Hearing and Basic Audiometric Concepts
The essentials of hearing evaluation and measurement are certainly an appropriatestartingpointforthischapter.Soundperceptionistheprimary reason forthe existenceof the complex sensoryorgans on eachside of the head. Awareness of the environment and the ability to communicate are keyelementstosurvival.Ofcourse,balanceandorientationaresignificant parts of ear function as well—these will be discussed in more detail later. The contents of this brief chapter might seem very technical to readers who do not specialize in ears, but familiarity with the basics of hearing is necessary for an understanding of ear complaints. Audiometric testing, represented in graphic form, is the easiest way to “visualize” a patient’s hearing, and this will be discussed first. Tuning-fork evaluation will be covered in the next chapter.
Pure Tone Audiometry Audiometryistheprecisemethodofhearingassessment.Itisperformedin a soundproof compartment by an audiologist, who uses an audiometer to introduce measured sound intensities of selected tones to the listener, usually through earphones. Pure tone thresholds are the minimal inten- sities of given tones (frequencies) that can be heard by the person being tested.Thesethresholdsareusuallyrecordedonagrid,liketheoneshown in Figure 1.1, to create an audiogram. This one has no responses recorded on it and is presented purely to give an idea of the frequencies and loud- ness of common sounds. On this graph, the vertical axis plots hearing level (HL) in decibels (dB), tiny units of loudness. These two abbreviations are usually used together (dB HL) to report a patient’s test results or to refer to the level of loudness of a given tone. Zero dB HL is near the top of the graph; this level is barely audible to a human with “perfect hearing.” The louder the sound needed for a test response, the greater a patient’s hearing loss. Thus, the scale is inverted—a lower number means better hearing. Above 0, there is a line for –۱۰dB HL, indicating “supranormal” hearing, something like 20:15 vision. At the other extreme, some very loud sounds can be 110dB HL or even greater, such as a nearby jet engine (roughly 125dB HL). Most audiometers only go to 110dB HL. For all practical purposes, an individual who cannot hear noises or tones at 100dB is considered profoundly deaf. Between these extremes, shown on the right side of Figure 1.1, are examples of everyday sounds at their approximate decibel levels. For in- stance,conversationalspeechisroughlyat45dBHL.Theborderlinenormal rangeofhearingisconsideredtobe25dBHLorlessforadultsand20dBHL orlessforchildren.Mostnormalsubjectswilltestclosetothe0line.Hear- ing loss that falls between 25 and 45dB HL is considered mild loss; be- tween 45 and 70dB HL, moderate loss; between 70 and 90dB HL, severe loss; and greater than 90dB HL, profound loss. Often, to patients, we refer to decibels of hearing loss as “percentage” loss, which is not completely accurate but a convenient approximation. Frequencies are displayed along the horizontal axis. The lowest fre- quency measured by the audiometer is usually 125Hz, or cycles per second. Actually, this is not a very low tone—it corresponds to the note C one octave below middle C on the piano. The fundamental vocal tone of an adult male, when producing vowel sounds, is about here. Each dou-blingof this number in Hertz correspondstoa frequencyone octave high- er. Thus 4000Hz, or 4 KHz, is the highest C on the piano.* The human ear perceives a vast range of frequencies, from 20 to 20 000Hz or more, but as the graph shows, the 125–۸۰۰۰-Hz range is
what we usually test audiometrically. The frequencies most important for hearing and understanding human speech are from between 500 and 4000Hz. Vowel sounds are at the lowend of this range or even lower (as previously stated, a deep-voiced male actually phonates at 125Hz). Consonant sounds tend toward the higher frequencies, especially the non-voiced ones like “s,” “f,” “sh,” and “th.”