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How do we measure sound?

I last wrote on this topic five years ago this month and decided to revisit the measurement of sound. Sound energy travels in waves that are invisible and is measured in frequency and amplitude. I will define both of these words.

Frequency is measured in the number of sound vibrations in one second. That measurement is tested by a machine called an audiometer and the scores recorded on a graph called an audiogram. This machine makes pre-programmed sounds at certain frequencies, pitches, or tones. The slower the vibration per second the lower the tone and the faster the vibrations per second the higher the tone.

The cycle per second is called a “Hertz.” A healthy ear can hear sounds of very low frequency, 20 Hertz (or 20 cycles per second), to a very high frequency of 20,000 Hertz. The lowest A key on the piano is 27 Hertz. The middle C key on a piano creates a 262 Hertz tone. The highest key on the piano is 4186 Hertz. Men typically speak in a range from 1800-2800 Hertz and women and children from 3000 to 4200 Hertz. This is because women and children have shorter and thinner vocal cords than men. The effect is just like piano strings in tone difference. Many patients with high frequency hearing loss will tell me they have greater difficulty hearing women’s voices and are missing a great deal of what their grandchildren are saying.

Amplitude measures how forceful the wave is. There are a number of scientific factors in sound amplitude or force, but we perceive it as loudness or volume. It is measured in decibels or the abbreviation dBA of sound pressure. 0 dBA is the softest level that a person can hear. Normal speaking voices are around 65 dBA. A household vacuum runs at about 80 dBA, a rock concert can be about 120 dBA, a jet plane taking off at 150 dBA, and a hunting rifle discharges at 165 dBA. Sounds that are 85 dBA or above can permanently damage your ears.

The more sound pressure a sound has, the less time it takes to cause damage. For example, a sound at 85 dBA may take as long at 8 hours to cause permanent damage, while a sound at 100 dBA can start damaging hair cells of the inner ear after only 30 minutes of listening.

“For every 3 dBs over 85dB, the permissible exposure time before possible damage can occur is cut in half. With the 85 db for 8 hour limitation, a lawnmower running at 92 db would cut that down to 1 hour and a leaf blower at 115 db down to 10 minutes. The need for ear protection with these tools is obvious. Many young people use headsets or ear buds with their media devices and crank the volume up past 100 dBA. I ride frequently on public transportation andoften hear the music clearly someone is playing into their ears from 10 rows away. That sound exposure level may be good for my industry as the “Ipod generation” will need vast numbers of hearing aids, but it is acoustical suicide for their ears.” (Audiology Online)

When a patient has been exposed to dangerous levels of industrial noise for long years of their employment, it often shows on the audiogram as a notch. The graph begins to go downward until it reaches a lowest point and then comes back up again leaving a valley or a notch showing were the noise affected them. If it were possible to back trace the origin of those sounds we would find the machine or perhaps a building full of manufacturing machines that were running at exactly the frequencies revealed in the notch which is usually 3000-5000 Hertz. The frequency could be identified, but it was the force or volume at that frequency that damaged the hearing.

Every year, approximately 30 million people in the United States are occupationally exposed to hazardous noise. Noise-related hearing loss has been listed as one of the most prevalent occupational health concerns in the United States for more than 25 years. Thousands of workers every year suffer from preventable hearing loss due to high workplace noise levels.

Workers are now required to wear ear protection but that was not always the case. It was not until 1981 that the Occupation Safety and Health Administration or OSHA federally mandated ear protection in the workplace. In addition to high noise exposure, hearing loss typically is a result of heredity, aging, and certain medicines that affect the inner ear. Sound perception ability, or a hearing test measures the frequencies (cycles per second) and the amplitude (force) of the sound wave the person can sense at a threshold levels and thereby the professional produces a graph that shows ability to hear and the impairment, or hearing loss.

On a different issue, if you have behind the ear hearing aids, be extra careful when you wear a face mask for the health restrictions. You could lose your hearing aid if the mask flipped it off. Put on, and take off your mask slowly.

If you have the symptoms of hearing loss let a professional help you find out why. The hearing professional will help you sort out the technology level to meet your need, your budget, and answer your hearing need questions.

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Jeffrey L. Bayliff, NBC-HIS is owner, Hear the Birds Hearing Aid Center, Lock Haven, PA

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