I’ve been reading Linkwitz’s site and got confused about the shape of open baffle loudspeaker. His LX521 has a keystone, or keyhole, shaped baffle which makes sense if you read his page. However, most other open baffle loudspeakers have very simple sheets of rectangular plywood which are relatively huge. Some builders attempt to round off the corners or have sweeping curves resembling Samurai Jacks kimono. My question is, does it matter? I imagine a smaller baffle will allow for more cancellation of the lower frequencies requiring a larger speaker for lower midrange, or you can use a smaller speaker with a bigger baffle for the same effect. I get that. Also, as the frequency goes up the panel should narrow. -?
You have a good grasp of the tradeoffs with OB. The LX521 is a sort of "smaller baffle" design. This has advantages off axis compared to large baffles. Some designers (MJK for example) only care about the front radiation on axis, and others (like me) are concerned with what is happening on and off axis, both in front of the speaker and to the rear, as well as how the loudspeaker's radiation interacts with the room and returns to the listener. A larger baffle will certainly have less low frequency loss than a narrower/smaller one, and this simplifies the design and makes it possible to use fewer bands in the loudspeaker sucessfully.
Does room size matter? If the baffle is too big it would be the same as installing speakers in the wall. I guess there is a baffle to room size relationship?
I also don’t understand how an open baffle woofer makes any sound whatsoever. I’ve played woofers, not in a box, and there is no bass. So what magic is going on if there is a minimal box with no front or back?
After all that, please post some links or references where I can study this.
OB systems need some space to work best, so if the room is on the smaller size with any dimension less than 15 feet or so it is probably not a good choice. The speakers need to be located away from walls and the listening position must be away from the rear wall, so this implies that that room must be a certain minimum size.
At very low frequencies, a dipole source has a lot of cancellation. This is because the different pathlengths between the front source (front of cone) and the rear source (rear of cone) and the listener is much less than the wavelength of sound. Therefore the phase difference of these two sources at the listening position is not much different than at the source itself, and with the rear and front sources out of phase you get almost complete cancellation. But as you increase frequency, the two pathlengths are different by an amount (e.g. the width of the baffle) that becomes larger compared to the wavelength, eventually reaching half a wavelength. This flips the phase difference from 180 deg out of phase to in phase and the two wavefronts actually constructively interfere (add in phase) creating a +6dB SPL at that position and frequency. In general the SPL from a dipole source is dependent on distance from the dipole, and the listening angle. Linkwitz's web pages contain some models of the type of response pattern that this generates and that is a good source for info.
In between very LF and the dipole peak, the relative phase angle is changing from 180deg out of phase to in-phase, and so there is a slow change in the SPL from the dipole source at the listening position vs frequency.