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Electronic stethoscope design can be any number of variations on the traditional acoustic stethoscope design.
Amplifying Stethoscopes
All of the electronic stethoscopes require some degree of power to run the circuitry components that allow their features to function. The types of power required may vary, but are generally batteries (AA, AAA, Lithium or rechargeable). Some of the stethoscopes offer visual indicators of the power level for the user to see on an LCD screen or may have an LED indicator on the chest piece or stethoscope module itself. Most have some type of audio or visual signal to the user when the power level is running low, which is necessary because most will not function without adequate power supply. The majority have a power button or switch that can be located on the chest piece, on a module incorporated in the tubing, or directly on the stethoscope module itself.
Digitizing Stethoscopes
These stethoscopes utilize AC 120 V power cords rather than batteries. They do have a light that indicates they are powered on. The two digitizing units on the market have no internal power supply and no on/off switching capability other than plugging and unplugging the units.
Amplifying Stethoscopes
Power consumption and battery life preservation are important features in the electronic stethoscope, which in general don’t work unless there is a power supply. Therefore, most of the electronic stethoscopes on the market allow for some degree of auto-shutoff as a power saving feature. The auto-shutoff time period can vary in length and in some models can allow for customization of the time period. For convenience and time saving, we found it to be most convenient when a “settings retained” feature was paired with an auto-shutoff feature. The retained settings functionality was extremely important in the units that had a set run time, at least when the unit was repowered the user could return to use in a relatively fast manor. We also found it more useful when an auto-shutoff period starts after the last button push or sound detected, as opposed to a set time interval. Models that have a set auto-shut off period, and in essence a set run-time despite current usage, were inconvenient and risked the unit turning off during a patient examination.
Digitizing Stethoscopes
These units utilize AC power and do not have auto-shut off and do not employ any power saving features. As they do not have batteries, digitizing stethoscopes do not have the same need to automatically turn off. The digitizing stethoscopes will never turn off mid-examination. If the plug should come unplugged during an examination, the user will know automatically because the sound will stop transmitting and the LED indicator will turn off.
Electronic stethoscopes take the collected acoustic sound energy from the chest piece and convert it into electrical energy. As the converted voltage is sent through the electronic stethoscope’s internal circuitry, it encounters an amplifier component. This amplifier has the ability to take in small amounts of energy and convert the signal to increase the sound output. The majority of the electronic stethoscope units have volume buttons or a volume dial. It can be located on the chest piece, on a module incorporated in the tubing, or directly on the stethoscope module itself. Some of the modules offer a visual or audio indicator of the volume level currently in use, while others do not.
In some models amplification can be turned on and off. Some models incorporate the amplification process as a part of increasing or decreasing the unit’s volume, while in other models it is a separate function. As amplification level increases so does volume level. In general, we found more ease of use for the user in the units where amplification and volume increase/decrease buttons were a combined process. The single process allows for one less variable when the user is attempting to assess a patient. Manufacturers offer varying degrees of sound amplification, most commonly they claim sounds are 10-50 times amplified.
A current shortcoming that we noted with the electronic stethoscopes is that movement of the chest piece on clothing or skin surfaces may produce painful sound artifact. Care must be taken when placing the chest piece or moving the chest piece to avoid abrupt changes in sound and/or volume level. A feature that we would love to see in the future would be some sort of sound limiting mechanism that blocks sudden increase in sounds, perhaps at certain frequencies or decibel levels.
Most stethoscopes will typically have some sort of button or switch to electronically switch between frequency filtering (listening modes). Some electronic stethoscope chest piece may also utilize a tunable diaphragm or may require physical changing of the bell and the diaphragm. This switching of modes is simply filtering the frequency being heard, which is explained further in the frequency section. There is one model of electronic stethoscope on the market that requires the user to plug and unplug the chest piece module from the stethoscope unit to change between transmit and receive modes.
Electronic stethoscope tubing is remarkably similar to acoustic stethoscope tubing in general; however there are some notable variations. Some models incorporate modules that are mounted in or around the tubing, where the circuitry for the sound conversion is located. The basic principles of tubing length and thickness are the same in electronic stethoscope as they are in acoustic stethoscopes. Other models’ tubing may internally contain additional circuitry and transducers for sound conversion. Some models do not use a traditional tubing design at all, opting instead for electronic cords.
Ear tip designs in electronic stethoscopes often utilize the same principles as their acoustic counterparts; however, some stethoscope designs are significantly different. The most outstanding difference is that some models of electronic stethoscope do not even contain ear tips. Some of the electronic stethoscopes allow the user to use headphones. It is important to note on this topic that most of the headphones included with the stethoscopes currently on the market have sub-optimal sound quality. This is especially true for the “ear bud” style of headsets that are inserted into the ear.
The investment in a quality pair of headphones goes a long way to significantly improved sound output quality. For all of our testing, we utilized the full-sized, professional-quality R80 stereophones by Koss. These headphones have a frequency response of 16-20,000Hz, a 60 Ohm impedance level, and a less than a 0.2% distortion rating. We found that the models of stethoscopes that utilized headphones mostly allowed for interchangeability, however there was one stethoscope manufacturer that did not. That manufacturer did offer two models of Koss headphones for purchase; however they had customized cabling that make it necessary to purchase the headphones directly from them.
For the models that utilized ear tips, in general they offered additional sizing and replacement options with a variety of types of ear tips. The ear tips ranged from silicone soft sealing to molded plastic. There was one model that had built in functionality in the tubing to tighten ear tip insertion pressure.