Speaker Guide: 40-55 mm speakers

PUI Audio is excited to offer new micro speakers in the 40-55mm size range. These micro speakers meet the needs of applications that increasingly demand better performance in smaller footprints. Key models use strong magnets along with a mylar diaphragm to help deliver reliable sound reproduction that improves user experience. Most of these new products are ingress protected and can withstand high temperatures up to 85 ℃, which makes them ideal for harsh and hazardous environments. To ease installation, some of the featured speakers include wire leads and flange mount. Key applications include diagnosis & monitoring medical devices as well as various Industrial & consumer devices.

Functional Description

The basic speaker model is a cone, voice coil driven permanent magnet speaker assembly. The enclosure is designed to optimize the speaker assembly frequency response and match the acoustic impedance of the speaker to the surrounding media to obtain maximum efficiency and sound pressure level output. The enclosure main goal is to prevent the speaker’s back-wave from interfering with its front wave and creating partial acoustic cancellation. Additionally and equally important, It does allow higher possible drive voltages: the air contained within the enclosure acts as a restorative force that works to return the diaphragm to its quiescent position. As the drive voltage increases, producing increased excursion, the air’s spring-force increases, applying ever greater restorative force.

Defining Operating Power Level

These new speakers have a maximum power rating of up to 12W. This simply means that the product of the applied RMS voltage and RMS current PLUS any steady state DC voltage and current must not exceed 12W.

Exceeding this operational limit will cause power dissipation in the voice coil that could result in the coil to mechanically distorting. This distortion could result in mechanical interference between other parts of the speaker components: the magnet structure or the frame. Further, the speaker diaphragm’s peak-to-peak excursion limits could be violated, resulting in mechanical deformation of the speaker diaphragm’s suspension, which can be unrecoverable. Perceived audio distortion may be noted and continued operation outside the speaker electrical and mechanical limits will typically cause voice coil destruction.

General Driving Considerations

The speaker assemblies may be directly driven from many circuits with DC coupling. However, any DC offset voltage or current must be continuously dissipated in the speaker. Even more important, any DC voltage present at the speaker connection terminals will produce a constant diaphragm offset. This constant offset will impact the speaker’s sound pressure level (SPL) (reducing it) negatively and introducing nonlinearities in the speaker’s acoustic output (increasing THD).

A polarized capacitor may be used if the AC coupled output is always referenced to ground. This is the case with any driving circuit operating from a single supply (V+ and GND). If a split supply (V+ and V-) powers the driving circuit, a non-polarized capacitor can be used to ensure that any input offset voltage that is amplified by the circuit is blocked at the output. This type of capacitor is required because the voltage across it will reverse when the AC output waveform crosses the zero axis and the most polarized electrolytic capacitors can only withstand a 1 to 3 volt reverse polarity.

The AC RMS current should be computed or measured and compared to the capacitor manufacturer’s permissible RMS ripple current specification. Selecting a capacitor with a ripple that matches or exceeds the measured or computed value contributes to a long operating life for the capacitor. The ripple current will act with the capacitor’s Equivalent Series Resistance (ESR) value to generate heat.

Aluminum electrolytic capacitor will meet most requirements while providing a range of ripple current specifications necessary to drive the 8Ω speaker at full power.

The optimum capacitor value should always be chosen to maximize circuit operation. If the application only requires the generation of a single tone or limited bandwidth audio signal, then the capacitor should be sized at the operating frequency.

Analog Amplifier Applications

Analog amplifier circuits are those that amplify a continuous analog waveform and drive a speaker, producing acoustic output. The acoustic output is proportional to the instantaneous amplitude of an signal applied to the amplifier’s input. The most cost-effective means of driving a speaker is to utilize one of the many inexpensive monolithic power audio amplifiers that are currently available from several domestic and foreign sources.

Bridge-Tied-Load (BTL) Drive Circuits

Ease of design, single 5V supply, an output voltage swing that is twice that achieved by a single amplifier, and elimination of a coupling capacitor between the amplifier output and the speaker makes a BTL amplifier one of the best choices to drive speakers. Internally, this amplifier topology consists of two inverting amplifiers in operating in series, with the speaker connected between the two outputs.

This amplifier is able to force a1W with no more than 1% THD into an 8Ω speaker load while operating on a single 5V supply voltage.

Higher Power Analog Amplifier

For those applications that require more than 1W of drive power, the circuit shown in Figure 5 will meet that need. This amplifier can deliver up to 20W while operating on a 45V supply voltage. The output power scales with supply voltage: if 10W is desired, the supply voltage is a nominal 32V.

Class D Amplifiers

Class D amplifiers, when compared to the Class AB amplifiers shown above, have the major benefit of higher efficiency when compared to analog amplifiers operating at the same output power level. While historically these amplifiers required output filters to suppress the switching frequency present in the output signal, many current designs are filterless, further increasing the system efficiency through reduced parts count. This amplifier operates on a single supply (applied to pin B1 and B2) with a range of 2.5V to 5.5V. At VDD = 5.0V, the output power into an 8Ω load is a nominal 1.46W and 2.57W into a 4Ω load.