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Sound and Silence

Hearing is believing. Due to the popularity of sound-offs and the flood of new high-end components, extremely loud auto sound systems are in. Drive through almost any town in the nation and you'll stumble upon at least one car stereo "fanatic." Not surprisingly, sound-off contestants thrive on car-shaking bass and systems that pin SPL meters with relative ease. Unfortunately, few of these listeners pay attention to the impact loud music has on the most important sound-measurement device of all: the human ear. As a long-time participant in sound-offs, I know I've become more concerned about my hearing – ever since I awoke one morning with a ringing in my ears. It wasn't especially loud, but it certainly was annoying. And I hear it to this day.

My condition (known as "tinnitus") is a permanent one, and undeniably it is the result of sheer ignorance. I had been so busy tweaking my system so it would outplay all others that I never stopped to consider what I was doing to my ears. Winning was everything. I hope that by sharing what I've learned – the hard way – you'll become more conscious of protecting your hearing. The first step is to develop a basic understanding of the nature of sound and how the human ear works.
The Nature of Sound

Sound can be described as any variation in air pressure that can be detected by the human ear. In a music-reproduction system, these variations are created by the excursion of speakers as they vibrate. Two important quantities are used to describe sound: frequency and amplitude. Measured in hertz (Hz), frequency is the number of pressure variations that occur per second. For most music listeners, frequency is associated with musical pitch – high-frequency sounds have a high pitch and low-frequency sounds have a low one. The normal range of hearing for a healthy young person is 20 to 20,000 Hz; hearing deteriorates with age and with exposure to unsafe volume levels.

Amplitude is the size of the fluctuations in air pressure. The softest sound a human ear can detect has an amplitude of twenty millionths of a Pascal (20 uPa) – some five billion times less than normal atmospheric pressure. (A pressure of 20 uPa is so minute that it causes the eardrum to move a distance of less than one tenth the diameter of a single hydrogen molecule.) Healthy ears can tolerate sound pressure that is more than a million times higher. The range of human hearing capability is so vast, in fact, that the Pa scale is not very good for expressing it – most useful figures are very large, unmanageable numbers. The decibel (dB) scale is much more manageable, and it has become the accepted way of expressing sound-pressure level (SPL.).

The decibel is not an absolute unit of measurement; rather, it is a ratio between a measured quantity and an agreed reference level. The dB scale is logarithmic, using the hearing threshold of 0 dB (20 uPa) as its reference level. Multiplying sound pressure by ten, using the Pa scale, is equivalent to an increase of 20 dB; thus, 200 uPa corresponds to 20 dB, 2,000 uPa to 40 dB, and so on. Remarkably, a range of twenty million uPa is compressed into only 120 dB.
Range of Human Hearing

The hearing range of the healthy human ear is depicted in Figure 1. This range is bound by the "threshold of hearing" on the bottom and the "threshold of feeling" on top; the boundaries on the left and right represent the lowest and highest frequencies a listener can perceive, respectively. Note that speech occupies only a very small portion of the entire audible range. Even music, with its wide frequency response and dynamic range, does not cover the entire auditory spectrum. To give you a feel for different levels, the sound of a jet taking off 300 feet away measures about 125 dB, average street traffic measures about 85 dB, and sounds in a library measure about 35 dB.

Although our ears may appear to work very well, their response actually is far from "flat." To complicate things further, this variance in hearing sensitivity is most pronounced at low sound-pressure levels. The boundaries depicted in Figure 1 belong to a family of curves known as "equal loudness contours," and they can be thought of as the inverted frequency-response curves of the ear. Because the frequency response of the human ear is not equally sensitive to all frequencies, it is necessary to use a weighting network when measuring SPL. (The frequency responses of the three standard weighting networks – A, B, and C – are shown in Figure 2.) In simplest terms, these networks are electronic circuits whose sensitivity varies with frequency, reflecting the way the human ear works.

Since A weighting is thought to provide the most accurate model of how the human ear perceives loudness, and noise that causes hearing damage correlates most closely to the A curve, the A network is usually specified in federal government regulations established by the Occupational Safety and Health Administration (OSHA). In contrast, B weighting is rarely used and C weighting is used only by sound-off organizers – primarily because using it results in higher SPL readings. Because the ear's response is flatter with high SPL'S, there is little difference between C weighting and no weighting at all.
The Human Ear

To understand how hearing impairment occurs, you must appreciate the complex workings of the human ear. The ear's role is to convert sound waves into nerve impulses, which are transmitted to the brain where they are interpreted as sounds. (See Figure 3 for a cross-section of the ear.)

The human ear consists of three major parts: the outer ear, the middle ear, and the inner ear. Sound waves entering the outer ear travel through the ear canal to the eardrum, a thin membrane that stretches across the inner end of the canal. Changes in air pressure cause the eardrum to vibrate; these vibrations are transmitted to a chain of three tiny bones in the middle ear called ossicles. These bones conduct vibrations across the middle ear to a second thin membrane known as the oval window, which separates the air-filled middle ear from the fluid-filled inner ear. Deep inside the inner ear, and resting on the basilar membrane, is a spiral structure called the cochlea that houses the essential organ of hearing – the corti. When the basilar membrane vibrates, tiny, sensory hair cells inside the corti bend, which stimulates the transmission of nerve impulses to the brain.
Hearing Loss

When the ear is exposed to high sound-pressure levels even for short periods of time, the listener experiences a type of hearing loss known as "temporary threshold shift." If you've ever been to a loud rock concert, you know what I'm talking about: When you step outside the arena, everything seems unnaturally quiet. But after awhile, your threshold of hearing returns to a level very close to its original level and you can hear again.

Prolonged exposure to high sound-pressure levels kills the very fine hair cells, progressively impairing hearing. Damage to a few may not cause noticeable impairment, but as more of them are killed off the brain will not be able to compensate for the loss of information. This results in a condition known as "permanent threshold shift." Usually by the time the listener becomes aware of the loss, considerable and irreparable damage has already occurred. The result: a form of sensory-neural hearing loss.

The symptom most common to this kind of impairment is the inability to distinguish between different sounds at normal listening levels – words seem to run together, speech and background noise mesh, and music sounds muffed. While hearing aids and other devices can alleviate some of these problems, full hearing capability can never be restored.

Tinnitus, the ever-present ringing in the ears that I suffer from, is another telltale sign of hearing loss. De- pending on the number of damaged hair cells, ringing can vary from mild to severe. Extreme cases require medication and/or sound-making devices, which pump white noise into the ear to mask the ringing.

Aside from damage to your hearing, exposure to high sound-pressure levels also can produce physiological side effects – including disorientation, diarrhea, and chest pains – especially when very low frequencies are prevalent in the music.
What's Dangerous

To provide a basis for determining safe sound-pressure levels, OSHA has established the following guidelines: For sound-pressure levels of 90 dBA (A-weighted decibels), the maximum allowable continuous exposure time is eight hours. For every 5-dB increase in SPL, the allowable exposure time is cut in half. Note that OSHA regulations prohibit exposure of any duration to sound-pressure levels above 115 dBA unless hearing protection is used. Unfortunately, levels of 120 to 130 dBA are fairly common at sound-off competitions.

You can protect your hearing, however. First, use common sense. If you're listening to a system and the volume hurts your ears, turn it down. But if you really must feel the music as well as hear it, opt for one of two basic kinds of hearing protectors: ear-plugs made of cotton, foam rubber, or other similar materials, or head-phone-like ear muff's that completely cover the ears.

Due to their size and simplicity, earplugs are inexpensive and unobtrusive. Earmuff's, on the other hand, are more effective, although they are pricey and tend to be bulky. A combination of earplugs and earmuff's afford the best protection – but they also provide an experience of near deafness. My personal favorites are the disposable ZEE N.R.R. 2 earplugs from ZEE Medical (800-621-5891). Made of a foam-like substance, they conform to the contour of your ear canal and cost only 50 cents a pair. Before purchasing either type of protector, however, note that the degree of protection varies from model to model and manufacturer to manufacturer. Check manufacturers' specifications: the higher the attenuating factor (expressed in dB), the better the protection. Typical headphones and earplugs will offer between 15 and 30 dB of attenuation when the devices are used properly.
SPL Measurement

To measure SPL accurately, you need an SPL meter, an instrument that responds to sound in approximately the same manner as the human ear. The basic components of an SPL meter are a microphone, sound-processing circuitry, and a readout (usually a digital one). The Term-LAB SPL meter is the standard for measuring SPL in the auto sound competition arena. A Term-LAB setup is available from WHE Inc.

The SPL meter's microphone converts incoming sound into a corresponding electrical voltage. After preamplification, the signal passes through the A-, B-, or C-weighting network, which electronically simulates the response of the ear. The resulting signal is fed into a root-mean-square (rms) detector, which extracts a value that is directly related to the amount of sound energy being measured. This value is then displayed as a decibel figure on the meter's readout.

Before making sound-pressure-level measurements, the SPL meter and microphone assembly must be calibrated with a device that attaches to the end of the microphone and produces a steady tone at a specific reference level. When the tone first sounds, the SPL meter should be adjusted so the figure displayed on its readout corresponds to the reference level produced by the calibrator. Once the equipment is calibrated, all SPL measurements are based on an absolute reference. The actual measuring procedure for an A-weighted figure goes like this: First, set the meter's weighting network to the A position. Next, verify that the meter is set to make "average SPL" readings. Then, carefully place the microphone inside the vehicle in a position that closely approximates the location of the driver's ears.

Here's where hearing precautions come into play: Whenever you're dealing with a system of unknown output, proceed with caution and always wear earplugs or earmuffs – at least until you determine the system's maximum SPL. When you're ready to take an SPL reading, close all of the vehicle's windows and doors and select program material with a wide frequency range. To avoid damaging the system's components, bring the volume up slowly until you reach maximum output.

There's no question about it: Prolonged exposure to sound-pressure levels above 90 dBA is detrimental to the human ear. And the longer the exposure, the greater the risk of sustaining permanent hearing damage. In fact, studies show that prolonged exposure to even moderate sound-pressure levels can cause even more damage to the ear than brief exposure to high levels. So the next time you're in the driver's seat reaching for the volume knob to crank it up, remember those tiny little hair cells deep in your ear – without them, what's the point?
References

Glen Ballou, Handbook for Sound Engineers (Indianapolis; Howard W. Sams & Co., 1988)
Johnson/Walker/Cutnell, The Science of Hi-Fidelity (Dubuque; Kendall/Hunt Publishing Co., 1986)
G. Rasmussen, "Human Body Vibration Exposure and Its Measurement," (Technical Review, No. 1-1982)
Industrial Noise Control and Hearing Testing (Denmark; Bruel & Kjaer Instruments, Inc.)
Measuring Sound (Denmark: Bruel & Kjaer Instruments, Inc.)



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