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Modifying the 2012 Implala Fascia...for Maximum Siren Output
At Kerr Industries we feel siren speaker performance should not only be measured in anechoic chambers as most siren manufacturers do now, but it should also be measured based on in-vehicle installations because this represents real life use.
This is a more difficult task than what it might seem at first thought. There are many factors that affect the performance of a siren speaker beyond the speaker itself. These include the length of time the siren has been running, the air temperature and humidity, as well as the siren amplifier driving the speaker.
These reasons along with many others are why siren manufacturers test in an anechoic chamber in the first place. That it is the only way of removing variables from the test to ensure the results are accurate and repeatable every time.
Logically, a siren speaker will always perform best when it is mounted as far forward in a vehicle as possible and is free from obstructions blocking the speaker output. Practically speaking, this is hard to do on most if not all modern police sedans that are not fitted with a push bumper where the siren speaker can be placed outside the vehicle.
For that reason, many agencies are forced to mount their siren speakers behind the front grille of the vehicle where the sound output will be reduced from the manufacturer’s anechoic chamber tested output because of the mounting location in the vehicle.
At Kerr Industries, our minimum output standard of a class A siren system installed in a vehicle is 115dB in yelp or wail mode measured on the centerline of the vehicle from a distance of 3 meters from the forward vertical surface of the front bumper. Our test includes a siren warmup to stabilize the output of the siren, but it does not consider temperature or humidity because in real-life use, the weather cannot be controlled anyway.
Our test siren amp is a Whelen 295HFSA6. The siren amp plays a large role in speaker performance and the 295HFSA6 siren system has been in service for many years by lots of fleets large and small. Compliant to most known and used testing standards, it is a good middle-of-the-road choice for our purposes.
Our standard is based on the NIJ 500-00 siren guide but allows for a 5 percent drop off in maximum A weighted SPL (sound pressure level) output performance over SAE J1849 and CCR Title 13 requirements due to obstructions caused by the placement of the speaker in the vehicle.
This is a very technical way of saying we will accept a 5 percent drop in anechoic chamber performance to a minimum of 115dB if the speaker is mounted behind the vehicle grille and is driven by our test siren amp.
GM has improved front brake cooling in the 2012 Impala by adding ducts in the fascia that better direct the air to the front rotors and calipers of the car. In order to best control the airflow to the ducts, the fascia no longer has open slots in it as it did in the 2011 and prior model years.
In the 2011 model year, we mounted our siren speakers on the driver’s side of the car forward of the front wheel well just behind the slots in the fascia. For 2012, without the slots, a siren speaker mounted in the same location will not produce enough sound output to meet our standard.
We like the siren speaker location we used in the 2011 Impala because it makes for easy installation and servicing of the speaker so we wanted to retain the location for the 2012 Impala. After a few failed attempts at changing the angle of the speaker mounting itself, we realized the only way we would be able to use the 2011 Impala speaker location would be to develop a modification to the front fascia of the 2012 Impala to allow our siren speaker to meet our internal standards.
Our engineering team decided the fascia modification had to meet the following requirements: 1) 115dB minimum output using a Class A siren system compliant to SAEJ1849 and CA title 13; 2) no reduction to the brake duct air flow; 3) minimal impact to the vehicle value (when it would be taken out of active police service); and 4) not detract from the looks of the vehicle.
The engineers at Kerr started by testing the speaker output in a 2012 Impala without modification, meaning the speaker output was completely blocked and then again with the fascia duct completely removed. By doing so, the engineers were able to bookend the minimum and maximum dB outputs of the speaker installation and develop a tolerance we could work within.
Without a fascia duct, the speaker output was measured at 121.8 dB and with the fascia duct installed it was 110.7 dB. The engineers then agreed on the aesthetic direction the opening in the duct would take and began working backward by removing the minimum amount of material from the brake duct needed to gain the 115dB output required.
They would remove some material, test the output with the speaker in the car, and then remove more duct material until they hit the target. During this process the actual duct in the fascia trim was not altered, so air flow to the brakes was not affected.
Once we consistently reached our siren output target, we cleaned up the shape of the duct modification and manufactured tooling to allow us to reliably cut the required shape cleanly and accurately every time. Our tooling design allows us to modify the Impala fascia duct without ever having to remove the duct from the fascia. When the car is ready to come out of service, the duct can be replaced in the car in a matter of minutes.
The engineering team at Kerr Industries achieved all the required goals, and now every siren speaker Kerr installs in a 2012 Impala comes with the modification to the left front brake duct. With this small modification the siren speaker of a 2012 Impala police sedan can have every bit as much siren speaker sound output as the ones installed in the 2011 Impala’s, plus the 2012 Impala has better brake cooling.
Published in Police Fleet Manager, Mar/Apr 2012
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