21" reflext enclosure needed

At this time many rental companies are trying to off load some of their gear in order to survive the crisis.

Here is one example of a pair of very solid double 18 inch box that goes for very little money.

Bit late to the party........

This is a Josh Ricci Ckram (without front chamber attachment)

Loaded with B&C 21DS115, built by a friend of mine upon my suggestion.

Roughly about the same size as the beyma plan, but with a slot port with roughly twice the surface area, and nearly 1m port lenght in L shape. The tuning is exactly 30 Hz. 

It goes flat to 30 Hz, according to Joshs akabak simulation about 92 -93 dB sensitivity at 30 Hz! Averages about 95-96 dB in the upper band. Without processing. 

Box is mostly rebated, hasnt been glued up yet, currently assembled for testing with screws only. With most of the bracing in place, it is already quite impressive. We run it on a bridged yamaha h7000, and have not managed to get the amp to clip in the small workshop we are currently confined to by the weather. We have also not been able to get the ports to chuff yet. It sounds balanced,  very round and punchy, and plays all the sub 40 Hz stuff with authority. 

I assume the Beyma design you were talking about is this:

If you check the Beyma pdf, except for the unrealistic peak spl figure (without frequency reference!!!), the response of the box is 6-7 dB down at 30 Hz.

 This means  that  

1) the actual sensitivity of the beyma design at 30 Hz is 99 -6 = 93 dB AT BEST , but probably lower, plus the likelyhood of strong port compression and  chuffing

2) you will have to apply serious eq to get a flat response down to 30 Hz.

 To add further smoke and mirrors, their response graph is normalized, not telling you the true sensitivity of the design.

There is also a chart of eq settings, with a 2,5 dB boost at 45 Hz. 

If you look at the lenght of the triangle ports it is very obvious that the box is tuned very high, not lower than 45-50 Hz. 

Based on my experience i would strongly recommend the ckram in standard reflex configuration over this beyma design. I don't think the beyma design is any good if you need flat response down to 30 Hz.