Choosing acoustic transducers for today’s electronic designs

Choosing acoustic transducers for today’s electronic designs

The ability of electronic systems to emit warnings, alerts, and

instructions has become an expected part of their technical repertoire

BY IRWIN FRIEDMAN
International Components (ICC)
Hauppauge, NY
http://www.icc107.com

Audible-alerts devices have become essential to today’s electronic designs. Whether in robust-sounding security alarms or soft beeping barcode scanners, these products have become a standard part of our lives.

A diverse range of audible alert devices has been developed to meet different design requirements. The devices vary in size, electrical power, and sound coverage, and may be referred to as buzzers, beepers, sounders, sirens, or speakers.

Essential considerations

To select the best audible transducer for a particular design there must be close collaboration between the mechanical and the electronics engineers. Unlike other electronic components, the audible device sound characteristics and physical size have a major effect on each other.

The electronics engineer chooses the desired frequency and sound output level (SPL) based on the distance the warning sound must cover to be heard and acted upon. The mechanical engineer must determine if the space allotted is sufficient and which type of termination is best for production. Once these issues are decided, the power source must be considered. Will it operate for an ac, such as line voltage, or dc, such as a battery.

The next step is to determine which of the available technologies — electronic buzzer, electroacoustic, or piezoelectric type (see Table 1 ) — is best suited for the criteria established above.

Table 1. Acoustic transducer types and typical specifications

Type selection

An engineer designing a process control for a production line might choose to use an electronic buzzer, such as the BR2818L-12 or a similar type, because of the low frequency (400 Hz). This type of sound reaches a wide area and can be heard in a noisy factory environment. Also, when designing a gas station’s fill pump control, this low frequency is preferable because of the station’s open-air location. In both applications, the flange construction and the flying leads provide many options as to where this type of device can be positioned.

For medical instruments, an electroacoustic device such as the BRT2821P-12 is better suited, because the 2,800-Hz frequency is directional and covers a narrow area. It can be heard locally at a hospital bedside and will not disturb other patients in the same room. This equipment usually has the necessary power available, so that a current drain of 40 mA is not a problem. The pin-type termination is suitable for pc-board insertion, but models with flying leads are also available.

While a piezoelectric device such as the BRP3018P-12-C has a lower profile than the electroacoustic type, which may be useful in the product’s mechanical design, the device’s low current drain of 10 mA may be more important. This suits battery-operated devices such as portable handheld meters and similar products. The 2,000-Hz frequency is considered a near-perfect sound for this type of application, because it is not annoying to the user or those nearby.

Within-type selection

Within each technology type mentioned, there are many other choices. SMT designs are available in piezoelectric types as small as 11 x 9 x 1.7 mm with an SPL of 80 dB at 10 cm and electroacoustic types as small as 5.5 x 5.5 x 2.5 mm with an SPL of 87 dB at 10 cm. These are used in barcode readers, handheld medical instruments, and other equipment requiring a device that is small and can be flow soldered as part of the desired production run.

Larger-sized piezoelectrics are used in security alarm equipment. Various sounds are available, such as sirens, intermittent and pulse tones, and many others. The decibel level is normally over 100 dB at 1 m, and there are many that exceed that level.

The electroacoustic type is often referred to as a speaker because it uses the same basic technology — coil, permanent magnet, and diaphragm — to develop its sound. Therefore this type of technology has the potential to produce a single alert or dual sounds at different frequencies, very much like a regular speaker. However it is limited; if the product requires voice prompts in addition to alerts at multiple frequencies, then a miniature speaker can be used.

There are many miniature speakers available in sizes 30 mm and smaller, as seen in Table 2 . This speaker comparison shows some of options available in sizes similar to the transducer described in Table 1. In addition, there are many other types ranging from Micro Miniatures of 15 mm for cell phones to 102 mm for security alarm applications. Many variations in mounting methods and terminal types are available.

Table 2. Variations within a speaker type

Application possibilities

In addition to the previously mentioned applications, there seems to be endless applications for audible devices. In our kitchens the microwave oven alerts us that our food is ready, the refrigerator lets us know when the door has been left ajar, our oven reminds us that the roasting temperature has been reached, and the cooking range alarm lets us know that the cat jumped on a burner or a child has put his or her hand in the wrong place. In the laundry room, an alarm tells us that the wash is finished so we can empty the washing machine and load the dryer. The dryer alarm notifies us when it has completed its cycle so we can remove the clothes and fold or hang them so they remain wrinkle-free.

Of course, we are familiar with these situations because they are part of our everyday lives, but consider the number of unusual applications. An acupuncturist uses a device that locates a pathway where needles which are intended to relieve pain should be placed. An ophthalmologist uses a handheld gauge to quickly check a patient’s eye pressure after certain types of treatments. The dentist uses a timer to alert him when the cement curing time for a crown has elapsed.

When we are out of our homes and away from our doctors, there are other applications that use common sounds to alert us that a situation must be addressed. Busy restaurants now use a device with a dual alarm that can be carried a distance from the restaurant. This device will announce when “your table is ready” and at the same time, make a beeping sound. There are automobiles that have a dashboard backup warning system that beeps faster as the car nears an object. This has helped avoid inadvertent personal injuries as well as damage to the vehicle. The same concept is used with golf carts and trucks, but these require the alarms to be very loud and mounted outside the vehicle.

As new equipment is designed using audible alerts, the manufacturers of these devices continue to perfect their technology to meet the demand of this ever-evolving market. ■For more on acoustic transducers, visit http://electronicproducts-com-develop.go-vip.net/controls.asp.