Subwoofer build Drivers

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I bought 2 Monocor SPH -255 drivers to build two low powered 10"subwoofers, what does the board think of these drivers.I want to build 2 subs to support my bookshelf speakers either side of my projector screen.Maunfacture reckons 30ltr sealed 50 ltr bass reflex.Its a 10" HIFi Bass woofer driver but has a fs of 23hz.Im going to drive these with a NHT SA-2 mono sub amp that has built in cross over capability to 120hz rated at 200w RMS @ 8ohms


SPH-255 - Monacor 10 inch HiFi bass speaker 120Wmax 8Ω - Europe Audio

Monocor SPH 255

Technical Facts Speakers
Impedance (Z) 8Ω
Resonant frequency (fs) 23Hz
Max. frequency range f3-3,000Hz
Rec. crossover freq. (fmax.) 2,000Hz
Music power 120WMAX
Power rating (P) 60WRMS
SPL (1W/1m) 91dB
Suspension compl. (Cms) 1.19mm/N
Moving mass (Mms) 42g
Mech. resistance (Rms) 2.1kg/s
Mech. Q factor (Qms) 2.86
Electr. Q factor (Qes) 0.26
Total Q factor (Qts) 0.24
Equivalent volume (Vas) 205 l
DC resistance (Re) 6.1Ω
Voice coil induct. (Le) 1.5mH
Voice coil diameter 50mm
Voice coil former aluminium
Voice coil winding height 17mm
Air gap height 8mm
Linear excursion (XMAX) ±4.5mm
Eff. cone area (Sd) 350cm2
Volume displacement (Vd) 157.5cm3
Force factor (BxL) 11.7Tm
Reference efficiency (No) 0.9%
Magnet diameter 140mm
Magnet weight 39oz.
Mounting cutout Ø 235mm
Mounting depth 108mm
Dimensions Ø 257mm
Weight 3.22kg
Rec. net cabinet volume
Closed 30 l
...what does the board think of these drivers.I want to build 2 subs to support my bookshelf speakers either side of my projector screen...

Hi Ted,

I vote for ported boxes if the low frequency cut-off is acceptable, but there are other options too:

b :)


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Hi Bjorno,
Thanks for the reply.Could you send me a larger copy of the drivers measurments.And if possible could you give me an idea as to what would be the best size enclosure for these drivers I will go with two single 10" front firing front port vented enclosures for these drivers if thats what your measurements are saying.I wasnt sure as to go with sealed or ported.I have the room for two 50ltr boxes but would like to keep the hieght of the boxes under 500mm.They will be passive boxes of course.I only have Boxnotes and the program is telling me the best size enclosure at 50ltr is 490D x 490H x 326W.The port vent is 75mm X 490mm and is too long for the box.Does your measurments give you the best size enclosure for this drivers specs.I have tried to seach around for specs for these drivers to see if other people have used them but come up with diffrent quoted enclosure sizes and recommendation.This is my first speaker build and I want to get it right the first time.


Hi Cal,
Thanks for the tip.I tried saving the thumbnail and blowing it up didnt work.

Ok I have had a look at the measurements and to be honest Im little lost.Could someone be so kind as to let me know if I have worked this out right.

The graphs on the left I gather is for a ported enclosure, the right looks like a tube of some decription.The vented box at 50ltrs graph on the left shows the frequency will go down to 15hz at 0db and will climb to a frequency of 40Hz with a db of 105.Flatening off and maintaining around 105 db to round 200hz then fading off till reaching around 400hz .

I guess the cone diagram is suppose to represent a sealed enclosure.The sealed enclosure looks like it will start at 14hz with a 0 db and climb to 24hz at 98db then flattens off to 100hz at 102 db and then the sound drops off a cliff ie a cutoff.

My thinking is I would be best to go with a ported enclosure as recommended by Bjorno thanks bjorno, and adjust my crossover on the NHT SA2 mono amp to around 100hz.

My book shelf speakers are Polk RT25i's and range from 50hz to 20,000khz.So crossing over around 100hz should blend in well, but I wont know till I build these subs and try them out in my room.The mono amp will allow cross over from 35hz to 150hz.

Nac thanks for your reply, your help would be great.Music,well I like everything classical, rock ,Jazz,instrumental,blues you name it I love the lot,my other half is the same we love all music.

My home theather is used for everything cable, Blue ray movies music Free to air TV. PS3 gaming The projector is my TV and music centre and game console.The room is basically 3.6W x 5.6L x 2.7H Mtrs the projector throws onto the screen 3.6mtrs from a hole in the wall.

I know the projector layout should be pointing at the 5.6mtr length but we have windows doors and open areas.So the setup is a bit weird.I use a Rotel RSX- 986 reciever amp and 4 Polk RT25i speakers as surrounds and I have a CS 245i centre in the entertaimnet unit just under the screen 20 inches/500mm.

I have 2 bookshelf speakers on stands either side of the screen which is 2.5mtrs across.They stand 600mm high off the floor.I was thinking of installing the Sub's just under each bookshelf speaker both left and right hence the 500mm hieght.And point them slightly towards the seating kind of the same as my front speakers.

The rears are also on stands 600mm high but they are against the back wall in the corners and point to your ears when seated for surround effects whilst gaming or for movies.I just use just the fronts for sterio music sounds much better for CD's.I have a rotel CD player RDV - 995 to match the amp.

So what do I envisage these subs will do.Well I would like these subs to fill in the bass the bookshelf's cant produce.I am more intrested in clarity rather than room shaking fear.But if I can build a set of subs that would fill in the bass missing when listening to sterio music and offer some home theather feeling for movies when using the surround sound I will be happy.

So anyway if anyone can help me design a sweet sounding a set of low powered subs to do the job I would be truly grateful.I have a wood shop ready to go for the build.And Im very keen to start.

Big Ted
Well After alot of hunting around and reading I think I have worked out how to build these subs.I found another site that had the manufactures recommendations and I have decided to go with the 55ltr ported as suggested not 50ltr as suggested on another site.Heres my build plan.If you think it will work ok let me know.If I have gotten something wrong let me know.I only guessed the port tuning but I down loaded WinSD and I think 33hz looked ok, but I really dont know what im looking at.

Driver single 10" Monocour SPH 255 woofer sub project. Passive bass reflex 55ltr.

Enclosure Dimensions 55Ltr 490D x 490H x 353W made with 19mm MDF
Internal Volume 61.6Ltrs
Working Volume 55Ltrs
Double thickness Front Panel Thickness 19mm x 2
1 x Double Flared Port 101mm Diameter Length 313mm Wall thickness 2mm Flare 30mm mounted flush and screwed.
Enclosure Tuned to 33hz
Port Hole centre 187 mm from bottom on front panel centre line.

Driver Mounted 2mm off centre Flush not recessed
Drive Mount Hole Centre 170mm from top on front panel screwed
Internal brace installed at 294mm from front panel
Internal brace has 5 holes 100mm each with routered rounded edges both sides
19mm MDF brace rebated 5mm into all internal walls
88.9mm Square cut into rear of enclosure 60mm off bottom panel in centre for Square Binding post cup.Screwed.

All internal walls lined with 10mm dense foam sponge glued to walls.
All Panels Dowelled and glued.
Enclosure Lamanted all four sides with Black gloss lamanate trimmed edges.
Four black 10mm rubber feet fitted to each bottom corner.
Four Grill plugs fitted to each corner of front panel.
Black Square grill 15 degree edge 490mm x 353mm frame with black grill cloth and grill pegs.

Heavy gauge speaker wire insulated soldered to terminals.

Looks good to me, good bracing, double thick baffle, etc. MDF is not air tight on its own (this can be easily proved) so you need to apply some kind of air tight coating to all interior surfaces. I would also recommend spikes, or something functionally similar. That can make a world of difference with a subwoofer. Also think about what happens if they dont like the placement you have envisioned. Its almost impossible to predict where they will work best. Also, dont glue the port. If the response is off you can easily change the effective cabinet volume bigger or smaller, and the port size, but you will need to take the port out to do it.
Thanks Nac,
I decided to go with rubber feet rather than spikes as we have wood floors.I will take note about sealing the inside walls maybe with an expoxy black paint.What do you think about the port tuning will 33hz be ok for this appllication or would I be better to try and get the port tuning Hz lower.

The lowest I can go with the port tube I can source, length 17 inches, is 29hz.With the port tuning @ 33hz Winsd is telling me I can get 70db @ 20hz to 90db @ 100hz not much diffrence going down to 29hz maybe faster response time, I dont know.I dont think their is much content below 20hz and I dont think I would be able to hear it with, my system is very low powered at 60w per sub.

How much room do I need to leave between the back wall and the port tube to be safe without restricting airflow.I have allowed 158mm currently.

Once I build these sub I dont think I will be taking it apart, so I have to get the measurements right the first time.And I do understand about room placement.

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