Designers may, however, be faced with standards that set limits on noise. If a plant does federal contract work under the Walsh-Healy Act, and if it has an operation that produces substantial noise levels of 90 dB or more, the noise standards for federal contractors that took effect on May 20, 1969, apply. If these regulations are not met, the government can cancel the contract, buy the goods elsewhere, and charge for the difference. Fast changes in velocity and pressure and the collapse of bubbles are the principal sources of hydraulic noise. Pumps and motors are the most common contributors to these factors in hydraulic equipment. The internal loading producing sound also produces pump vibration. These loading noises deflect the pump structure, so that component masses are displaced. The noise caused by air or gas entering the pump is an abnormal sound.