Several principles can be applied when choosing the right buzzer or speaker. Designers should consider the size of their product and its terminal type. Will it connect with a wire or directly to the board? Other characteristics to consider for buzzers and speakers are volume, frequency, voltage, and current consumption. Lastly, consideration should be made for any special requirements, such as RoHS directives, water-resistance, or even bug resistance.

Some audio transducers and buzzers are programmable, provide volume control, or include a visual alarm to alert employees who work in areas with high noise levels. Internally driven transducers use self-contained circuitry with inductively coupled coils, and produce an average sound output of 85 Db or 10 cm/min. Externally driven transducers have a single coil and a magnetic circuit composed of a permanent magnet, an iron core, a highly permeable metal disk, and a vibrating diaphragm.

Panel mounted devices have a partially threaded body, a matching nut, and can be either flush or flange. Screw-down devices have flanges with holes. Socket mounted devices plug into a receptacle, typically an integrated circuit (IC). Surface mounted components are soldered to a flat pad on the face of a printed circuit board. Typically, this pad is coated with a paste-like formulation of solder and flux. With careful placement, components mounted in this way remain in place until elevated temperatures, usually from an infrared oven, melt the paste and solder the mount’s flat terminals to the circuit board’s pad.

Piezo Buzzers
Piezo buzzers, like the ones below,are typically high frequency (2-4Khz), with pleasant sounding pulsing or solid tones at low sound pressure levels and piercing tones at higher outputs. Usually buzzers sound pressure levels range from 80dB to 105dB or more, with larger buzzers required to produce louder sound.

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Piezo Buzzers with Internal Circuitry	FEATURES * SPL output–75 to 115dB * 9, 12, 24 VDC * Low profile, compact design * Consistent, high reliability * Long operating life * Low power consumption * Variety of frequencies * Lightweight	APPLICATIONS * Fire & Burglary Alarms * Smoke & Gas Detectors * Home Appliances * Medical Instruments * Auto Backup Warning * Dashboard Gauge Alerts * Fish Finders * Industrial Equipment
Surface Mount Piezo Buzzers	FEATURES * SPL output–75 to 90dB * 6, 9, 12, 24 VDC * Low profile, compact design * Available on Tape and Reel * Long operating life * Low power consumption * Variety of frequencies * Extra Lightweight	APPLICATIONS * Hand Held Portable Equipment * Medical Devices (Testing, Diagnostics) * Home Appliances * Medical Instruments * Auto Backup Warning * Dashboard Gauge Alerts * Fish Finders * Industrial Equipment

General Purpose Speakers
Designed for multimedia PC and television applications, these speakers feature specially constructed magnets and shields to eliminate leakage that can change monitors, floppy disks and other data storage equipment. Available in round, square and oval designs, for voice or music reproduction. Their size permits use in speaker boxes providing excellent high fidelity sound.

General Purpose Metal Frame Speakers

FEATURES * SPL output–80 to over 115dB * 6, 9, 12, 24 VDC * High tone quality * Wide range of products * High power * Variety of frequencies

APPLICATIONS * Computer Peripherals * Telephones * Radios/Recorders * Intercoms * Multimedia

Transducers
Transducers incorporates a piezoelectric element, which converts electrical signals into mechanical vibrations (transmit mode) and mechanical vibrations into electrical signals (receive mode). Many factors, including material, mechanical and electrical construction, and the external mechanical and electrical load conditions, influence the behavior of a transducer. Some transducers are specially fabricated to be more efficient transmitters and others to be more efficient receivers.

Electro-Acoustic Transducers

FEATURES * Small And lightweight * Dependability and long life * No electronic noise due to mechanical * Wide temperture range * As small as 9 mm * low voltage- as little as 1.5VDC

APPLICATIONS * Pocket Pagers * Cellar Phones * Cameras * Alarms/Digital Clocks * Typewriters

Sources: http://video-equipment.globalspec.com/LearnMore/Video_Imaging_Equipment/Meters
_Readouts_Indicators/Audio_Transducers_Buzzers
http://www.sourceresearch.com/intervox-buzzers-speakers-tranducers.cfm
http://www.globalspec.com/FeaturedProducts/Detail/InternationalComponents/Shielded
_Low_Leakage_Speakers/22216/0
http://www.ndt-ed.org/EducationResources/CommunityCollege/Ultrasonics/EquipmentTrans
/characteristicspt.htm