As I traveled to places later in life, anytime I had the opportunity to hear this phenomenon, I either yelled aloud in canyons or honked my horn in tunnels. I just loved listening to the effect such acoustic reflections had on the way the sound changed. Sometimes I could hear exactly what I said and other times it was delightfully jumbled; that's why I did it because I never knew what the result would be.
Unfortunately, while entertaining in the outdoors, the single most destructive force in the clarity of a high-quality playback system occurs from what is called the first reflection, a form of an echo. When sound strikes a surface, its behavior is much like that of billiard balls on a pool table; the sound is reflected at the same angle it strikes the surface. The reflection in itself is harmless but what happens after the reflection is not.
As the reflected sound bounces back toward the source, constructive and destructive interference patterns rise literally altering the subsequent sound coming from the speaker. The closest object to a speaker capable of creating a reflection is the source of the first reflection. In speakers, this may be the screws that attach the driver to the baffle board, the edge of the grill, the baffle board recessed cutout for the driver, or the closest object to the speaker.
The speaker on the left of the above two pictures has smooth surfaces everywhere on the baffle board and gentle rounded edges on the corners to avoid any first reflection issues. The speaker on the right, on the other hand, has ignored the importance of the first reflection problem. Other examples of avoidable first reflections are non-recessed drivers, protruding mounting screws, overly recessed holes for mounting screws, and the like. The point is that there should be literally nothing nearby a speaker driver that could potentially cause a first reflection.
The desirable effect I observed in that old church where echoes bounced around the cieling is totally undesirable in a high quality playback system. If you have any way to eliminate built-in first reflections on your existing speakers, you should try to remove them.
If you have a removable grill, see how you system sounds without the grill off. If you have protruding screw heads, change them out for flat head screws. If you have over-recessed holes in speaker drivers, fill them flat with wood putty or spackling (the stuff used to fill nail holes in walls). Although you may be tempted to fill the gap between the speaker and the baffle board cutout with wood putty, do not do this; it creates more problems than it solves.
I have also used a trick to eliminate first reflections on the backs of my speakers where the speaker wires connect to the terminals and you can too. Cut out a piece of thick polyster batting (the stuff used to make quilts) to fit neatly behind the speaker and tape it to the back panel.
What you will hear as these first reflections are eliminated is the vacuum cleaner effect where things that appeared to be garbled are no longer so. Complex nuances in instruments come forward to your attention instead of hiding in the background. The size of the sound stage expands and the position of instruments in three-dimensional psycho-acoustic space stabilizes. And the front-to-back depth improves, sometimes in a startling fashion.
While echoes are appealing in cathedrals, caves, and canyons, first reflections should be well controlled in your playback system. Taking the time to investigate and eliminate them can be very rewarding.