Overpressure and Noise Due to Airbag Deployment

An overview is presented of the overpressure and noise due to airbag deployment in a passenger car. Overpressure is the low frequency compression of the air in a closed compartment, and noise is the higher frequency soundof bag inflation. The overview is timely, because there is now an accumulation of medical evidence to indicate that the overpressure and noise resulting from airbag deployment can be a threat to hearing, while, at the same time, the growing use of multiple airbag systems increases the threat. This problem can be counteracted by using aspirating airbags that draw in air from the passenger compartment as they deploy. There are two types of aspirating bag: one using suction that requires a compartmentalized bag and the other using entrainment that does not require a compartmentalized bag. The relative merits of suction and entrainment are discussed in the paper. To date compartmentalized bags have not been used. However the bag that appears to be most effective is one with a compartmentalized structure, called the breathing bag originally developed at GM, which draws air from the passenger compartment into an inner cavity, as it deploys. Since less gas is used to inflate the outer structure of the bag, this can be considered to be a form of depowering. Results show that the breathing bag provides sufficient restraint for a vehicle occupant, while reducing overpressure and noise significantly. Also the compartmentalized structure of the breathing bag provides additional safety features. One of the principal purposes of the paper is to indicate the advantages of this bag. SAE standard J247 lists the requirements for accurate measurement of overpressure and noise due to airbag deployment. Instrumentation that satisfies these requirements is described in the paper.