Adrian
They tune off of the first reflective surface, Which in our case is the first baffle, There are many types of configurations, one hole 2 holes 4 holes, many tiny holes.
And Yes there are 2 nd , 3rd, and sometimes 4th order waves coming from the other baffles and the rear of the pipe
We have made and tried literally 100s of differant types of baffles and pipe configurations
Regards
Randy