[SoundStage!]Getting Technical
Back-Issue Article

September 2003

Loudness

Some people love their music loud.

At stop lights, your car vibrates like a defective washing machine because of the deep, thundering bass of the custom speakers in the car beside you. Leaving rock concerts, you try to discuss the show with ringing ears and a grin on your face. In dance clubs, you are unable to pursue the most elementary getting-to-know-you conversation because 130 beats per minute of house music drowns out all but the loudest shouting.

In addition to the ringing in our ears, we’ve heard something else because of our allegiance to loud music. Particularly while growing up, and particularly from our parents -- or are you one of those unfortunates, who, while trying to provide an evening's entertainment for your friends, have had the unpleasant sensation of opening your front door to police officers? In either case, the refrain is the same: "Turn that stereo down!"

Not all people appreciate -- let alone love -- loud music.

Sound

Before I discuss loudness any further, though, an understanding of the basics of sound is required. What is sound, and how do we hear it?

Sound is created by pressure changes in the air that are made by a vibrating body such as a vocal cord or guitar string. These pressure changes travel in waves much like ripples in a pond caused by a falling stone. The energy of the waves spreads from air particle to air particle. (This is why "in space no one can hear you scream." There are no air particles to propagate the spread of the wave.)

Sound waves have three characteristics: amplitude, frequency, and complexity. Loudness is determined by amplitude and -- due to the way in which our hearing system functions -- frequency. The amplitude of a sound wave is the distance covered by the vibrating particles between their lowest and highest points. The greater the intensity of the particle's vibration, the greater the amplitude of its wave and the louder the sound. Loudness is considered the psychological perception of this intensity.

The decibel

Common decibel levels
  • Threshold of hearing in quiet: 0dB
  • Whisper: 25dB
  • Normal conversation: 60dB
  • My auto: 80dB
  • Subway: 100dB
  • Rock concert: 110dB
  • Thunder: 120dB
  • Pain threshold: 130dB

Intensity of a sound is measured in pressure. Pressure describes the vibration of an object and is a measurement of force upon a unit of area, specifically newtons per square meter. The human ear is extremely sensitive to pressure differences and is able to detect pressure differences that range from one to an order of billions. Therefore, loudness is measured in a logarithmic scale of pressure ratios. In honor of Alexander Graham Bell, this scale has been named the decibel (dB), and its scale neatly converts the range of perceived pressure differences or loudness from 0 to 160dB. A sound’s pressure is measured against a reference pressure, which is the lowest pressure required for the human ear to hear a tone of 1000Hz. This brings us to the role played by sound frequency with respect to loudness.

Loudness and frequency

To determine the frequency of a sound wave, we measure the number of cycles that occur in one second, deriving a unit of measure called a Hertz (Hz). A cycle is the distance between the highest point of the wave, down to the lowest point, and back up to the highest again. Human hearing perceives frequency as pitch and is able to perceive frequencies from 20Hz to 20,000Hz (20kHz).

Not all frequencies are created equal. The human ear has frequency favorites, with 3000Hz being the most preferred. At this frequency, the quietest- or lowest-intensity sounds can be heard and they are close to being the sound given off by the movement of air molecules. It is believed that it’s not accident that the ear is most sensitive to this frequency -- 3000Hz is the frequency of a human cry.

Therefore, sounds of different frequency have different intensity threshold values. For sound with frequencies between 1000Hz and 4000Hz, the quietest or lowest-intensity sounds are heard. Outside of this range, the intensity must increase in order for the sound to be heard. Due to this differential sensitivity to frequency, some amplifiers have a "loudness" control. When listening to low-volume (low-intensity) sound, adjustments must be made to compensate for the fact that the low and very high frequencies of the music require higher intensity to be heard with equal loudness.

Fletcher-Munson curves

In 1933, American scientists H. Fletcher and W. Munson studied the relationship between loudness and frequency. Their graph demonstrates that sounds with different intensities and frequencies are perceived as being equally loud. The contours on the graph represent sounds that are heard as being of equal loudness. For example, a sound of 300Hz played at 40dB is as loud as a 1000Hz sound played at only 30dB.

The ear

Why loudness is dependent on the pitch of a sound can be explained by the physical properties of the human ear, a miraculous acoustic system with numerous tiny, intricate parts, all of which are designed with painstaking care to work together within microscopic tolerance ranges.

The ear has three sections: outer, middle, and inner. The outer ear, the pinna or ear flap, and the ear canal are all considered part of the outer section, and function to capture sound waves and direct them into the middle ear. While the convolutions of the ear flap or pinna seem to act as a good repository for piercings, in actuality, they are evolved to capture sound waves. Particularly in the case of sounds with high frequencies, such as 5000Hz, the convolutions amplify them before they enter the ear canal. The pinna also prevents objects from entering the ear canal where they may damage the delicate eardrum.

The ear canal's physical shape also acts as an amplifier for incoming sounds, especially those with frequencies of 3000Hz. Together, the ear canal and pinna can increase the intensity of sounds by as much as 15dB.

Like the pinna, the ear canal also plays a role in protecting the eardrum. Small hairs in the canal serve as broom-like "cleaners" and the constant production of ear wax in this region provides further protection.

The middle ear begins at the eardrum and includes the three smallest bones of the human body: the malleus, the anvil, and the stapes. The air-pressure changes of the incoming sound cause the eardrum to vibrate, which in turn sets the middle bones into motion so that the vibrations are passed along to the oval window of the inner ear. The eardrum and the middle ear bones also amplify low-intensity sounds. By the time a sound reaches the inner ear, the combination of the outer and middle ear may have increased its loudness by as much as 20dB.

It's all about the hair cell

Our sensory system's basic role is to transform physical stimuli such as sound, light, touch, or temperature into electrical nerve impulses that are then interpreted by the brain. It is the hair cells of the inner ear that perform this very important function. These hair cells are contained within the inner ear's cochlea, a snail-shaped structure less than a centimeter in size. The cochlea contains three lengthwise, fluid-filled chambers (canals). Vibrations occurring at the inner ear’s oval window, passed along by the middle ear bones, cause movement in this fluid. The frequency and intensity of the varying sounds, and hence vibrations, will cause the type and amount of fluid movement to differ.

The canals are divided by two membranes: Reissner's and basilar. It is the basilar membrane that is most involved in the perception of sound. Upon it sits the organ of Corti, which contains the hair cells and nerve fibers that innervate them. The vibrations in the cochlear fluid move the hair cells and the flexible basilar membrane, with different hair cells and different patterns of hair cells being stimulated depending on the type of fluid movement caused by the sound. This is because each ear has roughly 24,000 hair cells -- each with 100 microscopic hairs or stereocilia and connecting to some 50,000 nerve fibers -- and not all of these hair cells respond equally to the stimulation caused by the movements of the basilar membrane and surrounding fluid. Some hair cells work together to stimulate just one nerve fiber and are therefore suited for the detection of quiet sounds. Other hair cells have hundreds of nerve fibers innervating each one. These detect louder sounds.

The position of the hair cells on the organ of Corti is also of importance, since frequency discrimination is determined by which hair cells are stimulated. At one end of the organ of Corti, the hair cells signal low frequencies. As you move to the opposite end, they signal higher and higher frequencies. This organization -- like frequencies being positioned near like frequencies -- continues all the way into the higher processes of the auditory cortex of the brain.

What about the brain?

The nerve fibers attached to the hair cells bundle to form the auditory nerve, which leaves the inner ear and enters the brain. In the brain, the signals pass through various neural structures such as the cochlear nucleus, inferior colliculus, and eventually the auditory cortex. At this point, the listener becomes conscious of the signals as "sound," patterns are recognized, and the location of the sound is determined. The brain basically functions to make sense of all that noise.

Turn that stereo down!

As observed at the beginning of this article, anyone serious about music has listened to it loud at least some time in his or her life. Sometimes very loud. In fact, turning up the volume way up and watching the subwoofer dance seems to be a rite of passage for some.

Ironically, this practice damages the organ most precious to the audiophile: the ear. And the damage is widespread. In North America, one out of every ten individuals suffers from hearing loss. And, while increasing age is the most common cause, the second most common is overexposure to loud noise. Hearing loss of this type cannot be cured, and hearing aids provide an imperfect solution.

The damage to hearing occurs in the inner ear, and occurs quickly, with degeneration occurring in 30 seconds or less for sounds of 140dB. Loud sounds -- music, industrial, or otherwise -- are represented by extremely intense vibrations on the eardrum, middle ear bones, and cochlea, significantly displacing the cochlear fluid. The basilar membrane moves wildly in response, and the hair cells are over-stimulated again and again, until they are irrevocably damaged -- or even physically ripped from the organ of Corti. Without functioning hair cells, no sound can be transformed into the electrical impulses necessary for the brain to perceive sound.

After listening to loud sounds, sensitivity to loudness decreases. This auditory phenomenon is called Temporary Threshold Shift, or auditory fatigue, and is caused by damaged hair cells and/or fatigue of the stapedius muscle. This middle-ear muscle moves the stapes, which is the inner bone directly connected to the cochlea. Before turning the volume up, it should be noted that fatiguing the muscle prevents the stapes from responding to loud sounds and passing any further dangerous vibrations onto the inner ear. Temporary Threshold Shift occurs within two minutes of the loud noise and may take up to two days to fully recover.

Rock'n'roll is the devil's music!

Your average rock concert plays between 110 and 130dB. Below is a list of victims with permanent hearing damage.

  • Bill Clinton
  • Pete Townsend
  • Ozzy Osbourne
  • Eric Clapton
  • Barbara Streisand
  • Phil Collins
  • Neil Young
  • Lars Ulrick
  • Ted Nugent
  • Paul Schaffer
  • Cher
  • Sting

The ear does its best to protect permanent damage from occurring, and once again the stapedius muscle is involved. When loud noises (100 to 130dB) are first heard, the stapedius muscle tenses. This action effectively decreases loudness by as much as 20dB, by preventing the stapes from reacting to the eardrum's vibrations and thereby subjecting the cochlea to dangerous vibrations. But it’s an imperfect defense mechanism that doesn’t excuse the listener from exercising the required caution. While the stapedius muscle acts quickly -- within half a second -- hearing damage can still occur, because sound travels so quickly.

Another protective measure restricts blood flow to the hair cells, especially those responsible for responding to changes in loudness. This suppresses them from over-stimulation caused by any further loud noises. It's as if the ear pulls the plug before the motor over-heats. Eventually, time passes and blood flow returns and the hair cells function normally again.

Despite these mechanisms, if the human ear is subjected to loud sounds for a long enough period, hair cells will eventually die and cease to respond to any amount of stimulation. This permanent hearing loss is called Permanent Threshold Shift, and it may be accompanied by an on-again/off-again ringing in the ears (tinnitus) that lasts indefinitely. Permanent Threshold Shift may take years of chronic industrial noises -- or may be caused by listening to music that's too loud for just a few hours, which is why many rock'n'roll musicians are afflicted. Simply put, the human ear should never be exposed to sounds of 140dB for any period, no matter how brief.

Ultimately, you want to continue to enjoy music and its reproduction. To do this, you clearly need to do whatever it takes to protect your ears. These irreplaceable acoustic systems are as advanced -- and delicate -- as any high-tech audio components, and they deserve the same care. The next time someone says "Turn that stereo down!," maybe that person is just helping you keep your equipment working properly.

...Alison Aulph
alisona@soundstage.com 

 

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