If a certain noise is given a loudness of 30 dB, it means it’s 30 dB above the thresh-old of hearing, or 1,000 times as loud as the quietest detectable noise. A noise at 60 dBis 1,000,000 times as powerful as the threshold of hearing. Sound level meters are usedto determine the dB levels of various noises and acoustic environments.A typical conversation might be at a level of about 70 dB. This is 10,000,000 timesthe threshold of hearing, in terms of actual sound power. The roar of the crowd at a rockconcert might be 90 dB, or 1,000,000,000 times the threshold of hearing.A sound at 100 dB, typical of the music at a large rock concert, is 10,000,000,000times as loud, in terms of power, as a sound at the threshold of hearing. If you are sittingin the front row, and if it’s a loud band, your ears might get wallopped with peaks of 110dB. That is 100 billion times the minimum sound power you can detect in a quiet room.The rheostatA variable resistor can be made from a wirewound element, rather than a solid strip ofmaterial. This is called a rheostat. A rheostat can have either a rotary control or a slid-ing control. This depends on whether the nichrome wire is wound around a dough-nut-shaped form (toroid) or a cylindrical form (solenoid).Rheostats always have inductance, as well as resistance. They share the advantagesand disadvantages of fixed wirewound resistors.A rheostat is not continuously adjustable, as a potentiometer is. This is because themovable contact slides along from turn to turn of the wire coil. The smallest possible in-crement is the resistance in one turn of the coil. The rheostat resistance therefore ad-justs in a series of little jumps.Rheostats are used in high-voltage, high-power applications. A good example is ina variable-voltage power supply. This kind of supply uses a transformer that steps upthe voltage from the 117-V utility mains, and diodes to change the ac to dc. The rheo-stat can be placed between the utility outlet and the transformer (Fig. 6-11). This re-sults in a variable voltage at the power-supply output. A potentiometer would bedestroyed instantly in this application.The rheostat1096-11Connection of a rheostat in a variable-voltage power supply.