The Didgeridoo and its Acoustic Properties:

Physical Variation and its Effect on Frequency and AmplitudeYolnan Chen, Andrew ParkerFrontiers of Science InstituteUniversity of Northern Colorado

Image from: didgeridoowebshop.com

Hypotheses

We predict that when the physical attributes of an air cylinder that is closed at one end are altered, the frequency and amplitude of the energy produced are altered as well.

Increase length→decrease frequencyIncrease diameter→increase amplitude

BackgroundDidgeridoo● Originates from Australia● Tree branch or root

hollowed out by termites● Decorated (painted,

engraved or stained)● Simple instrument

Amplitude: ‘size of vibration’Frequency: ‘speed of vibration’(Hz)f = f = frequencyƛ = wavelengthn = resonant numberv = velocityOscillation: vibration of 2 sources against each other1 Hertz = 1 oscillation per second

Pre

ssur

e (P

a)P

ress

ure

(Pa)

Time (msec)

Higher Frequency

Lower Frequency

Time (msec)

Pressure is measured in Pascals (Pa)

Pressure is directly related to amplitude

More Dense

Less Dense

Longitudinal Wave● particles temporarily displaced, return to original position● one particle transport energy to another (horizontally)

Standing Wave● 2 waves, opposite direction, same frequency

Reflection● Can cause standing waves

Sound Wave Absorbing Materials● Hard→very reflective● Soft→less reflective

Acoustic Impedance: resistance of air to flow ● Some energy transmitted● Some energy reflected

Other Terminology:● Fundamental Frequency

-The lowest frequency of a periodic waveform● Harmonics

-The following frequencies at intervals of 4 (harmony/chords/pitch/octave)

Fundamental Frequency

Harmonics

Am

plitu

de (d

B)

Frequency (Hz)

MethodsPlay the PVC pipes simulating didgeridoos of various lengths and diametersHuman didgeridoo playerSound insulated box reduce background noiseSnapshots of spectrogram & oscilloscope

Materials● PVC pipes● Particle Board● Insulation foam● USB microphone● Didjeridu● Funnel

Results (Length Changes)

Didgeridoo 1length: 39.1indiameter: 1.25in

Didgeridoo 2length: 48.9indiameter: 1.25in

Am

plitu

de (d

B)

Frequency (Hz) Frequency (Hz)

Results (Diameter Changes)

Didgeridoo 6length: 39.1indiameter: 2in

Didgeridoo 7length: 39.1indiameter: 0.75in

Am

plitu

de (d

B)

Frequency (Hz)Frequency (Hz)

Results (Bell vs No Bell)

Didgeridoo 8(with bell attachment)

Didgeridoo 3 (without bell attachment)

Am

plitu

de (P

a)

Time (msec)

Am

plitu

de (P

a)

Time (msec)

Bell Effect● Gradual change in

diameter● Decreasing impedance● Less reflection, more

transmission● Pressure inside air

cylinder different than pressure outside

Conclusion● Hypothesis correct

● Wish to add filling to inside of PVC pipe

Sources ● Wolfe, J. (2005). Didgeridoo Acoustics/Yidaki Acoustics. University of

New South Wales. Retrieved from http://newt.phys.unsw.edu.au/jw/didjeridu.html

● Sound Waves and Music. The physics classroom. Retrieved from http://www.physicsclassroom.com/class/sound

● Heller, E. J. (2013) . Why You Hear What You Hear. Princeton, NJ:

Princeton University Press.

● Serway, R. A., & Jewett, J. W. (2010). Physics for Scientists and

Engineers (8th ed.). Boston, MA:Cengage Learning

AcknowledgementsThanks to...● Christopher Courrejou (mentor)● Rebecca Kipf (Advisor)● Dr. Semak (Physics Department Professor)● Dr. Galovich (Physics Department Chair)● Christopher Krause, Sofia Simina (Residential Advisors)● Noble Energy (Sponsor)● Rollie R. Kelley Family Fund at the Denver Foundation (Sponsor)● The Edward Madigan Foundation (Sponsor)● University of Northern Colorado● Steve Anderson (Head of MAST)● Lori Ball (Head of FSI)

Image from: colourbox.com

Questions?