Shamanism:: Foundation for Shamanic Studies founded by Michael Harner

The Shaman is usually regarded as a healer as well as a mediator between the
normal - ordinary - world or reality, OR, and the spiritual world, the so
called
non-ordinary reality (NOR). He is an expert for communication with
the helping
spirits by changing his everyday consciousness to an altered
state and
"journeying" to NOR to retrieve power and information for his
clients and
tasks.

"Shamanism is a path of knowledge, not of faith, and that knowledge
cannot come from me or anyone else in this reality.
To acquire that knowledge,
including the knowledge of the reality of the spirits, it is necessary
to step through the shaman's doorway and acquire empirical evidence."
...

"The evidence was staring me in the face for a long time, but I didn't see it; that in perhaps 90 percent of the world's shamanic cultures they use a monotonous percussive sound to enter altered states of consciousness, rather than significant psychedelics".
Michael Harner

Using the empirical method, I agree with the idea that drumming works. Chanting, singing, whirling, dancing also works.
Maybe the critical ingredient is in the repetitive excercise