Building on knowledge gained in our quiet pavement work, Caltrans developed a rumble strip profile that lowers outside-vehicle noise levels. This new low-noise noise rumble strip is fondly referred to as a "mumble strip".
The design goal of the sinusoidal profile, or "mumble" strip, is to move some of the higher frequency excitation produced in a conventional rumble strip, with its abrupt changes in profile, to lower frequencies that can be associated with the repetition rate, or frequency, of a sine wave shape. The concept is to decrease the overall A-weighted exterior sound level while still maintaining (or increasing) noise and vibration disturbance inside the vehicle to alert the operator and occupants of lane departures.
Caltrans' typical ground-in rumble strip profile is a 5 inch cylindrical grind on 12 inch center with a 5/16 inch depth. The mumble strip profile is a sinusoidal shape with peaks on 14 inch centers and 5/16 inch depth.
For a single passenger car travelling at 60 mph, the typical reduction in passby noise is more than 6 dBA. To put this into context, a 6 dBA reduction is equivalent to doubling the distance between the noise source and the receptor.
In general, the interior disturbance levels created by the mumble strips were comparable to those generated by the conventional ground rumble strips. For light vehicles, the differences in interior noise, and seat track and steering column vibration were about 13 dB during travel on either the mumble strips or ground rumble strips compared with travel off the warning strips.
The report Design and Acoustic Evaluation of Optimal Sinusoidal Mumble Strips versus Conventional Ground-In Rumble Strips (PDF - 10.18 MB) presents the development and field evaluation of the new mumble strip design.