Hearing by Bone Conduction
Hearing usually involves receiving a sound in the form of vibrating air particles. This "regular route" is called air-conduction. It is also possible to hear by bone-conduction, which means that the sound is transmitted via vibrations of the bones of the skull. Bone-conduction can be initiated by air conducted sounds that are intense enough to set the bones of the skull into vibration, or by directly activating the bones of the skull with a vibrator made for this purpose. In clinical audiology, air-conduction signals are presented from earphones or loudspeakers, and bone-conduction stimuli are presented from a bone conduction vibrator. Air-conduction and bone-conduction result in the same cochlear activity, that is the initiation of traveling waves and displacements of the hair cell cilia.
The Nature of Normal Hearing
Audiology is concerned with the normal and abnormal aspects of hearing, its place in the communicative process, and the clinical evaluation and management of patients with auditory impairments. The area of science that deals with the perception of physical stimuli is called psychophysics. Hence, in addition to being an aspect of audiology, the science that deals with perception of sound is also known as psychoacoustics.
The Range of Hearing
Minimal Audible Levels
It is useful to conceive of a range of hearing that includes sounds that are audible and tolerable. The lower curves show the faintest audible sounds or thresholds of normal people in decibels of sound pressure level (dB SPL) as a function of frequency. The striking feature of these curves is that they are not flat. Instead, the SPL needed for a sound to be barely audible depends on its frequency to a major extent. Hearing thresholds are reasonably sensitive between ~100 and 10,000 Hz, and become poorer (i.e., the least amount of intensity is needed to reach threshold) in the 2000 to 5000 Hz range. The resonant responses of the conductive system are largely responsible for the lower thresholds in this most sensitive frequency range.
Upper Limits of Hearing
The upper levels of usable hearing depend on how they are defined. The threshold of uncomfortable loudness occurs at ~100dB SPL, although higher values of ~111 to 115 dB SPL have been reported.
Loudness and Pitch
Loudness and pitch refer to how we perceive the physical attributes of intensity and frequency respectively. It may seem odd to make this distinction since everyone knows sounds with greater intesnities are louder than sounds with smaller intensities, and higher frequencies have higher pitches. These generalities are true. However, we will see that there is far from a one-for-one relationship between the perceptual world of loudness and pitch and the physical world of intensity and frequency. For example, equal loudness or phon curves tell us that all of the following tones sound equally loud even though their SPLs are different:
- 78dB @ 50Hz
- 54dB @ 200Hz
- 43dB @ 500 Hz
- 40dB @ 1000Hz (the reference)
- 39dB @ 2000Hz
- 40dB @ 5000Hz
Recall that our actual hearing sensitivity in decibels of sound pressure level (dB SPL/20uPa) is not the same at every frequency. For example, the average normal person needs 26.5 dB SPL just to barely hear a 250Hz tone, but only 7.5dB SPL to just hear a 1000Hz tone. There are normal threshold SPLs when using typical audiometric earphones. We consider these values to be normal reference values-more technically, reference equivalent threshold sound pressure levels (RETSPLs)-because they are the physical intensities needed by normal people to reach the threshold of hearing. In fact, when we say a person has a "hearing loss," we really mean that she requires higher SPLs than these values to just hear a sound; the more the thresholds deviate from these reference values, the worse is the hearing loss.