A Simple Acoustic Sensor Test Platform for Respiratory and Cardiac Acoustical Monitoring

1.1. Stethoscopes

Precordial stethoscopes and esophageal stethoscopes are often used to listen to breath and heart, especially during surgical anesthesia. Precordial stethoscopes are typically employed on a weighted chest piece or a conventional stethoscope head in contact with the chest (Fig. 1), while esophageal stethoscopes are typically soft, thin-walled tubes inserted into the esophagus in the unconscious patient. Esophageal stethoscopes usually feature a distal cuff designed to optimize sound transmission and a male Luer connector for attachment to standard earpieces. Many models support core body temperature measurements as well. Both types of stethoscopes can be very useful in monitoring changes in breath or other bioacoustic phenomena. Examples include the detection of crackles, wheezes and unilateral ventilation resulting from deliberate or inadvertent endobronchial intubation.

As the science of acoustics progresses, a number of innovations lead to improved stethoscope designs beyond the original 1816 monaural design by René Laennec [9]. For example, important research by Rappaport and Sprague looking at the effects of tubing bore [10] and the quality of the ear fitting [11] lead to innovations in use. In addition to directly listening to breath sounds via an earpiece, further innovations have led to developments, such as the electronic transduction, amplification and transmission of breath sounds, as well as in providing methods for advanced computer-based analysis. However, one important aspect significant to such undertakings is to evaluate various stethoscope assemblies to which a microphone is to be attached. This was the aim of this report.

1.2. Research Platform

While it is relatively easy to calibrate a microphone for use with acoustical respiratory monitoring, it can be somewhat more difficult to compare the effectiveness of various stethoscope assemblies to which a recording microphone is to be attached. As noted, precordial stethoscope assemblies usually come in two forms, the first in the form of a weighted chest piece and the second in the form of a conventional stethoscope head (Fig. 1). In either case, a miniature microphone as shown in the bottom of Fig. (2) is attached to the stethoscope assembly through a short piece of tubing. The question then naturally arises, as to which stethoscope best transfers the vibrations of the connected microphone? This issue was addressed by developing a special test platform for this purpose.

As shown in Figs. (2 and 3) the test platform consists of a small loudspeaker coupled to a plastic vibration test surface and driven by an audio amplifier (located at the bottom right of Fig. 2), this amplifier, in turn, is driven by a signal source such as a function generator. Additionally, the relative sound intensity obtained at the stethoscope under test was measured using a RadioShack 33-3013 miniature omnidirectional electret microphone connected to the stethoscope head through a short piece of tubing and amplified using a second audio amplifier, a RadioShack miniature audio amplifier (Catalog # 2771008) was displayed on a USB oscilloscope (Picoscope 2205).

Table 1 shows the obtained results with a 1000 Hz test signal. Strikingy, there was an approximately 6-fold variation in the sensitivity of the various stethoscope assemblies when tested with a 1000 Hz sinusoid, with the diaphragm of the Littman Classic II SE being the most sensitive (relative sensitivity 1.0) and the bell of the generic model tested type being the least (relative sensitivity 0.16). It is worth noting that different results might be obtained with different stimulus frequencies or conditions where the assembly under test is made to contact the test surface more firmly, for example, using a weight placed on the unit.