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How Do Semi Truck Air Horns Work? (14 Cool Facts)

Air horns utilize pressurized air to create a loud sound.

It is the blasting sound we hear from semi trucks, fire engines, and trains. 

In this article, we will talk about how semi truck air horns work, how they can be installed, and whether or not they are regulated by law.

Here’s a short summary of how do Semi Truck Air Horns Work:

A semi truck air horn works when compressed air from the braking system is sent to the horn. The air causes the diaphragm in the horn to vibrate, and the sound is then amplified when is passes from the horn throat to the horn mouth. The horn is activated by a pull cord or steering wheel horn switch.

What are Semi Truck Air Horns and What are they Used for?

Air horns are called “air horns” because it needs pressurized air from the trumpet-like horns to produce sounds.

It is an essential component of a semi truck, and its primary purpose is to alert nearby vehicles of its presence.

The air horn in semi trucks is also used to alert pedestrians or shoo away wild animals on the highways during emergencies.

Its sound is so loud that people and animals could hear it even from afar.

According to the Federal Motor Carrier Safety Administration (FMCSA), large vehicles like trucks, buses, and tractors or any vehicle under driveaway-towaway operations must have air horns. Under the law, air horns should be used as a warning signal. 

What Components make up an Air Horn System in a Semi Truck?

The air horn system is not as simple as it may seem. It is composed of many components, and each one of them has corresponding tasks to perform.

It’s good to get familiar with its components so you’d know what to replace in case of wear and tear or troubleshoot should issues occur.

The air horns of a semi truck consist of three main components: the air horn, air tank, and air compressor.

Air horn

The horn is the reason for the air horn system kit. It gives out a high pitch but is not as loud as the train horn system. However, you can make it incredibly loud like train horns. As such, the sound is basically a matter of personal preferences.

Air tank

The air tank is responsible for the how long you can sound the air horn. After all, the sound comes from the air. 

The tank determines how much air you’ll have to work with. The more air stored in the tank, the longer you can sound the horn for.

Air compressor

The compressor is the muscle of the air horn system. It needs to be robust so it would have strong sound capabilities.

Moreover, the air horn system also has minor parts which are needed for its installation. These parts include the pressure switch, solenoid valve, safety blow-off, ad air gauge.

How do Semi Truck Air Horns Work?

Air horns work through the pressurized air located in a compression chamber.

The chamber is responsible for the vibration of the diaphragm. It also ensures that the horn will create sound instantly.

Air horns need to be activated by their switch or button. Once activated, the valve will open, and the air will come out from the compression chamber to the horn throat.

If you’re familiar with the brass or trumpet instrument, the air horn works this way. 

Like the bass or trumpet, air horns can be calibrated to produce a specific tone or note. 

In most countries, some notes are reserved for a particular vehicle. For instance, a semi truck horn should play an authentic semi truck chord.

How are Semi Truck Air Horns Activated?

Some semi trucks, especially 18-wheelers, are not equipped with a built-in compressor. To activate air horns in this truck, it relies on the brake system for the pressurized air. 

Meanwhile, most drivers may activate the air horn system through the steering wheel horn button or by pulling a ceiling-mounted cord.

Steering Wheel Switch Activation 

You can activate the air horn system through the steering wheel switch activation. This feature is available mainly on modern semi trucks.

To start, just push the activation switch just like what you do in a usual car horn.

Pull Cord Activation

For older models, activation may be done by pulling the activation cable.

To do this, the drive should pull the mounted cord in the truck. In this way, the valve will open from the airline leading to the horns, thereby producing sound.

Do Semi Truck Air Horns Work with Air Tanks and Air Compressors?

The air horns, air tanks, and air compressors are significant components of the air horn system. These parts work hand in hand to ensure that a truck is heard while on the road.

Air horns will not produce sound without the air tank and compressors. The air tank determines the capabilities of your air horn while the compressor is in charge of the loudness of the horn.

If you want a continuous or long blasting sound, the tank should be bigger than usual.

Meanwhile, if you want a louder horn, the size of the compressor should be appropriate for the size of your air tank. 

These two components should be in harmony with one another to produce better air horn sounds.

Where do Semi Trucks get the Air from to Operate?

A semi truck gets air from the compression or compression chamber. The chamber is a standard component of a semi truck. However, this part is not primarily available in heavy-duty trucks like 18 wheelers.

The diaphragm, which is mounted in the compression, needs pressurized air to vibrate. The vibration it makes will cause the air horn to produce sound. 

The compression chamber needs to be pressurized so the horn will work instantly.

However, 18 wheelers who don’t have a built-in compression, need the brake system to get air.

Why are Semi Truck Horns so Loud?

Besides the air tank and compressor that makes it loud, the law requires the air horn to be loud. 

It needs to be loud as its purpose is to signal pedestrians and vehicles of the truck’s presence. Horn is also used for emergency purposes, like what trains and fire trucks do.

Air horn laws in the United States are not the same. Some are actually vague. However, if we would construe the letters of these laws to determine why air horns should be loud, it goes this way:

First, air horns are meant for road safety. The driver of any vehicle should sound off their horns to avoid road accidents. 

Second, air horns should only be used when necessary. Its main purpose is to prevent the occurrence of accidents. 

As such, when you sound the horn at another driver because he’s not paying attention to the green light, you’re violating the law, regardless it’s a semi truck horn or not.

How to make an Air Horn Louder?

There are two requirements to have a louder air horn: the air tank and compression.

The amount of air stored in the tank will determine how long the sound will be. For example, once you fill 1-gallon of air tank to 150 PSI, the air horn system will produce 4-5 seconds of blasting sounds.

The compression, however, is responsible for building pressure on the air tank. It can produce air between 110-200 PSI. The tank will hold the pressure until the horn becomes triggered and create sound.

These two features can create a blasting sound for about 148-153 dB.

How Loud can a Semi Truck Air Horn be?

Semi truck air horns have 110-150 dB. Take note that a sound beyond 85 dB will hurt or damage your ear.

Thus, air horns are so loud that they may harm your sense of hearing.

Air horns are not a joke. It should not be used when you’re losing your temper on the road, for harmless pranks, or scaring someone.

Air horns can cause serious and permanent damage to one’s ear, even to someone who is using it.

How Long do Air Horns Last?

Most truck manufacturers claim that the air horns can last up to 1 to 3 years. 

But in reality, the lifespan of the air horn system depends on how frequently you use it.

If you want a tough air horn that can last for years, you go for a single-trumpet air horn.

What can Cause your Air Horn Not to Work Properly?

The usual wear and tear is the most common reason why the air is not working correctly.

If this is not the case, you have to check how the air horn is mounted on your truck. If it is exposed to road chemicals and rain, the horn may be damaged.

However, inoperative air horns are caused by a lousy air horn switch in the steering wheel, broken wire, damaged horn relay, or corroded ground.

Can you Install an Aftermarket Airhorn?

No law prohibits the installation of an aftermarket air horn in a semi truck. As such, you can have one in your vehicle whenever you want.

Most drivers install aftermarket air horns when they’re not satisfied with the sound of their built-in air horn. The air horn is either busted or produces a weaker sound.

So basically, an aftermarket air horn works as a replacement.

If you are planning to install an aftermarket air horn on your own, check out this video for a more detailed guide.

What do the Regulations say about Semi Truck Air Horns?

The US federal state laws are diverse but they have a common theme concerning air horn laws.

It’s as if they have copied the laws of one another as they all pertain to one thing.

Under state laws, air horns shall be used to avoid accidents, and drivers or operators must ensure that they are not unreasonably sound.

After all, the purpose of air horns is to make the road a lot safer than dangerous.

Once an officer catches you using air horns unreasonably, you might get a citation or ticket. It’s very unlikely that you’ll be issued a ticket, though. 

Just make sure you’re not using air horns to bother anyone.

Do Semi Truck Inspections Cover Inspection of the Air Horn?

Technically, semi trucks inspectors will less likely inspect whether or not you have a functioning air horn. 

While having an air horn in your vehicle is mandated by law, there’s a slim chance that you’ll bump into traffic enforcers or law enforcement officers only to check your air horn.

But we don’t suggest that you have to risk it and drive without an air horn. 

You don’t know when you might need it. Hence, the need to install a working air horn at all times.

However, some states conduct periodic inspections on semi trucks. They check every component, including air horns. Make sure that you have your air horn system installed during this time.