most walls are 8" by 8" logs
No one else seems to have caught this. Logs are curved, in other words your entire wall is essentially a diffuser panel. Sound hits the curves and is scattered. That’s why you hear no slap echo.
This is however a bit misleading. The sounds that are scattered are in a frequency range related to the curves of the logs. You can calculate this, or you can experiment and hear it. What you will find either way is the sound is really well dispersed at the high frequency range represented by the diameter of the logs. The typical log in other words that curves out one to three inches diffuses frequencies that are around 3kHz or about what you hear when you clap your hands. Because our hearing is very sensitive at those frequencies. Those logs however don’t do such a great job of scattering lower frequencies. Lower frequencies are longer wavelength and as such the logs "appear" flat to them. Since we aren’t nearly so sensitive to them though they don’t matter as much.
You do not need the stuff the others are talking about. They missed the whole point. Its better to scatter (diffuse) than absorb, as absorption can create a dead closed in sound while diffusion creates more of a sense of space and openness. This is of course a balance, and one only you can decide what is best. Experimentation is always recommended.
Just thought I would point this out as no one else seems to have caught on to the log thong being nearly as relevant as it is.