Impulsive Noise Control Strategies

A noise that occurs at intervals of greater than one per second; for example, the noise made by a metal shear, jackhammer, or a gunshot. A gunshot is 3-5 milliseconds in length.

The terrorist attacks of 9/11 led to the complex security contractors adding heavy weapons to their inventory. At the Savannah River Site (SRS), three of the weapons that WSI added were the following: Dillon M134D Barrett.50 caliber rifle FN M240 and Mk48 belt fed machine guns

Currently 10 CFR 851 states that contractors shall use the more protective standard between the American Conference of Governmental Industrial Hygienists (ACGHI) Threshold Limit Values (TLV) and OSHA 29 CFR 1910.

At the time of the upgrades the overwhelming opinion among DOE complex security contractors was that MIL STD 1474C regulated exposures to noise generated by these weapons. It was believed that MIL STD 1474C allowed for unlimited exposures with the use of double hearing protection.

Centerra questioned the intent of 10 CFR 851 in regards to which exposure standard should be used for impulsive noise from weapons fire. DOE HQ responded that MIL STD 1474C could be used.

In the note below Table G16 OSHA states that “exposures to impulsive or impact noise should not exceed 140 dB peak sound pressure level”. OSHA believes that a single one second exposure over 140 dB is the equivalent of exceeding the PEL. There are several interpretation and compliance letters on OSHA’s website. NIOSH has conducted extensive research on impulsive noise and has a wealth of information available.

Both standards set 140 dB as the impulsive limit. OSHA does not make allowances for sounds that exceed this limit. The ACGIH TLV (MIL STD 1474C) does allow exposures above 140dB as long as the three conditions are met. It is beyond state-of-the-art It is cost prohibitive Or that system effectiveness will be seriously degraded

OSHA's experience and the published scientific literature have shown that laboratory-obtained real ear attenuation for HPDs can seldom be achieved in the workplace. To adjust for workplace conditions, OSHA strongly recommends applying a 50% correction factor when estimating field attenuation. This is especially important when considering whether engineering controls are to be implemented. The equations are as follows: Single Protection: Estimated Exposure (dBA) = TWA (dBC) - [NRR x 50%], or Estimated Exposure (dBA) = TWA (dBA) - [(NRR - 7) x 50%] Dual Protection: Estimated Exposure (dBA) = TWA (dBC) - [(NRR h x 50%) + 5], or Estimated Exposure (dBA) = TWA (dBA) - {[(NRR h - 7) x 50%] + 5}

For impulsive noises that are measured on the “C” scale: Foam earplugs NRR 33 = [33 X 50%] = 16.5 Foam earplugs plus muffs = [(33 X 50%) + 5]=21.5

Most sound level meters and noise dosimeters are only capable of measurements to 130 dBA or 140 dBC. High dB sound level meters with ¼” microphones are capable of measurements up to 190 dB. Example, Larson Davis 831 with ¼” microphone.

The acoustic energy in a single report from a high- powered rifle or shotgun ( dB) is equivalent to almost 40 hours of continuous exposure at 90 dBA. In other words, 1 bullet equals 1 week of hazardous occupational noise exposure. An avid target shooter can be exposed to an entire year's worth of hazardous occupational noise in just a few minutes

At about 120 dB discomfort begins. At 140 dB pain occurs and possible damage. At 160 dB and above possible rupture of eardrums. –At another site within the DOE complex there was a reported burst eardrum by protective force member who fired a Barrett.50 caliber rifle.

Centerra has determined that most impulsive noises generated by weapons systems in the training environment can be reduced below a protected level of 140 dB by employing economical engineering controls. The following slides summarize WSI-SRS’s efforts.

Engineering controls that are used for impulsive noise: Barriers – Utilizing materials with good noise attenuation to construct walls or enclosures Sound absorbing materials Suppressors for the Barrett.50 caliber

An engineered noise barrier reduces exposures during training events from ±170 dB to ±157 dB. An added benefit is a reduction in airborne lead levels at the shooter/instructors position as evidenced by the smoke plume separation from personnel in the illustration.

Centerra explored many designs and variations of an engineered noise barrier with the pictured structure being the best performer. Measured impulsive noises were reduced from a high of ± 180 dB to ± 164 dB by employing this noise barrier.

Suppressors were acquired to reduce impulsive noise during training with the Barrett.50 below dB. Measured impulsive sound levels with the suppressor are ± 156 dB. An added benefit is the elimination of the blast overpressure felt by students/instructors.

Centerra historically trained using the Combined Tactical Systems (CTS) 7290 full charge diversionary devices. To reduce impulsive noise exposures to acceptable levels WSI now uses the CTS 7290R device. The CTS 7290R reduced impulsive noise measurements by ± 26 dB. Students and Instructors both report that the reduced charge device has sufficient effect to be used as a training aid without causing discomfort upon detonation.

Sound in Decibels (+/- 3 dB) No Controls Noise Barrier Suppressor Full Charge CTS 7290T CTS 7290R Weapons Diversionary Devices dB 140 dB dB = dB Protected Limit

Steve Stamper, Industrial Hygienist Centerra – Savannah River Site