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Tfive 24th June 2019 07:35 PM

The semi-infinite-baffle coax three-way project
 
4 Attachment(s)
Hi guys and girls!

I'm in the planning phase of a 3way semi-infinite-baffle speaker project.

The mid/high speaker chassis is currently the only thing that is set in stone, it will be a Thiel SCS4 chassis (because I own a pair of them already and I believe in the advantages of coaxes). The woofer I have in mind is the Alcone AC 10 HE. I want to use active crossovers, of course involving my own software Pulseaudio Crossover Rack :: T5! DIY Audio Software & Hardware .

Below I attached the planned layout in my living room and two renderings if the first rough cad model I did.

Regarding the speaker enclosures: I want to use MDF, 32mm thick walls, internally damped by bitumen and small 4x4cm pieces of floor tiles. Lots of crossbracing of course. The top is shown the way I plan to glue the 8mm thick panels together. Then they will be smoothed out completely by planing and sanding them.
Internal volumes will be around 6L for the SCS4 (still have to measure actual TSPs) and the Aclone wants to have around 26L for a Q of 0.7 - of course having a Linkwitz Transform Filter at hands i might go smaller even (as smaller volumes can be made stiffer more easily).

Regarding the setup in the living room: This is the only way I do not get in conflict with the door. The room is quite small an "echoy". In order to get decent imaging I realized that doing away with the space behind the speakers would probably be the best solution. The wall behind the listener (behind the couch) will be damped as much as possible.

What do you guys think of such a setup, can it work? What height above the ground for the woofer would you recommend - as shown pretty hight or rather lower? I plan to cross the woofer around 100Hz.

Looking forward to your comments...

Tfive 24th June 2019 08:02 PM

PS: Dimensions of the enclosure are roughly 80cm wide, 40cm deep and 130cm high with the center of the SCS4 around 110cm (my ear hight when sitting on the couch)

ewollowe 25th June 2019 11:15 AM

I like your design. Reminds me of something ejjordan used to recommend for their diy enclosures, flat against the wall with an angled baffle for correct loading and toe in.

I would have thought you'd be good to put the woofer closer to the floor as you're crossing over quite low, you should get a bit more boost and less floor bounce if I'm right?

Interesting looking software too!

Colin 25th June 2019 01:54 PM

With the right speakers, wall-mounting is a valid approach and gets rid of any need for BSC. You may need some kind of eq to cope with the corner position. Alternatively, we've found that aiming at a Q of 0.55 to 0.65 helps a lot with positioning near corners.

What is the crossover point between the SCS4 and the bass driver?

Tfive 25th June 2019 03:54 PM

1 Attachment(s)
Quote:

Originally Posted by ewollowe (https://www.diyaudio.com/forums/multi-way/339553-semi-infinite-baffle-coax-project-post5834026.html#post5834026)
I would have thought you'd be good to put the woofer closer to the floor as you're crossing over quite low, you should get a bit more boost and less floor bounce if I'm right?

I was thinking about the same reasons as you. Didn't do it as i thought it would look a little weird. But the longer I'm thinking about this topic, the more i like the idea, especially as then the two internal enclosures would be spaced apart vertically by quite a bit reducing vibration transfer. And I can use the floor wall as the bottom side of the woofer enclosure too, thus coupling it to the floor.

Quote:

Originally Posted by Colin (https://www.diyaudio.com/forums/multi-way/339553-semi-infinite-baffle-coax-project-post5834147.html#post5834147)
With the right speakers, wall-mounting is a valid approach and gets rid of any need for BSC. You may need some kind of eq to cope with the corner position. Alternatively, we've found that aiming at a Q of 0.55 to 0.65 helps a lot with positioning near corners.

What is the crossover point between the SCS4 and the bass driver?

I haven't measured TSPs on the actual SCS4s I have. Running a simulation with the provided TSPs yields a -3dB point of 96 Hz in a 6L enclosure with Q of 0.7. Maybe I will use this slope together with a 12dB/oct Butterworth filter to form a 4th order Linkwitz-Riley filter as highpass for the SCS4.

Then I would use a 96Hz LR4 filter for the woofer.

The AC10HE has a simulated -3dB point of 60Hz in a closed box, which I want to extend down to around 30Hz by using a Linkwitz transform filter. Q on the low end slope then is adjustable.

So what kind of EQ are you referring to regarding the placement in the corner? And why is a Q of around 0.6 better, any references?

Any recommendations on a 10" or 12" woofer? The Alcone was chosen at least 30% for its matching looks to the SCS4. Another contender is the Dayton DA270-8, also a 10" aluminium cone. I attached a new rendering with the speakers now in the correct left/right placement and the woofer pretty close to the floor. Also note regarding looks: the color will be in the light gray regime, 1) to match the speakers and 2) because it will blend in more nicely with the relatively bright room colors. Might even consider plain white.

kipman725 25th June 2019 04:39 PM

If your ceilings are not very high you may want to take this even further and mount the speakers in the upper corners. That way your minimising power input for a given bass SPL and ensuring your first damaging room reflection for imaging is as the maximum possible distance.

I wouldn't worry too much about the bass frequency response of the different drive units as this is the most easily correctable characteristic with DSP. A Q=0.6, 0.7, 0.5 bass alignment is immaterial compared to the room response. What is more important is the maximum power input, piston size and excursion as these three parameters define the maximum output at a given frequency for a closed box.

The choice of bass driver depends a lot on the intended cross over frequency. Most metal cone drivers need to be crossed over quite low to avoid the under damped breakup which manifests as a big spike in the frequency response.

The bass driver should be positioned within 1/4 of a wavelength at the crossover frequency if your aiming for wide dispersion. This is a fun program for seeing how two sources affect dispersion:
Tolvan Data

*just saw your planning to crossover at 96Hz, I wouldn't recommend this as it severely reduces the total power handling of your speaker and increases distortion. I would have thought 400 Hz and up would be more appropriate.

Tfive 25th June 2019 06:46 PM

What's the advantage of the top corner of the room vs the bottom corner?

Regarding the crossover frequency between woofer and midrange:
1. I want to handle the coax as much of the frequency range as possible for imaging reasons
2. The SCS4 is a quite capable speaker of it's own, it's used in full range shelf speakers and can handle quite some power.
3. I can space the woofer further away and have a more even baffle next to the coax
4. I'm not concerned about SPL at all. normal listening will probably not exceed 80dB, 90dB would probably to loud for my neighbours :D

Will look at the software you recommended once I get home today evening, thanks for that tip!

kipman725 25th June 2019 11:08 PM

The upper corner is away from clutter on the floor. I was referring to the speaker only having to radiate into an 8th of a sphere. I have heard this arrangement before and it works quite well as the human ear is poor at recognising the vertical height of sources.

Higher SPL capacity will result in lower harmonic and intermodulation distortion at normal listening levels. I use a 15" mid bass which crosses to 4*4" midranges which are hornloaded at 500Hz and can't discern the crossover frequency with an on axis frequency response measurement.

wolf_teeth 26th June 2019 10:02 AM

I'm with kip- 300-500Hz will likely blend better, and reduce IMD on the mids.
Wolf

Tfive 26th June 2019 02:55 PM

crossing around 400Hz (Where 1/4 wavelength would be ~21cm) and placing the woofer in a corner somewhat contradict each other, am I missing something?


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