Daryle Conlin

Conviently located on the upper level of the Amsterdam River Front Center.  2440 Riverfront Center Amsterdam ,NY 12010. Open by appointment.

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The Real Cost of Headphones

Hearing damage from headphones is probably more common than from loudspeakers, because many people exploit the acoustic isolation by listening at higher volumes. Moreover, the risk of hearing damage from headphones is higher than with loudspeakers, even at comparable volumes, due to the close coupling of the transducers to the ears. One of the benefits of headphone listening is the ability to detect musical details; just be aware of the volume and remember to keep it low.

Help for those who are hearing impaired--
from the hearing instrument specialist.

HOW WE HEAR

Hearing (or audition) is one of the traditional five senses. It is the ability to perceive sound by detecting vibrations via an organ such as the ear. The inability to hear is called deafness.

Your ears are extraordinary organs. They pick up all the sounds around you and then translate this information into a form your brain can understand. One of the most remarkable things about this process is that it is completely mechanical. Your sense of smell, taste and vision all involve chemical reactions, but your hearing system is based solely on physical movement.

When something vibrates in the atmosphere, it moves the air particles around it. Those air particles in turn move the air particles around them, carrying the pulse of the vibration through the air.

When you hit a bell, the metal vibrates -- flexes in and out. When it flexes out on one side, it pushes on the surrounding air particles on that side. These air particles then collide with the particles in front of them, which collide with the particles in front of them, and so on. This is called compression.

In this way, a vibrating object sends a wave of pressure fluctuation through the atmosphere. We hear different sounds from different vibrating objects because of variations in the sound wave frequency. A higher wave frequency simply means that the air pressure fluctuation switches back and forth more quickly. We hear this as a higher pitch. When there are fewer fluctuations in a period of time, the pitch is lower. The level of air pressure in each fluctuation, the wave's amplitude, determines how loud the sound is. In the next section, we'll look at how the ear is able to capture sound waves.

The mechanisms of sound interpretation are poorly understood, in fact is not yet clear whether all people interpret sounds in the same way. Until recently, there has been no way to trace the wiring of the brain, no way to apply simple stimuli and see which parts of the nervous system respond, at least not in any detail. The only research method available was to have people listen to sounds and describe what they heard.

LOUDNESS

The ear can respond to a remarkable range of sound amplitude. (Amplitude corresponds to the quality known as loudness.) The ratio between the threshold of pain and the threshold of sensation is on the order of 130 dB, or ten trillion to one. The judgment of relative sounds is more or less logarithmic, such that a tenfold increase in sound power is described as "twice as loud". The just noticeable difference in loudness varies from 3 dB at the threshold of hearing to an impressive 0.5 dB for loud sounds.

PERCEIVED LOUDNESS OF SOUNDS

The sensation of loudness is affected by the frequency of the sound. A series of tests using sine waves produces the curves shown. At the low end of the frequency range of hearing, the ear becomes less sensitive to soft sounds, although the pain threshold as well as judgments of relatively loud sounds are not affected much. Sounds of intermediate softness show some but not all of the sensitivity loss indicated for the threshold of hearing. At high frequencies the change in the sensitivity is more abrupt, with sensation ceasing entirely around 20 khz. The threshold of pain increases in the top octave also.

The ability to make loudness judgments is compromised for sounds of less than 200ms duration. Below that limit, the loudness is affected by the length of the sound; shorter is softer. Durations longer than 200ms do not affect loudness judgment, beyond the fact that we tend to stop paying attention to long unchanging tones.

MASKING

The threshold of hearing for a particular tone can be raised by the presence of another noise or another tone. White noise reduces the loudness of all tones, regardless of absolute level. If the bandwidth of the masking noise is reduced, the effect of masking loud tones is reduced, but the threshold of hearing for those tones remains high. If the masking sound is narrow band noise or a tone, masking depends on the frequency relationship of the masked and masking tones. At low loudness levels, a band of noise will mask tones of higher frequency than the noise more than those of lower frequency. At high levels, a band of noise will also mask tones of lower frequency than itself.

PITCH

People's ability to judge pitch is quite variable. (Pitch is the quality of sound associated with frequency.) Most subjects studied could match pitches very well, usually getting the frequencies of two sine waves within 3%.

TIMBRE

Recognition of sounds that are similar in aspects other than pitch and loudness is not well studied, but it is an ability that everyone seems to share. We do know that timbre identification depends strongly on two things, waveform of the steady part of the tone, and the way the spectrum changes with time, particularly at the onset or attack. This ability is probably built on pattern matching, a process that is well documented with vision. Once we have learned to identify a particular timbre, recognition is possible even if the pitch is changed or if parts of the spectrum are filtered out.

LOCALIZATION

We are also able to perceive the direction of a sound source with some accuracy. Left and right location is determined by perception of the difference of arrival time or difference in phase of sounds at each ear. If there are more than two arrivals, as in a reverberant environment, we choose the direction of the first sound to arrive, even if later ones are louder. Localization is most accurate with high frequency sounds with sharp attacks.

Height information is provided by the shape of our ears. If a sound of fairly high frequency arrives from the front, a small amount of energy is reflected from the back edge of the ear lobe. This reflection is out of phase for one specific frequency, so a notch is produced in the spectrum. The elongated shape of the lobe causes the notch frequency to vary with the vertical angle of incidence, and we can interpret that effect as height. Height detection is not good for sounds originating to the side or back, or lacking high frequency content.