Resonance and Musical Instruments

Main Points from Sound Module 2• Experiment #1

– Velocity of a wave stays the same as long as the medium stays the same

– Smaller metal rod has a smaller wavelength than a larger metal rod

– Give our equation: v = frequency x wavelength, if v stays the same and wavelength decreases the frequency will increase.

– As a result, smaller metal rod = higher frequency (pitch); longer metal rod = lower frequency (pitch)

• How does this relate to guitar strings and frets?

Main Points from Sound Module 2• Experiment #2

– Standing waves: waves created by sound reflection and interference. Have node regions (no movement) and antinode regions (lots of movement)

– Vibrating Air Columns: When you blow across top of bottle you create standing waves (with air molecules inside). Top of bottle = antinode (where air moves a lot) Top of water = node (dead end, molecules don’t move)

– As you add more water, you decrease the wavelength of the air column (results in increase in frequency). More water = higher pitch

– When you hit the bottle with a spoon, you hear the waves of the bottle and water (not air). This time, more water = slower vibration and lower frequency (like thick strings = lower frequency)

Main Points from Sound Module 2• Experiment #3

– Tapping the balloon drum creates a pattern called a Chladni figure.

– Tapping creates standing waves with node and antinode regions

– At antinode regions, sand moves up and down a lot

– At node regions, sand collects (this is where the drum doesn’t move)

– Video: • http://www.youtube.com/watch?

v=YedgubRZva8&feature=related• http://www.youtube.com/watch?

v=EprMFajNzfQ

Frequency of Objects

• Sound is produced b/c of vibrating objects (strings, vocal cords, eardrums)

• Nearly all objects, when disturbed, will vibrate and create a sound– Dropping a meter stick– Blowing over the top of a bottle– Plucking a guitar string

• Frequency in which each object vibrates when disturbed = natural frequency. If the amplitude of vibrations is large enough (&within our hearing range) it will be audible

Noise vs. Music

• Each object has its own natural frequency or set of frequencies

• The quality of sound produced by an object depends on these natural frequencies– Some objects vibrate at a single frequency producing a pure

tone. E.g. Flute– Some objects vibrate at a set of frequencies that are

mathematically related by whole numbers. These are said to produce a rich sound. E.g. Tuba

– Other objects vibrate at multiple frequencies that have no mathematical relationship. These objects are not musical at all and the sounds they create can be described as noise. E.g. meter stick/pencil dropping on the floor

– http://www.youtube.com/watch?v=tasMFxMFPo8

Natural Frequency

• Flute– 200 Hz

• Tuba– 200 Hz– 400 Hz– 600 Hz– 800 Hz– 1000 Hz

• Dropped Pencil– 197 Hz– 211 Hz– 217 Hz– 219 Hz– 287 Hz– 311 Hz– 329 Hz– 407 Hz– Etc.

Frequency of Objects

• Actual frequency of an object is determined by:– Speed– Wavelength

• Musician’s job is to control these variables to produce a desired frequency– Speed Guitar strings with different tension – Wavelength Shortening vibrating portion of string (i.e.

pressing on frets)

Musical Instruments• Trombone

– Wind instrument– The tube of a wind instrument acts as the

container for the vibrating air column– Air inside column starts vibrating due to

vibrating reed or vibration of musicians lips against mouthpiece• Unable to alter speed of sound…why?• Change length of air column by pushing tube

outward (lengthen) or pulling it in (shorten)– http://

www.youtube.com/watch?v=LOL8-qIYemg&feature=related

To Summarize…

• All objects have a natural frequency or set of frequencies they vibrate at

• Actual frequency depends on properties of medium and length of medium

• Goal of musicians:– find instruments that possess the ability to vibrate with

sets of frequencies that are musically sounding (mathematical relationships)

– Vary the lengths of (and properties) of instruments to create desired sounds

To Do

• Find/create 5 different things that create “music” and 5 different things that create “noise”

• Organize according to increasing or decreasing frequency

• Pick one object and demonstrate how increasing/decreasing wavelength affects the frequency.