The speed of sound depends on the medium that the waves pass through, and is a fundamental property of the material. The first significant effort towards the measure of the speed of sound was made by Newton.
When the Occupational Safety and Health Act passed in 1970, steel-toe boots became a standard in the American workplace, preventing thousands of on-the-job injuries. Today's technology has only increased the variety of work boots available, and it's important to understand the differences before purchasing a pair.
If the boots can withstand the 75 pound weight, they are labeled as I/75. The minimum impact rating for safety toe boots is I/50. The compression rating is the number of loads the shoes can withstand before cracking or breaking. For example, a C/75 will guard against compressive loads of up to 2,500 pounds.