Piano Hammers, Fading Strings and a Microphone

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Piano Hammers, Fading Strings and a Microphone

Use studio data to argue where energy goes when kinetic energy changes.

Part 1: The piano hammer

Read the passage and Table 1. Use them to answer questions 1–4.

Session Day at Eastside Sound

1Nia Abernathy plays keyboard in a teen band that records one song each month at Eastside Sound, a community recording studio. Between takes, the studio's sound engineer, Teodoro Salas, likes to show the band how their instruments work. "Every sound in this room starts with something moving," he told them, "and something has to get it moving."

2First he opened the lid of the studio's piano. When a key is pressed, a small felt hammer swings up, strikes a string and bounces back down. Mr. Salas filmed one hammer with a high-speed camera while Nia pressed the same key softly, medium and hard. He measured the hammer's speed just before and just after it hit the string, and a sound meter 1 m away measured how loud each note was.

3Next, the band's guitarist, Ruben Okafor, plucked the same note on two guitars: a hollow acoustic guitar and a solid-body electric guitar that was not plugged in. A laser sensor measured how far the middle of the string swung back and forth each second. For a string playing the same note, the farther it swings, the faster it moves, so a wider swing means more kinetic energy. Near the start of each note, the sound meter read about 78 dB for the acoustic guitar and about 58 dB for the unplugged solid-body guitar. In a third test, Ruben plucked the acoustic string and then rested his palm on it half a second later.

4Last, Mr. Salas pointed to the vocal microphone. Inside it is a thin, light disk called a diaphragm, attached to a tiny coil of wire near a magnet. When sound reaches the diaphragm, it moves back and forth, and the moving coil makes a small electric signal that the computer records. The singer, Amara Lindqvist, sang the same note at three loudness levels, and Mr. Salas read the size of the signal.

Table 1: One Piano Hammer (same key each time)
Key press	Speed before hit (m/s)	Speed after hit (m/s)	Loudness at 1 m (dB)
Soft	1	0.3	62
Medium	2.5	0.8	74
Hard	4.5	1.4	83

1.

Use Table 1. After the hit, the hammer moves back away from the string. Compare only the two speeds, not the direction.

In the hard press, the hammer was moving m/s slower after the hit than before it.

2.

Which observation is the best evidence that energy was transferred from the hammer to the string?

A. The sound meter was set 1 m away from the piano.B. The hammer slowed down in the hit, and the string began to sound.C. The hammer is covered in soft felt where it meets the string.D. The hammer moved fastest when Nia pressed the key hard.

3.

The hard press made the loudest note. Which explanation fits Table 1 best?

A. The hammer moved slowest after the hard hit, so the string kept more of its own energy.B. The fast hammer lost less speed in the hit than a slow one, so it kept its energy and the note grew louder.C. The fast hammer had more kinetic energy and gave more of it to the string, so the note was louder.D. The string makes more of its own energy when it is struck hard, so the note was louder.

4.

Look at the object named in each event. Sort each event by what happens to that object's kinetic energy.

Part 2: The fading guitar string

Use paragraph 3 and Table 2 to answer questions 5–8.

Table 2: How Far the String Swings After One Pluck (mm)
Time (s)	Acoustic guitar	Solid-body guitar	Acoustic, palm at 0.5 s
0	2	2	2
1	1	1.6	0.1
2	0.5	1.3	0
3	0.3	1	0
4	0.2	0.8	0

5.

Use Table 2.

At 2 seconds, the solid-body guitar's string was swinging mm farther than the acoustic guitar's string.

6.

Use paragraph 3 and Table 2. The acoustic guitar's note was much louder, but its string slowed down faster. Which is the best explanation?

A. The hollow body passed the string's energy to the air faster, so the string lost it sooner.B. The hollow body destroyed part of the string's energy, so less of it was left to swing.C. The solid-body string started with more kinetic energy, so it had more energy left later.D. The solid wood made new energy for its string, so that string kept swinging longer.

7.

In the third test, the string almost stopped within half a second after Ruben's palm touched it. Where did most of the string's kinetic energy go?

A. Into the string itself, which stored the energy as extra mass for the next noteB. Back into Ruben's plucking finger, which gave the string its energy at the startC. Into the laser sensor, which took in energy each time it measured the swingD. Into Ruben's palm, as the swinging string pushed and rubbed against his skin

8.

Complete the energy summary for one note on the acoustic guitar. Use words from the word bank. Two words are not used.

When Ruben plucks, energy moves from his to the string, so the string's kinetic energy . As the note rings, energy moves from the string into the guitar body and the , so the string's kinetic energy .

Part 3: The microphone

Use paragraph 4 and Table 3 to answer questions 9–11.

Table 3: Amara's Note at the Microphone
Loudness at the mic (dB)	Signal from the mic (mV)
60	1
70	3
80	10

9.

Which conclusion do Table 3 and paragraph 4 support best?

A. The diaphragm makes the singer's voice louder, so a bigger signal comes out at 80 dB.B. Louder sound gave the diaphragm more energy, so it moved more and made a bigger signal.C. The signal stays the same size, because Amara sang the same note each time.D. The magnet gives the diaphragm more energy when it is loud, so the signal grows.

10.

Amara steps back from the microphone and sings at the same strength, so her note reaches the mic at 70 dB instead of 80 dB. Which prediction fits the data?

A. The diaphragm would get less energy and move less, so the signal would be about 3 mV.B. The diaphragm would move faster to reach her, so the signal would be over 10 mV.C. The diaphragm would get less energy, so the signal would drop to about 1 mV.D. The diaphragm would get the same energy, so the signal would stay at about 10 mV.

11.

Ruben says, "When a guitar note fades away, its energy is just used up and gone." Do you agree? Write a claim, give evidence from Table 2 and paragraph 3, and explain your reasoning about kinetic energy and energy transfer.