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Semi-trucks have many distinctive characteristics, but one of the most identifying features is the very loud air horn. You’ll see a big rig truck cruising along the highway, and then a loud bellowing “honk honk” blares from its air horn. Why are semi-truck horns so loud compared to cars? Let’s take a closer look. 

Reasons why an air horn in a semi-truck is so loud

Blue semi-truck driving on a highway, highlighting why truck air horns are so load
Semi-truck | Zetong Li via Unsplash

The primary reason why a semi-truck horn is so loud is that it uses an air horn. Due to mechanical differences, an air horn can produce sound at a much higher decibel level than a horn in a car.

An air horn in a semi-truck works by the braking system sending compressed air to the horn. When the air reaches the horn, its diaphragm vibrates. The sound is then amplified when it passes from the horn throat to the horn mouth. To activate the air horn, the driver either pulls a cord or presses a switch, which is typically on the steering wheel. 

The end result is the air horn produces a loud, trumpet-like sound. Also, like a trumpet, one can calibrate an air horn to produce specific tones or notes, as detailed by Freight Viking.

Additionally, compared to horns in cars, air horns for semi-trucks don’t last very long. Manufacturers typically claim that they need to be replaced every one to three years. However, the lifespan of air horns also depends on how frequently truckers use them. 

What is the purpose of a semi-truck air horn?

Semi-truck driving behind a car, highlighting why truck air horns are so loud
Semi-truck | Josiah Farrow via Unsplash

The loudness of a semi-truck air horn isn’t merely for show, as often depicted in movies and TV programs. The main purpose of an air horn is to alert nearby vehicles. Also, the air horn is beneficial for alerting pedestrians. Additionally, truckers use an air horn to shoo away animals that run across the highway.

Air horn laws for big rigs

Another reason why a semi-truck horn is so loud is that it’s required by law. While air horn laws vary throughout the United States, they all basically state that air horns are for emergency situations and road safety. However, it’s illegal to use an air horn for other purposes, such as blaring at another driver that is slow to move forward when a traffic light turns green.

The importance of the legal restrictions on air horns is that because of their loudness, they can cause damage to people’s ears. Air horns typically produce sound at an 80-90 decibel range. And research has shown that exposure to sound at 90 decibels can cause deafness. 

As a result, laws were passed throughout the country, mostly from the 1950s to the late 1970s, restricting the use of air horns to only emergency and road safety situations, according to The Law Dictionary. Also, reducing noise pollution is another reason for the air horn laws.

So, now you know why semi-truck horns are so loud, along with the reasons for using them and the laws that govern their use. While it might be tempting to mime the “pulling the cord” motion to encourage a truck driver to blast an air horn, it’s best to avoid doing it. Using an air horn for anything other than emergency and road safety purposes is not only potentially dangerous, but it’s also illegal too. 


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