I'm new to speaker diy, been reading interesting project done by members here. Im interest in building an alpha 15a H frame with Dayton PS220. I know the H frame baffle dimension but not sure of the PS220 baffle dimension Th at go on top of the H frame. please help guide me to the path, Thanks
Welcome to AudioCircle Prosh629!
The larger the baffle, the lower the center frequency of the baffle step (diffraction loss) and dipole cancellation. Choose baffle size so that these acoustic phenomena complement the FR of the bass unit and the room. Whatever baffle size you choose will affect the design of the crossover and EQ correction filters. There's no correct baffle size, as long as it's big enough to support the bandwidth you're trying to get from the drivers. Both your drivers are pretty large cones for their purpose, so you don't have to worry about too small baffle. Large baffle is always good, but room reflections against the rear and WAF do take their toll on very large baffles. Also, no point using a very large baffle to extend frequency response if you crossover without needing that extra extension from the baffle. Pick baffle size that complements the whole speaker design, in particular the driver cone diameter and crossover points and crossover slopes. SOmetimes you can save money on crossover parts by using the natural rolloff of the dipole cancellation (at -12dB/oct) as part (or all) of your high-pass crossover filter. You would fine tune that dipole cancellation rolloff with the baffle width.
Typically you would just extend the Alpha baffle upward to allow room for the FR driver and put the high frequencies at whatever listening height you prefer, usually ~40".
Use sound absorber on one or both of the inward facing sides of the H baffles so they don't slap echo at midrange freq.
You might want to put the FR (full range) driver offset from the baffle edges to distribute the baffle step diffraction across wider bandwidth. You have 3 sides to work with, since the 4th side is the floor with the Alpha. So place the PS220 so that the difference in distance from center of FR to the left top and right edges differ by same amount, say 3". So if baffle is 18" wide, place the center of the FR driver 6" from the left, 12" from the right, and 9" from the top edge, while center of FR is also at ~39" height, so baffle will be 48" tall. This offset of the FR driver will make a smoother freq response, and require a flatter baffle step correction filter. Keep the shape of the top of the baffle rectangular as that contributes to the smoothing of the distances to the baffle edge because of the corner points. You can simulate this to aid driver placement using The Edge
Put the Alpha all the way down close to floor to minimize floor bounce cancellation with the FR driver.
The baffle itself will vibrate in sympathy with the drivers bolted to it, and this can ruin the sound of the speaker. Your H frame legs mostly control this, but if the center area of the baffle between the drivers is not supported it will vibrate so you may want to glue some steel angles across the baffle between the side frames. Or glue a big porcelain tile to that rear baffle open space, as big as will fit between the drivers and side braces.