Closed Pipe Pipe closed at ONE end: closed end pressure antinode air press. L = /4 L.

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Presentation transcript:

Closed Pipe Pipe closed at ONE end: closed end pressure antinode air press. L = /4 L

fundamental frequency of closed pipe: note: this is half the frequency of an open pipe of same length (octave below ) open end: pressure NODE (motion antinode) closed end: pressure antinode (motion node)

example: how long is a CLOSED organ pipe that plays A 1 = 55 Hz? (note: A 1 A 4 = 3 octaves) L = v/4f = (340/220) m = 1.55 = 5 ft Higher modes of closed pipe: need pressure NODE at open end need pressure BELLY at closed end open end pressure closed end for fundamental closed end for second mode closed end for third mode position along pipe

closed pipe L 1 st mode (fundamental) f 1 = v/4L (first harmonic) 2 nd mode (first overtone) f = 3f 1 third harmonic 3 rd mode (second overtone) f = 5f 1 fifth harmonic closed end

higher modes (overtones) of closed pipe Only ODD multiples of fundamental f 1 first mode (fundamental) 3f 1 second mode (first overtone) but third harmonic! 5f 1 third mode (second overtone) but fifth harmonic!

Sound Spectrum (Fourier Spectrum) Fourier: represent complicated periodic oscillation (period T) as sum of sinusoidal oscillations of frequencies f 1 = (1/T) and harmonics f 2 =2f 1, f 3 =3f 1 etc. amplitude of harmonic freq of harmonic (Hz) easy visualization of harmonic content (timbre) but contains no information about relative timing of overtones (phase).

Pitch perception Fourier Synthesizer….. produces frequencies f 1, 2f 1, 3f 1, 4f 1, 5f 1 6f 1, 7f 1 etc of adjustable amplitude and phase. listen to the following combination of harmonics: what pitch do you hear? f (x 440 Hz) A Hz = A 5 e.g. f 1 = 440 Hz = A 4 can synthesize any oscillation of period T=1/440 sec. but…. the fundamental is 880 Hz since all freq multiples of 880

but what if we add a bit of 3x440Hz?? f (x 440 Hz) A now suddenly the (theoretical) fundamental is 440 Hz but what do we hear? Demo: in some situations (missing or almost missing fundamental) there may be an ambiguity in perceived pitch

A f(x200Hz) what do you hear? 100 Hz of course! A f(x200Hz) and now? DEMO

Tone Quality (Timbre) In acoustic theory, what exactly is “timbre”? Timbre is that attribute that differentiates two tones of same loudness and same pitch. The Fourier Spectrum (frequencies and intensities of overtones) is only one aspect of timbre…….. Other aspect of tone quality: rise and decay An example of two tonal presentations which show importance of the tone envelope (attack and decay)