Changing a designed sealed to a vented enclosure question

[DeanP]

Hello
Well I finally decided on a DIY Ht speaker system: Audax HT.
Since at this time, as always, money talks, so I would like to change the sealed surround speaker to a vented design.
This is what I came up with using Unibox:
20.5L with a 3" by 8.5" long port.
It was a sealed 11L before.
So without redesigning the crossover,can this be done?
Here is the link to the original speaker specs:http://www.solen.ca/audsrd.htm
It is the Audax surround speaker.
Thanks in advance...
Dean

BTW I will build the Audax fronts if this is not possible...

EDIT:
I will be keeping the same baffle width as well as tweeter/woofer placement on baffle plus tweeter to top of baffle placement.

[JonMarsh]

This can be done, without changing the crossover, but as you found out, using a driver designed (with regards to T/S parameters) for a sealed box results in a much larger box and different tuning- but it will give you more bass extension. Do you really need that in a surround, if you have a subwoofer and normal redirection of LFE below the crossover point?

BTW, if you're doing the Audax HT project, get your drivers soon, if you haven't already, because Audax is withdrawing from the DIY market.

Best regards,

Jon

[DeanP]

Well I don't have Unibiox here at work to get my actual values, but the sealed doesn't go very low and I would use them as fronts for now. So I tuned them to about 49Hz in a vented design.
Is this too low?

I do have a question about port resonance, where should this freq be tuned at?

As for the drivers, I better get what I need so I don't end up with mismatched speakers.
When are they supposed to quit DIY?

Thanks Dean

[ThomasW]

So I tuned them to about 49Hz in a vented design. I do have a question about port resonance, where should this freq be tuned at?

One doesn't 'tune' a port resonance, as a matter of fact the goal is not to have any port resonances.

BTW this is a woofer that starts rolling off rather steeply at ~170Hz. So you really shouldn't be tuning them to 49Hz. With too much power they'll bottom and you'll destroy the driver.

My suggestion is build them sealed as per the plans. Then set the pre/pro to 'small' for the rear speakers and be safe.........

I believe they have quit DIY and people are now just selling off the remaining inventory. Harmon International bought them and is converting the factory for production of other Harmon products.

[DeanP]

One doesn't 'tune' a port resonance

In Unibox there is a box where it shows port self-resonance and as port size changes so does this freq changes, so I played with the port size until it was at ~644Hz.

Hmmmm, I guess a person has to have much more knowledge than just a software program! :W

I believe they have quit DIY and people are now just selling off the remaining inventory.

Because of this I may be persuaded into something else as I will be building the speakers as money exists.
Someone else suggested these as a good starter:AR.Kits

My only problem is that they don't have an actual centre channel speaker...so

Do you have any other kits that maybe I should look at?
I would like bookshelf type for mains and also a center channel match.
I do have a 54L sealed shiva now which I am going to make into a Darman clone this summer...

I think Andrew Pratt uses the M8a for a center but it would probably be a bit big too sit on my 32" tv. Plus the cost for all three would definitely put me over the top!
Someday I WILL build the M8As...

Thanks in advance,Dean

[ThomasW]

Yes the catch 22 is that you need to know how to design speakers in order to use the speaker design software....

And yes the port resonances change with changes in the length of the port. This is common sense.

The longer the port, the lower the resonance, the shorter the port the higher the resonance. Think about a tubular windchime. The long tubes make the lowest notes. Those notes are the resonances of the windchime's tubes and a tube is the same as a port.

Note that port resonances aren't really a problem unless the port is 30+" long. And there are techniques for taming them IF they are problematic.

The M8a/M8n can be put on it's side as a center, the only drawback is that the drivers aren't shielded. They can be shielded with bucking magnets and by lining the cab with sheetmetal. And they can certainly be built in much smaller cabinets. I built a sealed pair of the MKIII (Focal tweeter) in a significantly smaller box for my bedroom speakers. These work VERY well with a sub



With the new low buck M8n and the lowered cost for the XT25, there's nothing in their price range that will beat the M8-MKIV.

[DeanP]

With the new low buck M8n and the lowered cost for the XT25, there's nothing in their price range that will beat the M8-MKIV.

Yes,I believe you! But it is too bad that the only"N" series of the bunch that isn't sheilded is the M8N...

And they can certainly be built in much smaller cabinets.

Yes, but I do believe that they must be kept to a certain baffle width.
So what would be the baffle width? is it 10" or 12"? (M8a(n)MKIV) I don't have the magazine in front of me,though I did go thru the hoops to get all three issues.
Any size recommendations for a centre channel?
I would build the fronts ported...
Thanks again Dean

[ThomasW]

The box in the picture has external dimensions of Height-19", Depth-12" ,Width-10".

Where are these rears going? Because if they're close to a wall, a narrower baffle is ok since the baffle step compensation must be changed for near wall placement.

Although there's a link on the PE website titled 'shielded' M8n, it goes to an unshielded driver. There a no "M" series 8" drivers that are shielded.

We 'shielded' Tibor's MTM center using bucking magnets and lining the walls of the cabinet with the thickest sheet metal we could find. And we know it worked becasuse his TV is sitting on top of it.

[DeanP]

Okay Here is what I would like to do.
M8aMKIV ported for the fronts.
A ported, smaller baffle centre and
later I guess, M8aMKIII sealed for the rears.

My problem would be the space requirement for the fronts, that being too close to the wall; approx. 2' vice the recommended 6'.
Now that being said, maybe I'd better stick with the M8aMKIII sealed design for the fronts and centre, as I understand these were designed for being close to a wall.

The box in the picture has external dimensions of Height-19", Depth-12" ,Width-10".

I believe the above are the M8aMKIIIs, is this correct? If I was to port these, what size would I make the cabinet and also what port size?

Hmmmm? I wonder when PE is going to actually recieve the M8Ns? As I see another thread that they keep getting delayed...

Maybe I should look at making something that Pat Sun made, I understand that you helped him out with his fronts and centre channel. Is this correct? Besides, I would be able to get all the parts here in Canada...

Anyways, that is just a thought.

Thanks, Dean

[ThomasW]

First, no don't build any MKIII's. I just used that just as an example of cabinet size. Build MKIV's they use the XT25 tweeter.

Second, DO NOT port a box the size of the cabinet picture I posted. The box is simply too small. The XO components barely fit, so there's absolutely no room for a port.

The 'expected' availability date for the M8n hasn't changed for weeks. As a result I'm hoping that means 5/21 is it.

If the speakers are going to be less than the recommended spacing from the wall, it will be necessary to get Jon to spec a XO with different baffle step compensation. Otherwise the tonal balance will be off.

As I recall Jon had minimal input into Pat's speakers.

[DeanP]

ThomasW
I just reread AudioXpress again...I think that I might get in over my head with the M8aMKIV as this would be my first set to fully assemble.
I guess I should start off with something easier but at least an "upgrade" from my Athenas.
Thank you for all your help thus far. :T
Dean
EDIT: I took a look at Andrew Pratt's thread and saw the crossovers there and they didn't seem too complex. Looking at the magazine, I got confused as to where the resistor l-pad goes.
Being a newbie, I basically need a component layout diagram as to where the l-pad is.
Can you explain this to me?

[DeanP]

I noticed at PE that the xt25 comes in a shielded version, is this a drop in replacement also like the M8n was?

[Andrew Pratt]

For what its worth the single M8a I'm using for my centre channel isn't shielded and my Wega is sitting slightly above it on a shelf I made but I've never seen any magnet effects on it due to the M8a driver....the quad tempests on the other hand were a different story :W

To that end though my sealed units weight over 60 lbs so you wouldn't want to place that on any TV without some sort of shelf to help take the weight off it.

The XO's for these are intimidating for a complete noob but these were the first real XO's I built and its not as bad as it seems...besides you can always dry mount them and take a photo for us to look at to make sure they're correct

Here's some old shots of my XO's

Lowpass



Highpass sitting on top[ of one of the sealed units



And here's the front of the mains...

[ThomasW]

Dean

Sorry, between your emails and your posting questions on other threads I missed the question about the tweeter. Go to the PE website and compare the T/S parameters for the 2 tweeters. IF the parameters are identical then yes the shielded one is a drop-in replacement. If they're different then obviously it's not.

For Tibor's MTM we didn't use the shielded tweeter, and we didn't add bucking magnets to his tweeters. The cabinet shielding itself was enough to control the tweeter's magnetic field.

[DeanP]

Andrew:
Thanks for the photos! They will help alot! :T
I see you are using an adjustable L-pad...Thomas also suggested this instead of the resistor build up. I'll have questions later on in the build as to how to hook this up properly.

ThomasW:
Sorry about all the different thread replies(two,I think) and e-mails. I felt I bugged you enough, so I posted in the other thread as I thought this thread was dead. (i guess it wasn't)

I see Andrew installed the non-bazillion capacitors using only about two -three.
I checked the parameters at Solen for the tweeters and they spec the same. Thats good!

sealed units weight over 60 lbs

EEEEK!
I guess I'll figure something out, maybe suspend the centre from the ceiling!

Thanks Guys for the help! :T

[Andrew Pratt]

Regarding the cap clusters...its cheaper to use a ton of small ones...but its more practical to use a few larger ones where you can..its adds to the cost but for me the fewer caps to worry about the easier it is to build.

[DeanP]

Well, I just figured out the cost of building these babies and now I will definitely have to hold off for a while until I can gather up some more cash. :cry: Man,those capacitors really add up quick! Now I see why the guy building the dual M8n built up his capacitors using smaller values! I look again at these after some TDYs when I can get the extra cash not needed for the "house".
In the meantime ,I'll look at building something else for the other stereo system...Back to seeking out more cost saving ideas!
I want to thank you guys for your help and for when I come back to the vented M8a. :T
Dean

[Andrew Pratt]

Yeah the drivers aren't the expensive part...not that their cheap either :E

[Ten 99]

No, the drivers aren't in a class of what I'd call cheap, but they are definitely affordable. I'm certain that not only are they affordable, but in the not so humble opinions of Jon and Thomas, they are excellent performers for the money. One thing I've come to the conclusion of, is that one of the major parts of a speaker system is the cross-over. Not only in the design and tuning of it, but also in the quality of the components that it is comprised of. You'll be glad you saved up a bit.

BTW, this got me to thinking about something regarding these drivers and cross-overs. I have another question for another application, and I guess I'll post it in a new thread.