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Ears to You!

May 1, 2004

Like many artists, musicians can be obsessive perfectionists. Modern technology offers so much control over sound that you can spend hours or even weeks on achieving just the sound you desire. With such attention to detail, you naturally want to use studio hardware that will provide the finest audio quality possible. You might fret over whether a piano multisample affords enough Velocity cross-switching, or if a condenser microphone is the right choice for the job at hand. Even the most transparent studio monitors, however, can't compensate for deficiencies in that final filter before sound reaches our brains: our ears.

CAN YOU HEAR ME NOW?

That humans can hear and make sense of anything at all, much less something as complex as speech, stereo imaging, or the release contour of a guitar note, is truly mind-boggling. For us to hear the way we do, an amazing series of events must take place as our auditory systems transform vibrations into music.

Sound waves travel through the air and are collected by the outer ear, known as the auricle or the pinna. (Until recently, the function of the outer ear was largely unknown, but we now realize that the human ear's particular shape alters the spectra of incoming sounds in ways that help us localize sounds in three-dimensional space.) From the outer ear, sound travels down the external ear canal to the point at which it hits the eardrum, or the tympanic membrane (see Fig. 1). The tympanic membrane acts like the head of a drum and is stretched taut over the delicate contents within.

The eardrum, however, is no ordinary drum. The size of the vibrating portion of the eardrum averages only about 0.085 square inches. The drum's underside is attached to the first of the three hearing bones, or ossicles. These bones are the malleus (which resembles a hammer or mallet), the incus (which resembles an anvil), and the stapes (which resembles a stirrup). In a fascinating system of leverage and impedance matching, audible vibrations travel down the malleus to the incus and then to the stapes, which sits at the entrance of the cochlea, also known as the inner ear. (The term external ear refers to the outer ear, ear canal, and outer portion of the eardrum; the middle ear is the space between the eardrum and the cochlea.)

Within the cochlea — which is a snail-shaped structure encapsulated in dense bone — the fluids of the inner ear receive the mechanical movements of the stapes bone in much the same way that waves are produced when you throw a pebble into water. The incoming fluid wave stimulates neural hair cells that are spatially organized according to frequency. Somehow, all of the incoming sound is divided and received by the appropriate frequency-specific hair cell, which sends it to the auditory nerve.

What is the point of such a complex system? Why doesn't sound traveling through the air stimulate the fluid directly? This elegant system of conductive transmission matches the impedance of sound traveling through an air-fluid interface, which amplifies the original sound more than 20-fold to produce a gain of approximately 25 to 30 dB.

The process described here, however, is probably the least complicated part of the auditory system. How the brain makes sense of the signals coming up the auditory nerve — how it simultaneously extracts melodies, harmonies, rhythms, timbres, and lyrics — is a mystery that science is just beginning to unravel.

BREAK IT DOWN

Naturally, with such an intricate system for sound transmission, the auditory system is susceptible to degradation over time. The sad fact is that hearing tends to decline with age, primarily because human cochlear hair cells do not regenerate once they are damaged. (They do in some animals, such as birds.) You start out with a fixed number of auditory nerve fibers and hair cells at birth, and that number only decreases over time.

Physicians categorize hearing losses as one of two types: conductive or sensorineural. With a conductive hearing loss, the neural elements of the auditory system are in place, but sound is not getting to them. It's like having a mixing console that works, but trying to record with a broken microphone. The cause of conductive hearing loss can be as simple as an ear full of wax or a hole in the eardrum, or it might be more complex — a middle-ear infection, for example, or arthritis of the hearing bones (a condition known as otosclerosis). As a general rule, conductive hearing losses can be medically or surgically treated and improved.

In contrast to conductive hearing loss, sensorineural hearing loss is caused by the failure of the auditory system's neural elements. One analogy would be having a collection of beautifully functioning microphones and a broken mixing console; the sound might reach the mixer, but nothing would be recorded because the mixer isn't working. The causes of sensorineural hearing loss can be as diverse as aging, a congenital defect, acoustic trauma, or tumors of the auditory nerve. Generally, such losses can't be corrected by surgery unless they have already led to deafness.

Someone can also have a mixed hearing loss, in which both conductive and sensorineural components are present. Hearing loss has literally hundreds of causes, ranging from genetic syndromes to getting punched in the ear. All hearing loss, however, can be categorized as purely sensorineural, purely conductive, or a combination of sensorineural and conductive.

Although it is daunting to realize that some hearing loss is beyond repair, it's important to determine the cause of the loss and ensure that some underlying disease isn't responsible. In some cases, hearing loss is only the tip of the iceberg indicating a more threatening condition.

THE SOUND OF MUSIC

Most musicians like their music loud; it just sounds better. Although I am an ear surgeon and the treatment of deafness (and musicians) is my specialty, even I prefer to turn the volume up. Increasing the level lets you hear the nuances of a recording, to feel surrounded by the music. But how loud is too loud? (See the table “Noise Exposure.”)

Some musicians turn up the volume because they have so much hearing loss that they can't hear anything unless it is really loud. Acoustic trauma à la Pete Townshend is one of the most common causes of sensorineural hearing loss (see Fig. 2). The Occupational Safety and Health Association (OSHA) has set out clear guidelines for how loud is too loud. These guidelines state that anybody exposed to a sound-pressure level (SPL) greater than 85 dB is at risk for noise-related hearing loss and requires ear protection. OSHA has set firm guidelines for allowable noise exposures in the workplace (see the table “OSHA Guidelines”). These rules are intended to prevent hearing loss in people whose occupations place them at high risk, such as airport traffic workers who listen to airplanes take off throughout the day.

The justification for these rules is simple: acoustic trauma produces permanent hearing loss. Hearing loss first occurs by a process known as temporary threshold shift. After an acute exposure to a loud rock concert, most concert-goers know all too well the cotton-in-the-ears feeling they have. The auditory world is muffled, and the volume of all sounds is diminished. That muffling is the result of a temporary threshold shift, in which the threshold sound level at which hair cells fire is increased, thereby requiring the sound to be louder in order to be heard. Normally, this shift is most pronounced in the high-frequency region. Over the course of hours or even days, the feeling slowly subsides until hearing thresholds have returned to normal.

After repeated assaults on the auditory system and recurrent temporary threshold shifts, however, things start changing. The system loses its ability to rebound from the threshold shift. Eventually, the threshold shift becomes permanent and irreversible. No medication or surgery in the world can effectively undo such changes, and the affected individual will lose the full frequency spectrum of his or her hearing forever. For a musician or sound engineer, the consequences can be downright devastating.

To compensate for hearing loss caused by acoustic trauma, the natural tendency is to increase the volume again, causing the entire cycle to repeat. Acoustic trauma that occurs in short, loud bursts causes its worst hearing loss at about 4 kHz on an audiogram (a test that quantifies the frequency and severity of hearing loss), but this is variable. I have seen many patients with nearly complete high-frequency hearing loss caused by acoustic trauma. Even if both ears are not equally affected, the effects of single-sided hearing loss for a musician can be disastrous, as the ability to perceive sounds in stereo and localize sounds in auditory space is diminished.

THE SOUNDS OF SILENCE

It would be bad enough if hearing loss were the only consequence of acoustic trauma, but unfortunately, it isn't. Many people with hearing loss, particularly high-frequency loss caused by continued exposure to loud volumes, find that lost frequencies are replaced by something worse than silence: the phenomenon of tinnitus.

Tinnitus refers to the perception of a sound in the absence of external stimuli. At one time or another, most people have had short-lived bouts of tinnitus, which is usually described as a high-pitched ringing. With hair-cell loss, however, it is possible (and quite common) for tinnitus to become permanent. Tinnitus can even affect people who are deaf. Patients suffering from tinnitus have to deal with a constant whine that can affect either one or both sides. Also described as buzzing, humming, or whistling, the sound ranges in annoyance from a mild nuisance in the evenings to a constant, debilitating condition that can lead to depression or even suicide.

How do you cure tinnitus? That is a million-dollar question, and if a reliable cure for tinnitus is discovered, it will have a tremendous impact on humanity. As of now, no reliable cure has been found, although some measures are reported to help. Medical science is beginning to understand that tinnitus is not simply an auditory percept generated from aberrant firing of cochlear hair cells. Even if you were to cut someone's auditory nerve (which connects the inner ear to the brain stem), you wouldn't necessarily reduce tinnitus; in fact, you might make it worse, and you would definitely make the patient deaf on one side.

Tinnitus is an auditory percept generated from disorganized neural activity within the portions of the brain stem that are responsible for sound perception. One leading theory is that the loss of neural hair cells (say, from acoustic trauma) causes disinhibition (a loss of suppression) of auditory brain stem neurons that typically respond to that frequency range. This disinhibition causes an increase in the brain stem's neural activity that the auditory cortex (the higher brain structure responsible for sound processing) perceives as a sound — namely, tinnitus — despite the lack of external acoustic stimuli.

CHECK YOUR HEAD

As annoying as tinnitus can be, however, it usually isn't a sign of anything more dangerous. If severe tinnitus has a clear cause, such as a long history of standing in front of a Marshall rig, it is generally benign. However, certain other conditions, such as tumors of the hearing nerve, can cause tinnitus on the affected side. Although such ailments are certainly less common than tinnitus caused by acoustic trauma, they are much more serious and early diagnosis is the key to successful management.

So what can you do? Get checked out. Anybody who has one-sided tinnitus or tinnitus of a pulsatile character (in rhythm with the heartbeat) should be evaluated. In addition, anybody who recognizes (or has been told frequently) that he or she has hearing loss should be evaluated. Make an appointment with an otolaryngologist (ear, nose, and throat specialist), or better yet, with an otologist or neurotologist (ear specialist). Get your hearing formally tested by an audiologist and have your ears examined.

I'm continually amazed by how few musicians have ever had their hearing examined, even though they suffer from auditory complaints and their livelihood and joy are dependent upon their hearing. That's like a painter with blurry vision not seeing an ophthalmologist or getting eyeglasses. Perhaps some people are afraid of finding out they have a serious ailment, or they hate the idea of needing a hearing aid.

The one thing that anyone with hearing complaints must be sure to do is to see a medical professional skilled in audiology. Regardless of whether anything can be done about your hearing problems, it's still important to quantify the amount and the frequency range of any hearing loss. Whether you are a musician, an engineer, or a composer, you need to know if you have any hearing loss so that you can account for it when you listen to music and make critical decisions in the studio. If you know that you have a high-frequency hearing loss, you will learn to stop boosting the highs just so you can hear them, because what sounds right to somebody with hearing loss sounds bad to somebody with normal hearing.

PUMP DOWN THE VOLUME

Unfortunately, OSHA's guidelines for sound exposure don't translate well for musicians. First of all, musicians need to hear. You can't expect musicians to just put on protective earmuffs and be satisfied or even able to perform their jobs. If earplugs (even musician's earplugs, which are designed to attenuate frequencies evenly) were a perfect solution, they would be in much wider use; however, ear protection does reduce one's ability to hear accurately.

In addition, the type of noise exposure that musicians are subjected to is difficult to quantify and control. For example, do you have any idea how many decibels you're exposed to every day? No one can ensure that the drummer who bangs away in the basement or the guitarist who wails in front of an amplifier uses ear protection. It's a free country, and if you want to play at the loudest possible volumes, nobody (except maybe the police or your spouse) can stop you.

General guidelines can be established, though. As a very basic first step, accept that any sound that causes pain is too loud. If your ears hurt every time you gig or rehearse, you are certainly suffering from hair-cell injury that could become permanent. Turn down the volume. Get farther away from the speakers. Leave the mosh pit. At the very least, get earplugs to protect your ears when they hurt; otherwise, you won't be hearing for long.

If, however, you notice that your ears feel plugged up after listening or performing but they don't hurt, you are experiencing a threshold shift from assaulting your hair cells — one that will eventually become permanent. If that's the case, you can completely avoid permanent injury by paying judicious attention to ear protection, by lowering volumes, and by avoiding excessive noise. Like the hair that's lost from a balding head, hair cells don't come back.

For reasons that I have never understood, headphones lead people to turn volumes up loud. Maybe it's the crisp sound, the feeling of immersion, or the sense of isolation from the world. People at risk for hearing loss, however, need to be aware that just because the sound is coming from a tiny speaker (or even an earbud), it is no less damaging at high levels than larger speakers are. I encourage you to monitor through headphones only when absolutely necessary, and be mindful that headphone volumes can become intense but don't need to be (see Fig. 3). You will probably find after lowering the volume that you adjust to it rather quickly, and you will soon experience the same enjoyment you had previously with louder levels — with the full range of sonic immersion and crisp sound.

When monitoring through loudspeakers, keep track of how long you are listening and at what volumes. The same goes for musicians who rehearse or perform for extended sessions. If possible, try to measure the output in decibels, so you can get a sense of the amount of energy your ears are processing. Music tends to be loud in spurts, with peaks and valleys that correspond to sounds such as cymbal crashes and song-ending flourishes. Such peaks can damage your ears even if the average volume level is not excessive. If you are getting close to the OSHA recommendation (say, listening for four hours a day at 95 dB), you are at risk for hearing loss. You'll need to take breaks, turn down your levels, and get your hearing checked on an annual basis (see Fig. 4).

FIXING A HOLE

If you do have hearing loss, take heart; in most cases a lot can be done to help, though the prognosis depends mostly on the cause. The great majority of reasons for conductive hearing loss, including a hole in the eardrum, arthritis of the hearing bones, and obstructions in the ear canal, can be fixed with surgery. Although surgery is no fun, it's better than having no options at all. Some hearing losses are caused a by disease that would be dangerous to leave undiagnosed, such as cholesteatoma, a skin cyst that forms within the middle ear, destroys the hearing bones, and can eventually erode into the brain. These nasty cysts usually produce a combination of recurrent infections, drainage out of the ear, and hearing loss. In such cases, the first goal is not to restore hearing, but to remove the disease, as it does not improve on its own.

Like music technology, medical technology has accelerated at an astounding rate in recent years. In most cases, we are now able to treat complete deafness in both ears. Deafness, which is usually caused by complete sensorineural hearing loss in both ears, can be treated with a device known as a cochlear implant. This device, surgically implanted into the inner ear, bypasses the defective hair-cell system and stimulates the hearing nerve directly. The user of a cochlear implant wears a microphone and sound processor attached to the ear, which uses a magnet to communicate with the internal implant. Sounds crazy, doesn't it? What is really crazy about it is how well it works. The cochlear implant is the only successful neural prosthesis available to date. There is an active and growing community of cochlear implant users that have musical backgrounds and are now learning once again to enjoy the benefits of music through their implants. For people with less severe sensorineural hearing loss, hearing aids (basically a small microphone, amplifier, and speaker) remain the mainstay of rehabilitation.

THE EARS DON'T LIE

Take care of your ears! They're the only ones you have. Musicians' ears work overtime on a daily basis, and that can take its toll. Be aware of how loud things are, and try to recognize any signs of hearing loss. Avoid excessive volume levels and use ear protection when it is reasonable. If you think you might have a problem with your hearing, you're probably right. Don't ignore it — go see an ear specialist, because these things usually only get worse with time. In the end, the ears have it, and as long as they do, they'll give you that most perfect gift: the ability to hear.


Charles J. Limb, M.D. (climb@jhmi.edu) is a neurotologist at Johns Hopkins Hospital and the National Institutes of Health. He specializes in the treatment of hearing disorders in musicians.

Noise Exposure

This table shows common sound sources and average decibels. Because the decibel scale is logarithmic rather than linear, doubling the decibel level is not the same as doubling the volume. Instead, a 10 dB increase in sound is equivalent to a tenfold increase in total sound energy.

SOURCE AVERAGE VOLUME
Soft whisper 30 dB
Rainfall 50 dB
Normal conversation 60 dB
TV audio 70 dB
Toilet flushing 80 dB
Boom box, volume on high 100 dB
Shouting 110 dB
Symphony concert 110 dB
Rock concert 115 dB
Ambulance siren 120 dB
Car stereo, volume on high 125 dB
Percussion section, orchestra 130 dB
Airplane taking off 140 dB
Shotgun 170 dB

OSHA Guidelines

According to OSHA, noise exposures greater than the maximum allowed will lead to permanent hearing loss. Because the effects are cumulative, the most accurate way to estimate sound-level exposure is to consider the total throughout the day.

MAXIMUM SOUND LEVEL ALLOWABLE DURATION PER DAY
90 db 8 hours
92 db 6 hours
95 db 4 hours
97 db 3 hours
100 db 2 hours
102 db 90 minutes
105 db 60 minutes
110 db 30 minutes
115 db 15 minutes or less

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