Teach Yourself Electricity and Electronics, 5th edition

Stan Gibilisco

Explanations for Quiz Answers in Chapter 31

1. We define dynamic range as the ratio of the maximum power output to the minimum power output that a hi-fi system can deliver while maintaining acceptable performance with low distortion. None of the choices (a), (b), or (c) specify dynamic range, so we must conclude that the correct answer is (d), "None of the above."

2. For best performance, speakers should be able to handle at least twice the maximum rated amplifier RMS output power. In the case of a 100-W amplifier, therefore, the speakers should have at least a 200-W rating. The correct answer is (c).

3. When interference to radio receivers results from the heterodyning of signals from multiple transmitters in a congested zone such as the downtown part of a large city, engineers refer to the problem as intermodulation (or sometimes "intermod"). The correct choice is (a).

4. When engineers record a hi-fi compact disk (CD), they use a scrambling circuit (or scrambler) to distribute the pits physically around on the disk surface, rather than "burning" them sequentially along the recording track. The answer is (b).

5. Pitch is the musician's expression for the frequency of an acoustic disturbance (or note). In any particular medium such as the earth's atmosphere at sea level, a note's frequency depends entirely on its wavelength. The correct choice is (b). The waveform (a) or amplitude (c) can vary while the pitch remains constant, so neither of those choices work here.

6. The timbre of a musical note refers to its "tone quality," which depends on the waveform (shape of the acoustic wave). The correct answer is (a). The wavelength (b) or amplitude (c) can have a subjective effect on timbre as certain people hear it, but the main factor is the waveform.

7. A balance control allows you to adjust the ratio of the left-channel volume to the right-channel volume in a hi-fi stereo amplifier, whether the output goes to speakers or to a headset. The correct choice is (b).

8. In the CD recording (or "burning") process, digital signal processing (DSP) minimizes the amount of noise that gets recorded. Noise bursts generally have an analog nature, while CD data is digital. The DSP circuit "cleans up" the digital recording, discriminating against analog fluctuations including noise. Choice (a) is the correct answer.

9. A computer modem, a cathode-ray-tube (CRT) monitor, and a computer's central processing unit (CPU) can all cause electromagnetic interference (EMI) to a nearby FM tuner.  The correct choice is (d), "All of the above."

10. In a CD player, nothing physically touches the disk surface in order to facilitate sound reproduction (unless you want to imagine visible-light photons as physical objects). The correct choice is (d). This lack of physical contact explains why CDs last so much longer than vinyl disks did in their heyday. The turntable's stylus literally eroded the vinyl away, a little bit at a time, whenever anybody played the disk.

11. A young person with excellent hearing can detect sounds having frequencies in approximately the range 20 Hz to 20 kHz. As people age, their low-frequency and high-frequency hearing limits tend to converge as their ears' sensitivity at the lower and upper extremes decreases. Choice (a) is the only selection in this question that reflects that sort of change.

12. Ideally, your audio amplifier should have separate inputs for various devices such as microphones, CD players, and FM tuners. However, if your amplifier has only one input and you want to connect multiple audio devices to it, you'll need an audio mixer. The correct choice is (a).

13. If a listener sits at a point where a direct sound wave combines in phase opposition with a reflected sound wave of equal amplitude, then the two waves cancel each other out and the listener is in a node, also known as a dead zone. The correct choice is (b).

14. A tweeter is specially designed to convert AF current into sound in the upper portion of the audible spectrum. The correct choice is (c).

15. A moderate-sized FM receiver, capable of producing loud sound, is called a boom box. The correct choice is (d). The term comes from the exceptional ability of the device to reproduce sound in the bass (low-frequency) part of the audible spectrum, despite its relatively small woofers. Anyone who has heard the "thumping" noise characteristic of some popular music, coming from a boom box nearby, knows how well this term describes the effect!

16. If we want to calculate the wavelength of an acoustic disturbance in meters, we must divide 335 by the frequency in hertz in the earth's atmosphere at sea level. According to that formula, a sound wave at 670 Hz has a wavelength of 335/670 m, which equals 0.500 m or 500 mm. The correct choice is (c).

17. By definition, a component hi-fi system comprises several devices in separate cabinets. For best performance, and also for optimal protection against electromagnetic interference (EMI), these devices should be connected to each other with shielded cables. The correct choice is (a).

18. A graphic equalizer allows you to adjust the output volume from a hi-fi amplifier within several well-defined AF bands. In that capacity, the equalizer acts as a sophisticated tone control. The correct choice is (c).

19. For an acoustic disturbance in the earth's atmosphere at sea level (or in any other physical medium), the frequency and the wavelength vary inversely with respect to each other. Therefore, if we quadruple the frequency (increase it by a factor of 4), the wavelength becomes one-quarter (1/4) as great. The correct choice is (d).

20. As in the scenario of Question 19, the frequency and wavelength are inversely proportional for sound waves in any particular medium. If we double the wavelength, we cut the frequency in half. The correct choice is (c).