Brief answers to homework (not necessarily complete answers with justification or details of calculation)
Links to the other homework answers: 1 , 2 , 3

Chapter 18:
R1 Direct current (DC) continues to flow in the same direction.  Alternating current (AC) flows in back and forth, one direction then the other, at some frequency.
R2 Electrons move in an electric current (though they move inthe opposite direction that the current is said to flow)
R7 A charged capacitor stores energy in an electric field (current does not need to be flowing).  An inductor stores energy in a magnetic field, only when current is flowing through it.
R8 A capacitor passes high frequency current easily, becuase it's impedance is inversely proportional to frequency (a capacitor opposes buildup of charge, which bulds up only as long as current continues in the same direction).  An inductor does not pass high frequecnies well, because its impedance is proportional to frequency (an inductor opposes change).
R10  The resonant freq decreases when the inductance increases, becaue the inductance is in the denominator of the eqn for resonant freq (eqn 18.6)
R11 Low pass filters block high freqeuncy (by definition, they pass only low frequnecies)
R14 The level is too high, and the signal exceeds the maximum (and/or minumum) that is possible for the amplifier.  Thus the peaks get clipped off.
E3  Use eqn 18.6.  For part a, use L = 1X10-3 H, and C=3X10-6 F.  You get  2906 Hz.   For part b, use L = 5 H, and C=500X1-6 F.  You get  3.18 Hz.  
E9 The circuit on the left is a low-pass filter (the inductor does not pass high freq).  According to eqn 18.8, the cutoff above which not much signal gets through is fc = 7958 Hz. 
The circuit on the right is a low-pass filter, too (the capacitor shunts the high freq, and they don't go to the output).  According to eqn 18.7, the cutoff above which not much signal gets through is fc = 318 Hz.

Chapter 19:
R3 Dynamic speaker (definition, no justification required)
R5 Large speaker cone has low freq resonance (like a large drumhead membrane).  It will produce a lot of low freq sound.
R6 Small speaker cone has high freq resonance (like a small drumhead membrane).  A large speaker cone is heavy and difficult to accelerate quickly to make it oscillate quickly.
R9  A bass-reflex speaker has a port or a hole so the sound pressure wave coming from the back of teh speaker cone is not lost, but can come around to the front to be heard (hopefully in phase witht eh wave fromthe front)
R10 Baffle keeps the sound pressure wave from teh back of the speaker cone from immediately combining with teh pressure wave from teh front.  If they did, they would pretty much cancel, since when one is at hgihg pressure, teh other is at low , and vice versea (in other words, they are out of phase and distructively interfere)
R14 A horn speaker radiates sound more efficiently.
R16An  electrostatic speaker is based on the elctrical attraction and repulsion of opposite and like charges (rather than magnetic attraction and repulsion like a dynamic speaker does).
R17 See def in chapter 19 glossary
R18  Ambient sound it picked up by mike.  A sound wave opposite it is produced electronically. These two waves cancel in the headphones.
a
Q4  See R5 and R6
E3 a) The wave has to travel 0.5 m across the front then 0.5 m across the back -- total path is 1m.
b) 1m = 0.5 l, so l = 2m.  f = v/l = (343 m/s)/(2m) = 171.5

Chapter 20:
R4 Dynamic microphones (definition, no justification required)
Chapter 21:
R8 Digital signals are nearly impervious to noise.  Binary should only have one of two values (on&off, 0&1, whatever).  If noise causes that to vary (0.9, say), then that can be easily detected and corrected.
R10 If you sample at frequency f, then you limit yourself to picking up signals or waves with frequency f/2 or less.
R18 According the the answer to R10, you need at least 200 kHz sampling to pick up 100 kHz harmonic.
 
Chapter 27:
R11 Using  a variable voltage to control some property -- in a synthesizer, these properties are often frequency, amplitude, filter frequency, etc.
E4

Chapter 28:
R2 Time domain means looking at the waveform as a function of time.  Frequency domain means considering the spectrum and the amplitude of different frequencies.

Chapter 29:
R24 A polyphonic synthesizer can play moer than one note at once.
R33 MP3 compression is lossy -- some information about the wave is lost.  Hopefully it is information that doesn't effect the way teh sound sounds too much.