Speaker Mounting and baffle/enclosures.
The sound performance of any attached speaker(s) is greatly affected by the mounting system and required baffle or rear enclosure.
The baffle is used to isolate to speaker diaphragm front sound waves from the out of phase rear sound waves.
This minimizes sound cancellation, particularly at lower frequencies.
For most efficient sound generation, the volume of the baffle should be as large as practically possible, and the baffle walls should be acoustically rigid so not to allow acoustic interference.
Practical baffle materials are plastic, cardboard and even sheet metal.
Common items such as cardboard tubes or 35mm film canisters may be modified and trimmed to create reasonable baffles.
Commercial baffles are also available in many sizes to match many types of speakers, please see you local model railroad dealer for baffle availability.
Most HO or O scale or similar locomotives have limited internal volume within the shell, so the choice of speaker mounting requires some ingenuity for good sound performance and volume.
An example of a commercial molded baffle in a ready-to-run locomotive is the Kato HO F40-PH, where the 28mm speaker clips into an enclosure at the upper rear of the locomotive shell.