Maren strikes a tuning fork and holds it up. The fork hums.
Earlier in the course you saw that a wave is produced by a vibrating source that disturbs the material around it. This lesson follows that idea into sound: what exactly vibrates, and at what frequency.
The struck tuning fork's two prongs swing back and forth, hundreds of times each second.
Each swing of a prong presses on the air beside it.
The pressed air passes the vibration on, and a sound wave travels out through the air.
Count the two rates. The prongs vibrate at 440 Hz. The air beside them vibrates at 440 Hz too.
The sound wave carries that matching frequency away through the medium.
Every sound starts this way: a vibrating object, and a medium driven at the object's own frequency.
Noll taps a drumhead and it vibrates at 90 Hz. The air above the drumhead carries the sound away. State the frequency of the air's vibration.
A vibrating object sets the surrounding medium vibrating at the same frequency.
The drumhead vibrates at 90 Hz.
The air vibrates at 90 Hz, the same frequency as the drumhead.
A phone's small speaker vibrates at 1200 Hz. Pim asks what frequency the sound wave in the room's air has. State it.
The speaker is the vibrating object; the room's air is the medium.
The medium vibrates at the object's own frequency.
The sound wave in the air has a frequency of 1200 Hz.
Ottoline plucks a guitar string and it vibrates at 330 Hz. At what frequency does the air around the string vibrate?
Noll says: 'A buzzing speaker cone vibrating at 250 Hz makes the air in front of it vibrate at 250 Hz.' Is Noll correct?
Pim says: 'A bell vibrates at 600 Hz, but by the time the sound reaches the back of the room its frequency has dropped.' Is Pim correct?
A struck xylophone bar vibrates at 700 Hz in a school gym. Which statement is correct?
A cello string is bowed and a listener across the room hears the note. Which statement puts the events in the correct order?
A phone on silent buzzes against a desk at 180 Hz, and the desk surface passes the buzz to the air. At what frequency does the air vibrate?