My open baffle dipole with Beyma TPL-150

[mige0]

Originally Posted by catapult

That said, with 8 drivers, I like the idea of a double bass array (DBA) for dipole-like bass.

Find this DBA fascinating too - but until now I haven't seen a way to realize with dipoles


Michael

[mige0]

Originally Posted by Rudolf

Michael,
brute force for sure.
I see that Monte Kay does not get rid of the longitudinal 20 and 40 Hz modes. And at low frequencies those W-frame towers next to the wall don't work as real dipoles any longer IMHO. The back wave has a 1,5 m wide opening into the room, which is 1/7 of the wavelength of 40 Hz. I would consider this as a really big mass of trapped air behind the W-frames.

What Markus has achieved with the multisubs looks more impressive to me.

Rudolf

I wouldn't mind if *full* dipole operation is compromised in rooms.
I think this is unavoidable anyway.

The benefits of dipole - IMO - are two fold
1.) directivity control for free up to a certain degree
2.) *no* box coloration of sound

Having struggled for years to overcome this nasty box coloration with different - mostly pretty weird - designs before taking the OB route, the second point is at least as important for me as is the first one.

Until now I haven't see someone to be able to pin down this colorations by measurements though - so it remains a highly subjective judgment.

Michael

[StigErik]

Originally Posted by skorpion

What a disaster ! Cuibono is quite right when he directs you to MJK's worksheet. I had early in the thread a simulation that was based on a deeper U-frame than you probably built, and that simu pointed to massive EQ to reach high in low bass. So when you dumped the baffle altogether I was really surprised.

There are however good hints in what you have been doing here. Yours and others efforts points to that more investigation into 'optimal' baffle size is necessary. So far I don't think that 'No Baffle' is a proven answer.

I was also surprised that you when HP-ing the Excel unit so high as 350 Hz and steep enough (24 dB/octave ?) even without a baffle should have so big problems with EQ and that class unit. I can't really see the reason for this.

Before giving way to megalomania, I think the Beyma should be given a chance in an H-frame or as IB. In an H-frame with inner measures 22 x 22 x 22 inches including a resonable baffle thickness, the unit should produce 100 dB SPL at 25 Hz with about 7 mm cone movement with some 6 dB/octave EQ-ing from say 80-90 Hz downwards.

/Erling

Let me explain my choices a little bit.

The main reason I went for no baffle, is the lack of coloration. There is no baffle that can vibrate and potentially damage the sound. Yes I know - rear mouting without direct contact like Linkwitz does with the Orion+ would help, but it will not remove all vibration. We will also be left with some vibration in the mouting structure. The point is that with the hanging method, almost no energy is transfered from the driver frames. To my ears, this solution sounds so much cleaner. There is a layer of "grain" or "distortion" that is no longer there.

An other point is that I can operate both mid drivers below the dipole peak, and get better off-axis response.

For the new proposed dipole, I hope to be enable to operate the 15" woofers below the dipole peak as well.

Why cross over the mid at 350 Hz? The only reason is the floor bounce. The reflection from the floor creates a giant hole in the response between 200 and 350 Hz, which simply cannot the fixed by any amount of EQ. Since the woofer is located close to the floor, the null from the floor bounce happens at a much higher frequency.

I tried twin 8" midrange setup to see whether this could give me less problems with the floor bounce. I didnt, sadly enough.

When it comes to dipole low bass, I do understand that baffles are necessary. My 21" without baffle is rather helpless below 40 Hz. I would think that the 15" I'm planning to use will have to be crossed over around 60 Hz. The 21" did produce a lot of quality bass down to approx 30 Hz when I had the U-frames. The bad thing was that the upper bass and lower midrange was not all that good due to vibrations in the frame, cavity resonances and inferior off-axis response. The effective width of that U-frame was approx 1 meter. I would think that an even wider frame is necessary to get down into the 25 Hz range. By separating the low bass and the upper bass/lower midrange, I can get the bass qualities from the baffle-less speaker, and enough low bass from the H-frame. Any vibrations in the H-frames would not colour the lower midrange and upper bass. Best of both worlds!

To sum up all my design ideas right now:

25 - 60 Hz : four 18" in large H-frames
60 - 350 Hz: two 15", no baffle, operate below dipole peak
350 - 700 Hz: two 8", no baffle, operate below dipole peak
700 - 1600 Hz: two 5", no baffle, operate below dipole peak
1600 -> : Beyma AMT

[StigErik]

Ok people, here's an idea for a H-frame subwoofer, hopefully capable of 25 Hz? If someone with too much spare time would like to simulate it with those MathCad models, please go ahead!

There are two 18" woofers with 18 mm p-p Xmax in each frame. Will be two of them of course, that is four 18" total.

This proposal has the same width and height as my current closed box sub's, and a little less depth.

What do you think? Should they be wider, deeper?

[slowmotion]

Very interesting thread.

Wouldn't hanging the woofer from a frame and put a
floor standing baffle in front of it but not touching the woofer be a help
for low frequencies?
It the woofer doesn't move when suspended it could be placed really close to the baffle and still not transmit vibrations. Or not?

[StigErik]

Originally Posted by slowmotion

Very interesting thread.

Wouldn't hanging the woofer from a frame and put a
floor standing baffle in front of it but not touching the woofer be a help
for low frequencies?
It the woofer doesn't move when suspended it could be placed really close to the baffle and still not transmit vibrations. Or not?

Interesting idea, but.... Its a bit difficult to hang the driver and at the same time use a H-baffle. Either the baffle has to be rather shallow, or the wires holding the driver must go through the top of the H-baffle.

I'll think I try the easy route first - build very rigid baffles and hard mount the drivers in them. I cant possibly be worse than the closed box subwoofers I currently use.....

[slowmotion]

Yes, but I was thinking about a baffle for the 21" woofer.

Re sub, have you considered an infinite baffle arangement?

[StigErik]

Originally Posted by slowmotion

Yes, but I was thinking about a baffle for the 21" woofer.

Re sub, have you considered an infinite baffle arangement?

Yes, that would enable the 21" to go lower, but I'ill need subwoofers anyhow. Then why not build dedivated H-baffle sub's instead, and run the woofers without baffle?

IB is not possible with my room. Besides, I will get the same room problems as with regular box type speakers. IB is too much effort and too little to gain I'm afraid.

[cuibono]

I'll try simulating the 18" drivers this evening if you can get me a set of TS parameters..

[StigErik]

Originally Posted by cuibono

I'll try simulating the 18" drivers this evening if you can get me a set of TS parameters..

GREAT !

Beyma 18P1200nd is what I'm concidering, because I have four of them already. Fs might be a little too high, or what?

http://profesional.beyma.com/ingles/pdf/18P1200Nd.pdf

An alternative is the 21" Beyma:

http://profesional.beyma.com/ingles/pdf/21SW1600Nd.pdf