By Chris Daly
Overestimating the effective range of a siren is a common cause of fire apparatus crashes.
Studies have shown that the effective range of a siren at a 90-degree intersection is often less than 80 feet. This effective range may be less, depending on the design of the intersection and the soundproofing properties of an approaching vehicle.
While siren limitations are a common cause of emergency vehicle intersection crashes, few emergency vehicle operators course (EVOC) programs address the topic. The goal of this article is to provide training ideas that will help demonstrate the limited effective range of a siren.
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| 1 A class 2 sound level meter. (Photos by author.) |
Overview
A vehicle driving on a road will have a substantial amount of noise inside the passenger compartment of the vehicle. This noise is known as “ambient noise.” Ambient noise will depend on several factors, including the noise from the engine, the radio, the HVAC system, and the friction of the tires rolling on the road surface. The ambient noise inside a passenger vehicle traveling 45 miles per hour (mph) usually averages around 65 decibels (dB).
For a siren to be effectively heard by a civilian driver, it must penetrate the body of the vehicle and become louder than the ambient noise. Studies have shown that the siren level must rise approximately 10 dB above the ambient noise to effectively break the driver’s concentration. If the ambient noise inside the civilian vehicle is 65 dB, the siren must rise to 75 dB.
The structure of a modern vehicle is designed to keep sound OUT. On average, a modern vehicle will block approximately 30-40 decibels of noise from penetrating the passenger compartment of the vehicle. This is known as “insertion loss.” If a civilian driver requires 75 decibels of siren noise to react, the siren must arrive outside the driver’s window at approximately 110 decibels, assuming an average insertion loss of 35 dB.
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| 2 A sound level meter calibrator. |
The Problem
Most sirens are rated at around 124 dB when measured 10 feet in front of the siren. As the distance from the siren doubles, the sound pressure of the siren will drop by approximately 6 dB. This concept is known as the “inverse square law.”
It is important to understand that this 6-dB drop in sound pressure level assumes that the distance measured is directly in front of the siren. When sound pressure measurements are taken at a 90-degree angle from the siren, the 6-dB drop can be more significant. Studies have shown that the reduction in sound pressure level at a 90-degree intersection could be as high as 11 dB. This is an important teaching point, as intersection crashes occur when the fire apparatus and civilian vehicle are approaching each other at a 90-degree angle.
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| 3 The dBA/dBC setting on a sound level meter. |
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Posted: Apr 4, 2017
By Chris Daly
Overestimating the effective range of a siren is a common cause of fire apparatus crashes.
Studies have shown that the effective range of a siren at a 90-degree intersection is often less than 80 feet. This effective range may be less, depending on the design of the intersection and the soundproofing properties of an approaching vehicle.
While siren limitations are a common cause of emergency vehicle intersection crashes, few emergency vehicle operators course (EVOC) programs address the topic. The goal of this article is to provide training ideas that will help demonstrate the limited effective range of a siren.
|
| 1 A class 2 sound level meter. (Photos by author.) |
Overview
A vehicle driving on a road will have a substantial amount of noise inside the passenger compartment of the vehicle. This noise is known as “ambient noise.” Ambient noise will depend on several factors, including the noise from the engine, the radio, the HVAC system, and the friction of the tires rolling on the road surface. The ambient noise inside a passenger vehicle traveling 45 miles per hour (mph) usually averages around 65 decibels (dB).
For a siren to be effectively heard by a civilian driver, it must penetrate the body of the vehicle and become louder than the ambient noise. Studies have shown that the siren level must rise approximately 10 dB above the ambient noise to effectively break the driver’s concentration. If the ambient noise inside the civilian vehicle is 65 dB, the siren must rise to 75 dB.
The structure of a modern vehicle is designed to keep sound OUT. On average, a modern vehicle will block approximately 30-40 decibels of noise from penetrating the passenger compartment of the vehicle. This is known as “insertion loss.” If a civilian driver requires 75 decibels of siren noise to react, the siren must arrive outside the driver’s window at approximately 110 decibels, assuming an average insertion loss of 35 dB.
|
| 2 A sound level meter calibrator. |
The Problem
Most sirens are rated at around 124 dB when measured 10 feet in front of the siren. As the distance from the siren doubles, the sound pressure of the siren will drop by approximately 6 dB. This concept is known as the “inverse square law.”
It is important to understand that this 6-dB drop in sound pressure level assumes that the distance measured is directly in front of the siren. When sound pressure measurements are taken at a 90-degree angle from the siren, the 6-dB drop can be more significant. Studies have shown that the reduction in sound pressure level at a 90-degree intersection could be as high as 11 dB. This is an important teaching point, as intersection crashes occur when the fire apparatus and civilian vehicle are approaching each other at a 90-degree angle.
|
| 3 The dBA/dBC setting on a sound level meter. |
Read more
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- Article rating: No rating