And if an ATM also USES the keypads for entering other Numbers, such as the amount of money to withdraw, it contributes additional noise, says Meiklejohn.

Meanwhile, a group of experimental data of additional noise pressure level and fluid pressure loss generated by flow in a duct is presented to explain the readability of us...

Then there's increasing the number of rotor blades. This helps because each additional blade carries less of the weight of the helicopter, and thus produces less noise.