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Multi-Way Conventional loudspeakers with crossovers

My submission to the "Better Homes & Donga's" speaker competition
My submission to the "Better Homes & Donga's" speaker competition
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Old 10th August 2017, 08:50 AM   #1
Oldvinylplayer is offline Oldvinylplayer  Australia
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Default My submission to the "Better Homes & Donga's" speaker competition

Firstly, for non-Australians: In this country a Donga is a temporary accommodation building typically used at fly in fly out remote work sites. The rooms are like VERY small hotel rooms, with an en-suite bathroom, a beer fridge, and not much else.

These speakers were inspired by suzyj's noisePlank here: noisePlank - a simple folded TL with TG9FD10

I sometimes visit the same remote site as Suzy, I'm trevelling there now, and have the same need for good sound in my donga. I didn't want to just copy the noisePlanks, and owning and very much liking a pair of Linkwitz LXminis I thought it would be fun to try a similar layout using pairs of TC9FD-18-08 drivers. The donga rooms are built on a steel frame and there is minimal sound deadening between rooms, so SPL and particularly bass SPL were not a priority design requirement. But I wanted as good sound quality as I could get and particularly I wanted good stereo imaging.

The only similarity to LXminis is the vague visual likeness and the 90 degree two driver layout. The acoustic design is completely different. The 104mm ID PVC pipes are 760mm high, and about 3 litres in volume, they are ported at the base with a 25mm PVC tube 85mm long. They are designed to sit on the floor.

There's mild filling with acousta-stuf. I think this means the enclosure is a damped mass loaded quarter wave transmission line, but I stand to be corrected.

Because I must travel with these speakers in a suitcase they had to be small, and I needed to use a single small Class-D stereo amplifier, ie with passive crossover, not 4 channel amplification and miniDSP like the LXminis. I have briefly experimented with basic 1st order crossovers, mainly tuned by what sounds right, and I have done some measurements at home which I will post up over the next few evenings. I hope to get some in-donga measurements as well. I currently have a 0.47mH coil on the lower driver and a 1.5uF cap on the upper. With this quickly thrown together crossover they sound surprisingly good. Top notch imaging, flattish response and reasonable bass distortion at low volumes.

The plan over the next week is to run them in situ and get the crossover better if I can.

Has anyone tried something similar? Comments on these ones are welcome. I'm going to need some help with the crossover.

Graham.
Attached Images
File Type: jpg IMG_9596.jpg (76.1 KB, 335 views)
File Type: jpg IMG_9592.jpg (82.9 KB, 330 views)
File Type: jpg IMG_9595.jpg (51.0 KB, 329 views)

Last edited by Oldvinylplayer; 10th August 2017 at 08:55 AM. Reason: added stuff
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Old 10th August 2017, 09:30 AM   #2
suzyj is offline suzyj  Australia
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I like! You've solved the problem of getting good sound while moving around the room plus good sound while laying in bed by making them self supporting and mobile. If you pull your bed away from the wall a tad, then you can put these either side at pillow height. Brilliant!

Can't wait to have a listen.
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Old 10th August 2017, 09:32 AM   #3
planet10 is offline planet10  Canada
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My submission to the "Better Homes & Donga's" speaker competition
Quote:
0.47mH coil on the lower driver and a 1.5uF cap on the upper.
Looking at the TC9 impedance curve that looks like ~10 kHz HP and about 3500 Hz LP.

dave
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Old 10th August 2017, 02:35 PM   #4
HarryY is offline HarryY  United States
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My submission to the "Better Homes & Donga's" speaker competition
I was curious id you could make the path longer by
putting a smaller diameter pipe inside of the larger one
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Old 12th August 2017, 09:47 AM   #5
Oldvinylplayer is offline Oldvinylplayer  Australia
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Suzy, yes, like the real LXminis these are naturally good at sounding OK in front of and from the back, plus as you say they are easy to move around. I've yet to experiment with different positions, but they sound OK from the bed placed just off to one side (see pic below).

Dave, thanks for the numbers. I'd used an on-line calculator and got a similar result, but I realised that with the weird layout, with one driver pointing upwards, and the other a dipole, the actual acoustic slopes are quite different, so I didn't draw any conclusions from this about the crossover frequency. I'll post some measurements later which show the crossover.

Harry, that's a great idea. It would necessarily result in an abrupt step change in cross sectional area though. How would this effect the performance of the line? I've brought out some tube extensions and will add these when I get a chance. These extensions bring the column length to 760mm. Back at home in my large workshop space they sounded better with the extensions, but I'm not so sure here. The room is small at about 3x4m (10x13 feet) and there appears to be quite a lot of room gain, so the bass may be too strong with the longer tubes. I might need to pull apart one of my employer's pillows to raid some stuffing.

Here's a picture of the current set-up. They are sounding great, so much better than listening on the laptop speakers. I think they'll be staying here.

Graham.
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File Type: jpg IMG_9608.jpg (49.5 KB, 208 views)

Last edited by Oldvinylplayer; 12th August 2017 at 09:49 AM. Reason: grammar
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Old 12th August 2017, 10:06 AM   #6
Oldvinylplayer is offline Oldvinylplayer  Australia
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Here's some measurements that I took back at home. This was with a .056mH coil and a 1.5uF capacitor plus a 2.2 ohm resistor in series with the top dipole driver to bring the top end down a bit. The mic was 1 m away. My interpretation of these measurements is that the crossover is at 2250 Hz.

They measured flatter, and sounded better, with the upper dipole driver's phase reversed (as per the first plot), and I have no idea why this is. Maybe a time alignment problem? The second plot below, with the upper driver in phase, sort of hints at a reverse null appearing at the crossover at 2250 Hz, so I think the reverse phase is the correct one. Any suggestions about this?

These measurements were taken in my workshop, roughly in the centre of the space, with the speaker up on a high stool and the mic on a stand. The mic and speaker would have been 1.5 m from the (highly reflective) ceiling. The separation was 1 m. I think the bumps in the lower driver response curve might be ceiling bounce. My thinking is that the vertically pointing driver will beam heavily at the roof as the frequency goes up, so the ceiling bounce will be greater than with a conventional forward facing driver. Is this explanation valid?

Graham.
Attached Images
File Type: jpg Twin TC9 drivers separate.jpg (108.1 KB, 201 views)
File Type: jpg Twin TC9, tweeter in phase.jpg (98.5 KB, 66 views)

Last edited by Oldvinylplayer; 12th August 2017 at 10:25 AM. Reason: added more questions
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Old 12th August 2017, 10:16 AM   #7
Oldvinylplayer is offline Oldvinylplayer  Australia
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Here's some off-axis measurements. Same crossover as above. The plots are in numerical order; 0,30,60,90,120,150 and 180 degrees.

It looks like there is some sort of brief cardioid behaviour where both drivers are radiating equally at the crossover of 2250 Hz. The two greenish curves that dip down at the crossover frequency are the 150 and 180 degree measurements. Above that, the top driver does indeed appear to be working like a dipole, with the rear radiating 150 and 180 deg. measurements starting to rise back up.

Graham.
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File Type: jpg Twin TC9 360mm column, polars.jpg (121.6 KB, 74 views)
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Old 13th August 2017, 06:44 AM   #8
suzyj is offline suzyj  Australia
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Hi Graham,

I presume the off-axis measurements are in situ rather than in your garage? It certainly makes a huge difference to the bottom end. Looks like you're getting a decent response down to 60 Hz without Eq, which is very cool.

0 and 30 look lovely - not so sure about everything else though ;P
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Old 13th August 2017, 10:22 AM   #9
Oldvinylplayer is offline Oldvinylplayer  Australia
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Default In-donga measurements

No, those measurements above were taken back at home in my workshop.

The off-axis measurements might look a bit rough, but I don't think they are too different from those here of the LXminis by Linkwitz:- LXmini design I'm learning that measuring dipoles is problematic. It's difficult capturing a true summing of the front and rear responses without getting all the clutter from reflections.

Here's some in-donga measurements, taken just now. 360mm column, 0.47mH, 1.5 uF and 2.2 ohm series R crossover. No stuffing inside, as that's what seems to sound best. I tried with moderate stuffing but strangely the dips and bumps in the lower midrange got worse.

Comparing these measurements to the earlier ones, the amplitude response clearly shows the effect of the room gain in the lower bass. Or maybe the difference is just from the actual location in the room that I set up the measurement (500mm from a wall). But whatever, the plot mirrors what I hear - a distinct improvement in lower bass.

I reckon the impulse response is looking pretty good

Graham.
Attached Images
File Type: jpg Twin TC9 360mm column in donga.jpg (72.2 KB, 41 views)
File Type: jpg Twin TC9 36mm column in donga .jpg (90.4 KB, 40 views)
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