A Beginners Guide to Enclosure Alterations - diyAudio
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Old 7th January 2009, 02:28 PM   #1
Bork is offline Bork  Australia
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Default A Beginners Guide to Enclosure Alterations

Hello all,

Iíve been lurking the posts on this forum for a while now. I am new to audio DIY and feel like it would be a fun and satisfying pursuit. I am keen to try one of the (many) well established designs available on the net:

http://www.zaphaudio.com/
http://www.troelsgravesen.dk/Diy_Lou...r_Projects.htm
http://www.humblehomemadehifi.com/
http://www.rjbaudio.com/projects.html

(currently, I have not settled on a design but donít feel like I would have any trouble doing so - there is so much choice! Letís pretend the I/the noobs have gotten this far)

I recognise that the only way to ensure success with these designs is to stick to the plans religiously. I have absolutely no intentions of changing the chosen drivers or the crossover design. However... (I can feel eyes roll as you guys anticipate a routine beginner's urge ) ... how much creative input can a person put into their enclosure? To claim something as their own, Iím sure there are many beginners out there who want to put their own unique spin on an enclosure design.

There are little nuggets of information here and there that help out the novices but you have to read a lot to get there (often by coincidence) . It would help if there was a more densely packed post...

So Iíll try and get the ball rolling...

From my surfing, I think it is safe to say (?) you can achieve the intended acoustic results and do what you like with an enclosure IF your modified design:
- has a front baffle identical to the original design
- has an enclosure volume identical to the original design
- has port tuning identical to the original design, if applicable

all provided that the modified enclosure has an equivalent bracing/rigidity and internal padding to the original.

Assuming you guys agree to this point, what might be a more interesting topic for discussion is what you can change on the front baffle? I understand that baffle diffraction effects the resultant sound and that many crossovers are designed to eliminate some of the effects of this - clearly changing the front baffle can then take you down an undesired path. What are the limitations on this?

If the creative/foolish, maintain a volume could they then:
- Make the enclosure shorter or taller? (I think so, within limits?)
- Make the enclosure wider or thinner? (I believe not...)
[I guess you have to maintain the speakers distance from the top and side edges?]
- Change the distance between the speakers?
- Change the location of the port, if applicable? (I believe this is okay... although something might be said about moving from front to back and vice versa?)

Thanks guys. Please contribute additional 'dos and donts' as well

Cheers,
Ash
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Old 7th January 2009, 03:04 PM   #2
DcibeL is offline DcibeL  Canada
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Changing the front baffle will change the frequency response of the system due to baffle diffraction. You can visually simulate these changes using the Baffle Diffraction Simulator, found here. It requires MS Excel, OppenOffice won't cut it, and is pretty awkward to work with, but has a fairly well explained manual. One thing to note is that when you first enter the original design's baffle, depending on the design some of the baffle ripple may have been taken into account in the crossover, which is why changing the baffle can have detrimental effects on the resulting frequency response.

I would suggest that you play around with this software for a bit, it will give you a better understanding of just what the changes you intend to make will do. Be sure to simulate off-axis as well using the "axis ang" buttons.

Regarding the ports and volume, you can change this as well if you like. You can use another spreadsheet, Unibox, to simulate these changes. Changes to cabinet volume and port tuning will affect:
-bass roll off
-power handling due to driver excursion
-impulse response

Port location isn't incredibly important, just don't put it in a location that would be at the end of a standing wave (like 1/2 or 1/4 of the box height), and don't do something silly like put the driver at the very top of the box, and the port at the very bottom. If the port is on the rear of the cabinet, avoid placing it directly behind the mid/bass driver to minimize midrange sound leakage through the port.

I would like to stress the use of software simulation where ever possible, as seeing the changes you intend to make visually can answer a lot of questions, and give a better understanding of the various parameters, rather than just having a long list of "do's and don'ts". Good luck !

Here's some good info on using the FRD tools in aid of speaker design:
http://www.rjbaudio.com/Audiofiles/FRDtools.html
http://www.geocities.com/woove99/Spk...esigningXO.htm
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Old 8th January 2009, 12:39 AM   #3
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When deciding on a kit in Aus, you need to take into account local availability unless youíre prepared to import.

Re. Baffle step & how two compensate for it if you change baffle width:

http://sound.westhost.com/bafflestep.htm

http://diyaudioprojects.com/Technica...it-Calculator/

(often this is done with only an inductor in series with the woofer)
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Old 11th January 2009, 10:36 AM   #4
Bork is offline Bork  Australia
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Thanks for the advice guys,

I have been fiddling with The Edge. It doesnt seem as flexible as BDS but it feels like a more intuitive introduction. Its given me an idea of how speaker placement on a baffle can affect frequency response.

Im still curious about the distance between drivers in a 2-way. I thought i read somewhere (but forgot where!) that this dimension was not arbitrary.

Am I imagining things?

Thanks again,
Ash
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Old 11th January 2009, 11:05 AM   #5
Ron E is offline Ron E  United States
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Quote:
Originally posted by Bork
Im still curious about the distance between drivers in a 2-way. I thought i read somewhere (but forgot where!) that this dimension was not arbitrary.
There is a maximum practical distance, but there is no minimum. It is always better to have the drivers closer. Any more than a wavelength apart at the crossover and you are asking for vertical lobing problems. Coaxial drivers generally have very good dispersion characteristics.
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Old 11th January 2009, 11:30 AM   #6
AndrewT is offline AndrewT  Scotland
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agreed, closer the better for Mid to Treble.
Not quite so important for Bass to Mid, but depends on crossover frequency.

Even to the extent of cutting/machining the Treble fascia to allow the two drivers to overlap slightly.
Another tweak is to ensure that the distance from each edge to each driver is different, so that no single reflection distance predominates.
Radiused edges seem to minimise the reflections.
A felted surface helps reduce reflections from edges.
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Old 11th January 2009, 11:48 AM   #7
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regarding the port,....what difference does it make to put it at the front of the enclosure rather than at the back? ..also, can a given port can it be replaced by a flat port? -mantaining the volumeŅ?- just for asthetics ..

flat as in this
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Old 12th January 2009, 02:10 AM   #8
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This is generally known as a slot port and is perfectly acceptable.

Have a read of this:
End correction formula for rectangular port ?

as there is also an end correction factor that can change, but many people report using slot ports without correction. I imagine this is because often this correction will only change tuning by 1 or 2Hz which will have only a small effect on the sound.
Edit: While you're right in saying the volume should remain the same, it's important that both the cross sectional area and the length remain the same, not just maintaining the same volume

With regards to front/back ports... If the speakers are to be placed more than a foot from a wall, its best to put the port on the back as it helps to attenuate any midrange leakage from the port and can improve coupling to the room. If the speakers are to be placed very close to/against a wall then the port needs to go on the frontand that really isnt a big problem - works fine
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