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Old 18th January 2006, 04:27 AM   #1
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Default Dual Chamber Reflex Subwoofer Revisited

Ok guys,

I am out to build the ultimate high performance yet inexpensive subwoofer for an in house HT room of moderate size.

I have always wanted to do a dual chamber reflex subwoofer based on the experiences I have had from two dual chamber's speakers that I have built in the past which were early attempts at getting good sound.

For a long time I wanted to do a TL because I wanted the fast articulate bass. But it seems that the enclosure is going to be to big for me to deal with.

I have read nearly everything on this DCR (Weem's) speaker topology that I can find and I have come to the conclusion that it's strengths are that it can reproduce bass with less distortion not that it can produce more bass.

However, there are two ports coupling with the room air so it is possible that it might energize the room air more if the driver is up to the challenge of moving the air.

As such I will be using a dedicated subwoofer driver of modest cost.

I have read some papers that indicate that this design typically has a dip in frequency response in the 90 - 120 Hz range so that most speaker designers have found this unusable. At least for a two way design. Also, a three way design would have to have a very low crossover point to avoid this dip which is in a very important range for the lower mens voice. But many designers shy away from a really lower bass crossover point with passive crossover s due to some kinds of electrical considerations.

Yet in theory, this design should have lower mid bass and midrange coloration. So I have to wonder if it is more a manufacturing issue than anything else. I know of one manufacturer that designed a 8 inch two way but sadly I never heard that speaker.

In any case, with an actively low passed signal as in the HT LFE channel these limitations should not be a problem.

The first dual chamber that I built was many years ago when Weem's book first came out and was my first foray into speaker design. At that time I knew very little. It was a two way design with a cheap radio shack 8 inch woofer. It did not put out the kind of bass I wanted but in retrospect it may have been the crappy RS driver that I used in addition to the fact that I was asking too much from an 8 inch driver and was expecting to hear some kind of boom which the speaker did not have.

I few years later I tried to build the ultimate three way speaker. It used a 1 inch dome tweeter and 6.5 inch midrange in it's own large cabinet. The lower woofer was a 12 inch driver in it's own dual chamber reflex cabinet. The speaker was about 4 cubic feet total and three cubic feet were dedicated to the lower woofer and it's dual chamber configuration.

I ended up never finishing the speakers after building the cabinets because I determined that they were way to big for the room I was in and I just could not live with them.

I gave them to my friend who had some ambitions to do something with them but he never did anything with them. I also gave him the 12 inch drivers that I had which were some Gold sound woofers.

A year or so later we hooked the 12 inch drivers in the cabinets all by themselves and drove them with an old Sansui 40 watt reciever as the LFE channel from his HT receiver.

The room was rather large but they were tucked away behind a six foot tall entertainment center in the corner. So the space in the corner was pretty large.

THe room was a tall vaulted ceiling combined with the kitchen which was a pretty big space.

I will never forget watching the Sound of Music. I had not seen it since I was a kid and you would not expect an old musical to have much bass information but those speakers acting as subwoofers had the most profound effect on the movie since they were suddenly filling the room with high quality low fast bass which is rarely ever heard and which I could feel as much as hear.

I have wanted to make 3-4 foot dual chamber sub ever since and I think that this summer I may.

Here is what I want to do to make it rather inexpensive but high fidelity.

1. Self made cabinet of dual thickness .75 inch MDF extensively braced.

2. Good quality long Xmax DVC 12 inch subwoofer from Dayton.

3. I will drive both 8 ohm coils with a separate amplifier from an old stereo amp. Both channels with the identical signal. My theory here is that I will be pushing more current through the voice coils this way which will exert more control over the cone and have the damping effect of both amps.

4. The cabinet volume will be a little larger than optimum and have slightly lower tuning than butterworth to create a more gentle roll off below 30 Hz. This will more closely match with the room reinforcement which will be strong at about 20 Hz and will give me a stonger output at 20 Hz. The room reinforcement for this room should be nearly 6 db at 20 Hz so the way I figure it I should only be down about 1 db at 20 Hz in real world response. This should also improve the transient response of the driver a little by making it behave more like a driver in an infinite baffle.

5. The tuning as I recall from my modelling in WinISD should be about 19 - 21 Hz for the lower F and about 38 - 42 for the upper giving me much lower distortion in these areas.



My thinking is that the dual chamber idea might actually work better for a larger driver with lower tuning since it is at the really low Hz where the driver will create the most distortion. THe reduced cone excursion at the tuning frequencies should reduce the overall intermodulation distortion at the upper bass frequencies and help to make the upper bass frequencies in the 50 - 80 Hz region sound better.
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Old 18th January 2006, 06:52 PM   #2
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Hi Hezz,

I have read your post, good typist by the way and I don't see a question.
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Old 18th January 2006, 08:22 PM   #3
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Cal,

I guess I didn't explicitly state a question. But I am hoping to get some general feedback from any who might have possibly built a DCR subwoofer.

More subjective impressions of their sound. I don't really have any questions on how to build it. They are pretty straight forward. I guess more than anything I would like to have my ideas validated but I don't know how many others have experimented with this design as a subwoofer.

Most people making subs just want loud bass. I want strong but highly articulate bass that reaches down pretty low.
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Old 18th January 2006, 08:36 PM   #4
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Hi Hezz,
I built a pair around the same time you did. I can't remember the specifics, but they still sound great. Sicko that I am, they are in my bedroom.

They have tight bass and great extension. Most people search high and low for the sub woofer. I used a Marsland Image 8" at the time (Cheap) with a pair of Philips AD0163T8 textile domes. They aren't perfect, but I was proving the concept.

Others have built the same box, tuned lower using an 8" Peerless and Peerless tweeter. They sound much better (not a surprise). The bass can be devastating when a big bass line comes by.

I haven't had a chance to build more, my plan was to try the same thing in three way using the Peerless 10", tri-amped.

-Chris
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Old 18th January 2006, 11:45 PM   #5
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Anatech,

Thanks for the information. This makes me feel that I am on a good path. I wonder if Claudio ever finished his two way DCR to the point were he wa satisfied with the sound.

I just realized that so long ago when I built my first pair of 8 inch DCR that I was driving them with an old Zenith tube amp which was part of an old console. Now that I think about it, that console had really poor bass even though it had dual 12 drivers. My guess is that they were 16 ohm speakers.

I was driving 6 - 8 ohms with an old anemic tube amp that probably didn't genrate much current at all. This was not a good quality tube amp of vintage variety.

This could be another reason why my old 8 inch DCR's didn't have much bass output. That and I was expecting boominess.

I am getting hyped to build these things.
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Old 19th January 2006, 12:09 AM   #6
anatech is offline anatech  Canada
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Hi Hezz,
Hmmm, let's see. I've abused those over the years with a Caver Lightstar, Marantz 300, 1180DC, 500, MA500 ... , SAE's, Nakamichi PA-7 &TA-2 (current) and some tube amps.

They sounded good on all. I did bottom them out a couple times. Nasty sound.

I'd say you were heading in a good direction.

-Chris
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Old 21st January 2006, 05:08 AM   #7
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Ok,

I think I have come up with a subwoofer design. It is going to be unusual in that it will look like a large floor standing speaker. It will be in the corner and have two drivers so that I can balance out the bass levels.

THe lower driver will be a Dayton 12 inch DVC in a large volume dual chamber reflex. The upper speaker will be a 10 inch Dayton in an aperiodic enclosure. THe upper speaker should have slightly better transient response and upper bass articulation.

The lower speaker will move more air and really reach down low in the frequency range. I will be able to balance the level of each speaker independently to get the best possible sound.
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Old 11th February 2006, 03:43 AM   #8
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I have changed my mind and come up with a more practical design than the larger two driver tower that I had originally conceived of.

This is what I have come up with for the basic cabinet shape. It is designed to fit snugly in a corner and utilize the corner space for cabinet volume. It also has few parallel walls.

Now that I know the basic shape and size that I am shooting for I will start to build up a more accurate model.
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File Type: jpg single driver corner sub enclosure exercise1.jpg (14.6 KB, 358 views)
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Old 11th February 2006, 03:45 AM   #9
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Here is a picture from the side. It will be a bit of a challenge because of a couple of compound angles.

It will use a 12 inch subwoofer driver and the internal volume will be about 4 cubic feet.
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File Type: jpg single driver corner sub enclosure exercise2.jpg (13.1 KB, 339 views)
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Old 25th February 2006, 05:15 AM   #10
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OK guys,

Here is a snapshot of my more developed model. Two of the walls are cut away in this rendering which shows the inner vent arraingment.

I have decided on a tuning frequency of 23 Hz. The butterworth tuning point was about 23.79Hz with a 3.2 cubic foot box. I will have about a 3.5 cubic foot box and a little lower tuning that should work well with the room gain.

At this point I am using three inch vents which are about 18 inches long. I am still going to add more internal bracing and I have some elements of the model to clean up.

WinISD shows about a -3db at around 24Hz and with room gain I should be flat or maybe +1db at around 20Hz.
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File Type: jpg dual chamber sub 1.jpg (42.7 KB, 333 views)
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