Humans have been studying the impact of sound on our bodies and minds for thousands of years. Many Eastern and Indigenous traditions include the use of chanting, drumming, or tonal instruments to balance the body and the mind. I find sound baths, while lacking concrete scientific evidence, to be a fantastic way to reduce stress and ease tension.

You may have personally experienced sound as a physical sensation in the body and/or an excitation of structures around you. I recently spent the better part of a day wandering around the Tate Modern Art Museum and was drawn to a sonic installation entitled Begin Again by Abbas Zahedi. A collection of peculiar horns, instruments, and playback devices had been tied into the museum’s HVAC systems and building architecture to create a constantly shifting sound composition. Moments of harmony disintegrated into dissonance, which reconstituted into a pleasing chord. As I sat and listened, I was inspired to meditate on how the shifting sonic landscape impacted the way that I felt.

I also find this to be an interesting meditation during acoustic testing campaigns. The intensity and frequency content of an acoustic qualification test is obviously both objectively and subjectively different from a sound bath or an art installation. The overall sound pressure levels generated require technicians to wear hearing protection, and the low-frequency content of most test specifications makes the whole body an important receptor for sound waves. It has been my experience that different test specifications create different felt sensations in my body, exciting different physical components. This experience changes from person to person, but the felt impact of sound on the body is universal during acoustic testing.

Historically, the rise of guns, engines, and industrial machinery necessitated the study of how exposure to consistent or impulse noises impacts human well-being. Through this study, it is now well understood that exposure to elevated noise can have both auditory and non-auditory effects. Elevated noise levels can cause pain or hearing damage (without proper hearing protection!) as well as temporary feelings of nausea, dizziness, sensations of vibration, or chest discomfort.

U.S. publications regarding personnel noise exposure in military and industrial environments were first published in the 1950s, with the first federal OSHA Standard 29 CFR 1910.95 published in 1971. Since then, additional standards have been issued, and revisions published. Today, ARS uses the following industry-accepted standards and publications to quantify personnel noise exposure during DFAN campaigns:

• S. Occupational Safety and Health Administration (OSHA) Standard 1910.95, Appendix A and B
• National Institute for Occupational Safety and Health (NIOSH) Methods #1B1, #1B2, and #1B3
• Department of Defense (DoD) Design Criteria Standard, Noise Limits, MIL-STD-1474E
• National Aeronautics and Space Administration (NASA) Kennedy Space Center (KSC) Hearing Loss Prevention Program, KNPR 1820.3

These standards define four primary criteria for understanding noise exposure. ARS has built its company standards and recommendations around these key metrics.

Before reviewing these metrics, I think it’s important to touch on the difference between A-weighted and unweighted, Z-weighted, decibels. Test specifications are unweighted, as raw, unadjusted sound data is needed. A-weighted decibels adjust sound measurements to reflect human hearing sensitivity, effectively filtering out low and high-frequency energy. All of the industry-recommended metrics are in A-weighted decibels.

According to the above-referenced standards, there are two key calculated metrics: Noise Dose (D) and Time Weighted Average (TWA). Noise dose is defined as the measured amount of noise exposure relative to the calculated amount of allowable exposure, expressed as a percentage, with 100% being the maximum noise exposure over the course of an 8-hour shift. Time weighted average is the calculated sound level which, if consistent over an 8-hour exposure period, would result in the same noise dose as is measured. OSHA states that the maximum recommended dose, without implementation of hearing protection initiatives, is 50% with a maximum TWA of 85 dB(A). It is important to note that these two values are calculated without any regard for the attenuation provided by hearing protection.

Noise exposure standards also define two additional metrics: Permissible or Recommended Exposure Level (PEL / REL) and Noise Reduction Rating (NRR). The REL is defined as the maximum allowable noise level to which a worker may be exposed without requiring protective measures. If the sound pressure level exceeds this value, hearing protection should be used to attenuate the level to be less than or equal to the REL value. This metric can vary between standards but is usually defined as 85 dB(A). Any hearing protection used to attenuate the sound level to the REL carries an NRR value. NRR is a measurement of how effective a hearing protection device, like earplugs or earmuffs, is at reducing sound exposure.

From these four criteria, we’ve put together some recommendations for clients and facilities that are considering hosting DFAN testing:

• Make baseline predictions of OASPL in areas that will be occupied by personnel. Validate these predictions with a low-level run prior to proceeding with full-level testing.
• Ensure no personnel are stationed within the test area.
• Locate the system control in an isolated area, ideally behind a significant partition.
• Most importantly, use hearing protection to attenuate sound to an REL of 85 dB(A):
  – Under 103 dB(A): Single layer of hearing protection (over the ear) with a minimum NRR of 25
  – Over 103 dB(A): Double layer of hearing protection (in-ear and over-the-ear)

I think it’s important to recognize that the bulk of energy in some test specifications is low frequency (20–250 Hz). Because industry metrics use A-weighted decibels, the impact of this energy is essentially filtered out of measurements—but we know that infrasonic energy still has an effect on the body. While there are some studies on infrasonic energy and the human body, I believe the unique nature of DFAN testing requires us to conduct some study of our own. I’m excited for some promising partnerships on the horizon that will allow us to explore this gap in standard noise exposure protocols. I’ve experienced the impact myself, and I can’t wait to have data to back it up.

It’s important to understand that the duration of a standard DFAN test case is between 2 and 5 minutes. Conservatively, a standard day of testing could contain 15 minutes of elevated noise exposure over an 8-hour shift. These short bursts of elevated noise have very little impact on the calculated noise dose and time-weighted exposure. This is clearly illustrated in the case study below.

Case Study:

A DFAN test technician wore a personal noise dosimeter over the course of a 6.5-hour acoustic testing period. A test configuration comprised of 24 Neutron™ devices was located approximately 50 feet from the system control area. A full-height partition separated the test setup from the control area. The test technician was primarily stationed at the system control rack, located in a hallway adjacent to the test setup. The control area was subject to an ambient noise level from local building mechanical and HVAC systems of 70 dBA. Four discrete acoustic test segments were performed over the recorded shift.

This figure illustrates both the C-weighted Peak Sound Pressure Level (Lcpk) and the Maximum A-weighted Sound Level (Lasmx) measured for this technician over the course of the shift.

The apparent peaks in the Lcpk and Lasmx measurements correspond to the run log for the test cases defined above. While the maximum OASPL over the course of the shift was 95 dBA, with a maximum peak of 124 dBC, the noise dose (D) for the technician was 10% with a TWA of 75 dBA. Both the D and TWA values fall below the action levels defined by OSHA. While the technician wore over-the-ear hearing protection during each test case, the noise reduction due to hearing protection is not considered when calculating acoustic dose for personnel over a standard shift.

Conclusion

Understanding personnel sound exposure is crucial for ensuring the health and safety of workers during DFAN testing. While traditional metrics like TWA and noise dose are helpful, they often overlook the unique and sometimes underestimated effects of low-frequency sound energy on the human body. By combining industry standards with firsthand experience and proactive research, we can develop more comprehensive safety protocols. As DFAN testing continues to evolve, so too must our approach to managing its acoustic impact—ensuring both effective testing and well-being for all personnel involved.