably badger you again when it comes to the porting.
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Old 5th Mar 2018, 4:07 pm   #7
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Default Re: Speaker cabinet query.

It's all a bit of a juggling act. Yes, making the box bigger doesn't always bring the returns you'd hope for - but so much depends on the raw specs of the driver. Some drivers are optimised for ported boxes, others work best in sealed boxes, others are somewhere in-between and give ok-to-mediocre results in either!

Of the ported boxes I've made, I think luck was always the biggest factor! I like sealed boxes. As well as frequency response, there is transient response too, and a lot of ported boxes are quite bad in this department - in WinISD, look at the Group Delay graphs to see what I mean. Whether this matters is a question of personal preference, as some musical genres are affected more than others. Bandpass boxes are usually worse again in this area.

At the bass end, a flat frequency response isn't always what you want in a real room. The roll-off of a sealed box is usefully augmented by "room gain", but the same effect might make a boomy ported box might sound really quite muddy. There's still a lot of art in speaker design, and it's good fun experimenting. If you can, I recommend making up a box with a removable baffle (that can be replaced with cut-outs for different drivers) that is over-sized (fill with bricks or similar to reduce volume as needed). Once play-time is over, then knock up the final model, and use the "dev box" for the next project
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Old 5th Mar 2018, 5:31 pm   #8
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Default Re: Speaker cabinet query.

Hey Andy

In a previous life I used to teach this stuff, and work it out on paper. If you send me your Thiele-Small parameters for the driver, perhaps I can dig out my notes and work through it with you.

One thing to note is that to tune the cabinet, the equivalent acoustic compliance C of the cab is V/(rho*c^2) (V =vol in cubic meters, rho is density of air = 1.21 kg/m^3, c = speed of sound = 340 m/s). The acoustic mass M of the port is (and this sounds weird without the derivation, which I'm sure you don't want) rho*L/S - where L is length of port (m) and S is port cross section (in m^2). So if you make it longer it gets heavier - but if you make it wider, it gets LIGHTER

Then port resonance (Hz) = 1/(2pi) * sqrt(1/{mass * compliance})

Make a hissy-crunchy noise sound in the back of your throat by breathing through some flob. Then open and close your mouth. You can hear the bandpass filter going up and down in freq - down when you close your lips into an 'oooo' (small S, heavy port (large M) ) and up when you open your lips into an 'aahh' (large S, light port). Whistling works the same way, except then you keep M the same (lips don't change) but you mess about with C by moving your tongue up and down, changing the internal volume of your mouth and so moving the resonance.
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Old 5th Mar 2018, 6:20 pm   #9
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Default Re: Speaker cabinet query.

Thanks, some good tips. I've cut the MDF today, just have to fettle it some more then will glue it up, except the front baffle, then will experiment. I'll probably email you Mark # 2 if I get stuck with the port, which I probably will.

Your last paragraph cracked me up, been sitting here making weird noises, good job Molly's out. Unfortunately I have no flob to play with, I'll try smoking a weeks baca tonight and report back : )

Andy.
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Old 9th Mar 2018, 8:07 am   #10
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Default Re: Speaker cabinet query.

Built the cab just have to wire it up and tune it. You can see the size of it compared to my Mission 720's - over double the size. One thing there doesn't seem to be a lot of info out there on, is dampening. I presume it needs lining with an absorbant material and you add to taste. I also presume you tune it before adding said dampening.

I did think of making it an active speaker, I have various amp modules and hefty tfmr's, but as it's daft heavy anyway, a separate amp seems best idea. I have temporaraly screwed some castors on the bottom for ease of lugging. It can be propped up on bricks or whatever when in situ.

I've also been thinking about filtering and isolating the separate stereo signal's in. I have a few schematics somewhere but in the mean time I have a passive LC filter somewhere for testing.

Andy.
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Old 9th Mar 2018, 1:16 pm   #11
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Default Re: Speaker cabinet query.

Looking good

There are 2 types of damping: air damping and panel damping. The latter is fairly rare, and is usually used with BBC-style "thinwall" cabinets to improve the midrange - not an issue for a sub.

Air damping is there to reduce reflections inside the box. With no damping, you get strong standing waves between opposite panels at frequencies that relate to the spacing of the panels. This again tends to be a mid-range problem - with a sub, the wavelengths involved are much larger than the box (100Hz is about 11 ft).

But in addition to all this, damping makes the drive unit think it is working in a larger box. Perhaps up to 10% bigger for a fully stuffed box. Or to put it another way, you can make the box 10% smaller than you thought! That's handy for manufacturers, as every inch saved translates to significant cost savings.

But in a ported box, you can't fill the entire space with damping as that interferes with the reflex action (though some designs might deliberately choose to damp it). So you tend to just line the walls to reduce standing waves.

In short, you probably don't need any damping at all, but if you feel the need, just line the walls. A couple of inches of that fluffy white stuff (BAF wadding) or Rockwool works, though the latter really needs a fabric covering to keep the fibres in place.

Because the damping will alter the apparent box volume, you tune the box after the damping has been fitted (or check that the addition of damping hasn't altered the tuning significantly).

For a crossover, I'd definitely recommend doing it at line level, ahead of a dedicated amp for the sub. A passive crossover involves some rather large and expensive components. Take a look at Sallen and Key filters - this project gives a good starting point: http://sound.whsites.net/project155.htm - note the dual pot to adjust frequency, which is obviously important. It'll also need a level control and a switch to flip the phase - all easy stuff. You might even consider a steep high-pass filter to roll it off below the port tuning frequency. It can all be knocked up on a prototype breadboard and fiddled with - it's really good fun
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Old 9th Mar 2018, 1:22 pm   #12
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Default Re: Speaker cabinet query.

Andy,
That is one big lad’s subwoofer,I will probably feel it in Co. Durham.
Great stuff.
Regards,
David
Ps it has given me an idea how to re-purpose our old front loading washer !!
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Old 9th Mar 2018, 4:37 pm   #13
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Default Re: Speaker cabinet query.

I have the idea of a dodecahedron of 'speakers, minimal cabinet sides to worry about.
 
Old 10th Mar 2018, 7:23 am   #14
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Default Re: Speaker cabinet query.

Thanks Mark. I was looking in some old Elektors last night and found an article on active X overs, they were using Butterworth and had various circuits with 6, 12 and 18dB cut off. I'll also look at the ESP link, as you say, good fun.

Aye David, I made it too big by mistake, read inch's instead of cm's; it won't fit on the back mudguard of the motorbike now as intended : )

"I have the idea of a dodecahedron of 'speakers" That's ok if your good at woodwork Merlin, that's a lot of odd angled joints : ) I've had some mad idea's like using 2 x 3 concrete slabs, getting the trowel out and doing a bit of brickwork or re purposing the chimney breast as an enclosure. Michael Eavis famously used a gurt big concrete pipe with a speaker at one end to listen to and play music to his cows in the milking parlour.

Andy.
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Old 10th Mar 2018, 11:15 am   #15
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Default Re: Speaker cabinet query.

Quote:
Originally Posted by Diabolical Artificer View Post
"I have the idea of a dodecahedron of 'speakers" That's ok if your good at woodwork Merlin, that's a lot of odd angled joints : )
There are a couple of "instructables" for 3d-printed dodec speakers, or ones made out of fibreglass where the gloop seals all the odd angles for you.
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Old 10th Mar 2018, 12:56 pm   #16
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Default Re: Speaker cabinet query.

Quote:
Originally Posted by merlinmaxwell View Post
I have the idea of a dodecahedron of 'speakers, minimal cabinet sides to worry about.
That's where the 12 off, 12" ATC speaker drivers I have to dispose of, came from. Pentagon-based pyramids building into something well over a meter in 'diameter' for outdoor sound propagation tests, driven from a 1kW PA amplifier which I sold to a Serb (which, I joked, may have been causing the recent continental power generation problems centred on that area).

It sounded great with an MLS (maximum length sequence - Google it if you want to know more about deterministic signals which are 'random') signal going through it. Musique concrete blasting over the moors around Buxton, trying to amuse the sheep who nonetheless looked like they'd heard it all before. Sheep just seem to understand pseudo-random number sequences like that
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Old 21st Mar 2018, 2:51 pm   #17
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Default Re: Speaker cabinet query.

A little further forward on this project...

So far I've left the cab sealed and have tried a few test tones through it, it sounds "muddy", but I'm in the early stages of experimentation with filters. I've knocked up three so far. One out of Elektor 1986 which was supposed to have a cutoff of 200hz. In reality it passed higher frequencies and was tricky to build - lots of odd cap values like 120 and 180n.

So I built one blokes take on an ESP LPF - here - http://sound.whsites.net/project123.htm this worked well but the OP has very low gain so I added a non inverting gain stage. Now it's passing higher frenquencies, no worries, still mucking about. I will build the variable filter as suggested by Mark H.

One aspect is problematic, namely the port or vent. I've used two online calculators to calc the port length, both give very low figures EG 1.17" and 0.47" ! I tried your formula tother Mark, but got lost.

My Vb is 7 ft^3, f is 42.16hz and the pipe I snaggled out of a skip has an internal dia of 9cm. I used this calculator here - https://www.the12volt.com/caraudio/boxcalcs.asp and this one - http://www.calculatoredge.com/new/ventlength.htm#factor The latter uses a correction factor - K that has something to do with the vent's mating onto free air or somesuch.

The more I read up on this subject the more complicated it seems to get. Once I have the filter sorted I think I'll go for hands on approach of using my ears and varying the length of vent.

Andy.
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Old 21st Mar 2018, 3:38 pm   #18
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Default Re: Speaker cabinet query.

WinISD will tell you the port length based on the drive unit, size of the box and the diameter of the pipe - no need to do the maths yourself. You can see what frequency the port is working at by the impedance curve (see post #5).

As for the filter, I'd suggest that a 2nd order (12dB/oct) is sufficient. This has the advantage of being easily adjustable with just a stereo pot - higher order filters will require a more complex pot. Take a look at this project - http://sound.whsites.net/project78.htm - it's a 3-way crossover, but you just need to take A2 and the circuitry surrounding it. To make it variable, replace R1 and R2 with a ganged pot. Put an op-amp ahead of it to deal with summing L+R to mono, but that's it - job done
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Old 22nd Mar 2018, 2:41 pm   #19
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Default Re: Speaker cabinet query.

The L vent window/box in WINisd is faint, which I took to mean "no length given". So I mucked about with parameters to see if it changed, it did. So the length given was 0.011m which is 11mm; the baffle is thicker than that.

So I tried a different approach. I laid the cab on it's back and put a handful of black eyed beans in the driver cone, connected up my sig gen to a bass amp, and plugged the subwoofer in. At 35hz the beans stopped jumping, so this should mean the resonant frequency of the cab is 35hz. This is with an open 4" (4 and 3/8" ) hole in the baffle, no vent/pipe.

I then got a length of 4" drain pipe and cut several length's off it, EG 1", 2", 3", 4" and 7" . I put the 1 inch bit in, beans jumped, dropped frequency down till beans stopped jumping. Repeated this with all length. The result is roughly a 1-2hz drop in the resonance of the cab, per inch of pipe.

The next step is subjective; what frequency do we "tune" the cab to? So I got my bass guitar out and mucked about a bit. It seems a longer vent rolls off higher frequencies, which makes sense.

I popped the 3" vent in and am doing some listening tests. As I had no dual gang 10k pot, I built the circuit attached from ESP project 81 http://sound.whsites.net/project81.htm. You can alter the roll off from 90 to 200hz by swapping R's, C's stay the same.

I'm a bit uncertain how to connect the two input buffers to the filter. I used two 15k R's as they were on the bench. Another aspect I'm uncertain of is phase; the OP is about 270 deg out of phase with the IP. I swapped the +/- over on the back of the subwoofer, can't really tell much difference.

The bass amp I'm using has a graphic EQ, which means I can play with the sound. It's going to take some experimenting to get it right. The room is small too, another factor in the mix. I want the sub to complement the L/R channels, not swamp it out. However with the volume not up too loud, the bass hits you in the guts.

Andy.
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Old 22nd Mar 2018, 2:46 pm   #20
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Default Re: Speaker cabinet query.

So that I can sense-check all this, could you tell me the make/model of the driver (or list the main TS parameters) and tell me the volume of your box?
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