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Phil Starr

Easy 12" cab build

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It looks like a budget version of the SM212. There's no info on the Beyma website, but might well be a direct swap for the SM212, which has gone up in price by about 20 percent in the past few years.

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I've just put the specs through winISD and it looks like an improvement on the SM212. It has the classic Beyma smooth bass roll-off
and handles more power. It appears to be very flexible in it's cabinet volume requirements and maybe requires fewer vents / ports. This maybe a case of "what's not to like"!

I'll put some graphs up tomorrow.

Balcro.

 

Edited by Balcro
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Thanks for the correction on the parameters Phil. Lesson for today - always go to the manufacturers website.

The Thiele / Small parameters on the Blue Aran web-site are nearly all wrong! Where they got those figures from I don't know,
but they don't seem to found on any of the models in Beyma's new CMV2 series.

The CMV2 is marginally weaker on power handling through the overall frequency bandwith, but the difference to the the SM212
is quite small. With both models assessed on a 200 watt input & in a 50 Litre cabinet:-
The sm212 -  takes full rated power down to 37Hz.
The 12" CMV2- takes full rated power down to 41.5Hz

xMax in the low bass is only reached @ 35Hz whereas in the CMV2 it's 40Hz, a function of the SM212's capability of being tuned
to a lower frequency. The sm212 was auto-tuned to 45.45Hz by winISD.

However the CMV2 appears to have an extended frequency response up to 5 & 6K whereas the SM212 drops off quickly above 4K,
so maybe that's the real intention. A cheaper bass-mid speaker rather than a pure bass.

Balcro.

PS. Attached Frequency Response curves. CMV2 in green.

Beyma Freq Resp-212-CMv2.jpg

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Those graphs are interesting, and the differences are to be expected really. First of all the power handling: The cast chassis of the SM212 will conduct a little more heat away from the magnet than the pressed steel frame which explains the extra 30W of power handling.

Although similar these aren't the same speaker with just the frame changed. The voice coil is different and the net effect is that the damping of the cheaper speaker is less. That's why in a 50litre cabinet there is a peak of about 2dB at 90-100Hz. I suspect that would sound quite nice for bass guitar but not so good for FRFR use. I prefer to roll off deep bass in most of the spaces I play so I don't think you'd really miss the slightly earlier roll off. In fact though you might notice it if you had both speakers there I think it would be subtle enough you probably wouldn't notice, the 2dB peak would be the thing that would dominate bass response.

I'd imagine the 12 CMV2 would be less good in the 30l cabinet than the SM212 but I haven't modelled it yet.

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Guys, if you've already done an idiots guide to these graphs could you point me in the appropriate direction? If not it would be great to have a broad brush stroke explanation for myself and I suspect others around these parts.

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Ha ha you aren't asking for much :)

However I'm sat in a waiting room waiting for some test results so here goes.

To completely simplify the whole thing if you know all the forces acting upon the cone and it's mass you can calculate how far it will move at each frequency. That's essentially what winISD does. Once you know that you can say how loud it will be at each frequency and print that off as a frequency graph. The first thing to look at IMO is the frequency graph then. 

I'll copy one down as an example and explain

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6 hours ago, Chienmortbb said:

It models well at 30L tuned to 50 Hz.

 

 

 

New~Beyma12witgh 30Lcab-1.jpg

Here you go.First of all notice that only the lowest frequencies are showing. The upper frequencies aren't affected much by the cab. The average output of the speaker is set to 0dB on tis graph. Anything louder is above the nought line and anything below is quieter. We use dB's because it is close to how we hear things.

You can see that this speaker in this cab starts to roll off at just above 100Hz and is 10dB down at just below 50hz. You can also see that there is a 2dB peak just above 100Hz. That peak is going to warm up the bass but there won't be too much deep bass. The only way you learn that is by trying out cabs you know and listening out for what each sounds like but it helps to compare cabs. If you look up you can see in Balcro's charts how the same speaker behaves in a bigger 50litre cab. More bass and a flatter response, that's a useful bit of information.

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6 hours ago, Chienmortbb said:

Here are cone extension and max power graphs.

 

cone extension.jpg

maxpower.jpg

OK looking at the top graph it is excursion, how far the cone moves at each frequency. It's not there to tell you how the speaker sounds but if you look there is a red line at 7mm. That's the maximum the cone can move without distorting or Xmax. anything much above this and you will not just get distortion the speaker will be damaged. The other thing to notice is that the deeper the frequency the further the cone moves except at 50Hz where it moves less than you'd expect. That's the port tuning frequency. The air in the port is doing all the work and creating a back pressure on the cone stopping it from moving. Without the port the cone would be well outside the safe excursion limits and that is why we use ported cabs.

Looking at the last graph and that tells you the maximum power you can use. Above 40Hz you can use the full power of the speakers rating 320W. It'll get very hot but won't burn out. Below that you can see that too much power will move it out of the 7mm zone and it will get damaged or distort. If you put in a 20hz signal it will only take 20W without distortion and you'd blow it with 30W'ish. 

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That's brilliant Phil. Seriously thank you. I appreciate there is a greater complexity and nuance to this stuff but I feel I have a way in now.

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Looking up there, there's a 320W speaker cabinet. We all know that the engineers who designed the cab would say it's 250-300W to be safe. The marketing department are going to round that up to 400W+ though. 

 

It's probably worth pointing out that it's the ability to do this kind of math at home now that's helped to make the high pass filter the ubiquitous gadget de jour around here over the past year or so. We can do the math, it's 320W above 40Hz and only really 20W at 20Hz, which is why people are so up on the HPF to filter out the sound below a certain point. Not only to clean up the muddy tone but also to protect the physical speaker. If you were to give that cabinet the full 320W at every frequency (you would't anyway, it's bass guitar not sub-bass guitar) then you've got 320W at both 40Hz and 20Hz, bad. If you put a 10dB/octave high pass filter from 40Hz down, then you're at 32W at 20Hz, which is looking a lot better. Most HPF are either 12dB/oct or 24dB/oct, so you'd likely be safe either way round. 

Edited by Jack
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It would be good if bass amp makers fitted a high pass filter as a matter of course. It would only cost them pennies. I know some do, but none of them provides that information as far as I know.

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1 hour ago, stevie said:

It would be good if bass amp makers fitted a high pass filter as a matter of course. It would only cost them pennies. I know some do, but none of them provides that information as far as I know.

+1

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3 hours ago, Jack said:

Looking up there, there's a 320W speaker cabinet. We all know that the engineers who designed the cab would say it's 250-300W to be safe. The marketing department are going to round that up to 400W+ though. 

 

It's probably worth pointing out that it's the ability to do this kind of math at home now that's helped to make the high pass filter the ubiquitous gadget de jour around here over the past year or so. We can do the math, it's 320W above 40Hz and only really 20W at 20Hz, which is why people are so up on the HPF to filter out the sound below a certain point. Not only to clean up the muddy tone but also to protect the physical speaker. If you were to give that cabinet the full 320W at every frequency (you would't anyway, it's bass guitar not sub-bass guitar) then you've got 320W at both 40Hz and 20Hz, bad. If you put a 10dB/octave high pass filter from 40Hz down, then you're at 32W at 20Hz, which is looking a lot better. Most HPF are either 12dB/oct or 24dB/oct, so you'd likely be safe either way round. 

I'm possibly opening a can of worms here, but is there any rule of thumb for how the power handling graph relates to the real world where we have a fundamental tone and a bunch of harmonics?

I don't have the graphs to hand at the moment but I recall looking at a model of a Beta 12 in a box a bit smaller than the original Mk 1 cab and thinking it was a bit marginal because the maximum power at the low E was about 90W.  However I also recall seeing another chart somewhere showing that the first (and possibly second and more) harmonics typically had more amplitude than the fundamental, but then there's the fact that the power required (I may have got this wrong...) halves for every doubling of frequency...

Hence gut feel says that if you have, for example, 320W going into a speaker then there's only going to be a fraction of that used to produce the fundamental, but I'd guess it's probably quite a large fraction - around a half maybe?

Does that make sense or have I got completely the wrong end of the stick?! 

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1 minute ago, Gottastopbuyinggear said:

I'm possibly opening a can of worms here, but is there any rule of thumb for how the power handling graph relates to the real world where we have a fundamental tone and a bunch of harmonics?

I don't have the graphs to hand at the moment but I recall looking at a model of a Beta 12 in a box a bit smaller than the original Mk 1 cab and thinking it was a bit marginal because the maximum power at the low E was about 90W.  However I also recall seeing another chart somewhere showing that the first (and possibly second and more) harmonics typically had more amplitude than the fundamental, but then there's the fact that the power required (I may have got this wrong...) halves for every doubling of frequency...

Hence gut feel says that if you have, for example, 320W going into a speaker then there's only going to be a fraction of that used to produce the fundamental, but I'd guess it's probably quite a large fraction - around a half maybe?

Does that make sense or have I got completely the wrong end of the stick?! 

Yes, that is right, there's reference to that on one of our incredibly long 12" threads :)

For anyone with a bass to hand it's easy to see, pluck the E string at the 12th fret and it will primarily vibrate around that point, touch your finger against the string just above the 12th fret and it will go on vibrating but will vibrate at it's lowest at the 12th fret. In fact if you leave it alone and watch you can see dead spots (called nodes) forming and breaking down along the length of the string. Easiest to see under a flickering light BTW. https://www.physicsclassroom.com/mmedia/waves/harm2.cfm

The only way you'd get the full fundamental would be to pluck the string at the 12th fret and have the pickup directly under at that point. As any bassist will know when you pluck closer to the neck you get more bass (from the fundamental) and closer to the bridge gives you less bass. Somewhere there is a modeller which let's you see what happens to the mix of fundamental and harmonics when you move the position of the pickup. Again it's on one of the threads here. One of the genius things about a P bass is how the pup's are placed at a position with a great mix of fundamental and harmonics.

In practice you get way less than 10% of the signal as fundamental, that will be modified by any bass cut or boost and fx, especially octavers and synth effects. Most of the time with most players there won't be a problem but equally cutting everything below 40Hz won't be heard by anyone in a band situation and potentially will distort the sound so why amplify it.

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Indeed. I alluded to it with my 'not a sub-bass guitar' comment and Phil covered it way better than I could have but basically there's not a whole lot of fundamental in a bass guitar signal. An SVT 8x10" rolls off around 70-80Hz and sounds great.

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Duke le Jeune of Audio Kinesis says that a -3db point of twice the fundamental frequency works well. That coincides with your figures for the SVT. I must admit though that I would prefer a bit lower and then dial it out electronically if needed

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On 19/09/2018 at 10:09, stevie said:

It would be good if bass amp makers fitted a high pass filter as a matter of course. It would only cost them pennies. I know some do, but none of them provides that information as far as I know.

So anyone know if the Ampeg pf350 has a HPF fitted?!

I’m assuming it’s just the higher end stuff though.

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On 19/09/2018 at 08:50, Jack said:

Looking up there, there's a 320W speaker cabinet. We all know that the engineers who designed the cab would say it's 250-300W to be safe. The marketing department are going to round that up to 400W+ though.

Nobody complains louder about cab manufacturers' specs that me, but by and large their published power ratings seem to be on the level. They're provided with the power handling figures by the driver manufacturers and any attempt to fudge them would be tantamount to lying to their customers. As far as I can tell, there are very few commercial cab makers who feel they have to resort to that.

Frequency response and sensitivity - now that's a different matter.😀

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