Attila Barath wrote: Whats the ratio on open backs one 1/3 or smaller 1/4 open ?

This is a tough question ??? But let's think about the purpose of the back panel.

Think of it like this. The front of the speaker is 180deg out of phase with the rear. So if your ear is equally distant from the front and the back, you'll hear nothing. So the back panel limits the rear reflections and accentuates the front sound.

Another thing. Imagine you have a piece of string. When your speaker is in it's cabinet, the distance from the front of the speaker to the rear of the speaker determines the amount of bass you hear. The longer the string needed, the more bass your speaker has. An infinite long string would be needed to reach around a infinite large baffle, that's why sealed cabinets are called infinite baffle speakers.

The other thing to consider is this. There are many paths from the front of the speaker to the rear. The shortest route determines the the whole bass response of the speaker. So if your speaker is mounted on the edge of you cabinet, the amount of bass would be determined by the path length round the closest side panel. So that is why you see on rectangular cabinets, only back panels at the top and the bottom of the cabinet. It increase the path length from the front to the rear around the narrow section of the cabinet.

But more bass is not necessarily better. It takes more energy to amplify bass, so the more bass you cabinet can deliver, the less loud it sounds relatively. So if you want to cut through the noise, you need to limit your bass response.

Other aspects of the rear baffle size are determined by the limitations of the driver's design. All speakers have a limited distance it can move on the X axis. To stop your driver from jumping out of the magnets groove, you need to dampen the movement when it gets to extremes. This is why closed speakers are great for loud playing.

Then speaker drivers have resonance. It is the frequency it plays the easiest. On the Celestions it is about 70Herz. If you do not want bass boominess and the associated lack of power, you need to bring some out-of-phase sound from the rear to the front to cancel the resonance so you sound is equally loud on all frequencies :bopping:

Look at the VOX rear panel. Think of the speaker as a piston pushing air. The effective area on top of the speaker that pushes the air is a figure to keep in mind. A good starting point is to make the gap in the rear panel as large as the piston area of the speaker. The AC30 has two 12" and the gap is about the the double the size of the Celestion's effective piston area ?