One of the best ways to liven up a layout is to create scale-appropriate sound effects.  These can not only include the obvious sounds of locomotives and rolling stock whistles, horns and braking, but also ambient sounds – birds, water,, city traffic, and workshop sounds

Scale sound is a phrase referring to the differences in the sounds you should hear when associated with different scales.  For example, the sounds associated with an N scale layout should be significantly softer than those used for an O scale layout.  If your sound is too loud for N scale then the sound is “out of scale”, the same as an O scale building would be out of scale on an N scale layout.

The difference in scale sound is mostly a matter of volume; however, it is important to remember that when the volume is low, the human ear picks up only midrange sounds.  The low frequency bass and the higher frequency sounds may be lost.  There are certain ways to enhance these frequencies even when the volume is low.  For example the “bass boost” controls on many small stereo systems are just for that purpose. 

Many, if not most, locomotive manufacturers produce locomotives with sound units already included.  They can produce exhaust sounds, chuff, whistles, horns, etc.  This is the easiest way to get sound effects on your layout – just purchase a locomotive that already has a sound unit on board.