Tilt-back legs, as seen on this Fender Super-Sonic 212 rig, provide the dual benefits of letting a guitarist hear himself better and greatly reducing the “bass coupling” effect that often results in unwanted boominess.
If you didn’t already know about this, Tech Talk will let you in on a simple and handy tip for reducing unwanted stage “boom” from your guitar amp or bass amp—raise it off the floor.
When a sound source like your amp is directly adjacent to a reflective surface—like a floor—the low end gets boosted by about three decibels. This kind of interference is often called “bass coupling,” “stage coupling” or “floor coupling,” and it describes an acoustic phenomenon that is in fact called “mechanical coupling.”
In mechanical coupling, the stage or floor itself is excited by the vibrations produced by a sound source sitting directly on it; for example, a speaker enclosure or the output from the speakers themselves in such close proximity. The stage/floor itself then becomes a resonating surface kind of like a giant speaker cone or drum head. The stage/floor has its own resonance—its own characteristics for signal reinforcement and cancellation—and its own acoustic output is often most unpredictable. Low-end frequencies are especially affected by mechanical coupling, however, and a result that can often be counted on in such instances is an overly booming stage sound.
To get uncolored bass response from the amp and eliminate or reduce the effects of mechanical coupling, the best solution is to isolate the amp from the stage or floor. The simplest way to do it is to raise the amp off said surface, or at least reduce the amount of actual contact.
Elevating your amp helps reduce the floor reflections that create unwanted low-end boost. Even tilting the amp back greatly reduces this coupling effect. Fender was wise to this phenomenon pretty much from the very beginning; it’s part of the reason why tilt-back legs were introduced on many of its amplifiers.
Today, many kinds of amp stands and isolation platforms are available that are designed specifically to deal effectively with the coupling phenomenon and the low-end boost it often creates.