The sound performance of any speaker is greatly affected by where and how the speaker is mounted and whether an adequate speaker enclosure or baffle is installed.
The baffle is used to isolate to speaker diaphragm's front side 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. The baffle walls should be acoustically rigid to prevent acoustic interference.
Practical materials to use for the baffle are plastic, cardboard and sheetmetal. 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 N, HO and O scale locomotives have limited internal space inside the shell, so the choice of speaker placement and mounting method requires some ingenuity for good sound performance and generating an adequate volume level. 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.
If your sound decoder does not seem loud enough or the sound quality is not as good as you would like, we highly recommend that you condsider installing a speaker enclosure or baffle to improve the performance.