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I've always been an audio fanatic and hard-core DIYer all my life, and it was time to build yet again a new subwoofer for my stereo system. But what is a subwoofer??? Well to define that a subwoofer is basically a loudspeaker design to reach into and below the bottom octave of frequency's. Really, a subwoofer should be able to reach down to 20Hz to be any good. For SPL, I wanted something around 105db @ 20Hz. I wasn't quite sure on the budget yet, but it was around AU$1300 in the end.
I was looking at these subs in detail: Adire Audio Shiva, Peerless XLS 830500, TC Sounds HE12, lambda acoustics drivers and a few blueprint drivers. At the end of the day, I decided to go with the Peerless XLS 12" 830500 driver because of its very good value for money in Australia (to import a us sub to Australia ends up costing a LOT of $$$). At first thought it was going to be very difficult to have a vented design, as it was calling for huge 4 or so meter long ports. The idea of passive radiators did come to mind, but they were going to cost me AU$150 each, and I was going to use two. So thats AU$300 worth of stuff which is the same as a $10 PVC pipe - damn. In the end though I knew it was the only way to go. The great thing about these peerless xls drivers is their very small vas and low fs. This means that it can be used in a really small enclosure. The size for this project was just a small 50 litres! So here are some of the benefits of using a passive radiator in place of a port:
A. Aluminium Spacer The aluminium spacer serves as heat
sink for the coil to reduce power compression.
B. Black Anodized
Voice Coil The 2-inch 4-layer voice coil is wound on thick black anodized
aluminium for improved heat dissipation.
C. Stacked Magnet System
The twin stacked magnetic system is optimized by FEA (Finite Element
Analysis), to create a symmetric powerful magnetic field in the air gap,
and provide space for the 44 mm max excursion of the voice coil.
D.
Distortion Reduced Motor A long 4 layer voice coil normally results in
high self induction and impedance varying with excursion. Its many ampere
turns react on the magnetic field in the air gap. These two main factors
causing motor distortion in subwoofers are practically eliminated by the
combined impact of the Aluminium Short Circuiting Ring (D) and the
Aluminium Spacer (A) on the pole piece. At the same time they both
contribute as heat sinks for the voice coil, reducing power compression.
The result is unbelievable clean bass reproduction.
E. Vented Cone
To eliminate compression under the dust cap the cone is vented by 8 large
holes. This way the coil is cooled and there is no need for a bore in the
pole piece.
F. Nomex® Spider The spider is made of high tech Nomex®
material. This material is chosen for its high rigidity and long term
stability. The suspension will stay in shape for a very long time under
heavy load.
G. Rubber Surround The surround is made from SBR rubber
because of the wide operating temperature, low creep and long term
reliability.
H. Fibre Composite Cone The cone is moulded from a
propriety air dried wood free pulp with a blend of Nomex®, Kevlar® and
glass fibres bonded together by deep impregnation with polymers. This
creates an ultra stiff and relatively light cone that will stay stable
even under very large sound pressures.
I. Rigid Cast Aluminium
Basket The rigid cast basket with an aerodynamic profile provides the
necessary sturdy base for the magnet structure and suspension and allows
for the 44 mm max excursion of the cone. The spider is ventilated to
achieve the lowest possible compression and allow air to flow freely to
create a cooling effect for the voice coil.
This was to be an
active subwoofer (as passive subwoofers draw to much power from the main
amp) so I needed another amp for it. I ended up getting the AA0508 350wrms
4/8ohm plate amplifier. This deemed appropriate for the design, which will
provide the sub 109db SPL @ 30Hz in anechoic chamber. Below are the
Thiele/Small specs for this woofer. Notice the large Xmax, low fs and low
vas of the driver:
Thiele/small parameters |
Symbol |
Unit |
Free air |
Common |
Baffled |
Nominal impedance |
Zn |
ohm |
8 |
||
Minimum impedance/at freq |
Zmin |
ohm/Hz |
4.7/106 |
||
Maximum impedance |
Zo |
ohm |
64.2 |
||
DC resistance |
Re |
ohm |
3.5 |
||
Voice coil inductance |
Le |
mH |
4.2 |
||
Capacitor in series with 8 ohm(for impedance compensation) |
Cc |
uF |
38 |
||
Resonance frequency |
fs |
Hz |
18.1 |
18.1 |
|
Mechanical Q factor |
Qms |
3.70 |
3.70 |
||
Electrical Q factor |
Qes |
0.21 |
0.21 |
||
Total Q factor |
Qts |
0.20 |
0.20 |
||
Xmax (linear one-way) |
mm |
12.5 |
|||
F (Ratio fs/Qts) |
F |
Hz |
90 |
||
Mechanical resistance |
Rms |
(Kg/s) |
5.12 |
||
Moving mass |
Mms |
(g) |
166.3 |
166.4 |
|
Suspension compliance |
Cms |
(mm/N) |
0.46 |
||
Effective cone diameter |
D |
(cm) |
24.4 |
||
Effective piston area |
Sd |
(cm^2) |
466 |
||
Equivalent volume |
VAS |
(ltres) |
139.2 |
||
Force factor |
Bl |
(N/A) |
17.6 |
||
Reference voltage sensitivity Re 2.83V 1m at 106Hz (Measured) |
db |
90.6 |
|||
Magnet and voice coil parameters |
|||||
Voice coil diameter |
d |
(mm) |
51 |
||
Voice coil length |
h |
(mm) |
33 |
||
Voice coil layers |
n |
4 |
|||
Flux density in gap |
B |
(T) |
1.04 |
||
Total useful flux |
(mWb) |
2.50 |
|||
Height of the gap |
hg |
(mm) |
8 |
||
Diameter of magnet |
dm |
(mm) |
147 |
||
Height of magnet |
hm |
(mm) |
|||
Weight of magnet |
(kg) |
2.42 |
Thiele/small parameters |
Symbol |
Unit |
Common |
Resonance Frequency |
fs |
Hz |
10.4 |
Mechanical Q factor |
Qms |
15.2 |
|
Equivalent volume |
VAS |
(ltrs) |
169.6 |
Mechanical Resistance |
Rms |
(Kg/s) |
1.8 |
Moving mass |
Mms |
(g) |
425 |
Suspension compliance |
Cms |
(mm/N) |
0.55 |
Effective piston area |
Sd |
(cm^2) |
466 |
Xmax Peak (one-way) |
(mm) |
24 |
Components |
830500 12"XLS driver |
830548 12"XLS passive (X2) |
350wrms AA0508 amp module |
Enclosure |
2400mm x 1200mm sheet 1" MDF |
1200mm x 600mm sheet ¾" MDF |
Wood screws (X3) |
Self tapping screws |
Liquid nails (X2) |
Liquid nails dispenser |
Jarrah gloss stain |
Clear gloss stain |
Can "satin black" paint |
Inner front:
450*386*18
Inner sides: 450*350*18
Outer front: 450*436*25
Back:
450*386*25
Outer Sides: 450*393*25
Bottom: 350*350*35
Top:
500*475*34
All dimensions in mm (millimetre)
The above is in the
format of: HxWxT. Basically, take the inner front for example: 450*386*18.
This means you must use 18mm thick MDF (3/4"), and cut it to dimensions
450mm X 386mm
The final results were very outstanding indeed. It reaches
well to 20Hz, and hits very good SPL levels, considering the enclosure
size. This is the most bass I've ever made out of a 50 litre box. Here are
the SPL (sound pressure level) results:
SPL 104db @ 20Hz
SPL
109db @ 30Hz
SPL 112db @ 50Hz
All these SPL levels were measured
anechoic - this is so room gain will not get in the way of the results.
Indoors, its flat to 20Hz and plays at about 112db throughout its
passband.
If you wanted to cut costs a bit, you could use just one
passive radiator instead of two. But make sure you don't add any extra
mass to this - it must be massed to 435 grams.
Please
excuse the image quality my digital camera is a real piece of crap. The
sub looks WAY better than this in real life.
Check out the Peerless XLS 10" version of this project, its the same idea as the 12" sub on this page, except its 10", has one XLS 10" Passive Radiator and is only 20 litres in volume !!!
Please EMAIL me at macky888@wasp.net.au if you have any comments, suggestions, questions, etc. I would love to hear from you!
This project was copied with permission from http://www.geocities.com/adrian_mack/homepage.html
Here is a nice little sub, and is it small indeed, at 30 liters! It uses a Peerless XLS 10" driver (part code 830452) and two passive radiators, also made by peerless (part code 830468). This sub was designed to extend to ~22Hz in a very small enclosure. I personally did not make this sub, it was built by Mark Beaulieu, I did the design work for him (Mark can be contacted at markb1@cableone.net). Don't let the small box size fool you, this is a very capable sub. It is a 6th order design, as it uses active equalization to achieve a flat response taking into account the room gain. The bass is very clear, and for such a small enclosure, output is still over 110db. Output to 22Hz can even be made in as small as a 20L box if required. The sub is powered by a 250Wrms plate amplifier with remote from Parts Express.
The driver used is one from a favourite brand of mine, Peerless :) Features include extended pole piece, faraday ring in voice coil assembly for lower distortion, rubber surround, cast chassis, small box operation, and all the other well known features of the XLS series. See the 12" Peerless sub project for more info on driver choice.
Here are some pictures of the box being built, and the final pictures. They are rather large pics, and may take a while to load depending on what connection you have.
Here is a Microsoft Excel spreadsheet that I have made which models the response of vented box systems and sealed box systems. It is based on Brian Steele's spreadsheet (diysubwoofers.org), but I have modified it to provide more functions to aid in designing a subwoofer. Some of the functions of this modified spreadsheet are:
Alignment Comparisons - calculate box size/tuning for Thiele and Small vented/sealed box alignments such as SBB4/BB4, SC4/C4, QB3/SQB3, etc and different sealed box Qtc's
Compare vented box and sealed box response simulations on the same graph
Simulate almost all types standard active crossovers, user specified Q and center frequency for Linkwitz Riley, Butterworth, etc
Set Qa/Ql/Qp losses
Parametric and sallen key EQ modelling
Simulate effects of adding extra mass to the cone and shows new T/S parameters of driver when mass added
Simulate extra circuit resistance in cabling/xover/amplifier
Calculate frequency of 1st port resonance
Calculate usable xmax of driver based on voice coil length and gap height, with more complex calculation for cone overdrive
Compare real world to measured results, user enters measured frequency response data
Model the response contribution of car cabin gain and room gain
Model transfer function of vented and sealed systems including effect of active filters/EQ
Model phase and group delay of sealed and vented systems, including effects of active filters/EQ
Model electrical impedance of sealed and vented systems with circuit resistance effects and voice coil inductance
Model port air speed including effects of active filters/EQ
Model cone excursion of vented and sealed systems including effect of active filters/EQ
Model excursion limited SPL at given power level of vented and sealed systems including effect of active filters/EQ
Model port contribution to system SPL including effect of active filters/EQ and SPL caused by leaks
Model phase and group delay response separately of active filters/EQ
Model transfer function response of active filters/EQ
Calculate components required for 2cd sallen key active EQ and schematic provided
Help notes throughout entire spreadsheet - move mouse pointer over the red tag to see help note, more on this in Introductory tab in spreadsheet
It's an easy to use spreadsheet too. As stated above, there's also help notes right throughout the spreadsheet in case something doesn't make sense. Please contact me if you have a problem with it though.
Alternate Link excelsub.zip
You will need Winzip to extract the Microsoft Excel spreadsheet. You can download a free copy of winzip from http://www.winzip.com/
I would like to hear any comments, suggestions, etc about this spreadsheet.
Please email them too macky888@wasp.net.au Back to
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