1x12" Cab Design Diary

[LawrenceH]

Going back to the port thing, what muddies the water somewhat is that though there is some concern about output compression, more often people's main empirical test of size is based on unacceptable chuffing noise. Not the same thing (though related), and though lots of people report marked improvements in relation to chuffing, I'm not really clear as to what effect the flares have on output level of an otherwise undersized port.

[Chienmortbb]

I agree Lawrence. I think this is what Bill was saying. We have to think of the port as a spring, and varying either the lenght or area will alter resistance to the movement of the cone and hence the efficiency of the cabinet.

Using ports that are too small will offer more resitance to cone movement and increased chuffing. Flaring the port will reduce the chuffing but will not alter the "spring" of the port.

What might make sense is to use the correct port (17-20 m/s) but also add a flares. This will assure that even at full power there will be no turbulence. Unfortunately adding flares will lengthen the port as a whole and I suspect that the current design is tight on space for ports.

The other issue is that the response lower down is affected by the overal size of the baffle. This would suggest that the depth be as short as is practical. This reduces the space for a port mounted either on the front or the back.

I am learning all the time but is a side mounted port a possibility?

Edited August 11, 2014 by Chienmortbb

[LukeFRC]

I read in interesting paper about flaired ports and investigating them from a aero fluid dynamic POV. The main thing I can remember that in a straight exit port the air forms laminate layers at the length of the port. Flaired ports , ideally on both ends induce eddies that break up the laminates reducing resistance on air flow. A bit like the dimples on a golf ball or sharks skin

[Bill Fitzmaurice]

[quote name='LukeFRC' timestamp='1407772789' post='2523841']
I read in interesting paper about flaired ports and investigating them from a aero fluid dynamic POV. The main thing I can remember that in a straight exit port the air forms laminate layers at the length of the port. Flaired ports , ideally on both ends induce eddies that break up the laminates reducing resistance on air flow. A bit like the dimples on a golf ball or sharks skin
[/quote]If you want to see the effect of a port of insufficient area model the cab in WinISD Pro. Open the Advanced tab in the Box window, change the Qp value to 5 (BTW, it should be at 50 if the port models with port velocity less than 20) to see the effect of a seriously undersized port. It shows the result of friction. Dimples reduce friction, they're used by some duct manufacturers.

[Owen]

Anyone know of any decent plate power amps to bolt onto something like this?

[Chienmortbb]

[quote name='owen' timestamp='1408141170' post='2527645']
Anyone know of any decent plate power amps to bolt onto something like this?
[/quote]there are very few full range plate amps available commercially. Most are subwoofer amps.

Matrix do the GM50 ( http://uk.matrixamplification.com/amplifier-modules.html). Minidsp donthe PWR-ICE125 and 250. The matrix one is a Class AB amp and will get warm. The minidsp one is based on am Icepower Class D amp and will run cooler and also has DSP processing included. That may or may not be useful. I have heard that the Minidsp amps are limited to 6 ohm min when operating in bridge mode. I am trying to verify that at the moment.

Evotec (www.amplifier.co.uk) have Class D ones listed but you will have to contact them about purchasing.

However, owing to the hest issue, the extra weight and the difficulty in obtaining a good seal, I would not use a plate amp in a Bass Cab.

[Chienmortbb]

OK I have now checked and the MiniDSP amp can be used at 4 ohms but it is not recommended to run them at over 200W continuously at 4 ohms.

Edited August 27, 2014 by Chienmortbb

[Chienmortbb]

I was chatting to Blue Aran the other day re stock on the SM212 and told them about this thread.

After reading it they sent me this response;

"I had a bit of a read of the thread, looks very interesting.

There is a driver that is mentioned mid-way through: Faital Pro 12PR300
http://www.bluearan.co.uk/index.php?id=FTP12PR300AP
It won't be everyone's cup of tea for this design but should certainly be a contender - it has great mid-range and is about as lightweight as they come. Assuming it is indeed suitable for this design we can do a Basschat special on these: £115 inc VAT, free delivery. We tend to keep them

In order to get the price all you need to do is register on our website then email us with a link to that forum thread.

Unfortunately there isn't anything we can do on the Beyma SM212 but it's already fairly well priced."

I have no affiliation with Blue Aran and although the PR300 is a neo, it does not model as well as the SM212 so I will stay with that. For those that need a lightweight cab it might be an option though.

[Phil Starr]

Well done, I've thought about contacting Blue Aran, or the importers, to at least ensure there is some stock available.

With the plate amps it is actually cheaper to buy a cheap powered PA speaker and chuck the rest away or sell it on as a passive if you can fill the hole where the amp was!

I didn't list all the speakers we considered, a lot were rejected without even modelling them because of value for money or because the specs were out the range we thought was acceptable.

I'm afraid that there is a little pause in proceedings before I make up the final cabs. I caught my thumb in the bench saw last weekend and minced the tip. Can't even hold a pick with it yet.

[Owen]

If anyone is handy with a soldering iron there is a dead Berg IP310 on ebay at the moment which has a potentially really nice plate amp in it that has a "blown power stage due to water/ liquid ingress ".

http://www.ebay.co.uk/itm/281427447511?_trksid=p2055119.m1438.l2661&ssPageName=STRK%3AMEBIDX%3AIT

[LukeFRC]

Think that if the amp is rescueable then tracking down another speaker from Jim bergantino might be a better bet. I'm half tempted to explore it further

[Owen]

[quote name='LukeFRC' timestamp='1409741816' post='2542798']
Think that if the amp is rescueable then tracking down another speaker from Jim bergantino might be a better bet. I'm half tempted to explore it further
[/quote]

Do it for me!

[tauzero]

Well, someone's had it.

[Chienmortbb]

They are welcome to it. Once water gets into an electric circuit it just keeps corroding away until something fails.It does not do paper cones a lot of good either.

[LukeFRC]

[quote name='Chienmortbb' timestamp='1409780168' post='2543396']
They are welcome to it. Once water gets into an electric circuit it just keeps corroding away until something fails.It does not do paper cones a lot of good either.
[/quote] it had failed- I had messaged him asking why it wasn't working - and it had failed due to water ingress.... one of the speaker cones was shredded too...

[Chienmortbb]

Everyone take care when using power saws. Duke from Audio Kinesis had a fight with a table saw and lost. He is now only able to offer a Hi Three.

Duke has been designing and building cabs for many decades so it can happen to anyone,

[Phil Starr]

[quote name='Chienmortbb' timestamp='1410262228' post='2547547']
Everyone take care when using power saws. Duke from Audio Kinesis had a fight with a table saw and lost. He is now only able to offer a Hi Three.

Duke has been designing and building cabs for many decades so it can happen to anyone,
[/quote]

I was lucky in a way, I touched the back of the saw and it threw my hand upwards away from the blade, the tip of my thumb was mincemeat but I could have lost it. I had a gig four hours later and I usually anchor my thumb pretty solidly, still don't know how I finished the gig, I've only started practicing again yesterday 10 days on. I've been using power tools like this for over 40 years and should know better.

[Chienmortbb]

I realise that the design is not finalised yet but can I assume the the depth will be greater than 280mm. As I am currently planning my amp build, I don't want to have the amp overhanging the cabinet and the current favourite is 279mm deep.

[Phil Starr]

The prototype is 53x42x36cm (exterior dimensions) I don't think that will vary by more than a cm in any dimension in the final version. My rack mounted Hartke sits nicely on the largest face. Because it is a self build you can vary the shape to get the exact dimensions you want so long as the internal volume stays the same. I'm going to offer everything down to a cutting list to people who just want to make a perfect copy but the reason we are giving all the design details away is so you can tweak the design if you want.

[Phil Starr]

[size=4][attachment=172884:clip_image002.jpg][/size]



[size=4]OK update on the design process, which is back to where this thread was originally meant to be, I feel sorry for anyone who comes to this in the future to try to find information [/size]

[size=4]The biggest problem we are having is to select the port dimensions. Tuning the cab is easy as winISD will calculate all this for you once you have decided what outcome you want. the problem is deciding the outcome. It is always a compromise and all the cabs you might buy have had the compromises decided for you, however high end they are.[/size]

If the plot above comes out then it should show the cone excursion at 300W. The pink curve shows the cab tuned to 50Hz resonance and the yellow to 40Hz. the red horizontal line shows Xmax, the point at which the speaker coil leaves the magnet and the speaker starts to distort. You'll see that they both start to distort at 100Hz and the 40Hz tuning distorts more. Because Beyma use the worse case calculation for Xmax it is unlikely you would notice any distortion at all with the pink 50Hz tuning and in practice, probably none at the 40Hz tuning. So far, so good.

The problems come below the port tuning. Look at the 30Hz line and you'll see the excursion shoots up to 11mm and 18mm. Critically at 13.5mm for this speaker the coil hits the back of the magnet, and this will destroy the speaker. Low B is 31Hz. This means the speaker won't handle 300W of low B fundamental if we tune it to 50Hz.

So we tune it to 40Hz right? Well no, not necessarily, because this causes other problems. One you might be able to see on the image. The port length is bigger that the cabinet for a 40Hz tuning. More about this in my next post.

[Phil Starr]

file:///C:\DOCUME~1\dad\LOCALS~1\Temp\msohtmlclip1\01\clip_image002.jpg[size=4][attachment=172886:clip_image002.jpg][/size]

[size=4]OK if I've got this right this should show the frequency response of the two cabs. Pink is 50 Hz and Yellow 40Hz. You'll see that 40 is flatter and tails off more gradually and a 50Hz tuning gives a bit of extra bass between 200 and 50Hz. whether you see this as extra warmth or distortion is a bit of a matter of taste, and at only 1dB I doubt you'd notice it unless you are listening for a difference. The point is that mucking about with tuning affects the low frequency response, and it might be greater for other speakers and other cabs. Without the simulation you won't know.[/size]

[tks.se]

[quote name='Phil Starr' timestamp='1412149582' post='2566052']
Low B is 31Hz. This means the speaker won't handle 300W of low B fundamental if we tune it to 50Hz.
[/quote]

When playing a low B, most of the tone consists of harmonics to the fundamental. Here's the result of playing a low B and running the signal through a spectrum analyser:


If we "translate" those dB (logarithmic) values to watts (voltage swing would be better, but using watts makes it a bit more intuitive for most) and use say 200W as a total, you get the following "wattage distribution":


Different basses, fingers, plucking style etc give different responses/harmonic distribution, and this also changes over time (the harmonic distribution of a ringing note differs from that of the attack). The main thing to focus on is that a low B is never a 31 Hz only tone, most of the "energy" is distributed in the harmonics. Because of that a 50 Hz tuning doesn't mean a death sentence for a SM212 (nice speaker by the way!) even when playing a low B at full power.

Great initiative with the design diary!

[Phil Starr]

Final one this morning. For some reason the winISD plot won't go up. If I halve the port area of the 40Hz tuning I can use a shorter port, it now fits inside the speaker!! The plot you can't see
now shows the air velocity inside the port. The faster the air moves the noisier the port gets and you'll hear chuffing noises at somewhere between 15 and 17m/s. Now we have the port down to reasonable dimensions but with the same air moving through it the 40Hz tuning will start to make noises. So we have to compromise; tune to 40Hz and accept a noisy port at high volumes or 50Hz and have to de-rate the power handling for 5stringers or users of octavers.

The solution for us is to go for 50Hz tuning. I'm the one who hung out for the lower tuning but the other guys are right really. Why the hell would you think a single 12 is the way to go if you want a stack to handle full power fundamental from a 5string?

[Phil Starr]

[quote name='tks.se' timestamp='1412160565' post='2566219']
When playing a low B, most of the tone consists of harmonics to the fundamental. Here's the result of playing a low B and running the signal through a spectrum analyser:


If we "translate" those dB (logarithmic) values to watts (voltage swing would be better, but using watts makes it a bit more intuitive for most) and use say 200W as a total, you get the following "wattage distribution":


Different basses, fingers, plucking style etc give different responses/harmonic distribution, and this also changes over time (the harmonic distribution of a ringing note differs from that of the attack). The main thing to focus on is that a low B is never a 31 Hz only tone, most of the "energy" is distributed in the harmonics. Because of that a 50 Hz tuning doesn't mean a death sentence for a SM212 (nice speaker by the way!) even when playing a low B at full power.

Great initiative with the design diary!
[/quote]

Thanks

That's great information, do you have a link?

I suppose I'm designing for worst case, because someone somewhere is going to think they have a 350W speaker that can reproduce anything they throw at it, add 12dB of bass boost from a traditional tone control and the wattage distribution would look very different and my suspicion is that someone, somewhere will do this. I suppose the possibility of designing this hazard out was worth a look, in the end the compromises weren't worth the gains and I'd advise anyone with a fiver who wants to drive at extreme sound levels uses at least two speakers and preferably a filter like a Thumpinator.

[JTUK]

I run a DB750 into a 4ohm 212 for a 5...and it gets pretty loud on the dial.
What I am actually pushing out, I am not sure and I play pretty lightly but
I will be filling the room, for sure.
For bigger gigs, I'll add a 8ohm 210 on top, so the amp is potentially upto nearer
950w for its 2ohms load. Obviously, the 8ohm cab will bring that figure down.

I think people may have been seduced into what cabs will theoretically do
and how they actually do it. IMO, a single 12 with a 5 is a pretty low level application
and you could help yourself a lot soundwise by going to 2x112.
The carry will be still be easy but you'll cover a LOT of gigs