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.)|
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.|
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.|
It is also important to note that there may be times when the volume of the siren does not drop by 6 dB each time the distance from the siren is doubled. This is usually seen in urban environments where the siren is reflected off of buildings, asphalt, and other manmade surfaces. In these instances, the siren may not lose as much volume, but the reflectivity of the siren makes it difficult to determine where it is coming from. There will also be times where the siren will drop more than 6 dB each time the distance from the siren is doubled because of the directivity of the siren speaker and obstacles in the sound path.
Ambient noise, insertion loss, and the inverse square law explain the limited effective range of a siren. The physics of sound will reduce the volume of the siren as the distance from the siren increases. With today’s modern vehicles, the effective range of the siren at a 90-degree intersection is often no more than 80 feet.
Assume that a civilian driver requires 110 dB of siren noise outside the driver’s window to effectively hear a siren. Using the inverse square law, the sound pressure level of the siren will drop below 110 dB at approximately 80 feet.
|4 The high/low range setting on a sound level meter.|
If a civilian vehicle is traveling 45 mph, it will take the driver approximately 195 feet to perceive, react, and skid to a stop on a dry road once the driver hears a siren. If the driver hears the siren 80 feet away from an intersection and it takes 200 feet to come to a stop, the driver will have no time to yield the right of way should the fire apparatus pull out into the intersection. This concept explains the need for complete stops at negative right-of-way intersections.
Many students will ask, “Why not make the siren louder?” The reason sirens can’t be made louder is that a louder siren could lead to hearing problems for firefighters and disturb the surrounding public to an unacceptable level. For these reasons, the volume of an emergency vehicle siren must be limited.
It is important for EVOC instructors to provide hands-on training to reinforce the concept of siren limitations. This training can be accomplished using a sound level meter, a tripod, and an emergency vehicle siren.
- Sound Level Meter: A sound level meter can be purchased inexpensively from a safety supply store. There are different types of sound level meters, and each type ranges in price. A Class 1 meter is more accurate, but a Class 2 meter will suffice as a training tool (photo 1).
- Sound Level Meter Calibrator: It is important to calibrate the sound level meter before each training session using a sound level calibrator. While a calibrator can be expensive, it will ensure that the sound level measurements are accurate (photo 2).
- Tripod: Most sound level meters can be affixed to a camera tripod. A tripod will allow a student to set the sound level meter at a fixed point and retreat to a safe place while the siren is being measured. Do not stand in the sound field of the siren to take the measurements. Protect your hearing.
- Safety: Remember to make sure that each student is wearing proper hearing protection. Also, remember to make sure that everyone is standing BEHIND the siren speaker when the siren is being tested. Students should walk to the rear step of the apparatus to help shield them from excessive sound. Don’t stand in front of the siren!
|5 The “Max Hold” setting on a sound level meter.|
Sound Level Meter Settings
- dBA vs. dBC: Sound level meters often have two decibel settings: dBA and dBC. The dBA or dBC setting will determine how the meter filters sound frequencies. When conducting training exercises, use the dBA setting as this more accurately simulates how the human ear will hear the sound (photo 3).
- High/low: Another common setting on a sound level meter is the “high/low” setting. The “high/low” setting is based on the volume of the measured sound. When the meter is closer to the siren, use the “high” setting. As the meter is moved farther from the siren, switch the meter to the “low” setting. Refer to the manufacturer’s instructions to determine the appropriate high/low range for your meter (photo 4).
- “Max” setting: Sirens “sweep” up and down, which will cause the decibel reading on the display screen to constantly change. The “max” setting will hold the loudest siren reading during the test. This makes it easier to record data. In real life, the sound pressure level of a siren will only hit this max reading from time to time, based on how the siren sweeps up and down. Make sure the students understand that the “max” reading is the best case scenario for the fire apparatus operator (photo 5).
- Fast/slow: This setting will determine how rapidly the sound level meter samples the sound. Because of the rapid rise and fall of the siren, the meter should be in “Fast” mode when sampling a siren (photo 6).
|6 The fast/slow response setting on a sound level meter.|
To conduct the siren testing, find an open area approximately 300 feet long. I commonly use parking lots or little-used roads that can be easily shut down. Position the apparatus perpendicular to the roadway and measure 10 feet directly in front of the siren. This will be the “0” mark. From the “0” mark, students can examine several scenarios.
- 0-degree approach: A 0-degree approach measures the sound directly in front of the siren. This will simulate a civilian vehicle that is driving in front of a fire apparatus along the same roadway. To measure the 0-degree approach, take measurements directly in front of the siren. Measurements should be taken every 10 feet to demonstrate how the sound pressure level drops as the distance from the siren increases (photo 7).
- 90-degree approach: A 90-degree approach measures the sound along a simulated cross street. This scenario demonstrates the volume of the siren for a vehicle approaching the fire apparatus at an intersection. To measure the 90-degree approach, take measurements to the left or right of the siren. Measurements should be taken every 10 feet to demonstrate how the sound pressure level drops as the distance from the siren increases (photo 8).
- Siren sound field: Measuring the sound field will provide a picture of how the siren projects from the fire apparatus. Measurements are taken in a grid at 10-foot intervals. This allows students to plot the sound field and create a visual picture of the siren’s projection.
- Actual intersections: If the fire department can safely control traffic, sound pressure readings can be taken at an actual intersection. This allows students to see how the siren is blocked or reflected by buildings, trees, and other objects in the siren’s path.
|7 Measuring the sound pressure level of the siren on the 0-degree approach. Measurements are taken every 10 feet out to 300 feet. This scenario exemplifies the fire apparatus approaching a civilian vehicle from the rear.|
Once the sound measurement locations have been marked, attach the sound level meter to the tripod. The tripod should be set to around 3.5 feet, as this is a common height for a driver’s ear.
After ensuring that each participant is wearing hearing protection and standing behind the siren, have a member activate the siren. A single up and down cycle will suffice. When the siren winds down, students should examine the decibel reading on the sound level meter and record the reading. Repeat this process at each measurement mark.
Most fire apparatus are equipped with a mechanical siren, an electronic siren, and air horns. A common question students ask is, “Which siren is better?” Test each siren system individually and then test them all at the same time. More than likely, each of the siren systems will be similar.
Remember that most sirens will need to arrive at the driver’s side window of the civilian vehicle at around 110 dBA to effectively penetrate the vehicle and warn the driver. At what point does the siren system fall below 110 dBA? How far away from the fire apparatus does this occur? Would a civilian vehicle be able to perceive, react, and skid to a stop from that point?
Tying It Together
Having demonstrated the limited effective range of a siren, it is important to correlate this newfound information with apparatus safety. If the effective range of a siren is 80 feet, how does this relate to the stopping distance of a civilian vehicle? Will a civilian be able to stop in time if the driver doesn’t hear the siren until he is 80 feet away from the fire apparatus?
Having determined where the siren sound pressure level drops to below 110 dBA, mark the spot with traffic cones. Next, have someone drive toward the cones at approximately 45 mph. When he reaches the traffic cones, have the driver stand on the brakes and skid to a stop.
Where did the vehicle stop skidding? Was it past the fire apparatus? What would have happened if the fire apparatus had pulled out in front of the car or not come to a complete stop at the red light or stop sign? Providing this type of visual reference to emergency vehicle drivers is priceless.
|8 Measuring the sound pressure level of the siren on the 90-degree approach. Measurements are taken every 10 feet out to 300 feet. This scenario exemplifies a 90-degree intersection.|
Although the limited effective range of a siren is a common cause of fire apparatus intersection crashes, few EVOC programs address the topic. It is important that this issue be included in all EVOC programs as both a classroom discussion and hands-on demonstration. Providing fire apparatus operators with a real-life demonstration of effective siren range is a valuable facet of any driver training program.
CHRIS DALY is a 19-year police veteran, currently serving as a patrol supervisor in West Chester, Pennsylvania. He is an accredited crash reconstructionist and a lead investigator for the Chester County (PA) Serious Crash Assistance Team. In addition to his police duties, he has served 26 years as both a career and volunteer firefighter, holding numerous positions including assistant chief. He is a member of the editorial advisory board for Fire Apparatus & Emergency Equipment. Daly has also developed an emergency vehicle driver training program called “Drive to Survive,” which has been presented to more than 15,000 firefighters and police officers at more than 380 emergency service agencies across the United States.