Facebook Cajon Forum member Peter Wolf had some good observations about the downward facing sound hole: “… the sound depends on the floor, where you play. If there is concrete, the sound will be hard and loud, if there is wood, it will be softer and not so loud, an if you play on a thick carpet, there will not be very much sound. And if you rock with your cajon, the sound changes also.” Here is my edited response:
I agree that the design is not perfect. The floor does affect the sound. But a regular cajon has a sound hole facing backwards, away from the audience. If the situation is perfect the wall behind reflects the bass back to the audience and player. But if there is no wall? Or if it is metal or sheet-rock or brick, it will affect the sound. So both styles have sound hole placement issues. I play the bottom facing cajon mostly on a carpeted floor and prefer the sound vs on a wooden floor. I haven’t tried these on a cement floor or on a super-thick carpet. The thin carpet I use them on does not effect the sound much. I consider the floor issue as just a different variation of the same issue that a rear-facing sound hole has.
The other thing you mention, the effect of the player tilting backwards, I now consider to be a desirable feature. You can ‘tune’ these cajons by selecting the feet height that gives you the desired amount of bass. If you ‘tune’ it so they have just a little less bass when they are flat on the floor, you can add a little punch by angling the cajon backward as you play.
A big advantage of this design is the easy placement of the bass mic, it simply goes on the floor in front of the cajon, if the floor is wood I put a small piece of foam under it. Tilting the cajon has minimal effect on mic pickup and slight movements of the cajon have minimal effect on the mic. Whereas a cajon with a sound hole on the back has to have a mic stand of some sort and small movements in the cajon can effect or even hit the microphone.
With a mic on a backwards facing sound hole the player is mostly unaware of the effect of their movement on the mic placement. Dealing with rear sound hole issues has spawned a number of pricey, cajon specific mic’ing systems. With the downward facing sound hole you can always quickly see where the microphone is sitting. Any microphone will do and heavy bass microphones sit stable on a small piece of foam.
Perhaps the biggest issue I have with the downward facing sound hole is that it works too well, much of the sound is channeled forward. The cajon player doesn’t hear the full richness of the cajon. It’s easy to test this by simply leaning forward a little as you play, the tone and bass becomes more obvious. Sometimes I’ll sit directly in front of a low window or a solid door to reflect the sound back as I practice. This is a common issue with instruments that project their sound forward, trumpets, trombones etc. When evaluating the sound of any cajon it is best to also listen to someone else play the cajon.