The “C” network and the “slow” meter response of the sound level meter shall be used. Sounds of short duration, as from forge hammers, punch presses, and metal shears, which cannot be measured accurately with the sound level meter, shall be measured with the impact noise analyzer as manufactured by the General Radio Company, or its equivalent, in order to determine the peak value of the impact. For sounds so measured, the sound pressure levels set forth in Section 42-413 (Maximum permitted decibel levels) may be increased by six decibels.