“Rrrring, Rrrring” — when every telephone still had a cable that was exactly how a phone call would sound. “Ding-dong” was how the doorbell would herald the arrival of house visitors. Today, users have a choice of countless songs, sounds, and natural noises. Miniature loudspeakers with wide frequency ranges make this possible.
Audible communication in the form of speech or music is becoming increasingly important in human-machine interfaces (HMI), and this is also true of industrial applications. In this field, audible signals or announcements can supplement visual indications, especially when information needs to be conveyed despite a lack of visual contact with the device. There is a selection of several loudspeaker types for a wide variety of applications.
A conventional loudspeaker, also known as a dynamic or magnetic loudspeaker, basically consists of a permanent magnet mounted on a voice coil. These in turn are connected at multiple points. This structure is surrounded by a loudspeaker frame, which supports a taut diaphragm made of paper, fabric, or plastic.
Electrical signals (DC) create a magnetic field in the coil, which causes the diaphragm to oscillate. These oscillations create the acoustic signals perceived by the human ear.
The physics of this type of loudspeaker—in particular sound pressure levels (SPL)—require a certain geometry. This often makes the loudspeaker a limiting factor that prevents the device from being thinner.
Piezoelectric Speakers
Piezoelectric speakers, on the other hand, are available in thin forms. They do not require a voice coil to generate sound—instead, they use the piezoelectric effect, whereby the tried and true piezoelectric ceramic material is usually bonded to a brass or nickel plate and made to vibrate by supplying it with a voltage. Unlike dynamic loudspeakers, which measure at least four millimeters in height, these are a single millimeter tall at most.
Another design is that of the multilayer piezo loudspeaker. They are constructed not just from one ceramic element, but multiple layers, with each layer filtering different frequency spectrums. This allows them to generate superior audio.
The PiezoListen from TDK is among the thinnest speakers in the world. Measuring just 0.49 mm in height, it can be glued easily to smooth surfaces, which are then made to oscillate. This is how it transforms practically any surface into a loudspeaker, be it a display, a table, a mirror, or the plastic housing of an application. The PiezoListen generates a high sound level pressure even at low voltages of 24 V p-p or less.
Multilayer piezo loudspeakers are triggered and controlled in exactly the same way as a dynamic loudspeaker with an IC amplifier.
Amplifier Circuits
Amplifier circuits for loudspeakers need to consider three factors in particular: the short-term power, the impedance, and the frequency range.
The short-term power—the load that a loudspeaker can briefly withstand from an amplifier signal without being damaged—can range from several 100 mW to several watts in miniature speakers.
The impedance of magnetic speakers is usually 4, 8, or 16 Ω. It therefore has a significant impact on music output and also plays a major role in the amplifier circuit—if a speaker with a higher impedance than is recommended is connected to the amplifier, its output may be reduced (i.e. the sound becomes quieter). On the other hand, if the impedance is too low, the quality of the sound may be impaired, or the amplifier may even shut down due to overload.
While magnetic speakers are an inductive load, ceramic speakers are a capacitive one. This means that they have a much higher impedance, which diminishes with increasing frequency. The amplifier therefore needs to supply much greater currents and cannot limit the current while maintaining the same voltage if the loudspeaker is expected to receive signals with high-frequency content.
The frequency range is divided into higher power classes, so that the signal is sent to a tweeter speaker, mid-range speaker, and bass speaker. For low power levels, a single wide-range speaker is used to eliminate the need for audio crossovers.
Dynamic speakers have a relatively low efficiency level. Their voice coil can be seen as a permanent resistor connected in series with a high inductance. It produces ohmic losses with much of the active power being dissipated in the form of heat. The amplifier must therefore provide more power—which is a disadvantage in the case of battery-powered applications in particular.
In ceramic speakers, their capacitive nature means that the reactive power plays the greater role, generating little in the way of heat. However, the amplifier circuit in the output stage requires a large amount of active power, which dictates efficiency here. Instead of conventional Class AB amplifiers, other topologies such as Class G or D amplifiers are recommended nowadays to achieve higher efficiency.
A push-pull amplifier consisting of transistors is a suitable amplifier stage for the audio amplifier. A small high-efficiency speaker only needs simple standard transistors and electrolytic capacitors to achieve an acceptable volume level.
The BCP5616 and BCP5316 complementary transistors from Diodes are well-suited for medium-level output, while something like the EEEFK1V101XP from Panasonic is ideal as an electrolytic capacitor.
Integrated Circuits
Often, however, it is more effective to use integrated circuits to create simple amplifier circuits. These audio amplifier ICs control and boost the audio signal, providing a louder, cleaner, and higher-quality sound. They are available in small packages and are found in many applications such as televisions, computers, or home audio systems as digital amplifier or operational amplifier types with mono or stereo audio. Most audio amplifier ICs are specifically designed for dynamic loudspeakers (e.g. NJU8759 from JRC), although models are also available for piezoelectric speakers with integrated charge pumps (e.g. NJW1280 from JRC).
Loudspeaker Housing
Once the amplified audio signal is at the voice coil or the piezo element, sound pressure waves are generated. When the diaphragm moves forward, a slight positive pressure develops on the front side of the diaphragm, while a negative pressure develops on the rear side, and vice versa.
To avoid cancellations and thus to increase the sound level significantly, the front and rear sides of the speaker should be acoustically insulated, for which the housing must be designed accordingly. Ready-to-use miniature loudspeakers complete with housings provide a convenient alternative.
There is often little space available for the loudspeaker, in which case it is important to make optimum use of the available volume to achieve good audio quality with optimum sound pressure. This can be achieved with a housing that is as rectangular or cubic as possible, matching the speaker diameter to the front and rear. A horn or funnel also serves to amplify sound.
For the housing structure, the following rules apply in general:
- The loudspeaker diaphragm must not hit the front plate at its maximum excursion.
- Vibrations can be avoided using foam between the speaker chassis and housing. This also prevents the sound from spreading from the back to the front.
- A cavity of maximum size behind the speaker provides greater volume and superior sound quality. It is often worth having a well-designed construction to this end.
Conclusion
The choice of speaker type, the amplifier circuit, and the housing can all affect the size and audio quality of the speaker. But nowadays, there are solutions available for practically all requirements and construction size limitations.
