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In a bus, in theater, during your school assembly, and almost everywhere else, people can be seen singing songs or listening to songs. Indeed, singing and listening to songs is a favorite pastime for many of us. So how do we hear these songs? How do the songs come out of the loudspeakers (or speakers, as we sometimes call them)? How do the speakers work? What is a “good quality” speaker? In each of our ears, we have an ear drum. When the ear drum vibrates, our brain interprets it as sound. Why does the ear drum vibrate? Well, when the air pressure close to the ear drum increases, it pushes the ear drum inwards and when the air pressure close to the ear drum decreases; it pulls the ear drum outward. Sound traveling through the air creates these waves of compressed air (higher pressure) and rarefied air (lower pressure). So when these waves reach our ear, our ear drum also moves in and out at the same rate of frequency as these waves, and our brain recognizes the sound. They use the same principle to generate sound, except that the process is in reverse. A membrane or diaphragm in the speaker vibrates in and out and pushes and pulls the air near it. The pulsating movement of the air then travels outward as sound. Indeed, it is the same principle that we use when we talk. Our vocal cords vibrate and generate sound. The term “speaker” refers to something a bit more than what we usually associate it with. The typical cone that we see inside a radio or type recorder is actually termed a “driver”. One or more such driver and the enclosure/box they are mounted in together constitute a “speaker”. Why do we need more than one driver? The answer is simple, sound has a range of frequencies and a single driver may not do a good job of reproducing all the frequencies. So while a single driver can be used, usually two or three drivers are present in each speaker, each of which is optimized to a smaller range of frequencies. For example, high frequency noises, usually from stringed instruments, are better reproduced in small sized drivers. These small sized drivers are called “tweeters”. Their small size allows them to more easily sustain high frequency vibrations. Larger sized drivers may be more sluggish and less effective in sustaining high frequency vibrations. Mid-range frequencies, such as human voices, are better reproduced in medium-sized drivers, which are simply called mid-range drivers. Finally, low frequency noises, which also correspond to large wavelengths, such as those from deep sounding drums, are best reproduced using large drivers, called “woofers”. The box or enclosure the drivers are placed in also plays an important role in making sure that the drivers are held securely in place. Well-designed enclosures prevent unnecessary vibrations interfering with the sound from the speakers. What makes the drivers vibrate? Electricity that is encoded to carry the sound signal arrives at the drivers through wires. At the drivers, the electricity encoded with sound keeps changing the polarity of an electromagnet present at the center of the driver. These changes in polarity occur in a manner consistent with the sound encoded in the electricity. The electromagnet at the center of the driver is surrounded by a permanent magnet. Due to the interaction between the changing polarity of the electromagnet and the permanent magnet, the electromagnet moves back and forth. Attached to the electromagnet is the membrane that makes up the cone of the speaker and hence the cone vibrates as will and sound is produced