69 yr-old guy needs advice with 1st DIY sub

[Boomer1950]

After further consideration I've made another adjustment to my plan. If I leave the box @7ft^3 & change the tuning to 20Hz, the port length drops to 22.25" - removing the need to curve the ports.  Plus, blocking one port should take it into the teens for HT, if I've understood properly.

If I get this type of performance from my first DIY sub, I'll be thrilled. 

[SME]

I get an "Internal Server Error 500" when trying to load the link you posted.  The link may only work for you.

That design should work decently, though depending on the type of ports used (like flare characteristics if any), the length you gave might tune a bit lower than 20 Hz.  Also keep in mind that the vent area limits how much air can be moved which limits how much output you can get at the bottom before the vent overloads.  And it's home theater content that's more likely to push the sub to its limits.

One thing I'd consider changing is to add a 3rd 4" vent, increasing the lengths and cabinet depth accordingly.  I'm not sure you can do this with the Precision Ports I posted a link to, earlier in the post, and if you can, it'd probably be a (maybe ugly) asymmetrical arrangement.  Another option is to do a slot vent, which can be built right into the cabinet.  It should be braced every ~8" or so, which can also be used to the vent into individual chambers that can be plugged.  See for example the Skhorn and Skram designs posted here.

Do you think you'd be able to fit a second one of these subs somewhere?  Subs tend to sound better with at least two in different parts of the room.

[Boomer1950]

Thanks for your comments, @SME.  Because the problem you mentioned with arranging 3 Precision Ports on the front of my box is virtually unsolvable, I'm going to go with a slot vent.  This construction is a little more complicated than using pre-made ports, but I don't think it will be beyond my capabilities.  I've settled on a 7ft^3 box with a 70 sq.in. port that is about 40" long which I hope gives me a tuning of around 24Hz.  I will be dividing the port into 3 pieces for added support & to give me the possibility of closing one or two of the ports.  I assume dividing the 70 sq.in. port into 3 ports will lower the tuning, but that seems fine to me.

I plan to start working on the box in the next few weeks.  I'll keep you posted as to my progress.  If there are additional comments or suggestions, I'd love to see them.  Thanks again to everyone's help.

[Boomer1950]

About the suggestion of having two subs - I can't do it right now.  I understand the advantages, but it is unaffordable at the present time.  This sub will be such an improvement over what I've been listening to for the last 13 years that two subs are not very appealing to me.

[SME]

7 hours ago, Boomer1950 said:

Thanks for your comments, @SME.  Because the problem you mentioned with arranging 3 Precision Ports on the front of my box is virtually unsolvable, I'm going to go with a slot vent.  This construction is a little more complicated than using pre-made ports, but I don't think it will be beyond my capabilities.  I've settled on a 7ft^3 box with a 70 sq.in. port that is about 40" long which I hope gives me a tuning of around 24Hz.  I will be dividing the port into 3 pieces for added support & to give me the possibility of closing one or two of the ports.  I assume dividing the 70 sq.in. port into 3 ports will lower the tuning, but that seems fine to me.

I plan to start working on the box in the next few weeks.  I'll keep you posted as to my progress.  If there are additional comments or suggestions, I'd love to see them.  Thanks again to everyone's help.

Just to double check some things.  The 7 cuft should be the *internal* volume left over after subtracting stuff like the internal bracing and *especially* the port.  Also the driver takes up some space, though probably not more than 0.25 cuft or so.

The port dividers reduce the cross-sectional area of the port slightly, so you are right that this will drop the tuning a bit more than without them.  It shouldn't be hard to calculate how much area you lose ... For 2 dividers of 3/4" thickness in a 3" tall slot port you lose (2 * 3/4 * 3) = 4.5 in^2.  It's that simple.

There is one more important consideration worth mentioning.  Vents have a "pipe resonance" (like an organ pipe) at a certain frequency depending on its length.  Your 40" pipe is likely to resonate around 150 Hz.  That's a bit lower than we'd like but is "do-able", especially if you are crossing over at 80 Hz and not higher.  Certainly we wouldn't want a vent that's any longer.

As you can surmise when trying to make a low-tuned cabinet as small as possible, there is an inherent trade-off between getting vent area large enough to not overload at high levels while keeping the length short enough to fit into the cabinet and to avoid making the pipe resonance too low.  This is a trade-off that vented system designers have to routinely make, and it gets worse when either tuning lower or trying to use a driver with more output.

Your final design looks pretty good as far as balancing trade-offs.  For the length, area, and volume you specified, Hornresp indicates a tune of around 22.5 Hz.  A 3rd order high pass filter at 18 Hz appears to control excursion well and gives you a response that's roughly -6 dB at 20 Hz and -10 dB at 18 Hz.  Depending on what your room is doing, you may get useful output quite a bit lower than that.  If you plug a vent and use the same HPF, you lower the tune to 18.5 Hz, which gets you -6 dB at 17.5 Hz and -10 dB at 15 Hz.  Again, your room may help things get a bit lower.  In both cases, vent velocities are very reasonable, so you're not likely to see much compression.  While plugging two vents will technically get you a 13 Hz tune, you don't really gain any usable output or extension this way, so I wouldn't recommend using it this way.

Anyway, good luck with the build!  We look forward to hearing of your progress.

[Ricci]

23 hours ago, Boomer1950 said:

 I've settled on a 7ft^3 box with a 70 sq.in. port that is about 40" long which I hope gives me a tuning of around 24Hz.  I will be dividing the port into 3 pieces for added support & to give me the possibility of closing one or two of the ports.  I assume dividing the 70 sq.in. port into 3 ports will lower the tuning, but that seems fine to me.

Are you sticking with the box dimensions posted earlier of 20x26x29.35"? It sounds like you would be placing the driver and ports on the 20x26" face? 

[Boomer1950]

Ricci - the plan is to have the port & driver on the 20 x 26 side, as you thought.

[Boomer1950]

A couple of weeks ago I ordered my SI SQL-15 driver with dual 2 ohm voice coils.  Now I'm looking for a cabinet maker to give my cut list to.  I thought before I pay someone to start cutting MDF, I'd post my final (hopefully) subwoofer design. It hasn't changed much since my last post.

25" wide x 20" high x 32" deep - approx. net 7 cu.ft.  There will be 2 - 4" ports 28" long (including the Precision Ports flared port ends - outside & inside).  I'm planing to buy a Crown XLS1002 amplifier which is rated @1100W at 4 ohms bridged.

Under TV Subwoofer  25" wide x 20" high x 32" deep - using 3/4“ MDF.  Two 4" ports - 28” long (24” PVC pipe + flared ends)
I'm aiming for a box tuning of approx. 18Hz.

Top/Bottom - two - 25” x 32”

Left/Right - two - 30.5” x 18.5”

Rear - one - 25” x 18.5”

Inner Baffle - one - 25" x 18.5” - 13.8” hole - centered vertically - 3.5” left of center horizontally
Two - 6.25” port holes - Centered 5.25” from top/bottom & 5” from right side

Outer Baffle - one - 25” x 20”  - 15.4” hole - centered vertically - 3.5” left of center horizontally
Two 6.25” port holes - Centered 6” from top/bottom & 5” from right side

I uploaded a scale mock-up of the front of my sub that I made in Pixelmator Pro.  The large gray circle is the 15.4" hole for the driver.  The light gray circle around the port hole is the flange of the flared port.  The dark blue circle is the 6.25" hole cut into the baffle.  The light blue circle is the size of the port.

If I use a straight port, will I have enough room between the end of the port & the internal wall?

All feedback is appreciated.

 

[SME]

9 hours ago, Boomer1950 said:

Under TV Subwoofer  25" wide x 20" high x 32" deep - using 3/4“ MDF.  Two 4" ports - 28” long (24” PVC pipe + flared ends)

 

I'm aiming for a box tuning of approx. 18Hz.

Top/Bottom - two - 25” x 32”

Left/Right - two - 30.5” x 18.5”

Rear - one - 25” x 18.5”

Inner Baffle - one - 25" x 18.5” - 13.8” hole - centered vertically - 3.5” left of center horizontally
Two - 6.25” port holes - Centered 5.25” from top/bottom & 5” from right side

Outer Baffle - one - 25” x 20”  - 15.4” hole - centered vertically - 3.5” left of center horizontally
Two 6.25” port holes - Centered 6” from top/bottom & 5” from right side

I uploaded a scale mock-up of the front of my sub that I made in Pixelmator Pro.  The large gray circle is the 15.4" hole for the driver.  The light gray circle around the port hole is the flange of the flared port.  The dark blue circle is the 6.25" hole cut into the baffle.  The light blue circle is the size of the port.

If I use a straight port, will I have enough room between the end of the port & the internal wall?

All feedback is appreciated.

[...]

The design looks OK.  The ports definitely exit close to the cabinet wall, but I think that will be OK as long as you don't go any closer.  The wall will help lower the tune a bit more than the flared pipe alone would.

However if that's your cut-sheet list, you are  missing something very important: bracing.  Bracing helps stiffen up the cabinet walls so that they don't flex as much which causes some bass to be lost and causes the enclosure to resonate at somewhat higher frequencies,  coloring the sound.  The bracing design is a little tricky being that you have to clear the driver as well as the vents while trying to avoid letting any panels have more than a 6-8" span unbraced.  You might also want to build in some extra support for the vents, near the rear, being that they are quite long and presumably only attached at the front baffle.  (You might also want to put foam or cloth between the tube and brace to avoid buzzing sounds if they vibrate against one another.)  Ensuring no more than 6-8" of panel is unbraced is a good rough target, but often some compromise is necessary.  Either way, some bracing is much better than none.  You  will also want to add a light amount pf absorptive material (polyfill / pillow stuffing is popular as are denim scraps) around the inside panels to try to reduce resonances in the air cavity, which can also transmit through the panels.

After accounting for bracing, the cabinet volume will probably be a tad smaller, maybe 6.5 cuft (?) depending on design.  That should still tune at around 20 Hz.

[dgage]

1 hour ago, SME said:

Bracing helps stiffen up the cabinet walls so that they don't flex as much which causes some bass to be lost and causes the enclosure to resonate at somewhat higher frequencies,  coloring the sound. 

You probably didn’t mean higher frequencies but just to be clear for others, unsupported enclosure walls resonate lower and may start to get into the range of the subwoofer and then we might start to hear them as bad bass harmonics.  Think of it as larger drums play deeper and smaller drums play higher.  With bracing, we’re actually trying to make the unbraced areas smaller, which will push the panel resonances up into the higher frequencies and out of the range of subwoofer bass.  The other thing I’ll say about bracing is to purposely make them uneven.  If you place a long brace dead center in a panel, you’re breaking the panel into two halves with the exact same resonant frequency and then doubling the amplitude of that resonant frequency since there are 2 halves.  So work on unequal bracing with maybe 45/55 instead of 50/50.  And like SME stated, plan on bracing every 6-8”.  This will push the resonant frequencies of the panels to around 400-500 Hz and out of the passband of a subwoofer.

Looking forward to seeing this project come to fruition.

[Boomer1950]

Thanks for the quick responses!  I don't know why I forgot to post the braces I planned to use, but it's just as well, since I've made some changes after reading your comments.   I plan to use 2 braces 18.5" x 30.5" running from the front wall to the back wall (bright green).  Let's call them Long braces. I plan to put the Long braces 8.75" from the left & right walls.  Each arm of the brace is 2" wide.

Then there are "C" braces that are 8" x 18.5".  I plan to use 3 C braces on each side wall. The vertical piece of the C brace is 1.5" wide to allow clearance for the ports - the upper & lower arms are 2".  Putting foam between the braces & the ports seems necessary to prevent the ports from vibrating into the C braces, plus helping to support the ports.

The last graphic is a view from the front showing the C braces (magenta) meeting with the Long braces (bright green). 

[Boomer1950]

I am planning to install the C braces on the left & right walls at different locations relative to each other.

[Boomer1950]

I found someone to cut MDF into square pieces to my specifications. I didn't find anyone to cut the circles or make the braces (I didn't try very hard).  So... I'm buying a router & I'll learn to do it myself.  Novel concept.  This will slow me down, but I'm not in a big hurry.  It'll be a few days before I get the cut MDF.

I found a Crown CDi 1000 at a good price.  It produces 1400W bridged at 4ohms.  I'm very pleased. It's on the way here.

Things are actually happening.  Money has been spent.  Sometime in the near future I'll have a new sub. 

[SME]

Hey!  You need a jig to cut circles if you don't have one.  Parts Express sells one, or you might be able to buy one from a woodwork store near you too.

Definitely learn as much as you can before you start trying to route.  It's important to clamp the work tight.  Also always route in the direction that makes the spinning cutter bite the wood from behind.  When the spinning blade hits the wood, it deflects the router in the opposite direction, and it's always safer and more stable to be pushing against the deflection rather than trying to "ease it forward while keeping it from flying out of your hands".

Keep us posted.

[peniku8]

9 hours ago, SME said:

Hey!  You need a jig to cut circles if you don't have one.  Parts Express sells one, or you might be able to buy one from a woodwork store near you too.

Definitely learn as much as you can before you start trying to route.  It's important to clamp the work tight.  Also always route in the direction that makes the spinning cutter bite the wood from behind.  When the spinning blade hits the wood, it deflects the router in the opposite direction, and it's always safer and more stable to be pushing against the deflection rather than trying to "ease it forward while keeping it from flying out of your hands".

Keep us posted.

When "cutting" with a (fixed) router (like you do when cutting out circles), it doesn't matter which direction you go, since you're removing material with both sides of the bit.

When cutting along a guide rail or something, cut the way that the router pulls itself towards the guide (e.g. from left to right, when the guide is behind the router). You don't wanna climb cut unless you're using a CNC and you're removing less than say 60% of the tool diameter during a roughing pass. That's at least what I experienced so far.

[SME]

3 hours ago, peniku8 said:

When "cutting" with a (fixed) router (like you do when cutting out circles), it doesn't matter which direction you go, since you're removing material with both sides of the bit.

When cutting along a guide rail or something, cut the way that the router pulls itself towards the guide (e.g. from left to right, when the guide is behind the router). You don't wanna climb cut unless you're using a CNC and you're removing less than say 60% of the tool diameter during a roughing pass. That's at least what I experienced so far.

Thanks for helping to clarify things.  Though thinking about it now, when cutting circles I think you still want to cutter to enter "from behind" on the outside edge where there's slightly more material to be removed.  Another important detail when cutting circles is that the inside piece that's to be removed must be separately anchored, and directly clamping it is not usually possible.  Instead before making the cut, it should be anchored to a larger underlying sacrificial piece using a couple finishing nails, and the  sacrificial piece should be well clamped.

As an advanced technique, there are very exceptional cases to the rule of avoiding climb routing: specifically when exiting at the edges of BB plywood where climb cut of *just the last tiny bit* avoids unsightly blow-out.  Doing this safely requires appropriate technique to ensure control of the router is maintained.

CNC is a totally different thing because the router and/or table are mechanically controlled, so climb cutting is usually preferred for CNC to avoid blow-out.

[peniku8]

16 hours ago, SME said:

Thanks for helping to clarify things.  Though thinking about it now, when cutting circles I think you still want to cutter to enter "from behind" on the outside edge where there's slightly more material to be removed.  Another important detail when cutting circles is that the inside piece that's to be removed must be separately anchored, and directly clamping it is not usually possible.  Instead before making the cut, it should be anchored to a larger underlying sacrificial piece using a couple finishing nails, and the  sacrificial piece should be well clamped.

As an advanced technique, there are very exceptional cases to the rule of avoiding climb routing: specifically when exiting at the edges of BB plywood where climb cut of *just the last tiny bit* avoids unsightly blow-out.  Doing this safely requires appropriate technique to ensure control of the router is maintained.

CNC is a totally different thing because the router and/or table are mechanically controlled, so climb cutting is usually preferred for CNC to avoid blow-out.

The difference in circumference for a 12" hole is 2% (~1.6"), using a 1/4" bit, which is why I didn't adress this issue. Unless you're cutting a 4" hole with a 1" tool, it doesn't really matter 😉

Surface finish is a different topic thou. There might be differences.

For regular work I'd recommend a simple straight bit. Quarter inch works great for this kind of stuff, if your router can spin it up to 15-20k RPM. I simply used a very long screw as center axis for my circle jig, which also held the cut out piece to the board beneath. I clamped the actual work piece to the board with two bar clamps.

A dust collection system is also nice to have!

[Boomer1950]

Thanks for the advice about routing.  It's a major concern for me, since I have no previous experience using a router.  My plan is to use the router to cut away material from the braces I'm using inside the box.  After creating 6 side-wall braces & 2 longer braces I'm hoping the process will be smoother as I learn a little. I'm comforted by the fact that no one will ever see my "router practice" braces after I finish the box.  They are certainly structurally sound, but not exactly beautiful.  

An added benefit - every one of my braces will be somewhat different from the others reducing the chance of the same resonant frequencies being created (I don't know if that theory has any basis in fact, but it helps justify my irregular routing techniques).

I know I'll be buying a circle jig, but I haven't decided on which one.  There are a few that look like solid choices, but I am struggling with the selection of the jig.

I'm using a 1/4" straight bit now to cut the braces.  I think my best option is to be patient.  If I try to cut more quickly or more deeply, I run into problems.  With the circles I plan to move slowly & carefully with each pass - increasing the depth of the bit slightly each time.  Your advice about the direction of the router matches with other sources I've read.  Cutting circles should be in a clockwise direction.  I can handle that.  Also, making sure the piece being cut away is "clamped" in some way (nailed or screwed to the base board) was new to me.  Obviously, it's necessary to keep that center circle from moving at the end of the process.  Excellent point.

Whoa, I wish I had a dust collection method.  The amount of dust is stunning.  I'm working in my storage building, thank god.  There will be NO routing inside my house.  Tons of dust.

Thanks for the advice.  I need all I can get.

[dgage]

I don’t want to frighten someone away from using one but a router can be incredibly dangerous as it can jump when the bit catches, which is why you hear people telling you to cut in the direction where it pulls into the work.  When cutting a hole, the bit is touching both sides at the same time so it can pull in any direction, which is why you need to take multiple passes to get to full depth in a cut.  You also need to sometimes take multiple passes even when cutting in one sided the bit.  My best advice is to make sure the work piece is secured and will not move during cutting.  Make sure you personally have a solid, wide base to make sure you aren’t pulled off balance.  And finally, make sure you have the electrical cord pulled onto your shoulder or is otherwise out of your way.  You need your full concentration on routing, anything moving or getting in your way is dangerous as it will take away your concentration, which is when accidents happen.

 

The router is a fantastic tool but needs to be respected.  Please don’t be too scared by my writings, just be careful and focused.  Good luck in getting things done.

[Boomer1950]

@dgage don't worry. You didn't scare me, but you've confirmed what I've discovered so far. Concentration & small increases in depth seem to be key factors in routing success - especially for a beginner.
The brace I cut this morning (after reading all the good advice here) is the best I've done so far. I still have 2 braces 18.5" x 30.5" to finish. After those I should be ready to take on the circles on the baffles. The practice has been essential. As has been the advice. 

[Father Francis]

Try make some jigs to help you do some , of the repeat and alike holes and stuff saves you alot of time and gives you good practice too . 

[SME]

3 hours ago, dgage said:

I don’t want to frighten someone away from using one but a router can be incredibly dangerous as it can jump when the bit catches, which is why you hear people telling you to cut in the direction where it pulls into the work.

I think you meant "cut in the opposite direction where it pulls into the work" here.  It's much easier to control the router when the tendency is for it to be pushed towards you rather than pulled away from you.

5 hours ago, Boomer1950 said:

I'm using a 1/4" straight bit now to cut the braces.  I think my best option is to be patient.  If I try to cut more quickly or more deeply, I run into problems.

While it's OK and often a good idea to do shallower cuts, you don't want to move the router to slowly.  The spinning router bit rubs against the cut edge of the workpiece and generates a lot of heat.  This can overheat the bit, causing premature dulling and breakage, and/or burn the workpiece.  Moving the router slowly allows more heat to build up, and furthermore, while the bit is actually cutting, the material that's removed absorbs a lot of heat.

The higher the RPM, the higher the feed-rate should be.  Though you may need to make even shallower passes to avoid stressing the bit.  Contrary to @peniku8 suggestion, I recommend dropping the RPM a bit from the highest speed (if your router allows this) so you don't have to rush the cuts as much.  At a medium RPM, a feed rate of 2-3 inches per second is likely in the ballpark you want.  Try to avoid making "dust" and make small "chips" instead.  When cutting a 15" diameter circle, that means that making the full circle around should take ~20 seconds to do.

No worry about the braces looking ugly.  Mine often do too. 

Edit: Also note the sound and feel the of router.  Both should be relatively smooth.  A lot of vibration or screeching means something is likely wrong.  (I'm not sure if you get much screech when cutting MDF anyway, but it's something to pay attention for.)

[Boomer1950]

I had not associated the speed of the router with the speed I move the router. It certainly makes sense. I'll keep that in mind. 

I realize that when the router starts to vibrate or sounds odd, I need to stop immediately. That's finally getting through my thick skull. I kept trying to "power through" those moments.  I hope I know better now. 

The brace today went the best yet. One more to go. 

You'll notice I don't get much done each day. Prerogative of a retired person. 

[peniku8]

While you might burn the wood and dull the tool with too much generated heat (when moving too slowly for the set RPM), it's more likely that you get backlash when feeding too quickly with low RPM, which is more dangerous in my eyes. A firm grip on the router is important and imo the tooling should allow for almost any feed rate. 

RPM and feed rate  and their relation differ with material, tooling (diameter and shape, amount of blades) and eventually the desired surface finish. There are great feed rate/rpm calculators, which should get you a rough idea on your settings.

The chips should not be dust, it should be small grains.

[peniku8]

And btw, last time I manually routed an MDF sheet, I completed a full depth cut (1/2") for a 12" driver in probably less than 4 seconds at 20k rpm with a quarter inch bit. I've seen feed rates in MDF being up to 4x as high as in plywood, it cuts like butter. 

I have a 4" (two blade) tool for the Moulder, which the manufacturer recommends running at 9k RPM. When running at 2k, you'll get severe backlash and I literally had smaller workpieces fly through the shop because of low RPM. 

So in essence: start with high RPM and see what feed rate works best for you. Then lower the RPM until dust turns into small pellets and the wood doesn't burn anymore. 

Did you make plans on how to do the assembly already? Glue and clamp? Kreg screws (those will not work well in 1/2" MDF, I didn't catch what thickness you got)?