IAN LANG ELECTRONICS
No 8. The Loudspeaker .
Now we have demodulated our audio signal and amplified it to an audible level, we need to be able to turn the electrical signal to sound power. The device we need to do this is a loudspeaker, or speaker. There are several types but by far the most suitable and the most common one in small receivers such as ours is the moving coil type. A cross-section diagram provided by the US Navy is below:
As you may be able to tell it is exactly
like a dynamic microphone and indeed it is possible to use one as such, this being common practice in intercom systems. Our drawing here has a permanent magnet, others have an electromagnet but the power requirement for the magnet renders them unsuitable for battery operations. The cone is commonly made of paper but where a moist atmosphere is expected a mylar one is used. The frequency response is not usually as good in this case.
As you may know, where a current flows a small magnetic field is generated and the moving coil relies on this. The coil is mounted so that it is free to move in the air gap and is connected to the cone. As audio currents flow through the coil, the magnet repulses or attracts the coil, causing compression and tension in the cone, which in turn compresses and rarifies the air causing sound waves we can hear. All speakers have a characteristic impedance measured at a standard, usually 1 kHz. We must make sure we match our amplifier to this, and for the LM386 we have an output impedance of 8 ohms, and so must our speaker be too. Putting 16 ohms on the end will not be disastrous but will lead to intermittent and noticeable degradation of the audio.
Moving coil speakers are very good at reproducing lower frequencies, less so at mid range and much less so at high frequencies, unless they are specifically designed to do so in which case they will not be able to pass low frequencies well. For our AM receiver the highest audio frequency we will have to cope with is 4.5kHz and so one standard speaker can cope quite well. In HI-FI systems it is common to have three speakers coping with low, mid and high ranges and fed by a crossover network. The volume of sound depends on the the current passing through the coil but can be improved by clever mounting and design. This is a science in itself, encompassing acoustic absorption and waveform theories.