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Open-Baffle Speaker Concept No.1- Part 2

The Open-Baffle And The Cross-Over


Now let’s look at how this all fits together to make a speaker. As I said, one needs an idea of how an open-baffle affects the frequency response to make heads or tails of this speaker’s design. I therefore present three graphs- the first illustrates how the open-baffle affects all the drivers in the speaker, the second illustrates the effect of the cross-over and the third gives the combination of the two above.

If you look at Graph 1 you will see a representation of a frequency graph for the given speaker dimensions, where the frequency will roll off below a specific frequency (50 Hz in this case) at a rate of 6dB/octave. If you look at the open-baffle graph I have included, you will see the sizes and corresponding frequencies for different sizes of speakers or baffles. This chart was found in Sound Practices, but this chart was found on the Bona's webpage (see link below).




The Bona's Audio Webpage

With the cabinet size, and looking at the graph just above, it is clear that the roll-off will be at approximately just below 50 Hz (I will include a more detailed description of how I calculated the baffle width for this speaker at the bottom of this article).


Now, for good low frequency response, and to make the general design easier, we will take a point of two octaves below the open-baffle roll-off as our lowest frequency. Two octaves below 50 Hz is 12.5 Hz and at this point and first-order X/O is instituted. The effect is shown in Graph 2 (ignoring for a moment the effects of the open-baffle).


If the responses of the two graphs are combined, the net result will be a flat response between 12.5 Hz and 50 Hz.


The response will be 12dB below the response of the main driver at the X/O point though if the drivers are of the same (combined) efficiency as the main full-range driver. The PHY-HP KM30 driver is 97dB/W, so this makes the choice of bass drivers difficult. The unit used needs to have a very low Fs, as the Fs is usually the lowest point I’d advise to use a driver to in an open-baffle system. The best of these I know of are the TC Sounds 8 Ohm, 15”, underhung voice-coil units. They have a Fs of 15 Hz, which makes them very nice for these purposes.

Damping Of The Rear Wave


This actually quite important, as we don’t want too much mid and high frequency energy radiating into the room. I have very little input to give here save to suggest some longhair wool be used. The volume and density can only be determined through experimentation, but I would lean to a light amount of stuffing, so that you do not interfere with the open-baffle roll-off effect too much.

Some Musings On Possible Bass Drivers


As I said before, a pair of TC Sounds 15” underhung coil drivers might be good in this application, but others like the drivers from TAD-Pioneer and JBL-Pro drivers can also be used.

Other driver to consider are units from Fane. The Fane Colossus XB series are very nicely built, but these, as in the case of the JBL-Pro drivers*, are only available in 15” and 18” drivers, and if the 18” drivers are used the cabinet must again be resized (* there is one JBL-Pro bass driver of 12" diameter, but it has a high Fs of over 50 Hz, which makes it less than suitable for this application).

I might have missed some very worthy candidates, but this is where individual builders themselves will have to decide for themselves.

Good luck and enjoy.