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Phil Starr

1x12" Cab Design Diary

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[quote name='stevie' timestamp='1446054831' post='2896570']
I would have posted this a lot earlier but I hit a snag. When I started measuring one of the cabs I have here, it didn’t measure the same as the one I had had here a few weeks ago. I spent hours trying to figure out what was going on. I even started recalibrating all my measuring gear. Eventually, I pulled out a speaker that I know measures flat and tried it: it was flat.

The penny dropped and I put the second cab on the bench and measured it. There was a problem with one of the Beymas. This makes a bit of a mockery of our comparison test at the Bash. This is how they measured. I’d say one of those was definitely not to spec. The red one is the good one, by the way.


More to follow shortly.
[/quote]

Quick update, we swapped the speakers last night and the plots stayed the same so the big 700Hz ish resonance is a cabinet problem, not a problem with the drivers. A bit more investigation needed but it was partially tamed by a bit more wadding in the cab. There's also a big peak in the output from the slot port though it doesn't coincide with the resonance we were investigating. A little more work on damping materials needed before I release the final design. I'm also going to order up a couple of better quality grilles to give a better impression of the finish you might achieve.

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Yes, the 18mm slotted which is the green one on the graph. Stevie's initial thought was that it was the driver, since the volume of the box and the tuning were the same. It now looks like it is something about the box which is creating the resonance. Frustrating but I'm glad we found it before publishing the design.

We are investigating.....

I'm hoping it is something simple.

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[quote name='Phil Starr' timestamp='1447337716' post='2906836']
Yes, the 18mm slotted which is the green one on the graph. Stevie's initial thought was that it was the driver, since the volume of the box and the tuning were the same. It now looks like it is something about the box which is creating the resonance. Frustrating but I'm glad we found it before publishing the design.

We are investigating.....

I'm hoping it is something simple.
[/quote] forgive me for daftness - and maybe it's something simple - but maybe the ports and 15mm are a better method of construction? (and lighter)

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[quote name='Phil Starr' timestamp='1447337716' post='2906836']
Yes, the 18mm slotted which is the green one on the graph. Stevie's initial thought was that it was the driver, since the volume of the box and the tuning were the same. It now looks like it is something about the box which is creating the resonance. Frustrating but I'm glad we found it before publishing the design.

We are investigating.....

I'm hoping it is something simple.
[/quote]

Taking a simplistic approach, the wavelength at 750Hz (which is where it looks like the resonance peak is) is about 46cm, so are there any bits of the cab that measure 46cm apart, or possibly 23cm?

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[quote name='tauzero' timestamp='1447363858' post='2907161']
Taking a simplistic approach, the wavelength at 750Hz (which is where it looks like the resonance peak is) is about 46cm, so are there any bits of the cab that measure 46cm apart, or possibly 23cm?
[/quote]
Not simplistic at all. The baffle is about 45x40cm with the shelf of the port at the end of the baffle, so that is the simplest explanation and that critical point had no damping material at the time of the test so that will be my first port of call. I think we have set up some extra standing wave resonances in the changes we made to this cab. It's a problem with small cabs with biggish drivers that you end up with all the dimensions determined primarily by the need to get everything in and not by acoustic considerations.

I'm also gong to put in some simple cross bracing into the 18mm cab. I had particularly wanted to compare an unbraced 18mm cab with a fairly heavily braced 12mm cab. Interestingly the 12mm cab vibrates far more than the 18mm cab and you can both hear it and feel it with your fingers. A single test on similar but not identical cabs proves nothing of course but I've always argued that mass is an important factor in panel resonance. What I've found so far bears this out.

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[quote name='Thunderpaws' timestamp='1447353666' post='2907030']
What type of woodwork joint did you use for the slot port at the sides and with the bracing? Could it be vibrating at certain frequencies?
[/quote]

I think it's probably a standing wave problem as explained above. The joints are all reinforced butt joints, chosen to be easy to replicate for a home builder with limited tools.

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[quote name='LukeFRC' timestamp='1447357499' post='2907082']
forgive me for daftness - and maybe it's something simple - but maybe the ports and 15mm are a better method of construction? (and lighter)
[/quote]

Not daft at all and I'm trying to modify the design by absorbing people's comments so they are all welcome :)

I've given details of both so people can make a choice if they want to construct one of these. I'll include notes of both in the final write up too. Personally I prefer the 18mm construction with the tube ports, but I have a proper hole saw to cut the ports. Both cabs are an easy carry anyway.

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I agree with the standing wave explanation. The addition of the slot port makes the internal vertical and horizontal dimensions fairly similar, which will further reinforce the standing wave which, as tauzero points out, is at the frequency you would expect from those dimensions.

As we don't have this problem with the round port version, I'd suggest that we follow Luke's pragmatic approach. Another benefit of the round port option is that a tweeter could easily be substituted for one of the ports if desired - either straight away or at a later date.

Edited by stevie

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On the matter of cutting the holes for the port tubes, I bought an adjustable hole cutter recently for less than a fiver which may not be the last word in pro tools, but will do the job perfectly well. It might take a few tries on some scrap pieces of wood to get the size spot-on, but it's not too difficult to do. http://www.ebay.co.uk/itm/Handy-Adjustable-Metal-Hole-Saw-Round-Circle-Cutter-DIY-Tools-Accessory-/252069704389

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[quote name='stevie' timestamp='1447511926' post='2908204']
On the matter of cutting the holes for the port tubes, I bought an adjustable hole cutter recently for less than a fiver which may not be the last word in pro tools, but will do the job perfectly well. It might take a few tries on some scrap pieces of wood to get the size spot-on, but it's not too difficult to do. [url="http://www.ebay.co.uk/itm/Handy-Adjustable-Metal-Hole-Saw-Round-Circle-Cutter-DIY-Tools-Accessory-/252069704389"]http://www.ebay.co.u...y-/252069704389[/url]
[/quote]

I use one of these http://www.toolstation.com/shop/p45494?table=no either look good for the job. Building the slot port design was slightly trickier than building the tube ported design so I'd agree with the decision to go that way. I'll offer notes on how to build the slot ported design as others may prefer to build that. It does look very nice IMO but I'd rather go the trouble free route.

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Another benefit that occurs to me of going with rounded ports is that altering the cabinet volume becomes a lot easier than with a slot port. It should be a fairly simple job to recalculate the port lengths for a (slightly) smaller box using the usual free software.

Edited by stevie

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[quote name='stevie' timestamp='1447519480' post='2908297']
Another benefit that occurs to me of going with rounded ports is that altering the cabinet volume becomes a lot easier than with a slot port. It should be a fairly simple job to recalculate the port lengths for a (slightly) smaller box using the usual free software.
[/quote]

That was what I was thinking... if the the plans were for 18mm and someone bought 15mm ply then it's a relatively simple set of calculations with tube posts - with a shelf port it's way more complex....

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Funnily enough you've caught me messing around on WinISD with smaller cabs. A 35l cab looks interesting, roughly 1dB up from 100-200Hz but 3dB down at 40-70Hz. I think if I really wanted to go for a lightweight cab then reducing the size and compromising on deep bass looks interesting, you'd lose about 3kg and the panels would be inherently more rigid. I find that size affects portability just as much as weight.

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[quote name='stevie' timestamp='1447511926' post='2908204']
On the matter of cutting the holes for the port tubes, I bought an adjustable hole cutter recently for less than a fiver which may not be the last word in pro tools, but will do the job perfectly well. It might take a few tries on some scrap pieces of wood to get the size spot-on, but it's not too difficult to do. [url="http://www.ebay.co.uk/itm/Handy-Adjustable-Metal-Hole-Saw-Round-Circle-Cutter-DIY-Tools-Accessory-/252069704389"]http://www.ebay.co.u...y-/252069704389[/url]
[/quote]

These work well (I have one) but I personally would only use it in a pillar drill at low speed. I have not had a close shave with mine :). Some do have a safety guard, it would be quite easy to make a guard (for non-professional occasional use) using a plastic food tub.

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[quote name='Phil Starr' timestamp='1447523436' post='2908343']
Funnily enough you've caught me messing around on WinISD with smaller cabs. A 35l cab looks interesting, roughly 1dB up from 100-200Hz but 3dB down at 40-70Hz. I think if I really wanted to go for a lightweight cab then reducing the size and compromising on deep bass looks interesting, you'd lose about 3kg and the panels would be inherently more rigid. I find that size affects portability just as much as weight.
[/quote]

Is there an issue fitting in enough port area with that volume? I've noticed when playing around with WinISD that it can be a bit of a juggling act with smaller cabinets.

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[quote name='Beer of the Bass' timestamp='1447534959' post='2908455']


Is there an issue fitting in enough port area with that volume? I've noticed when playing around with WinISD that it can be a bit of a juggling act with smaller cabinets.
[/quote]
Yes, though at a practical level the potential length of the ports is at least as tricky the port can end up longer than the cab! i ended up with quite small ports which potentially give you a bit of port noise at low frequencies. It's all about which compromises you choose to make

The proof of the cab is in the hearing

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[quote name='Phil Starr' timestamp='1447518180' post='2908286']
I use one of these [url="http://www.toolstation.com/shop/p45494?table=no"]http://www.toolstati...p45494?table=no[/url] either look good for the job. Building the slot port design was slightly trickier than building the tube ported design so I'd agree with the decision to go that way. I'll offer notes on how to build the slot ported design as others may prefer to build that. It does look very nice IMO but I'd rather go the trouble free route.
[/quote]Any thoughts on whether to port forward or backwards also do you think that the round ports break up the standing waves. What I mean is that the four round ports in the original prototype would cause waves to be reflected in a quasi-random fashion?

Edited by Chienmortbb

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I think that is quite likely to be the case.

Edited by stevie

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I have a question about tube ports and their lengths. Am I right in assuming that standard downpipe means the 68mm stuff? I've noticed that winISD and a couple of online calculators come up with quite different lengths than the ones used in the prototype cab, quoted below. The exact figure varies depending on the end correction selected (I used "one end flanged"), but WinISD is suggesting a length of 298mm for a 50Hz tuning in 50l, rather than 190mm. That's with four 68mm diameter ports. I'm trying to figure out if I've missed something in the process.

[quote name='Phil Starr' timestamp='1436343308' post='2817185']
If anyone wants to start a build before I've had time to write this up and get drawings done it shouldn't be too difficult. The external dimensions are 360x424x530mm the baffle front is set back 30mm from the front of the cab so internally the depth is 306mm. The four ports are made of standard guttering downpipe 190mm for a 50Hz tuning, and 260mm for a 40hz tuning.
[/quote]

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[quote name='Beer of the Bass' timestamp='1448895807' post='2918936']
I have a question about tube ports and their lengths. Am I right in assuming that standard downpipe means the 68mm stuff? I've noticed that winISD and a couple of online calculators come up with quite different lengths than the ones used in the prototype cab, quoted below. The exact figure varies depending on the end correction selected (I used "one end flanged"), but WinISD is suggesting a length of 298mm for a 50Hz tuning in 50l, rather than 190mm. That's with four 68mm diameter ports. I'm trying to figure out if I've missed something in the process.
[/quote]II m sure that Phil, Stevie or Lawrence to answer this in full but in my view, WinISD and the other speaker design packages all aim for HiFi performance. In WinISD Pro, you can plot many things including the frequency response (low end extension), cone excursion and the velocity of the air in the port. The port size, affects all of theses and you can tweak for all these even though they are not optimal (optimal for HiFi).

If you allow the cone to go slightly over its nominal max travel (XMax) and accept that there may be a little chuff at full power, you can get a smaller cabinet with more manageable port sizes.

Why can we make these compromises? The Beyma Speaker has a thermal power rating of 350 wats. Most 500 Watt amps have an 8 Ohm power output of 250 watts so there is already a safety margin. Of course if you use a bigger amp the speaker may struggle but then maybe this is the wrong cabinet design for you. Finally if you are hitting 350W the small chuff will be lost in the mix anyway.

This compromise is tiny compared to most commercial designs (Barefaced, TKS and other Boutique makers excepted). Of course is you do not use a bass reflex/ported cabinet design (i.e. most of Bill Fitzmaurice's designs) then these statements are invalid

Edited by Chienmortbb

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I think I haven't expressed that very clearly. When I try and calculate the length required for four 68mm ports to achieve a tuning of 50Hz in a 50 litre cab, my results are very different from those used in the prototype cab. I am trying to work out whether I'm doing something incorrectly.

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[quote name='Beer of the Bass' timestamp='1448895807' post='2918936']
I have a question about tube ports and their lengths. Am I right in assuming that standard downpipe means the 68mm stuff? I've noticed that winISD and a couple of online calculators come up with quite different lengths than the ones used in the prototype cab, quoted below. The exact figure varies depending on the end correction selected (I used "one end flanged"), but WinISD is suggesting a length of 298mm for a 50Hz tuning in 50l, rather than 190mm. That's with four 68mm diameter ports. I'm trying to figure out if I've missed something in the process.
[/quote]

You aren't going mad, first of all the pipes internal diameter is 64mm the outside is 68mm. secondly it depends how you enter the parameters into winISD as sometimes the manufacturers don't always use the same techniques for measuring as are assumed by win ISD. Then the box isn't exactly 50l as I built it oversized to allow for the volume of bracing the ports and speaker and any modifications I would make as the design developed, I didn't use all the extra volume I allowed. We also found two sets of data for the SM212 out there and then took our own measurements of the basic parameters which differed slightly from those published. If you go back to post #291 you'll see we actually measured the tuning of the cabs and the port dimensions are measured from the cabs we tested.

Well spotted though, and I had to double check all this so you are keeping me on my toes :)

Ultimately that's why you try and build prototypes and test them.

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One last post tonight, if you go back to post #479 you'll remember Stevie scaring me with a very significant peak at around 700Hz in the slot ported cab. We swapped drivers and it was definitely the cab. Well the peak is tamed pretty much with wadding especially on the top edge of the port but I'm going to make a new baffle and move the speaker off centre which I think will help further. I'm happy we know what is going on there and have a solution.

However I'm now pretty happy with the basic design of a 18mm cab with four pipe ports tuned to 50Hz so the slot port won't be a problem as we won't be recommending it. The availability of cheap hole saws means cutting the holes for the ports will be easier than forming the slot which was the only reason I went that direction. I found the wooden slot was tricky to form if you don't have plenty of clamps to hand.

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