My open baffle dipole with Beyma TPL-150

[jerome69]

Why don't you choose a driver with a really low Fs ? In my opinion pro drivers don't fit. It will be difficult to go as low as 25Hz if your driver has a higher Fs. You should choose hifi drivers or car drivers( Notice a lot of car driver are design to work in free air )
For example

Dayton Titanic MKIII

DaytonRSS390HF-4

Aurasound NS18-992-4A.pdf

[skorpion]

Stig-Erik, you are a great inspiration for many of us, I'm certain.

Here is the simu for the 21" in the H-frame 22 x 22 x 22 inches that I mentioned earlier. Crossover is set to 60 Hz 24 dB/octav Linkwitz-Riley. First 1 watt response:



It will be sufficient to apply dipole 6 dB correction from 50-60 Hz.
Next picture is of cone movement when 100 dB SPL is reached at 25 Hz:



Quite impressive !

/Erling

[StigErik]

Nice! I could go with a slightly larger H-baffle than the one you simulated here.

Is it possible to show the excursion all the way down to 20 Hz? The graph just goes through the roof here....

[skorpion]

As said before, better wider and higher than deeper, but I don't think the difference will be very great. I'm afraid that the whole diagram from MJK's model is copied, so you will have to extrapolate yourself.

/Erling

[LineSource]

Originally Posted by skorpion

I'm afraid that the whole diagram from MJK's model is copied, so you will have to extrapolate yourself.

/Erling

A linear extrapolated Xmax seems low.

[mige0]

Would translate to ~ 105dB @ 20Hz @ Xmax-lin ?

Michael

[Rudolf]

Originally Posted by skorpion

I'm afraid that the whole diagram from MJK's model is copied, so you will have to extrapolate yourself.

Erling,
you can scale the diagrams at will. If you double click on the lone "5" left to the y-axis, you can edit it to the value you want. Same for the lower value "0" and the x-axis.

[cuibono]

Also, consider how MJK calculates "rms displacement": to convert Xmax (a peak to peak value), divide by 2 (to get the peak value), then multiply by .707 to get the rms value. For a woofer with 18mm Xmax, the 'maximum' rms displacement would be 6.4mm.

There are two important points about these type of calculations - first, each company defines/measures/estimates Xmax different ways - so it is impossible to know if it represents reality. The other point is that music isn't pure tones, and so music may be played much louder than just pure test tones. As a final check, I always measure non-linear distortion of the final design at high levels (although this is tough in the bass for a number of reasons).

So I consider these type of calculations rough estimates, but still useful.

[CLS]

When installed onto a baffle, the fs of the woofer (or now the system) drops.

The fs of my woofer is measured 30Hz free air / 20Hz on baffle. These are not very precise numbers, but close enough.

[cuibono]

Okay, here are the sims I did.

First a couple of things - MJK's models only allow you to use one driver per H-frame, so you have to estimate what happens when you add another. Basically add 3-6dB to the SPL graph. It depends on how you wire the drivers and if you use one or two amps (Re of 5.3 is low to put them in parallel, but could happen).

Also, the there will be two H-frames, so add another 6dB, for a total of 9-12dB more than what the graphs show.

I simmed the Beyma 18P1200ND. Its spec sheet says Xmax is 9.5mm, and that is equal to 3.4mm rms displacement, which is how Martin shows it. Again, caveats apply here.

So one set of graphs is with 1W input, the other was with 32W input, which is what would get you to Xmax at about 25Hz. I think the graphs look promising - like 90dB/1W/1m at 25Hz! You will reach 'Xmax' at about 106dB SPL at 25 Hz. But the max spl at higher frequencies will be much higher.