Brief answers to homework (not necessarily complete answers with
justification or details of calculation)
Links to the other homework answers: 1
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
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
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
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.
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
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
R4 Dynamic microphones (definition, no justification required)
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
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.
R11 Using a variable voltage to control some property -- in a
synthesizer, these properties are often frequency, amplitude, filter
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.
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.