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Old 17th October 2012, 03:54 PM   #1
Doug47 is offline Doug47  United States
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Talking Small Tower Subwoofer Project

This project will be my second DIY speaker. The first was a kit (Overnight Sensation) where my personal contribution was moving the port location to the front so I could wall mount the speakers (and use them for the surround sides in my 7.1 setup). I very much enjoyed the project, so I want to do one “on my own”.

My front speakers (a pair of Polk 10s from the early 80s) do a good job, so I didn't want to replace them. Same thing with the center and surround back speakers (not all Polks, but all decent speakers). The only speaker I don't have currently is a dedicated subwoofer (although the Polks do a decent job on the low end).

So the decision was made to build a subwoofer. However, that desire is complicated by the rather major restrictions put on said subwoofer by what I've read here as the WAF.

The first restriction is that the subwoofer must fit into a location that is only 10” wide, and not much deeper. So no 15” subwoofers for this design. The second restriction is that I utilize my old NAD 743 amp that was replaced by the modern (Marantz) 7 channel system. I have access to 3 power amp channels of the old NAD unit, each with about 50-70 Watts (depending on which power rating I use). The third restriction is that I do not have access to a real shop. I have a carport, saber saw, circular saw, a drill, and I know how to use sandpaper. So all wood cuts must be simple. Oh yea, and “breaking the bank” is very much a no-go. Again, the WAF comes into play.

So the initial design has a set of three small subwoofer speakers with power handling ability around 50-70 Watts (to match my NAD output). I found a speaker that seems to fit the bill: Tang Band W6-1139SIF. Given 3 of those speakers, the box volume comes out to 1.263cu.ft., with a port 3” diameter by 10.2” in length. That should get me a response down to 33 Hz (which I can live with). With the restriction that the front be only 9” wide using ¾ birch plywood, the outer box dimensions come out to be about 38” tall and 11.25” deep (I doubled the front speaker plate for stiffness and because I had enough wood left in a 4 x 4 sheet to do so). I will include pictures of my design if I can figure out how to do so. It ends up looking like a small tower speaker, not the usual cube shape of a subwoofer. In my drawings of it, it kinda looks cool (which helps the WAF).

The only major thing left to do in the design is to locate the port. I have three choices: front, bottom, or back. Putting the port on the front or back means I have to curve the port as there is not enough depth in the cabinet to fit the port. Home Depot/Lowe's have 3” 90° PVC elbows that should work. If I put it on the bottom, I don't need to curve it, but I will need to set the speaker up off the floor so the port exit isn't too close to my carpet (I've been reading some threads here about that very issue, so you've helped me already).

Each speaker will be driven by a single channel of the amp I have from the subwoofer (line level?) output of my new system, so there should be no crossover or impedance issues to worry about. The NAD will drive the 4 Ohm loads without a problem.

I do have some questions and hope to get some suggestions about how to solve any remaining problems, such as:

1. Sanity check. I've gone over everything I can think of, but since this is my first real “scratch” speaker project, I wonder if there are some “simple” things I've not thought of (“Doh!” moments). I have a degree in physics (PhD) and do research for a living. I understand the “theory” of all of this stuff, but I know from experience that theory and practice are two different things and I have VERY limited experience making speakers.

2. Where should I put the port? I see advantages/disadvantages with each of the three locations. For aesthetics (and from reading horror storied about things getting put in the ports), I am leaning towards the bottom or back placement.

3. Corners. Due to my lack of proper tools and skills (I work electronics and software, not woodworking), I am going for “butt” joints, but I'd love a suggestion as to how to get rid of those plywood end grains (without having to buy new equipment or learn new skills in too short of a time).

4. Feeding the speaker. My intention was to take the “pre-amp/line level” subwoofer output of my Marantz SR-5007, split it into three lines, feed them into the power amp connectors (which I assume are high impedance, so splitting the line out should not be a problem). Then take the speaker level power amp outputs and feed them to the three speakers. Good sounds should happen, unless I really screwed up something basic (and yes, I've screwed up “something basic” in the past).

5. Speaker box material. I first was going to use MDF and then use vinyl to make it look good, but due to various issues, I moved to birch plywood which I will stain either oak, walnut or cherry (depending on the WAF). I am hoping that is not a really dumb idea.

6. General issues with the design. I don't even know what questions to ask here.

This project will not get started until after the New Year as my weekends are booked. Okay, I might be able to get started during November as that month is somewhat free of major travel for me. I figured I could get some knowledgeable feedback from DIYAudio folks that might actually make my somewhat strange idea work (I've never seen a “tower subwoofer”).

Thanks in advance for any helpful hints.
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Old 17th October 2012, 05:22 PM   #2
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Keep in mind that with the aspect ratio you are proposing the box will have at least started to transition to an ML-TL and as such and sims assumming it is a bass-reflex will be off.

dave
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Old 17th October 2012, 08:12 PM   #3
Doug47 is offline Doug47  United States
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Quote:
Originally Posted by planet10 View Post
...started to transition to an ML-TL...
That's the type of information I would NEVER have thought of. At this point, I would not even be able to figure out how such a transition would affect the sound. Any suggestions as to what it would do (lower/increase the f3 point, etc.) and or how to stop it from happening (add insulation, subtract insulation, etc.)?

I am looking for a "tight", "flat" subwoofer as most of my listening is music (with the occasional Avenger's movie).
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Old 17th October 2012, 08:20 PM   #4
Doug47 is offline Doug47  United States
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Post Pictures

Here are some pictures. The first picture is what it may look like using my drawing package (with 3 different finishes and the 3 different port placement). The second is a sort of "three view" of the parts & shape of the box. Most of the design is in my head, but I need little drawings like this to make sure all the parts fit together. There are 3 braces that I intend to make out of the "scrap" bits used to make the 6 sides. The grey things are the speakers, and there is the port in the bottom. Again, for scale, the unit is about 38 inches tall, 9 inches wide, and 11 1/2 inches deep on the outside.

I hope this upload goes well...
Attached Images
File Type: png small_subwoofer_E.png (132.9 KB, 814 views)
File Type: png small_subwoofer_A.png (69.1 KB, 789 views)

Last edited by Doug47; 17th October 2012 at 08:40 PM. Reason: Fix text to match pictures
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Old 18th October 2012, 04:40 AM   #5
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Here are a couple of very important things to consider

1. That TB driver is not all that efficient, with 50W input, a single driver will only get you about 97db @ 30Hz @ 1m - with the three coupled in the enclosure would then get you to about 103db @ 30Hz @ 1m.
So that raises the question, how much room are you trying to fill ?

Also those TB drivers are quite mechanically noisy compared to other drivers, I know I own several - lol
If they are really starting to dig deep and loud and you are sitting relatively close facing them, you will definitely hear spurious noise from the cone movement that is not part of the music....

2. Even if your Marantz has a reasonable bass management feature it is still lacking features that even most of the cheapest Sub Amps have
- missing a subsonic(rumble) filter - usually at around 15>18Hz - if you don't have one your NAD amps may consume most of their power just trying to amplify LF rumble...
- missing a variable "sub phase" control - really necessary to properly integrate the sub with your mains
- most of the built in bass management systems only provide a shallow 2nd order crossover so your sub may end up seeing a lot of higher frequency content unless you set it at the very lowest crossover frequency which then may not end up integrating well with your mains.

3. Location, location, location - have you done any listening tests to determine if the location you chose was actually going to work for the sub ?
It could be that you have, due to WAF, chosen the worst possible location for the sub, in that, due to room modes the sub will sound boomy, sloppy or weak and ineffective etc.
Ultimately your listen room and the placement of the sub in that room will have more to do with how great or how poor it sounds no matter how you try to EQ or modify the sub

Last edited by Cokewithlime; 18th October 2012 at 04:58 AM.
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Old 18th October 2012, 05:26 AM   #6
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Quote:
Originally Posted by Doug47 View Post
That's the type of information I would NEVER have thought of. At this point, I would not even be able to figure out how such a transition would affect the sound. Any suggestions as to what it would do (lower/increase the f3 point, etc.) and or how to stop it from happening (add insulation, subtract insulation, etc.)?
You can model with Martin King software. Note that F3 is a pretty much meaningless spec. Look at F6 & F10

Quote:
I am looking for a "tight", "flat" subwoofer as most of my listening is music (with the occasional Avenger's movie).
Then you want to take into consideration room gain. A slow roll-off that starts at 50-70 Hz with F10 near your target low frequency. Of course there is a very large variation depending on room placement, room placement, and rom size

dave
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Old 18th October 2012, 02:41 PM   #7
Doug47 is offline Doug47  United States
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Quote:
Originally Posted by Cokewithlime View Post
Here are a couple of very important things to consider

1. That TB driver is not all that efficient, with 50W input, a single driver will only get you about 97db @ 30Hz @ 1m - with the three coupled in the enclosure would then get you to about 103db @ 30Hz @ 1m.
So that raises the question, how much room are you trying to fill ?
The room is fairly large. However, the current setup (Polk 10s I mentioned in a prior post) with the Marantz do a "decent" job right now. I am not looking for "Earth shattering", I am looking more for enhancement of what I've already got. Also, the NAD indicates it might be able to do more than 50W over short periods of time (we shall see what that really means), so it still might be sufficient.
Quote:
Also those TB drivers are quite mechanically noisy compared to other drivers, I know I own several - lol
If they are really starting to dig deep and loud and you are sitting relatively close facing them, you will definitely hear spurious noise from the cone movement that is not part of the music....
THAT could be an issue. Are there any alternatives (same approximate size) that you might recommend? I want the sub to ADD to my enjoyment, not subtract. I will be sitting about 8-10 ft from the sub.
Quote:
2. Even if your Marantz has a reasonable bass management feature it is still lacking features that even most of the cheapest Sub Amps have
- missing a subsonic(rumble) filter - usually at around 15>18Hz - if you don't have one your NAD amps may consume most of their power just trying to amplify LF rumble...
- missing a variable "sub phase" control - really necessary to properly integrate the sub with your mains
- most of the built in bass management systems only provide a shallow 2nd order crossover so your sub may end up seeing a lot of higher frequency content unless you set it at the very lowest crossover frequency which then may not end up integrating well with your mains.
The Marantz has "Audyssey MultEQ XT" for speaker management. From what I've read, it will take care of phasing via delay. As for NAD and a rumble filter, I will need to look that up. I thought it had one,but I am not sure. I do know it will go down to 5 Hz, so rumble could be an issue. Now that I think about it, the Marantz may have a rumble filter built in. As for integrating with the mains, those Polk 10s again cover the low frequencies pretty well, so the lowest crossover might be a way to go. Let the Marantz do all the "audible" signals and let the NAD concentrate on the "subaudible" components.

Since none of my sources are analog (DVD-A, Blueray, SACD, mp3, FLAC,internet radio, etc.), will rumble still be a problem?
Quote:
3. Location, location, location - have you done any listening tests to determine if the location you chose was actually going to work for the sub ?
It could be that you have, due to WAF, chosen the worst possible location for the sub, in that, due to room modes the sub will sound boomy, sloppy or weak and ineffective etc.
Ultimately your listen room and the placement of the sub in that room will have more to do with how great or how poor it sounds no matter how you try to EQ or modify the sub
Sadly, there I have almost no control. Our living room is already "too busy" and I think I am doing well to replace the vase with a subwoofer. Once I get it built, and if the sound is not what I wanted, I MAY be able to negotiate some movement. As for listening tests, no sub I've seen would fit where I need this one to fit. The test will be when I get it built and fired up.

I do thank you for your input. It's given me a lot of things to think about. DIYAudio needs a "taking notes" smiley. I've seen and read about some of the setups folks here have, and my whole system is MUCH lower in power and cost. It's actually rather intimidating. Right now, this is going to cost me about $200 (or so), which is a good budget for me (I have 3 kids in college, THAT's where all my money goes).

Last edited by Doug47; 18th October 2012 at 02:57 PM. Reason: Clarificaiton of my poor English
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Old 18th October 2012, 02:55 PM   #8
Doug47 is offline Doug47  United States
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Originally Posted by planet10 View Post
You can model with Martin King software. Note that F3 is a pretty much meaningless spec. Look at F6 & F10
Okay, maybe our terminology is getting confused. I thought "F3" was the 3 dB frequency (half power). "F6 & F10" would then be the 6 and 10 dB down frequencies. There goes my bias. In my work, the 3 dB frequency tells all. Those others are just "in the noise".

I will look up the "Martin King software" and see if I can understand it. It would be funny, given my profession, that I would build a transmission line speaker (my dissertation used a transmission line model, but for lightning).
Quote:
Then you want to take into consideration room gain. A slow roll-off that starts at 50-70 Hz with F10 near your target low frequency. Of course there is a very large variation depending on room placement, room placement, and room size
That is where I am severely limited. It will go where it goes. Hopefully, the Audyssey MultEQ XT will be able to fix the majority of the issues.

Again, thanks for the inputs. These give me many things to think about and do to make the best sub I can, given the limitations of the project.
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Old 18th October 2012, 03:54 PM   #9
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Quote:
Originally Posted by Doug47 View Post
Okay, maybe our terminology is getting confused. I thought "F3" was the 3 dB frequency (half power). "F6 & F10" would then be the 6 and 10 dB down frequencies.
Your terminology id correct. F3 is heavily used, is from the filter theory that current box alignment techniques. But Floyd Toole & co have shown that it means very little to how we perceive a loudspeaker.

dave
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Old 18th October 2012, 04:28 PM   #10
schmeet is offline schmeet  United Kingdom
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Below attachment is a sim of your design assuming it is a ML-TL. I have modeled your drivers as though they are at the start of the line but it has little effect being along the length (maybe a bit less ripple in the higher frequencies).

What frequencies will these be playing up too?

Also, be careful of port noise. you have over 10m/s there (@50w) and you don't want to be going much higher.
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