Passive Sound Examination (Stethoscope)
Vibrations, such as those caused by the closing and opening of
heart valves or air moving through the respiratory passages, travel through the body
as sound waves. Sound is generally conducted better and more rapidly in liquids
than in gases. Thus, bronchial breath sounds are heard better when the bronchi are
surrounded by lung consolidation. When sound waves reach a sudden change in density,
some of the energy is reflected.
Some simple facts about sound waves can facilitate an understanding
of the reflection and scattering process in the body. First, all sound waves can
be represented as a summation of sinusoidal waves of various frequencies and amplitudes.
This process of Fourier or power spectral analysis has been discussed earlier in
this chapter. A note from a musical instrument thus consists of a sine wave at the
"fundamental frequency" plus many "harmonics." The fundamental is the frequency
that describes the pitch of the tone: middle C is standardized at 256 Hz, for example.
Harmonics are sine waves at frequencies that are multiples of the fundamental frequency.
Fortunately for our ears, all of these many frequencies propagate
at the same speed, the speed of sound (called "a"). For ideal gases, the speed of
sound is proportional to the square root of temperature. The speed of sound in air
at room temperature is 344 m/sec, or 1129 ft/sec or
Figure 30-24
Doppler effect. A, When
a listener is moving toward a stationary sound source, the frequency increases because
the listener traverses more waves per unit time than a stationary listener does.
B, When a sound source is moving toward a stationary
listener, the wave fronts "stack up," thereby causing an apparent increased frequency.
770 miles/hr (mph). At an altitude of 13,000 m (40,000 ft), where the standard temperature
is -57°C, the speed of sound is only 295 m/sec, or 661 mph. The speed of sound
is much higher in liquids than gases. For example, the speed of sound through water
at 15°C is 1450 m/sec. This value approximates the speed of sound through most
of the solid parts of the body. In solids, the speed of sound varies greatly, with
a range of 54 m/sec in rubber to 6000 m/sec in granite. Reflection of sound occurs
at interfaces where the product of the tissue density and the speed of sound (ρ
× a) changes suddenly. Larger changes in this "acoustic impedance" result
in greater reflection and less transmission. In the human body, the largest changes
in acoustic impedance occur at gas-tissue boundaries: the lungs and the gastrointestinal
tract. Reflection of sound thus makes it difficult to auscultate heart tones through
an air-filled, emphysematous chest. For the same reason, a transthoracic echocardiograph
provides less detail than the transesophageal technique does; in the former case,
the lungs are in the way.
The first attempts to gather information about the inside of the
patient by using sound involved placing one's ear directly on the patient. Although
this procedure had many limitations, it led to development of the modern stethoscope,
which is based on the physical principles of sound transmission. The stethoscope
uses a large diaphragm to transmit and concentrate the sound energy. The bell acts
as both an amplifier and a low-pass filter to transmit low-frequency diastolic rumbles.
The physics of stethoscopy were described in depth by Rappaport and Sprague.[15]
In a simple stethoscopic examination, errors are introduced by
"signal processing," that is, lack of appropriate knowledge on the part of the listener.
Because it is a nonpowered, nontechnologic device (the energy levels come from the
phenomena themselves), an esophageal or precordial stethoscope has additional value
in being a continuous monitor during power outages. Physical limits to the technique
include air space disease (in itself an informational finding), inability to place
the monitor appropriately, and lack of quantifiable data.
