New Speaker build

[TBL]

What started off as an OB project has quickly turned into a sealed box design. I've done the basic design below and am currently talking to my local cnc shop to get the shapes cut in 25mm ply. The idea is that the entire speaker will bw made form 10 "slices" of ply glued together so that the whole speaker apears as on solid piece of wood with an internal cavity all the way through to the base.

I've left an internal gap in the base so that the crossover can be housed inside or I could also go for an external crossover at a later date. I've also allowed for the option of porting, either to the front or out through the rear of the base.

Front on you will see a traditional column shape with exposed end grain of the ply. Side on is as the drawing below.

Hoping to do the whole lot for well under a grand (£350 for drivers £500 for wood and cnc work)

http://i1127.photobucket.com/albums/l637/alphatoner/Speakers_1.jpg

I'll post more pics once the build actually starts

[aquapiranha]

hi Ian, that looks like an interesting build. did you calculate the profile? Do you have any details on the drivers etc?

[goraman]

Ian, Nice Cobra speakers.
Here is a link that shows you how to tame them during break in.
And reduce the annoying hiss.
http://www.youtube.com/watch?v=bcPeY-jXNtw&feature=related

[John]

I agree with Steve nice looking design, what drivers are you using.

[goraman]

They are cool but they are shaped like a King Cobra.
Sealed would most likely give him very tight bass.


The perfect veneer http://www.exoticwood.biz/snakewood.htm

Then rub them down with this to bring out the finish http://www.google.com/products/catalog?pq=snakewood&hl=en&cp=7&gs_id=8&xhr=t&q=snake+oil&gs_upl=&bav=on.2,or.r_gc.r_pw.,cf.osb&biw=1440&bih=799&um=1&ie=UTF-8&tbm=shop&cid=9899671758440457357&sa=X&ei=xHWWTpH8L4bkiALN6MjYDQ&sqi=2&ved=0CFkQ8wIwBA

[TBL]

Drivers arrived this morning

Scan Speak Revelator 6.5" mid woofer

http://i1127.photobucket.com/albums/l637/alphatoner/DSC_0014.jpg

Arum Cantus G2 Ribbon tweeter

http://i1127.photobucket.com/albums/l637/alphatoner/DSC_0013.jpg

[TBL]

hi Ian, that looks like an interesting build. did you calculate the profile? Do you have any details on the drivers etc?


err No :scratch:

[TBL]

These are the temporary crossovers I'll use untill I can sort out something better.

Monocor DN2618

http://i1127.photobucket.com/albums/l637/alphatoner/229149.jpg

[John]

What are you going to do around bass

[Reid Malenfant]

I'm not sure i'd shape the cabinet quite like that in all honesty ;) Depending on how thin the shaped lower section is you may find that if you introduce a reflex port out of the back at the bottom, you might not be able to tune it. You see that section will likely act like a kind of long thin port anyway & you'll have no way of altering it's length :( Or width come to that...

What model is the 6.5" revelator (part number)? I'll see if i can find it's specifications & advise on enclosure volume. You'll find it'll be different (depending on the driver Qts) for a reflex or sealed box to get the best performance, unless it's stupidly low that is (0.25 or lower)..

[TBL]

Mark,

The Scanspeak is this one

http://www.wilmslow-audio.co.uk/scanspeak-revelator-18w-8531g00-682-p.asp

At the moment all I've done is come up with a design that is aesthetically pleasing (at least to me) Although I fully accept that it is not a design you would choose. But I'n not a great fan of box style speakers, however good they sound. But if there is some change I can make to the internal chamber that would work better with the Scanspeak then I'd happy to make some adjustments. The thickness of the external walls will be 25mm and the gap down the column will be about 50mm deep by 200mm wide. In part that was done for practical considerations to allow the cables to go from the drivers to the crossovers in the base.

You might have learnt from some of my previous posts that I am completely non technical. So I have treated this project like all the other stuff I've designed (furnitiure) to look good and not much else comes into it. But if you can offer any advice that I would understand and stays within my parameters of what looks good I would be very grateful.

I've done a quick VAS calculation to see if needs porting and it falls into could be either camp (70). Given that I'm not into heavy bass I'm inclined to keep it as a sealed box anyway.

[Reid Malenfant]

Hi Ian, cheers for linking to the driver. I'll do some modelling for you tomorrow & see what i reckon, then present you with the various outcomes ;)

This will be box volume calculations & tuning frequencies, or not in the case of a sealed box. 50 x 200mm might start looking like a slot port to the driver & enclosure but obviously not if it's sealed. If you ported out the back of the main body you'd have no problem, you might if you did out of the base at the back though - difficult to say :scratch:

[TBL]

Thanks Mark, I appreciate the effort.

[Reid Malenfant]

Hokay, we have some results ;)

There are two enclosures modelled in these graphs, the blue trace is the bass reflex enclosure, the yellow is a closed box.. They differ in volume, te reflex being approximately twice the internal volume of the closed box. If you look at the pics you'll see all the relevant stuff like box volumes & the enclosure tuning frequency for the reflex.

I'll see if i can explain things as i go along :)

First up is the transfer magnitude response or frequency response if you like. You can see that the reflex will have a -3DB response at about 32.5Hz (quite deep) & the closed box manages just under 60Hz. You can see that i had the port set to 6.8cm diameter (more of this in a bit) & it is 25.11cm long to tune the box to the relevant frequency to get this performance.

http://i939.photobucket.com/albums/ad240/speakermark/Revelatortransferfunctionmagnitude.jpg

Next up is the maximum sound pressure level the driver can produce in each enclosure. That rather large scoop out of the reflex curve is due to the driver running out of linear excursion. However it has a mechanical limit of 11mm so it'll tend to fill a good bit of the scoop shape in in reality. As you can see, it will produce a good bit more output than the closed box ;) Looks like it'll be capable of nearly 100Db right the way down to 30Hz & all of it linear excursion. With two in a room you might as well add 12Db on top for the extra driver (6Db) & room gain (another 6Db approx).

http://i939.photobucket.com/albums/ad240/speakermark/RevelatormaximumSPL.jpg

Next is driver excursion & you'll see where that scoop shape comes from. There is a thin red line accross the graph at the 6.5mm point & this is the linear excursion limit. You can also see the enclosure volume of approximately 38.5L & tuning frequency of approximately 30.6Hz.

http://i939.photobucket.com/albums/ad240/speakermark/Revelatorconeexcursion60W.jpg

Finally the rear port air velocity using that 6.8cm port. 26M/s is getting a bit of a move on & unless the port has rounded edges there will be turbulence generated & you'll hear it as chuffing. I'd suggest a bigger port, possibly 8cm diameter, but it'll need to be longer :rolleyes: Don't forget though that you'll only get these kind of port velocities when stuffing full power into the speaker at the frequency in question, probably a rare occurence so maybe a 6.8cm port will be ok. You can also now see the box volume of the sealed enclosure, around 21L..

http://i939.photobucket.com/albums/ad240/speakermark/Revelatorportairvelocity60W.jpg

If it was me i'd build a reflex, these drivers look as though they will give excellent performance :eyebrows:

[TBL]

Thanks Mark,

Will take me time to wade through that but a couple of questions jump out at me to do with volume.

1. How have you calculated the internal volume.

2. Why would a reflex box be twice the internal volume of a sealed box?

3. In the last graph you show a volume of 21.33L for the sealed box, I've done a very rough calc on mine and allowing for the curves it comes out at about 28L how would this effect things? and would increasing the volume be an improvement or not. It wouldn't take much to add another 10L to the volume.

Thanks once again.

[Reid Malenfant]

1. How have you calculated the internal volume.

2. Why would a reflex box be twice the internal volume of a sealed box?

3. In the last graph you show a volume of 21.33L for the sealed box, I've done a very rough calc on mine and allowing for the curves it comes out at about 28L how would this effect things? and would increasing the volume be an improvement or not. It wouldn't take much to add another 10L to the volume.

Thanks once again.
1. I used the straight QB3 (Quasi Butterworth) response that winISD gave me as a starting point. As the driver has a Qts of 0.36 which is close to ideal for drivers in closed or sealed boxes it comes up with a good box size & tuning frequency with no prompting.

2. It's twice the size approximately as you need a lower box Q for the reflex to work properly. If you fitted it in the same size box as the closed enclosure there would be a big peak in the response unless you tuned incredibly low. This would not only be a waste of time but very difficult as the port would be very long indeed. I can show you this on a graph if you like, but you wouldn't like to attempt to build it as the port would be so long (you'd have to go round corners) that the extra volume taken up by the long port will almost make the enclousure internal volume the same as the QB3 reflex anyway :lol:

3. The bigger volume will lower the resonance frequency of the bass driver giving you a slightly lower cutoff point. But, & it's a big but, you'll have reduced power handling & even less bass output than the closed box system in the graph. It will be slightly cleaner though. What you need to remember is you'll probably need to stuff some kind of sound absorbent into the enclosure to damp things & attempt to lower front to rear reflections etc. If you use rockwool you could get the same bass resonance in the sealed box with only 80% of the enclosure volume, that'd be only 17L... So one that is 28L along with some stuffing is going to be massive for the driver & result in less bass output & power handling, but a slightly extended & cleaner bass...

E2A:- the reason you get less power handling with a bigger sealed box is due to the fact that the box has much more of restoring force for the driver than it's own suspension. The air pressure inside the enclosure predominates. When you make the enclosure a bigger volume there is less restoring force (air pressure difference between the inside & outside) & consequently the driver cone can move further in or out for the same power being applied, so less power is needed to move it "X" distance. This is at a lower frequency though as you have lowered the driver resonance in the box by increasing it's volume. So while you can go a bit deeper a bit cleaner, you'll suffer with reduced power handling & a reduced overall SPL output in the deep bass.

Hope that makes some sense :)

[Macca]

I remember when you had to do all these calculations with the equations, a biro and the back of an envelope :)

If it was me I would go for the sealed bass enclosure, make it larger to avoid 'bounciness' - what you lose on bass SPL you will gain with more natural sounding bass and much better timing.

[goraman]

Ian, Would this be a good match? http://www.madisoundspeakerstore.com/approx-8-woofers/audax-hm210c0-8-carbon-fiber-woofer/

[goraman]

If you really want some bass
http://www.madisoundspeakerstore.com/pro-sound/audax-pr330m0-13-professional-woofer/

[Reid Malenfant]

I remember when you had to do all these calculations with the equations, a biro and the back of an envelope :)

If it was me I would go for the sealed bass enclosure, make it larger to avoid 'bounciness' - what you lose on bass SPL you will gain with more natural sounding bass and much better timing.
Yes i remember how bloomin laborious it was to :doh:

The closed box is fine & yes it will give slightly better quality bass, but not by much. You see the driver has a nice lowish Qts so there shouldn't be much if any overhang in a well built & tuned enclosure. As for the closed boxes, power handling reduces to 15W @ 40Hz with the 22L box & if it was increased to 33L or 28L stuffed that drops to only 10W :rolleyes: I don't think this driver was designed to be run in a sealed box unless it was used as an upper bass - lower midrange, with something taking over below approximately 100Hz or slightly higher.


This is just a suggestion, if i had a way of making a fancy shaped enclosure as per the OP i'd not make the bit at the bottom as it is, what i'd do is concentrate on the enclosure & then using a decent diameter port i'd make a built in stand & have the port exiting out the bottom of the stand ;) With a bit of fiddling about it'd be possible to get the tweeter at ear height & the right length & diameter port to complement things nicely :cool:

In fact you could probably do it quite similar to the OPs present design, but with more thought again :hmm:

[TBL]

Ok Starting to get a bit more of an understanding now (not everything, but some) so a couple of what if questions.

First if we assume sealed box

With the current design 28l and the cavity going down through the column, would I be better sealing of the top and making that a 20l (appx) enclosure and making the column and base solid.

in either case should I fill with rockwool.

for the upper cavity would it be better to have straight internal surfaces rather than curved , this would lose some of the volume (about 2l) so should I also increase the size to get the volume higher or would 20l be enough

Regards

Ian