Week 3 January 19th

Week 3 January 19th (Late Start + no school Monday)














What does this mean?

You’re sitting in a car outfitted with an audio system where everything is perfectly synced and tuned correctly. No distortion at low bass, no fuzz at high trebles. The 600W audio system was painstakingly designed to distribute sound equally throughout the cabin of the vehicle.
THX engineered interactive menus give you totalitarian control over the bass, balance, fade, and direction of the music. The THX-patented configurable speaker array, a network of 6 small, powerful speakers that constantly adjust themselves to deliver natural sound, despite road noise or genre of music. Jay-Z sounds as perfect as Mozart. Really, we tried it. There are other features too that add to the sonic experience. Deep Bass Extension from the dual sub woofers yield deep lows without any perceptible distortion.

Objectives:
1. Students will be able to write a long constructed response on what the above paragraph means.
2. Students will be able to graph a sine wave.

Click here to watch videos of how speakers work.

http://videos.howstuffworks.com/science-channel/31523-deconstructed-how-speakers-work-video.htm

Agenda:

1. Video How speakers work
2. Kinematic Equation with clickers.
3. Music Powerpoint: slides 61-62 myth busters brown note .
3. Sound cancelling from the music ppt. slides 13-19
4. Graphing Sine Waves ws
5. Graphing Sine and Cosine waves clickers
6. Visit Mines, CU or DU campus
7. Guest on how stereo speakers work
8. Wave clickers (Make into clicker questions)
10.Record voice
11. Motion Sensor Lab with TI Calculators


Website:

http://phet.colorado.edu/simulations/sims.php?sim=Wave_Interference

http://phet.colorado.edu/simulations/sims.php?sim=Sound

Goto physics-animations.com for website that shows demonstrations about waves.

Notes:

The physics of the loudspeaker need to be understood before one can create a high quality sound system. Elements such as frequency response, damping, quality factor, efficiency, and impedance work together to determine the characteristics of a loudspeaker. Each factor needs to be considered in detail in order to bring out the full potential of a high-fidelity system. Frequency response, damping and quality factor effect one another very dramatically. Basically, a flat frequency response requires high damping and a low quality factor. Loudspeakers are terribly inefficient, but this does not cause a problem in most applications. Though efficiency is not a major concern, a precise impedance match is needed in order for the speaker to produce a high quality sound. As the final stage in a stereo system, the loudspeaker must adhere to these criteria in order to perform with precision.

When dealing with frequency response, a flat response is best. Every frequency, from the lowest that the woofer can produce to the highest that the tweeter can produce, needs to be equally represented. If the speaker is designed poorly, the response will be uneven and certain
frequencies will resonate more than others. A poor response can be corrected through the use of
an equalizer which can flatten large resonant peaks or boost weak frequencies (Borwick, p. 314).
It is often said that the “smoothness of response is more important than range” (Weems, p. 14),
though frequency range also needs to be considered. The range over which frequency response
is most important is 50Hz and 10kHz (Borwick, p. 369). To achieve such a range, a woofer is
needed for low frequencies and a tweeter for high frequencies. Both the quality factor (Q) and
damping have a large effect on the frequency response of the speaker. Frequency response often changes when operated at various Q values (Weems, p. 64). If the device has a low Q, the
frequency response will be relatively flat and even, but if the Q is high there will be resonant peaks. In turn, the Q is affected by the damping of the speaker. With the proper damping, the Q
of the speaker can be lowered and a flat frequency response is achieved. Damping is basically the “loss of energy of a vibrator, usually through friction” (Rossing, p. 31). When it relates to speakers, damping is a good thing. There are two kinds of damping in speakers: mechanical damping controls the suspension and air load in the cabinet, and electrical damping consists of the magnetic system (Weems, p. 17). If a speaker has poor mechanical
damping, the diaphragm motion will get out of control and the sound will become blurred. The damping restores the vibrating mechanism after the signal ends, and prevents this from
happening (Weems, p. 17). The damping material, usually acoustical fiberglass or polyester batting, “absorbs [the] sound that would otherwise bounce around the box…” (Weems, p. 58).
Thus, a good damping mechanism with the proper material allows for a clear sound and an even frequency response. Electrical damping relies on magnets to act as the restoring force in the
speaker. Cheap, small magnets result in multiple resonance’s in the diaphragm because they are
too slow in restoring the vibrating mechanism to its equilibrium point. This causes a poor
transient response (Weems, p. 18). Damping is most closely related to the Q of the speaker, in
that by adding the proper damping material, the Q of the system can be lowered (Borwick, p.
254).

The quality factor is “a parameter that specifies the sharpness of a resonance” (Rossing,
p. 553). A high Q indicates a peak at the resonant frequency whereas a low Q implies a more flat
response. The sound is very focused when the Q is high, but it is also hard to control because of
its loose coupling. In this case, the diaphragm is able to vibrate very freely at a certain
frequency. This is undesirable in speakers because every frequency needs to be reproduced
uniformly. Q is defined as the center frequency over the bandwidth (Q = f/Δf). The center
frequency, also known as the resonant frequency, is where Q has its maximum amplitude. There is a range of 3dB around this frequency, which is called the bandwidth. Some typical Q-levels are 0.7 for closed-box systems and 0.2-0.4 for vented-box systems (Weems, p. 22). These Q values are extremely low when compared to those of acoustic instruments such as the violin,
which has a Q value of 30-50. These instruments tend to have many resonant frequencies that give them a singing quality, but this is not desirable in speakers. Thus, a low Q value is favorable in loudspeakers.

Speakers are seen as the weak link in a high quality sound system because they have very
low radiation efficiency. They convert the electrical energy from the amplifier into mechanical
energy and then radiate this energy acoustically. Unfortunately, 90-99% of the energy in this
system ends up as heat (Rossing, p. 413). This poor radiation of energy makes speakers
inefficient. While cone speakers can only radiate at 0.5-5% efficiency, horn speakers able to
convert 10-50% of the electrical energy into acoustical power. (Borwick, p. 92). But Rossing
says that “horn loudspeakers are much more efficient than cone speakers, but low-frequency
horns are considered too large for most home sound systems” (Rossing, p. 414). When dealing
with cone speakers, the efficiency is determined by such factors as the magnet strength, and cone area, to name a few. Efficient speakers will have a powerful magnet and a large cone diameter (Rossing, p. 414). Efficiency is most important at large gatherings such as music concerts and public speeches. Musical instrument speakers, such as Marshall guitar amplifiers, are designed to have high efficiency, but at the expense of a clear sound (Weems, p. 31). As long as the basic idea gets through at an extremely loud volume, most rock musicians are happy. Though
efficiency is necessary is in some fields, most are satisfied with low efficiency for the time being.

Impedance in loudspeakers is similar to that of acoustic instruments. Acoustic impedance relates the “pressure to [the] volume velocity at the surface of the source” (Borwick, p. 4). Likewise, electrical impedance is the ratio of voltage to current, and the input impedance in loudspeakers is the current that is drawn from the source (Rossing, p. 416). The voltage forces the current to flow, as the lips of a trumpet player force wind through the instrument. Though the most common impedance values are 8Ω and 4Ω (Borwick, p. 431), these are only nominal values. The impedance actually “varies depending on the frequency of the music, and may range from 3 Ohms up to 20 Ohms across the auditory spectrum”
(http://www.djsociety.org/Speaker_3.htm, Impedance). For example, in the high frequency
range of a woofer the impedance rises with frequency because of the energy stored in the voice
coil (Weems, p. 143). Just as musical instruments require a certain amount of impedance to
function properly, loudspeakers must be correctly impedance matched with the other equipment in the system.

As the last component in a high-fidelity sound system, the loudspeaker determines the
quality of the final product. Such elements of the speaker as the frequency response, damping
and quality factor work together to provide an even response through the entire frequency
spectrum. A high efficiency is desirable, but not always achievable, and good impedance
matching is crucial. When properly implemented, these aspects of physics can yield a high
quality sound.

Bibliography
Borwick, John. Loudspeaker and Headphone Handbook. London: Butterworths, 1988.
Rossing, Thomas D. The Science of Sound, 2nd ed. Reading, Massachusetts: Addison-Wesley,
1990.
Secrets of Home Theater & High Fidelity. Everything You Wanted To Know About Speakers,
“http://www.djsociety.org/Speaker_3.htm”, 1998.
Weems, David B. and G.R.. Koonce. Great Sound Stereo Speaker Manual, 2nd ed. New York:
McGraw-Hill, 2000.