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How to determine the correct enclosure size?  

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kanaaudio
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27/08/2020 5:04 am  

Hi there. Just a quick question about determining the correct enclosure size for speaker drivers.

In the specs of the speaker drivers I am using (Dayton Audio ND91-8) the recommended sealed volume is 0.02 ft³ and the vented volume is 0.04 ft³. 
Since I am using two drivers, is it as simple as multiplying by 2, so the sealed volume should be 0.04 ft³?

Also, is it better if the enclosure is divided in two, so each driver has its own compartment?

And finally, is the volume of the enclosure calculated in "free" air, in so that only the volume of air counts. So one would have to take into account the space used up by amp cards, battery packs and the speaker drivers themselves?
Or is it calculated so that the needed volume is just the outline of the inside of the speaker enclosure, regardless of what else is filling up the space.

Thanks!

 


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ronL
 ronL
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27/08/2020 10:23 am  

Hi,

You might want to have a look at Unibox. ( removed link ) This is an Excel-based tool that helps you to play around with the box sizes and the behaviour of the boxed speaker (closed, ported etc.)

You need to put in the TS-parameters and with this you are able to calculate the low frequency response of the speaker. Recently I designed a new box for some old Mission 75 speakers. I measured the TS parameters of the woofers with REW (added mass method) as I didn't have this data from Mission of course.

Hope this helps.

 

edit: I just saw that the link was removed. You can search for "unibox speaker design" and you will find the site. The tool was written a couple of years ago by Kristian Ougaard from Copenhagen. Good luck!


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TVOR-Ceasar
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27/08/2020 11:45 am  

@ronl The link removal is due to your low post count at the moment. I believe that once you hit 5 posts, you should have that ability. Definitely by 10 posts. Until then, you can look here for that program.

@kanaaudio Generally, you calculate the box and it's associated extras for what will be included inside the common space. So, if you have 2 drivers in the same open space, you need to add the Vas together, keep the Fo/Fs and Qts the same, depending on configuration. (btw, the bare minimum requirements to calc a box are Vas, Qts and Fo/Fs) After you get your box volume, you have to account for added objects in the space such as the drivers themselves, and any other electronics that may intrude into that space and then add that to the base box volume. Wires (and stuffing - aka fiberfill) are not really any concern, since in reality most boxes and designs are pretty forgiving, somewhere in the +-10% range (personal preference).

I find that one of the best explanations out there are from HexiBase, starting with his Isobaric loading, and Thiele -Small parameters Part 1 and Part 2. I know, I know, I love Nick's (123Toid) videos, but Pete (HexiBase) really clarifies these points well.

To the division of the space inside (2 separate boxes in one), that is a personal preference choice. It can make it easier to calculate, but possibly more difficult to build. There's really not a big difference between the 2 types, so whichever strikes your fancy at that moment is the one to use. Of course, if you could use the extra bracing, then that will help.

Also, those published box sizes may or may not be optimized for the drivers. Most times I take them as rough suggestions and then run my own numbers, usually with a different result from what's published.

-Charlie


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kanaaudio
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28/08/2020 4:56 am  

Thank you so much, @ronl and @tvor-ceasar, for your replies. I need to study up on this further and familiarize myself with these different values and see if I can figure this out. I will update the thread with what I find.

@tvor-ceasar, one question to get me going - you mention 3 values needed as a minimum (Vas, Qts and Fo), but I can seem to find information on the Fo value, is it the same as the Fs value?

Thanks again,


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TVOR-Ceasar
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28/08/2020 6:27 am  

Yes, Fo=Fs. I've been playing around with this stuff for a long time and tend to hold onto some of the old descriptions. Sorry about the confusion.

**Edit - I've updated the post to include both descriptors since you may come across the old one if you are looking at older drivers.

-Charlie


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kanaaudio
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07/09/2020 8:52 am  

@tvor-ceasar I have been working with the excel sheet provided. I did what you said, added Vas together and kept Fs and Qtc the same. However, I am stuck on the value Rs, under external components. For most of the other drivers already loaded into the sheet, the value is either 0,10 Ohm or 0,60 Ohm - Which should I choose? The difference in box volume is relatively significant, because of my small enclosure size. I am using the Dayton Audio KAB-250v3 amp board and two ND91-8 drivers.

I have enclosed the excel sheet, if it makes it easier! Thanks in advance!

 

 


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TVOR-Ceasar
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07/09/2020 10:07 am  

@kanaaudio

I'm on vacation right now and limited to my phone. Can't read the XLS file. Rs is not one I recall offhand. Can you describe what it is / how it's derived? I should be able to figure it out from there. (Or maybe I need to rewatch the HexiBase videos again 😉 )

-Charlie


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kanaaudio
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07/09/2020 10:44 am  

@tvor-ceasar Oh, no rush, have a great vacation! The description for Rs says "Additional series resistance caused by speaker cabling and amplifier output resistance".


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TVOR-Ceasar
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07/09/2020 11:22 am  

Okay, that makes sense.

To me, that means that an amp that can drive lower impedances and shorter/larger gauge wire /higher quality (ie, lower resistance/ft or meter) would go with the lower Rs. So, using 24 ga. cheap wire that is rather long and an amp that will only go down to 8 ohm outputs should use the 0.6. Shorter runs with thicker ga., higher quality wire and an amp that can go down to 4 or 2 ohms should use the 0.1.

If anyone else knows how to properly use Rs, please chime in. This is purely conjecture on my part, and I'd like to know if I'm on the right track.

-Charlie


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