When most people think of microphones, they think of the ones singers use or you would find in a karaoke machine, but they might not realize that much smaller microphones are all around us. Current smartphones have about three or four microphones that are small. The miniaturization of microphones is therefore a desire in technological development. These microphones are strategically placed to achieve directionality. Directionality means that the microphone’s goal is to discard undesirable noise coming from directions other than the speaker’s as well as to detect and transmit the sound signal. For hearing implant users this functionality is also desirable. Ideally, you want to be able to tell what direction a sound is coming from, as people with unimpaired hearing do.
But dealing with small size and directionality presents problems. People with unimpaired hearing can tell where sound is coming from by comparing the input received by each of our ears, conveniently sitting on opposite sides of our heads and therefore receiving sounds at slightly different times and with different intensities. The brain can do the math and compute what direction sound must be coming from. The problem is that, to use this trick, you need two microphones that are separated so the time of arrival and difference in intensity are not negligible, and that goes against microphone miniaturization. What to do if you want a small but directional microphone, then?
When looking for inspiration for novel solutions, scientists often look to nature, where energy efficiency and simple designs are prioritized in evolution. Insects are one such example that faces the challenge of directional hearing at small scales. The researchers have chosen to look at the lesser wax moth (fig 1), observed to have directional hearing in the 1980s. The males produce a mating call that the females can track even when one of their ears is pierced. This implies that, instead of using both ears as humans do, these moths’ directional hearing is achieved with just one ear.
Lesser wax moth specimen with scale bar. Image courtesy of Birgit E. Rhode (CC BY 4.0).
The working hypothesis is that directionality must be achieved by the asymmetrical shape and characteristics of the moth’s ear itself. To test this hypothesis, the researchers designed a model that resembles the moth’s ear and checked how it behaved when exposed to sound. The model consists of a thin elliptical membrane with two halves of different thicknesses. For it, they used a readily available commercial 3D printer that allows customization of the design and fabrication of samples in just a few hours. The samples were then placed on a turning surface and the behavior of the membrane in response to sound coming from different directions was investigated (fig 2). It was found that the membrane moves more when sound comes from one direction rather than all the others (fig 3), meaning the structure is therefore passively directional. This means it could inspire a single small directional microphone in the future.
Laboratory setup to turn the sample (in orange, center of the picture) and expose it to sound from the speaker (left of the picture). Researcher’s own picture.
Image adapted from Lara Díaz-García’s original paper. Sounds coming from 0º direction elicit a stronger movement in the membrane than other directions.
Daniel Fink – djfink01@aol.com
Twitter: @QuietCoalition
Board Chair, The Quiet Coalition, 60 Thoreau Street Suite 261, Concord, MA, 01742, United States
The Quiet Coalition is a program of Quiet Communities, Inc.
Popular version of 3pNS1-What is the safe noise level to prevent noise-induced hearing loss?, presented at the 183rd ASA Meeting.
Ear structures including outer, middle, and inner ear. Image courtesy of CDC
If something sounds loud, it’s too loud, and your auditory health is at risk. Why? The safe noise exposure level to protect your hearing- to prevent noise-induced hearing loss (NIHL) and other auditory disorders like tinnitus, also known as ringing in the ears, might be lower than you think. Noise damages delicate structures in the inner ear (cochlea). These include minuscule hair cells that actually perceive sound waves, transmitted from the air to the ear drum, then from bones to the fluid in the cochlea.
Figure 1. Normal hair cells (left) and hair cells damaged by noise (right). Image courtesy of CDC
[A little detail about sound and its measurement. Sound is defined as vibrations that travel through the air and can be heard when they reach the ear. The terms sound and noise are used interchangeably, although noise usually has a connation of being unpleasant or unwanted. Sound is measured in decibels. The decibel scale is logarithmic, meaning that an increase in sound or noise levels from 50 to 60 decibels (dB) indicates a 10-times increase in sound energy, not just a 20% increase as might be thought. A-weighting (dBA) is often used to adjust unweighted sound measurement to reflect the frequencies heard in human speech. This is used in occupational safety because the inability to understand speech after workplace noise exposure is the compensable industrial injury.]
Many audiologists still use the industrial-strength 85 dB noise level as the level at which auditory damage begins. This is incorrect. The 85 dBA noise level is the National Institute for Occupational Safety and Health (NIOSH) recommended occupational noise exposure level (REL). This does not protect all exposed workers from hearing loss. It is certainly not a safe noise level for the public. Because of the logarithmic decibel scale, 85 decibel sound has approximately 30 times more sound energy than the Environmental Protection Agency’s 70 decibel safe sound level, not about 20% as might be thought.
The EPA adjusted the NIOSH REL for additional exposure time- 24 hours a day instead of only 8 hours at work, 365 days a year instead of 240 days- to calculate that 70 dB average noise exposure for a day would prevent noise-induced hearing loss. This is the only evidence-based safe noise level I have been able to find.
But the real safe noise level to prevent NIHL must be lower than 70 dB. Why? EPA used the 40-year occupational exposure in its calculations. It didn’t adjust for lifetime exposure (approaching 80 years in the United States before the COVID pandemic). NIHL comes from cumulative noise exposure. This probably explains why so many older people have trouble hearing, the same way additional years of sun exposure explains the pigmentation changes and wrinkles in older people.
My paper explains that the NIOSH REL, from which EPA calculated the safe noise level, was based on studies of workers using limited frequency audiometry (hearing tests), only up to 4000 or 6000 Hertz (cycles per second). More sensitive tests of hearing, such as extended-range audiometry up to 20,000 Hertz, shows auditory damage in people with normal hearing on standard audiometry. Tests of speech in noise- how well someone can hear when background noise is added to the hearing test- also show problems understanding speech, even if standard audiometry is normal.
The actual noise level to prevent hearing loss may be as low as 55 dBA. This is the noise level needed for the human ear to recover from noise-induced temporary threshold shift, the muffling of sound one has after exposure to loud noise. If you’ve ever attended a rock concert or NASCAR race and found your hearing muffled the next morning, that’s what I’m talking about. (By the way, there is no such thing as temporary hearing loss. The muffling of sound, or temporary ringing in the ears after loud noise exposure, indicates that permanent auditory damage has occurred.)
55 dB is pretty quiet and would be difficult to achieve in everyday life in a modern industrialized society, where average daily noise exposures are near 75 dB. But I hope that if people know the real safe noise level to prevent hearing loss, they will avoid loud noise or use hearing protection if they can’t.
The American Public Health Association states, “Noise is unwanted and/or harmful sound.” Noise not loud enough to damage hearing causes high blood pressure, heart attacks, and strokes. The Federal Aviation Administration (FAA) considers noise an annoyance but does not acknowledge the adverse health effects of aircraft noise. Based on the Schultz curve, the FAA adopted 65 dBA Day-Night Level (DNL) as “the threshold for significant aviation noise, below which residential land use is compatible.” The FAA’s recent Neighborhood Environmental Survey found that many more Americans are annoyed by noise than previously known.
Schultz Curve and Neighborhood Environmental Survey results, showing that many more Americans are annoyed by noise than the Schultz Curve showed. Source: FAA
[I have to tell you a little about the science of sound or noise measurement. The words sound and noise are used interchangeably. Sound is measured in decibels (dB). The decibel scale is logarithmic. This means that a 10 dB increase from 50 to 60 dB indicates 10 times more sound energy, not merely 20% more. Because noise disrupts sleep, DNL measures noise for 24 hours but adds a 10 dB penalty for noise between 10 p.m. and 7 a.m. A-weighting (dBA) adjusts sound measurements for the frequencies heard in human speech. A-weighting is not the right measure for aircraft noise because aircraft noise has lower frequencies than speech. A-weighting also reduces unweighted sound measurements by about 20-30 dB.]
According to the Environmental Protection Agency (EPA), though, safe noise levels are only 45 dB DNL for indoor noise and 55 dB DNL for outdoor noise. The World Health Organization (WHO) recommends lower aircraft noise levels: 45 dB Day-Evening-Night Level (adding a 5 dB penalty for noise between 7-10 p.m.) and 40 dB at night. Both EPA safe noise levels and WHO recommended aircraft noise levels are obviously much lower than the FAA’s 65 dBA DNL, especially because they use unweighted dB.
Being annoyed or disturbed by aircraft noise is stressful. Stress increases heart rate and blood pressure. Stress increases blood levels of stress hormones. Stress causes inflammation of the blood vessel lining. in turn causing cardiovascular disease, including hypertension and heart attacks, and other adverse health effects. Scientific experts think that the evidence is strong enough to establish causality, not merely a statistical association. Epidemiological studies demonstrating these effects have been confirmed by human and animal research. The biological mechanisms are now understood at the cellular, subcellular, molecular, and genetic levels. Aircraft noise also affects poor and minority communities more than others. Children are also more sensitive to damage from noise, which also interferes with learning.
The FAA insists that more research is needed, but no more research is needed to know that aviation noise is hazardous to health. The FAA must establish lower noise standards to protect Americans exposed to aircraft noise.
The University of Texas at Austin, Marine Science Institute 750 Channel View Drive, Port Aransas, TX 78373
Popular version of paper 1aAB7 Presented Monday morning, November 5, 2018 176th ASA Meeting, Victoria, BC
Photo credit: Tyler Loughran
The location and frequency of spawning (reproduction) in fish has a direct effect on the abundance, stability, and resilience of a fish population. Major storm events, such as hurricanes, provide a natural experiment to test the ability of a fish population to withstand disturbances. Acoustic monitoring of Spotted Seatrout spawning revealed that these fish are extremely productive, spawning every day of the spawning season (April – September), including during a category 4 hurricane. These results illustrate the amazing resilience of estuarine fishes to intense disturbances and their potential to cope with projected increases in extreme weather events in the future.
Spotted Seatrout and many other species of “drum fish” make characteristic sounds during spawning (figure 1), which can be heard on underwater microphones, or hydrophones. This allows us to remotely monitor when fish spawn and how long they spawn for, which is especially helpful in murky water, where it is difficult to see. Seatrout spawning can be identified within the audio recordings by analyzing the intensity of the sound within the specific frequency range (250-500 Hz) of the Spotted Seatrout calls.
Figure 1. Recording of male Spotted Seatrout drumming sounds during spawning.
We monitored Spotted Seatrout spawning from April to September 2017 at 15 sites within the estuaries of South Texas, to see how changes in environmental conditions affected spawning. Our study also coincided with a category 4 hurricane. Hurricane Harvey made landfall 9 km east of Rockport, Texas on August 25, 2017 at 17:00 h CST. The eye of the storm was 28 km wide, maximum sustained winds were 59 m s-1 with gusts up to 65 m s-1, and the storm surge caused water levels to rise 3.8 meters above ground level.
The sound pressure level within the frequency range of seatrout spawning sounds peaked every evening between 20:00 and 21:00, indicating that spawning was occurring on a daily basis. During the hurricane wind-associated noise masked any potential spawning sounds, except at two stations that were directly in the path of the hurricane. When the eye of the storm was directly overhead those stations, wind-associated noise decreased, and spawning sounds were audible (figure 2). The time that spawning began shifted two hours earlier for five days after the storm, which may have been partly caused by the decrease in water temperature.
Figure 2. Spectrograms of recordings during Hurricane Harvey showing storm noise at 21:55 and seatrout chorusing at 22:25 within the 250-500 Hz bandwith (dotted lines).
Species that live and spawn in estuaries must deal with conditions that can change rapidly and unpredictably. It is important to understand how those changes impact spawning activity in order to maintain sustainable populations for the fishing industry. Further, understanding how fish respond to environmental disturbances in these environments may offer insight on how fish will respond to climate change and other human impacts elsewhere.
Lesser wax moth specimen with scale bar. Image courtesy of Birgit E. Rhode (CC BY 4.0).
Lesser wax moth specimen with scale bar. Image courtesy of Birgit E. Rhode (CC BY 4.0).
Laboratory setup to turn the sample (in orange, center of the picture) and expose it to sound from the speaker (left of the picture). Researcher’s own picture.
Image adapted from Lara Díaz-García’s original paper. Sounds coming from 0º direction elicit a stronger movement in the membrane than other directions.
Photo credit: 
