5.18 Medical Instruments for Detecting Sound
153
Fetal heart sounds are detectable around the tenth week of gestation with an
ultrasonic fetoscope. A regular stethoscope or ‘Pinard’ horn can detect fetal heart
sounds after about 18 weeks. The normal fetal heart rate is 120–160/min.
Breathing sounds include clicking, bubbling, and rattling (collectively called
rales, due to opening of air spaces), and rhonchi, stidor and wheezing (due to
restrictive air passages). Lack of expected sounds is also a symptom.
5.18.2 Medical Microphones
Microphones are used in medical procedures to augment hearing, monitor heart
sounds, hear low-level auscultation effects, and to listen for echoes from ultrasonic
devices. When used externally to listen to internal sounds, the microphone’s
impedance must come close to that of the skin tissue. This can be done with a horn
(a conical shape with the large end on the skin surface and the small end next to the
microphone) or, if a piezoelectric microphone is used, by a gel on the skin.
Hearing aides are active devices (requiring a source of power) which transmit
amplified sound to the inner ear. They can be made to fit the external ear, or made to
be implanted in order to vibrate the cranium near the pinna (with a Faraday induction
rechargeable battery), or to connect directly by electrical stimulation to the nerves
in the cochlea. As electret microphones (see Sect. 5.16.1) can be miniaturized, they
are used for implantation devices. Filtering of tissue noise in the mastoid due to
vocalization, chewing, etc. helps in voice recognition.
5.18.3 Phonocardiogram
A phonocardiogram (PCG) is a recording of the amplified sounds made by the heart
(Fig. 5.12). The dominant sounds come from the closure of heat valves, and is often
characterized by the sound of “Lubb-bidub”. In the cycle of heart activity, the first
sound, S1, (the “Lubb”) is produced by the closure of the atrioventricular valves
(first mitral, then tricuspid) at the beginning of systole (contraction), and the second
sound, S2, (the “bidub”) is the sound of the closure of the aortic valve and then the
pulmonary valve at the end of systole. Both S1 and S2 are similar in their spectral
frequencies below 150 Hz, but are distinguishable above. A third low frequency
sound, S3, may be present, particularly in children and in patients with congestive
heart failure. S3 occurs in early diastole, due to increased atrial pressure causing
increased flow rates. A low frequency sound (S4) just after atrial contraction may
indicate an overly stiff left ventricle. Abnormal sounds such as murmurs, will also
show.
153
Fetal heart sounds are detectable around the tenth week of gestation with an
ultrasonic fetoscope. A regular stethoscope or ‘Pinard’ horn can detect fetal heart
sounds after about 18 weeks. The normal fetal heart rate is 120–160/min.
Breathing sounds include clicking, bubbling, and rattling (collectively called
rales, due to opening of air spaces), and rhonchi, stidor and wheezing (due to
restrictive air passages). Lack of expected sounds is also a symptom.
5.18.2 Medical Microphones
Microphones are used in medical procedures to augment hearing, monitor heart
sounds, hear low-level auscultation effects, and to listen for echoes from ultrasonic
devices. When used externally to listen to internal sounds, the microphone’s
impedance must come close to that of the skin tissue. This can be done with a horn
(a conical shape with the large end on the skin surface and the small end next to the
microphone) or, if a piezoelectric microphone is used, by a gel on the skin.
Hearing aides are active devices (requiring a source of power) which transmit
amplified sound to the inner ear. They can be made to fit the external ear, or made to
be implanted in order to vibrate the cranium near the pinna (with a Faraday induction
rechargeable battery), or to connect directly by electrical stimulation to the nerves
in the cochlea. As electret microphones (see Sect. 5.16.1) can be miniaturized, they
are used for implantation devices. Filtering of tissue noise in the mastoid due to
vocalization, chewing, etc. helps in voice recognition.
5.18.3 Phonocardiogram
A phonocardiogram (PCG) is a recording of the amplified sounds made by the heart
(Fig. 5.12). The dominant sounds come from the closure of heat valves, and is often
characterized by the sound of “Lubb-bidub”. In the cycle of heart activity, the first
sound, S1, (the “Lubb”) is produced by the closure of the atrioventricular valves
(first mitral, then tricuspid) at the beginning of systole (contraction), and the second
sound, S2, (the “bidub”) is the sound of the closure of the aortic valve and then the
pulmonary valve at the end of systole. Both S1 and S2 are similar in their spectral
frequencies below 150 Hz, but are distinguishable above. A third low frequency
sound, S3, may be present, particularly in children and in patients with congestive
heart failure. S3 occurs in early diastole, due to increased atrial pressure causing
increased flow rates. A low frequency sound (S4) just after atrial contraction may
indicate an overly stiff left ventricle. Abnormal sounds such as murmurs, will also
show.