(49 CFR Part 229). The U.S. Department of Defense (DOD) specifies that engineering controls are preferential when reducing the noise levels at the source. The use of hearing protective devices is considered an "interim protective measure" while engineering controls are developed. The goal of these controls is to reduce ambient steady-state noise levels to 85 dBA regardless of TWA exposure and to reduce impulse noise levels to below 140 dBP. The DOD requires that personnel be entered into a hearing conservation program when continuous and intermittent noise levels ale grs greater than or equal to 85 dBA TWA, when impulse SPL are at or in excess of 140 dBP, or when the personnel is exposed to ultrasonic frequencies. The DOD integrates all sound levels between 80 dBA to a minimum of 130 dBA when determining an individual or representative noise dose. When used, hearing protectors must be capable of attenuating worker noise exposure below 85 dBA TWA. Hearing protection is required to be carried by personnel who work in designated noise areas, such as those exposed to gunfire or ordnance tests and Service musicians. The DOD defines a significant threshold shift as a 10 dB average decrease in hearing thresholds at 2000, 3000, and 4000 Hz in either ear, with no age corrections. It is further specified that a shift in 15 dB at 1000, 2000, 3000, or 4000 Hz is an early warning sign for an STS; follow-up retraining is required in this case. (DOD Instruction 6055.12). The European Union (EU) requires a hearing conservation program be implemented when the worker exposure levels exceed 80 dBA TWA. Note that this is more strict than hearing conservation regulations in the United States. The EU specifies several different exposure action values: a "lower" value of 80 dBA at which the employer must make hearing protection devices available to the employee; an "upper" value of 85 dBA at which the employee is required to wear hearing protection; and an "exposure limit" value of 87 dBA, under which the individual's noise exposure shall be limited to preserve hearing. The directive also defines a weekly noise exposure level which is applied to individuals working in circumstances of inconstant noise exposure. Finally, the EU also recommends a variety of noise reduction methods, including administrative controls to reduce worker exposure duration, the provision of quieter equipment, and adequate maintenance of machinery and other noise sources (European