By Michael G. Brown
Twin air horns on fire trucks and other emergency response vehicles are one of the best ways we have to warn people of large, oncoming, fast-moving trucks. They are simple, are inexpensive to purchase, and deliver sound in excess of 125 decibels. But, like every other piece of equipment, a certain amount of maintenance is required to keep them working at their optimum level.
A perfect pair (or even trio) of air horns is designed to create a melodious, synchronous wave of ear-splitting urgency. This phenomenon is called acoustic harmony and can be detected by electronic instrumentation and (with multilevel hearing protection at about 100 feet) can be heard by the trained human ear. Unfortunately, most air horn sets are way out of acoustic harmony because of mechanical wear on the pneumatic diaphragm, mileage, vibration, and road/weather wear.
Fortunately, the trumpet (mouth) of the horns can be twisted to adjust their contact pressure with the diaphragm altering the pitch of the horns. If the diaphragm is in good condition, the air pressure is set to factory specifications, and the trumpets are properly “tuned,” the horns will make a wave-like, melodious sound that at normal sound levels is pleasant to the ear. At 125 decibels, they can create an autonomic human response and be understood in the brain instinctually, causing nice people to get the heck out of the way in a hurry.
Note: The air warning horns on speeding trains have long been recognized in Canada as dangerous and deserving of the ominous, threatening quality of a minor chord. In the United States, speeding trains are nostalgic and reminiscent of the “good old days” and therefore are given lighthearted, happy, major chords for their warning horns. (Author unknown)
MICHAEL G. BROWN is a program manager for Applied Marine Technology Inc. in the Homeland Security Division. He retired in May 2003 as a battalion chief in the Virginia Beach (VA) Fire Department after more than 27 years in the fire service. He is a task force leader with FEMA’s Urban Search and Rescue (USAR) program on VA TF-2 and a USAR specialist on the IST-Blue Team. He teaches the technical rescue disciplines, particularly Structure Collapse Technician and Rope Rescue. Brown is the author of the textbook Engineering Practical Rope Rescue Systems (Delmar Publishing).
