New Strategies in Interpreting Noise Measurements
Dr. Roger Hamernik and associates at SUNY, Plattsburgh, New York suggests we approach the evaluation of hazardous noise exposures in a new way. He suggests energy-based metrics commonly used to evaluate the adverse effects of a noise exposure on hearing (e.g., Leq or the equivalent noise level) are suitable only for Gaussian, random noise while many industrial/military noise environments are non-Gaussian. Gaussian noise is defined as one that has the same averaged spectrum and sound pressure level across its frequency range. A non-Gaussian noise then involves random frequency-specific noise bursts or impulse noises occurring across the averaged spectrum and SPL. In other words, the noise has random spikes occurring over its time period. We typically take our noise measurements, with noise dosimeters and sound level meters, using the A and C frequency weighting networks. Dr. Hamernik proposes we use instead a statistical metric, kurtosis, to more accurately determine for any noise environment its potential for causing NIHL. So what is kurtosis? Well, it’s a statistical function that allows for the measurement of the different temporal and peak (impulse) distributions of noises. Noises can have the same arithmetic mean and variance, but can be very different in terms of the actual distribution of energy. When taking our normal noise measurements, we may underestimate the damaging potential of the kurtotic noise on hearing. We may have two noise sources that have similar or the same average noise level or dB (A), but they may be quite different in terms of damage to hearing. By taking kurtotic noise measures (which we cannot do with our current noise measuring equipment in the Navy), we can better assess potential damage to hearing and make better recommendations regarding proper hearing protection for that environment. As we learn more about the value of such measures, we will hopefully see the development of commercial equipment to perform such measures.
Odds and Ends
Hellstrom and Dengerink of the Swedish Armed Forces performed an interesting study where individuals were either exposed to noise centered at 200 Hz, to noise centered at 2K Hz, or to both noise bands. They wanted to study the assumption we make performing noise measures using the dB (A) and (C) scales, which assume harmful effects of noise to vary with the frequency of the noise exposure. They found the TTS after exposure to 2K Hz centered noise was significantly greater than to 200 Hz centered noise. The TTS seen after exposure to the 200 Hz and 2K Hz noise in combination was significantly less than the shift after exposure to the 2K Hz noise only. They concluded that low frequency noise exposure might protect against NIHL from high frequency noise exposure. This implies that some type of sound conditioning may have noise protective effects, but needs further study to confirm and better understand. It also raises the issue of whether or not the dB (A) scale is the most relevant for calculating the risk of NIHL, as it does not measure the low frequency region to the same degree as the dB (C) scale.
Captain Jonathan Clark, MC, USN, of the NASA Johnson Space Center, Houston, TX gave an interesting presentation concerning noise issues in human space flight. NASA is concerned about the acute effect of sound on crew performance on the International Space Station (ISS) and is looking at strategies to evaluate and reduce acute, chronic, and delayed effects of sound. Crewmembers have complained that noise levels on the ISS have caused impaired speech intelligibility of non-native language (Russian Cosmonauts also work on the ISS.) and often complain of sore throat from talking loudly in noisy environments. On the space shuttle, acoustic dosimetry revealed maximum noise levels to be around 70 dB (A). No hearing loss occurred, but the crew reported significant effects on performance and communication. TTS and PTS have not been observed in the U.S. shuttle program. TTS has been reported for 100% of Russian Cosmonauts and PTS has been identified in 27 of 33 Cosmonauts (82%). Long-term space flight has produced 30-50 dB HL of NIHL in the 4K-6K Hz range. On the International Space Station, there is constant background noise in the 56-60 dB (A) range. There is concern that the Astronauts and Cosmonauts are not able to obtain “auditory rest.” NASA recommends hearing protection when noise exceeds 60 dB 24 hour Leq. The use of insert earplugs and passive or active noise reduction headsets are recommended. Captain Clark recommends longitudinal studies are needed to address what co-morbidity factors, i.e. radiation, toxicology, microgravity effects (fluid shift), and aging are involved with hearing loss in space. Wow, who would like to be the first audiologist in space!?
I know a lot of this must sound very “theoretical” and “futuristic.” Well, some of it is. The notion of using moderate levels of noise to protect the ear against NIHL from higher noise levels, performing noise measurements using spectrum analyzers and statistical tests to better predict the harmful effects, using chemical agents to stop outer and inner hair cell death so hearing abilities lost can be restored, are new concepts that require further research. The use of otoacoustic emissions to better identify NIHL and assist in monitoring individuals in the HCP is not so far fetched, and neither is the use of chemical agents to protect or restore hearing. The thing that impressed me the most about the conference was that so many people, from so many different countries (acoustic scientists, physicians, audiologists) are working so diligently to better protect people from the harmful effects of noise. I was fortunate to be there, to listen and soak up all of that knowledge. I also gained a greater appreciation of what hearing conservation is all about and why it is so important to me as a Navy audiologist. We should pat ourselves on the back a little, we have made significant improvements in the effectiveness of the Navy’s HCP and DOEHRS-HC will add to those improvements. However, we need to keep up with new advances in research that could affect future clinical and HCP practice. We can continue to do better in our fight against NIHL. I would like to see Navy Audiology, all of us, become part of future treatments for this occupational disorder. After all, thirty years ago tympanometry was not thought so highly of when first introduced, nor was the otoacoustic emission test when first introduced by David Kemp in the late 70’s. I hope you all got something out of my rambling summary of the meeting. It was fun meeting so many of these people and also a real blast to actually get to visit the Tower of London, London Bridge, Big Ben, and Buckingham Palace. And then there was that British ale…
CDR Keith S. Wolgemuth, MSC, USN
Naval Submarine Medical Research Laboratory
Box 900, Diving Bio-effects/Hearing Conservation
Groton, CT 06340