The whirly tube, corrugaphone, or bloogle resonator, also sold as Free-Ka in the 1960s-1970s, is an experimental musical instrument which consists of a corrugated (ribbed) plastic tube or hose (hollow flexible cylinder), open at both ends and possibly wider at one end (bell), the thinner of which is rotated in a circle to play. It may be a few feet long and about a few inches wide. The faster the toy is swung, the higher the pitch of the note it produces, and it produces discrete notes roughly belonging to the harmonic series, like a valveless brass instrument generates different modes of vibration. However, the first and the second modes, corresponding to the fundamental and the second harmonics, are reported as being difficult to excite. To be played in concert the length of the tube must be trimmed to tune it. According to the modified Hornbostel–Sachs organological system proposed by Roderic Knight it should be numbered as "A21.31" (twirled version) and as "A21.32" (blown version), described as "a corrugated or ribbed tube that produces overtones through turbulence" . In spite of being an aerophone, it is usually included in the percussion section of "sound effects" instruments, such as chains, clappers, and thunder sheets.
Sound
Hopkin describes a single whirled corrugaphone as capable of producing three or four different pitches. Crawford describes harmonics two through seven as reachable while whirling, though seven takes, "great effort." Hopkin describes that with a corrugahorn, "with tubes of suitable length and diameters, the range extends well up the [harmonic] series, where the available tones are close together and you can, with practice, play quite melodically." In fact, since each sounding mode plays throughout a range of speeds (rather than at one specific speed), it is difficult to skip over harmonics, as this requires a jump in speed (rather than gradual change), though this is easily done using one's tongue and throat to interrupt the air flow with a corrugahorn. Many sales offers describe the tubes as producing up to five distinct notes (presumably the bugle scale: close to the harmonics 2, 3, 4, 5, and 6 ), and while higher modes may be possible, if hard work, dissonant adjacent harmonics may sound simultaneously, such as 15 and 16. The modes of a corrugated tube are usually lower than those of an uncorrugated tube of the same length and diameter, and, "audible vibration in the whirly tube appears only when air flow velocity exceeds a certain minimum, which may preclude the sounding of the fundamental or lower harmonics." The timbre of the notes produced by the whirly tube are, "almost all fundamental," according to Fourier analysis (similar to sine waves). Tubes longer than many feet may have one end whirled while held near its middle or may be held out a car window. The equations describing the sound produced when the tube is whirled, as proposed by F.S. Crawford in 1973, as follows, proposes that the air flowing through the corrugations should produce a sound similarly to a scraping instrument, such as a "reco-reco", in which a stick is scratched against a surface with regularly spaced grooves. This would be the rationale for the formulas below. However this tentative model is not experimentally demonstrated or supported by the theory of sounding pipes in acoustics. On the contrary, the present theory of sound production in corrugated pipes refutes the assumptions by Crawford (1973).
frequency = bumps sec = bumps inch × ( air flow velocity in inches sec ) {\displaystyle {\text{frequency}}={\frac {\text{bumps}}{\text{sec}}}={\frac {\text{bumps}}{\text{inch}}}\times \left({\text{air flow velocity in }}{\frac {\text{inches}}{\text{sec}}}\right)}
flow velocity = cm sec = cm bump × bump sec = corrugation distance × bump frequency {\displaystyle {\begin{aligned}{}\\[1pt]{\text{flow velocity}}&={\frac {\text{cm}}{\text{sec}}}={\frac {\text{cm}}{\text{bump}}}\times {\frac {\text{bump}}{\text{sec}}}\\[6pt]&={\text{corrugation distance}}\times {\text{bump frequency}}\end{aligned}}}
Thus the faster the tube is swung or the more dense the corrugation the higher the pitch of the note produced.
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