A Medical Electronics Manufacturing Fall 1997 Feature

AUDIO COMPONENTS

Selecting Proper Audio Components for Medical Electronics

Mikulas Hlinka and Niv Amdur

Making the right choice between transducers and buzzers is key to achieving just the right tone in medical devices.

Avariety of sounds can be heard in a hospital, from verbal communication among doctors, nurses, and patients, to tears of friends and families of those who are sick. Other sounds within hospital walls can be as important as a doctor's orders: namely, the sounds made by the medical devices and instruments used to monitor the health and safety of patients.

A surface-mount audio transducer on a printed circuit-board.

Audio components are designed into medical equipment to verify that a patient is stable or to alert staff that care must be administered. Audio components can also emit sound to monitor air cleanliness, act as an alarm in case of emergency, or signify completed treatment. Because of the diverse uses of audio components in the medical industry, it is important to understand the makeup of the audio products and how certain frequencies (i.e, sounds) signify specific conditions.

Sound from medical instruments is typically emitted from either a transducer or a buzzer. Transducers and buzzers each offer distinct advantages in certain medical instruments, including sterilizers and infusion pumps, and equipment such as air monitors. Engineers select the type of transducer or buzzer based on sound pitch, response time, voltage, sound output, and size.

Buzzers and Transducers

Before deciding which audio component is best suited for a certain medical instrument, it is important to understand the difference between buzzers and transducers. Even though each emits a sound, the design of the components as well as their tone and pitch vary greatly.

Buzzers. An electronic buzzer is compact and produces high sound pressure levels with minimal power consumption. Operating voltages typically range from 1 to 30 V while sound output can be as high as 75 dB at 1 m. Most buzzers operate at low frequencies, typically 300­500 Hz, and use an oscillating hammer to resonate a membrane. An electromagnetic assembly controls the hammer, which vibrates once a current flows through a coil within the buzzer. A second coil detects the vibration and provides feedback to a transistor so that the oscillator becomes synchronized with the vibrating hammer. The result is a designated tone.

Low-frequency buzzers produce a very loud sound.

Tone output can be selected using two methods. The first is simply to listen to the tone. A second method, called fast Fourier transform (FFT) analysis, is perhaps more accurate because it visually collects a body of frequencies rather than just one. Such analysis can determine the tone's effectiveness compared to other sounds.

Transducers. Electromagnetic transducers generate either a single continuous sound or intermittent tones and are available with either self-contained or external drive circuitry. Transducers with self-contained drive circuitry have inductively coupled coils. This type of transducer produces an average sound output of 85 dB or 10 cm/min.

Automatic dispensing units use audio transducers to alert patients to take medication.

External drive circuitry transducers are designed differently. Rather than having two coils, these transducers feature a single coil and a magnetic circuit composed of a permanent magnet, an iron core, a highly permeable metal disk, and a vibrating diaphragm (see Figure 1). The disk and diaphragm are attracted to the core by the magnet's magnetic field. When an oscillating signal moves through the coil, it creates a fluctuating magnetic field that vibrates the diaphragm at the drive signal's frequency. A sound is produced based on the frequency applied.

Figure 1. Construction view of standard miniature audio transducer with external drive circuitry.

Rated voltage for externally driven transducers varies from 1.5 V dc to as high as 12 V dc. They offer wide voltage and temperature ranges, which are important considerations when designing medical equipment. Washable and nonwashable transducers are available in side-port or top-port sound emission constructions. These transducers are usually small, and some weigh as little as 1 g.

By contrast, much larger piezoelectric transducers use an older technology than electromagnetic transducers, making them less reliable. Additionally, piezoelectric transducers are less durable because they use ceramic packaging, which makes them more apt to crack or break.

The type of enclosure is critical because the mechanical design of the component can affect the sound. The enclosure can actually alter the sound emitted by the buzzer or transducer. By designing an enclosure with a resonance chamber in front of the sound-producing device, it is possible to produce specific tones, increase sound pressure, improve response in a wider range of frequencies, or soften sound output. Such a resonator is called a Helmotz resonator, and it can be designed using the following equation:

where fv = resonant frequency of resonator (Hz), V = volume of resonance chamber (mm³), D = diameter of sound emission hole (mm), L = depth of sound emission hole (mm), C = speed of sound equal to approximately 34,400 (mm/sec).

Including a Helmotz resonator allows a unique sound to be emitted that can differentiate a particular medical instrument from other audio signaling devices.

The mechanical design of audio devices makes them susceptible to vibration. Another variable that influences sound output is how a buzzer or transducer is assembled onto a PC board. If a component is not securely assembled to the board, the device may emit an improper sound, creating confusion for those who rely on the sound for monitoring.

Getting the Proper Sound

The performance of audio components is crucial to selecting proper components because many medical instruments have become synonymous with certain sounds. Heart defibrillators, for example, use buzzers because their low frequency produces a very loud sound.

Buzzers, however, are not the best choice for many other applications. In fact, more and more medical instruments feature transducers with external drive circuitry. In some instances, the same hospital room may have as many as a dozen different medical instruments. Recognizing the individual sounds of each of these instruments can be difficult. External drive circuitry lets the designer manipulate the sound output. Custom-developed sounds can differentiate various instruments.

Two other important performance characteristics--decibel level and frequency output--must be evaluated to determine appropriate audio components for medical devices. Greater emphasis is being placed on the need for everyone--including patients and visitors--to hear a medical instrument sound in time of emergency. As a result, transducers and buzzers with frequency levels below 3000 Hz are preferred for medical devices. This is primarily because it has been shown that most elderly people cannot clearly hear frequencies higher than 3200. The most desirable frequency range for medical products is between 2000 and 2600 Hz, although lower frequencies are sometimes used.

Manipulating sound output through external drive circuitry has another advantage. An instrument requires only a single transducer because the transducer can be designed to send out a variety of signals. This eliminates the need for additional transducers, thereby saving valuable board space as well as reducing cost.

Electromagnetic transducers with self-contained drive circuitry. Photo courtesy of North American Capacitor Co.

With external drive circuitry, multiple frequencies or a combination of frequencies can alter the transducer's output. A certain frequency can produce a safety tone, a second frequency can generate a cautionary tone, and a combination of the two frequencies can sound an alarm for immediate attention.

Surface-Mount versus Through-Hole

Surface-mount technology has certainly made its mark in the medical electronic marketplace, as it has in the general electronics industry. That does not mean, however, that traditional through-hole audio devices have gone the way of the medicine man.

Because they are packaged in tape and reel, surface-mount audio components are better suited for mass production. They are also more compact, an important consideration for crowded PC boards. Through-hole devices are less expensive, however, which makes them a more logical choice for low-volume products.

Surface-mount technology is used for devices such as dialysis machines, blood glucose monitors, sterilizers, and infusion pumps. Devices such as defibrillators and products such as air monitors for checking for biogens often use more cost-effective through-hole devices.

Conclusion

Choosing the proper audio component can affect the lives of patients. Selecting the wrong component can result in poor performance, including devices that fail to alert to potential problems. An equally important consideration is the method used to assemble the component onto the board. A thorough evaluation of all factors results in medical instruments that can emit a variety of sounds so that doctors, nurses, other hospital staff, and visitors can better monitor the health of patients.

Mikulas Hlinka is audio product manager and Niv Amdur is inside sales representative for Star Micronics America (Piscataway, NJ).