WATCH YOUR SOUND DOSAGE

If you don't watch your sound dosage, you might become hard of hearing. Research now shows that exposure to excessive sound, not aging, makes people lose their hearing.

NOTE: See:

• Addendum: New discoveries and special cautions on low frequency and infrasonic sound.
• Appendix A: Understanding the various weightings
• Appendix B: How hearing damage occurs
• Appendix C: How human hearing works

CHECK FOR SYMPTOMS OF DAMAGE

Look for the following symptoms of hearing damage:

1. PAIN - If you feel pain from the sound, get away from there immediately!
2. TEARING - If you hear a tearing sound in one or both ears, get away from there immediately!
3. BUZZING - If the sound makes a buzzing sound in one or both ears, get away from there immediately!
4. DISTORTION - If the sound as you hear it is distorted get away from there immediately!
5. MUFFLED SOUND - If the sound is suddenly muffled, get away from there immediately!
6. RINGING IN EARS - If you have ringing in your ears that lasts longer than one day, you probably have hearing damage.
7. WRONG DIRECTION - If you hear sounds of visible events coming from the wrong directions, you have damage.
8. LOSS OF BALANCE - If you have loss of balance and have just listened to loud sounds, you probably have hearing damage.

MEASURE THE SOUND PRESSURE LEVEL (SPL)

COMPARING THE STANDARDS

Comparison between the standards

CALCULATING YOUR 24-HOUR EXPOSURE

Now, how do you figure out your exposure if you are exposed to different levels of these sounds for different periods of time?

Here is one way to calculate your total dose to damaging sound (using the NIOSH levels):

• Find the level and duration of each period of exposure in a 24 hour period. Ignore periods where exposure is less than 75 dB SPL.
• Calculate the dose (in damage units) for each of the periods, using the following formula:

  NIOSH dose:
  N_dose = time (hr) * 2 ^ ((SPL - 94) / 3)

  Note: If your equipment gives you the peak level, average level, and minimum level for a given period, calculate the dosage for each figure for the period, and then average the three dosages found. DO NOT average the dB SPL levels.

  Note: If you know only the peak level, you must use that. If you know only an average level, add a 10 dB safety margin to it.

• Sum the calculated doses for the 24 hour period.
• The level of dosage is safe only if the result is less than 1.

ADDENDUM

NEW DISCOVERIES ABOUT HEARING LOSS

Recent research has indicated two new facts about hearing loss:

1. NO SUCH THING AS AGE-RELATED HEARING LOSS

   Age related hearing loss is now thought to not exist. Instead, what was thought to be age-related hearing loss has now been found to be acquired hearing loss from cumulative exposures to the loud sounds of steam trains, firearms, and factories.

2. HEARING DAMAGE FROM LOW FREQUENCY AND INFRASONIC SOUND

   Extremely low frequency sounds do not follow the tables above, because the weighting curves used for the sound measurements above are based on audibility. But it is now thought that frequencies under 60 Hz, and especially those approaching DC impulse transients, cause hearing damage, even though they are not very audible. This damage begins in the "rock range", starting between 2 KHz and 6 KHz, and expanding in frequency range as exposure continues. So the following precautions should be taken:

   • Infrasonic filters rolling off at 30 Hz should be inserted in all subwoofer, house, and monitor feeds.
   • Subharmonic generators (bass enhancers) must not be used.
   • If the sound is felt in a person's chest, it is too loud in the infrasonic region. It is causing hearing damage even though it is not heard to be that loud.
   • Infrasonic filters should be inserted in the instrument outputs of certain instruments. For example, some Yamaha instruments are known to output DC transients.
   • A new weighting curve, Z weighting (dBZ), has been created for SPL meters so this hazard may be measured.
   • Often the damage is done to people farther from the speakers. A 20 Hz wave does not fully form until the sound is 28 feet from the speaker.

Links:

• MIXING YOUR OWN SOUND
• HOME

APPENDIX A: UNDERSTANDING THE VARIOUS WEIGHTINGS

Several different weighting systems are used for several different purposes.

The chart is repeated so it can be seen without much scrolling.

The following list shows the correct weighting for each use:

• A Weighting (dBA):

  NEVER use the A weighting to evaluate whether a hazard to hearing exists.

  Uses:

  1. All sound and noise below 55 dB SPL
  2. Measuring noises that are annoying, but not dangerous (e.g. sounds that keep people awake)
  3. Measuring the annoyance factor of most machinery noises.
  4. Measuring the annoyance factor of traffic
  5. Measuring the annoyance factor of commerce
  6. Ambient noises inside an auditorium or concert hall
  7. Level of quiet classical music
  8. Measuring louder sounds at the source, where the listener will be far enough away to hear it below 55 dB SPL
• B Weighting (dBB):

  NEVER use the B weighting to evaluate whether a hazard to hearing exists.

  Uses:

  1. Most sound and noise between 55 dB SPL and 85 dB SPL
  2. Measuring noises that are annoying, but not dangerous (e.g. sounds that keep people awake)
  3. Level of medium loud classical music
  4. Measuring louder sounds at the source, where the listener will be far enough away to hear it below 85 dB SPL
• C Weighting (dBC):

  NEVER use the C weighting to evaluate whether bass notes are a hazard to hearing.

  Uses:

  1. Most sound and noise above 85 dB SPL not containing infrasound or frequencies below 100 Hz
  2. Measuring most machinery noises that can be hazardous to human hearing
  3. Measuring loud music that can be hazardous to human hearing
  4. Level of loud classical music
• D Weighting (dBD):

  NEVER use the D weighting to evaluate whether a hazard to hearing exists.

  Uses:

  1. Measurement of the annoyance level of distant aircraft noise
  2. Measurement of the annoyance level of distant factory noise
  3. Measurement of the annoyance level of distant military jet noise
• G Weighting (dBG):

  NEVER use the G weighting to evaluate whether a hazard to hearing exists.

  Uses:

  1. Measurement of the annoyance level of wind turbines
  2. Measurement of the annoyance level of blasting
• Z Weighting (dBZ):

  ALWAYS use the Z weighting to evaluate whether a hazard to hearing exists.

  Uses:

  1. Most sound and noise above 85 dB SPL which does contain infrasound or frequencies below 100 Hz
  2. Measuring machinery noises that can be hazardous to human hearing
  3. Measuring music that can be hazardous to human hearing
  4. Level of loud classical music
  5. Measuring louder sounds at the source
  6. With a tone generator, finding the frequency response of the sound system.
  7. With a tone generator, finding the frequency response of the room.
• CCIR 468 Weighting (ITU-R 468):

  NEVER use CCIR 468 weighting to evaluate whether a hazard to hearing exists.

  Uses:

  1. This is the European version of what the US uses A Weighting (dBA) for.
  2. Measuring the annoyance factor of most machinery noises.
  3. Measuring the annoyance factor of traffic
  4. Measuring the annoyance factor of commerce

APPENDIX B: HOW HEARING LOSS OCCURS

There are several mechanisms of hearing loss. The curves show the effect on hearing from each one:

• The dBC curve is an approximation of the response of normal human hearing at SPL 90 dBC.
• The dBhl1 curve is a plot of hearing loss caused by excessive exposure to loud music.

  Notice the dip in the response near 3 KHz caused by damage from excessive sound pressure. The hair cells in the cochlea in the 3 KHz area are those most susceptible to damage from loud bass notes.

• The dBhl2 curve is a plot of hearing loss caused by more extensive exposure to loud music.

  Notice the wider dip in the response near 3 KHz caused by more damage from excessive sound pressure. The hair cells in the cochlea are damaged over a larger area.

• The dBhl3 curve is a plot of hearing loss caused by massive extensive exposure to loud music.

  Notice the even wider dip in the response near 3 KHz caused by even more damage from extensive sound pressure. The hair cells in the cochlea are damaged over a much larger area.

• The series of dots representing dBHT does not represent a hearing loss, but an enhancement caused by a stenosis of the helicotrema, a congenital defect in the cochlea.

  There is a little bypass port at the end of the spiral of the cochlea called the helicotrema. Its job is to equalize pressure between the chambers in the cochlea. It also lets frequencies below 15 Hz bypass the membranes of the cochlea. Under normal circumstances, the ear is not sensitive to frequencies less than 15 Hz because the helicotrema lets them bypass the sound-sensitive cells.

  In a person with stenosis of the helicotrema, this bypass port is either greatly narrowed or it is completely closed. So the very low tones (below 70 Hz) have no place to go but to be reflected back into the cochlea. This has these effects:

  1. It changes the cochlea at low frequencies from a half-wave pipe open at both ends to a quarter-wave pipe closed at one end. This doubles the longest wavelength the cochlea is sensitive to.
  2. Doubling the longest wavelength means the lowest frequency the cochlea is sensitive to is now an octave lower. Instead of cutting off at 15 Hz, it now cuts off at 7.5 Hz.
  3. Because the low frequency energy is not vented by the bypass port, it exerts more than normal pressure on the hair cells, making the normally audible sounds louder than normal (hyperacusis).
  4. Because this low tone has more energy and sounds louder than normal, the threshold of pain and the threshold of damage are at lower levels. Infrasound can damage the ears of someone with stenosis of the helicotrema. Sounds that others can not even perceive cause pain to such an individual (hyperacusis).
  5. Because the low frequency behavior of the cochlea is a closed pipe instead of an open pipe, the lowest subharmonics of bass notes sound out of tune with the fundamental notes and with the rest of the music being listened to.
  6. Earplugs do not stop loud infrasonic sound or pain caused by it, because bone conduction also carries infrasound to the cochlea.
• The dBHd curve represents hearing loss in a person with stenosis of the helicotrema who was exposed to excessive infrasonic sound. The damage is in the same area as that caused by loud music.
• The series of dots representing dBHT also can represent a condition caused by bone loss next to the semicircular canals. But there are the following differences:
  1. It does not change the cochlea from a half-wave pipe open at both ends to a quarter-wave pipe closed at one end. Thus, it does not double the longest wavelength the cochlea is sensitive to.
  2. The frequency it does affect depends on the size of the area of bone loss and the location on the semicircular canals.
  3. Because the energy enters the semicircular canals instead of the cochlea, it has no tonal quality other than that detected in the cochlea. Thus, what is heard is not changed except in level.
  4. Because the sound is imparted to the semicircular canals, it is detected there as a level, but not a pitch. If the sound is loud enough pain is caused in the semicircular canals. This is also called hyperacusis.
  5. Because this tone puts energy into the semicircular canals, it can damage them. It knocks the otoliths loose from their hairs and damages hair cells. The loose otoliths roll around in the semicircular canals, causing momentary loss of balance with head rotation.
  6. The extra energy in the semicircular canals can lower the threshold of pain and the threshold of damage to lower sound levels (hyperacusis).
  7. Earplugs do not stop loud infrasonic sound or pain caused by it, because bone conduction also carries infrasound to the semicircular canals.
• Infrasound can also damage the cochlea of a normal person at the same point it causes damage in the person with stenosis of the helicotrema.
• Infrasound can also damage the semicircular canals without any damage to hearing by knocking otoliths loose. This causes a loss of balance without any other symptoms.

APPENDIX C: HOW HUMAN HEARING WORKS

THE STRUCTURE OF THE EAR The path of the sound from the open air to the brain: The pinna (earflap, A) changes the amplitude and phase of the sound depending on which direction the sound is coming from. This helps the brain determine the direction the sound came from. The sound enters the ear canal (B) and heads toward the tympanic membrane (eardrum, C). The sound vibrates the tympanic membrane (eardrum, C). The tympanic membrane (C) separates the outer ear from the middle ear. The vibration of the tympanic membrane (C) causes the malleus (hammer, D), a tiny bone (ossicle), to vibrate. This converts the air pressure vibrations in the sound to mechanical vibrations in the bone. The malleus (D) causes the incus (anvil, E), another tiny bone, to vibrate. Muscles control how much of the motion of the malleus (D) is transferred to the incus (E). Ligaments connect the bones together. The incus (E) causes the stapes (stirrup, F), yet another tiny bone, to vibrate. Muscles control how much of the motion of the incus (E) is transferred through ligaments to the stapes (F). The stapes (F) causes the oval window (G) to vibrate. The oval window is a membrane that separates the air-filled middle ear from the fluid-filled inner ear. The oval window (G) transmits the mechanical vibrations of the stapes (F) to vibrations in the fluid in the vestibular canal (N) of the cochlea (J). The cochlea (J) sorts the sounds by frequency and stimulates different nerves in the auditory nerve (K) for each frequency. This will be explained in detail below. Sound passing through the entire cochlea (J) exits through the round window (H) and is absorbed by the air in the middle ear. The semicircular canals (L) are not used for hearing. They are used for balance, keeping track of any motion of the head. But information about motion of the head is needed to keep motion of the head from being heard as sound through motion of the cochlea as the head moves. The Eustachian tube (M) goes from the middle ear to the nose. It equalizes pressure between the middle ear and the outside air during a yawn. How the cochlea sorts sound frequencies: The vestibular canal (N) carries sound waves in fluid from the oval window (G) to the membrane assembly(R). After the sound vibrates the membrane assembly (R), it returns to the round window (H) through the tympanic canal (P). The membrane assembly (R) is a group of several membranes and sensory organs. It contains the hair cells that turn sound energy into nerve impulses. The membrane assembly (R) is tapered, with the narrow end near the base of the cochlea, where the oval and round windows (G & H) are, and the wide end is at the apex, where the helicotrema (S) is. The narrow end is stiff, while the wide end is floppy. These two properties, combined with the distance along the canals from the base, determine which parts of the cochlea are responsive to which frequencies. Each position is resonant to mainly one frequency, and vibrates the most with that frequency. The highest frequencies (20 KHz) are detected at the base of the cochlea. The lowest frequencies (63 Hz) are detected at the apex. See the frequency diagram at right. Since the nerves can't pulse faster than about 100 Hz, the frequencies must be sorted and given separate nerves. Nerve pulsing speed encodes loudness, not pitch. Frequencies between 20 Hz and 60 Hz vibrate the entire membrane assembly (R). The sound is detected, but not given a sensation of pitch (frequency). Frequencies below 20 Hz normally pass through the helicotrema (S), bypassing the membrane assembly (R). They are not normally detected. Frequencies above 20 KHz normally can't vibrate the membrane assembly (R), so they are not normally detected. The brain then reconstructs the nerve impulses into what we actually perceive. Consonant (pleasant) sets of tones make the membrane assembly (R) vibrate gently with inactive nodes between the vibrating parts. Dissonant (unpleasant) sets of tones cause the membrane assembly (R) to vibrate harshly, with parts of it that are close together moving in opposite directions.

A. Pinna (earflap) B. Ear Canal C. Tympanic Membrane (eardrum) D. Malleus (hammer) E. Incus (anvil) F. Stapes (stirrup) G. Oval Window H. Round Window J. Cochlea K. Auditory Nerve L. Semicircular Canals M. Eustachian Tube N. Vestibular Canal P. Tympanic Canal R. Membrane Assembly S. Helicotrema The frequency locations are approximate, since various people's cochleas are made differently.

Notes on the structure and function of the ear:

• A single membrane could do the sound decoding job of the membrane assembly (R), except that the nerve cells need to be in a nourishing fluid, while the sound waves need to move through a different kind of fluid.
• The width of the membrane assembly (R) is its width between attachment points. The sizes of the vestibular canal (N) and tympanic canal (P) are not dependent on this width.

Links:

• MIXING YOUR OWN SOUND
• HOME