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Old 8th January 2004, 04:59 PM   #1
Tensop is offline Tensop  Lebanon
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Smile P17WJ speaker box check

Hi all. I've ordered 2x p17's and 2x D25ag tweeters to build a set of monitor speakers to go on my table.

I'd just like to check with you guys to make sure i havent broken any hidden rule when it comes to designing speaker boxes.

The measurements would be as followed:


VIFA P17WJ + 25AG speaker combination.
External Measurement: 440(h)x220(w)x295(d)
Internal Measurement: 420x200x275 = 23.1 Litres.
-12dB @ 31.2 Hz
-06dB @ 38.3 Hz
-03dB @ 43.7 Hz
-00dB @ 64.8 Hz
Port Value:
Tuning point: 43 Hz
1x .066(66mm JAYCAR) @ length: .188(118mm) @ mach .11


And also i have a question, is 10mm MDF thick enough for a speaker box?

I'm building them to look similar to Click the image to open in full size.

Regards,

Tensop
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Old 8th January 2004, 05:41 PM   #2
rabbitz is offline rabbitz  Australia
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You should use 16mm MDF as a minimum and I'd make the box 200mm wide maximum (outside dimension) and bring the woofer closer to the tweeter.

The port looks a bit short. For the internal volume of 23.1 litres would come down to about Vb=21.7 litres allowing for the port, driver etc and a Fb=43Hz, I calculated a 66mm ID port 207.8mm long (using formula in LDCookbook).

Nice driver combination BTW.

Hope this helps.

Cheers
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Old 8th January 2004, 06:12 PM   #3
Tensop is offline Tensop  Lebanon
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16mm diamatre MDF is going to reduce internal volume quite alot
Is there any particular reason why the width shouldent be 220?


Compensating for 16mm MDF instead of 10mm would =

440(h)x220(w)x295(d)
-
to a new value of:
408(h)x188(w)x263(d) equalling a new value of 20.1 Litres. Taking into account roughly 1.4(your value) litres of volume used by the woofer and other guts this would then equal 18.7 Litres


with a 66mm ID port @ 243mm Long this would equal a 47.2Hz -3dB point and 81.3Hz 0dB point, with a vent mach of .11 Tuned to 43hz
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Old 8th January 2004, 06:26 PM   #4
rabbitz is offline rabbitz  Australia
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Stay with your original volume but just make the box taller and deeper.

As far as baffle width, I always keep it as narrow as possible.
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Old 8th January 2004, 06:34 PM   #5
Bricolo is offline Bricolo  France
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Quote:
Originally posted by rabbitz


As far as baffle width, I always keep it as narrow as possible.
Any reason for this?
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Old 8th January 2004, 06:43 PM   #6
Tensop is offline Tensop  Lebanon
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Thankyou for the help so far rabbitz.

Reducing the width to 200mm. Extending the height to 450mm, and depth to 320mm... the Dimensions are quite awkward but honestly its as large as i'd want it on my table.

So..

450(h)x200(w)320(d)
-
418x168x288
= 20 litres. 18.7 after mods.

Taller, Deeper.. but basicly the same volume

However i easily have the space for 22cm wide.

which would be

450x220x320
-
418x188x288
= 22.6 litres booyah!

Makes for an awkward looking monitor though


At 45.47 -3dB with a .66 port @ 210mm
Thankyou for your help rabbitz.
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Old 9th January 2004, 01:59 PM   #7
rabbitz is offline rabbitz  Australia
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Hi Bricolo

A narrow baffle is supposed to give better imaging. How many fat speakers do you see these days like a 6 1/2" driver with a 18" wide baffle?

A good example is the Peerless HDS drivers with the side cut basket to allow narrow baffles.
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Old 10th January 2004, 01:41 AM   #8
shipai is offline shipai  China
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Quote:
As far as baffle width, I always keep it as narrow as possible.
With this question, as my understanding the advantage is, 1, reduces the disturbance of the baffle diffraction effectӣ2, to general cubic box, The narrow baffle means narrow backboard, so, There is lesser disturbance of reflection with the woofer

Tensop,
Actually the box volume is does not need precise, ”Ą10% is normal, even 30% is ok.
The volume by software is usually bigger, and the design of box involves the Q value. However 18mm MDF is needed.
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Old 10th January 2004, 02:11 AM   #9
shipai is offline shipai  China
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Cool The above post is replaced by the following.

Refering this question, the advantage of my understanding as follow:
1, to reduces the disturbance of the baffle diffraction effectӣ
2, to general cubic box, The narrow baffle means narrow backboard, then there is lesser disturbance of reflection with the woofer

Tensop,
Actually the box volume does not need precise, ”Ą10% is normal, even 30% is ok.
The volume calculated by software is usually bigger, and the Q value should be considered in the design of box . However 18mm MDF is needed.
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Old 10th January 2004, 06:39 AM   #10
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Tensop,

I would consider 18mm to be the minimum advisable. It's not necessary, but it's a good idea to also include bracing, even if it is just 1" thick pieces of dowell, which takes up so little volume that you won't really need to make the box any bigger, the difference won't be noticeable. Also, don't forget to include damping on the walls of the enclosure, such as open cell foam which you can attach with contact adhesive.

As has been mentioned, the driver placement in the image is not optimal. It is generally considered best to place drivers as close to each other as possible. The ideal, which is never achieved in reality is to get as close as possible to a point source across the whole range of frequencies covered. This is most important for a tweeter. I would use a 10mm gap between the tweeter and the woofer. The placement of the port is not so critical, and more a matter of aesthetics.

Regarding baffle width. The baffle acts as a sounding board that reinforces the treble and midrange to a point that is determined by the baffle width. At a point which can be calculated, there will typically be a -3db point called a baffle step, below which the baffle no longer reinforces the response. On a 200mm wide baffle this would be about 500 Hz. This will affect the design of the crossover, as will the placement of the speaker, whether it is against a wall or placed further into a room. Therefore a wider baffle is actually an advantage. However, to get this point down to 100 Hz would require a baffle over a metre in width, not acceptable.

I believe boxes are narrower for reasons of fashion rather than acoustic performance. I have yet to see why a narrow box should image any better. As I understand, a very wide box will have a narrower sweet spot due to the resultant beaming effect, but I don't think the difference between a 200mm and 220mm wide box are going to be significant. I think it should be considered in this case more of a choice based on aesthetics. On that note, I do think narrower boxes often look better, and more elegant.

I tend to make my boxes fairly deep as I find due to perspective effects, the height and width of a box has more bearing on the perception of the size of the box ie. making a box deep is a cheats way of making it bigger without making it look much bigger.

Also remember that the width of the box can relate to the difficulty in making the grille. If you use 12mm MDF for the grille and have a very narrow box, the grille will have more of a detrimental effect on the sound due to the effect of the edge being close the to the driver.

Regarding vent volume, I hope that your vent is not 118mm, as that would be tuned to 52 Hz in a 23L box with a 66mm diam vent. If it is in fact 188mm then it should be fine for 44 Hz tuning.

Are you designing the crossover or do you have a kit or are you following a design from someone else?

regards,
Paul
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