P17WJ speaker box check

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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
An externally hosted image should be here but it was not working when we last tested it.


Regards,

Tensop
 
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
 
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
 
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! :smash:

Makes for an awkward looking monitor though :)


At 45.47 -3dB with a .66 port @ 210mm
Thankyou for your help rabbitz. :)
 
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 effect¡£2, 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, ¡À10% 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.
 
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 effect¡£
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, ¡À10% 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.
 
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
 
paulspencer said:
Tensop,
*snip*

Hi paul, indeed the first design was supposed to be 188mm ,and i pressed the one key twice by mistake. :)
I'm building the crossovers on my own design and will be relatively simple(compensation for impedence and crossover frequencies only)
as for the front grill you are right.. a 200mm wide front baffle will create problems with interference in the path of the sound from the wofer, as it is 170mm wide.

as for tweeter and woofer placement.. will there be a reduction in quality if i was to move the woofer up towards the top of the box?

as for reinforcement inside the box i will use a U shape peice inside. Is 1 in the middle sufficient.. or should i go for two?

Thankyou for your help paul,

regards,

Tensop
 
Tensop,

Basically I'd suggest put in as much bracing as you can within the limits of acceptable size and achieving your target volume.

Yes you can move the midwoofer up.

I have a speaker using the Vifa P17 driver MTM and I use a width of 210mm. I find it works quite well. I'd say 200mm - 230mm is a good range to work with. If you go narrow, then don't make your grille too deep - say 6mm MDF instead of anything thicker.

Is there any reason why you chose to design the crossover? Those are some of the most commonly used drivers on the net, you should be able to find a good design already done for them.

regards,
Paul
 
I think you might have misunderstood my question. I was not suggesting you bi-amp them. I was suggesting that you look for a design for a passive crossover for these drivers. This is simpler than coming up with your own design.

To design the crossover, you need measument software and a microphone. You need to measure the drivers. You need to simulate the crossover. Then you need to try a number of crossovers, see what you like, tweak, make changes. It is much simpler to find a design someone has come up with having done all that. Even an acceptable microphone will cost you more than the speaker you are planning to build, unless you make a DIY microphone, which will further add time, effort and comlexity.

regards,
Paul
 
Well, my father actually has microphones for this specific use.. but they have not been used in a good 10 years and their condition could be quite questionable.. however while browsing through the forums i found this one..
 

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I've seen this schematic before and have listened to some speakers that used it with the same driver combination (one of our members). Works well.......1st order on the woofer and 2nd order on the tweeter.

I think there is something about changing the resistor values if the sound is too bright.........found it........3 ohm to 4 ohm and 15 ohm to 8.2 ohm.
 
It's hard to comment on the condition of those mics. Also you need to know how flat their response is, and if they have been calibrated. You can build a DIY microphone with an electret, there is one that has a very flat response, but you need to make a preamp for it, and put it in a case.

To buy a mic using this cartridge (worth only a few dollars) costs over AUD $300.

regards,
Paul
 
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