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Cello Takes and a Vocal Booth: Measuring Sound Wave Energy

Grade 8 sound wave practice: use a recording studio's cello and vocal takes to find wavelengths, read peak-to-peak meters and relate amplitude to energy.

Grade 8ScienceInteractive · Printable

Aligned toMS-PS4-1

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What’s included

Single worksheet
Auto-graded online
1 NGSS standard
Printable answer key
12 questions
Classroom-ready

Details

Format: Interactive · Printable
Grade: Grade 8
Subject: Science
Question types: Fill in the blank, Multiple choice, Sorting, Short answer

About this worksheet

What students practice

Students use wave speed = frequency × wavelength for a wave on a string and a sound wave in air.
Students find amplitude from a peak-to-peak reading and use it to predict the energy a sound wave carries.
Students explain why doubling the amplitude of a wave makes its energy four times as large.

How to use

Read the studio notes together and sketch one wave showing amplitude and peak-to-peak swing before students start. Calculators are helpful for Part 1.

Curriculum

NGSS standard covered

MS-PS4-1

Use math to describe how a wave's amplitude relates to the energy it carries, and read wavelength and frequency.

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Cello Takes and a Vocal Booth: Measuring Sound Wave Energy

Use studio meter readings to link amplitude, energy, frequency and wavelength.

Part 1: The string and the air

Read the studio notes, then use them and Table 1 to answer the questions.

Session Day at Brightwater Sound

1Every spring, the Lakeview Youth Orchestra records a short album at Brightwater Sound, a recording studio across town. This year eighth graders Tomasz Ekwueme, who plays the cello, and Liesl Varga, who sings, are recording a song together. The studio's engineer, Ms. Ottilie Branch, lets them watch the meters and keeps a log of every take. She says that engineers who understand waves can set up microphones faster and waste fewer takes.

2Sound is a mechanical wave. When Tomasz draws his bow across a string, the string vibrates and pushes on the air next to it. The air is squeezed together and then spread apart, over and over, and each bit of air passes the push along to the next bit. The air itself only moves back and forth, while energy travels from the cello to the microphone. In the studio's air, sound travels at 343 m/s.

3The vibrating string carries a wave too. Each of the cello's strings has a vibrating part 0.69 m long. When a string is played open, with no finger pressing it, the wave on the string has a wavelength of twice that length, 1.38 m. The string and the air around it vibrate with the same frequency, but the wave moves along the string at a different speed than sound moves through the air.

4The microphone measures how much the air pressure rises and falls as each wave passes. The amplitude of a sound wave is the largest rise in pressure above normal air pressure, measured in millipascals (mPa). The studio meter does not show the amplitude directly. It shows the "peak-to-peak" swing, from the lowest pressure to the highest, which is twice the amplitude.

5For Table 2, Tomasz played his open D string four times with the microphone 1.0 m from the cello, bowing a little harder each time. For Table 3, Liesl sang one steady note at the same loudness three times while Ms. Branch moved the microphone farther from her mouth. A computer on the mixing desk worked out how much sound energy reached the microphone each second in every take. It uses the same energy units in both tables, so the takes can be compared.