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www rutronik com Committed to Excellence Powered by 1 2021 11 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 circuitif 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 highfrequency content The frequency range is divided into higher power classes so that the signal is sent to a tweeter speaker midrange speaker and bass speaker For low power levels a single widerange 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 powerwhich is a disadvantage in the case of batterypowered 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 Gor Damplifiers are recommended nowadays to achieve higher efficiency Apushpull amplifier consisting of transistors is a suitable amplifier stage for the audio amplifier Asmall highefficiency speaker only needs simple standard transistors and electrolytic capacitors to achieve an acceptable volume level The BCP5616 and BCP5316 complementary transistors from Diodes are wellsuited for mediumlevel 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 higherquality 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 Readytouse 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 Ahorn or funnel also serves to amplify sound General rules for the housing structure •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 •Acavity of maximum size behind the speaker provides greater volume and superior sound quality It is often worth having a welldesigned 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 ■ Ceramic piezo loudspeaker Dynamic loudspeaker Multilayer piezo loudspeaker Advantage Disadvantage Advantage Disadvantage Advantage Disadvantage High efficiency High driver voltage Tried and true lowcost technology Poor efficiency No holes are required for audio output Costly technology Reproducible production process Capacitive behavior Consistent frequency behavior Deep construction Suitable for IP67 case Thin construction Poor handling of bass tones Ultrathin construction Advantages and disadvantages of various speaker designs TDKs PiezoListen can use its multilayer technology to transform anything into a loudspeaker Figure TDK