The nominal length of the resonating Air column in a wind instrument. In the case of an open organ flue pipe, the speaking length is commonly taken to be the distance between the centre of the mouth and the upper open end. In the simplest acoustical view of such a pipe, each of these points is taken to be a pressure node; if this were indeed the case, the speaking length would be equal to half the wavelength of the sounded note. In reality, the sounded note has a lower pitch and a longer wavelength than predicted by this simplified model. The ‘effective length’ of the pipe is defined as half the wavelength of the sound actually generated in the pipe, and is thus greater than the speaking length.
Similar considerations apply to the flute, and to conical-bore instruments such as the oboe; in these cases, the speaking length is taken from the centre of the embouchure hole or the reed tip to the centre of the highest open finger-hole. For stopped organ flue pipes, and for the clarinet and other cylindrical reed instruments, the simplified theory predicts that the speaking length of the tube should equal a quarter of the wavelength of the note sounded without overblowing, so the effective length is defined as a quarter of the wavelength of the note actually sounded. In all cases the effective length is longer than the speaking length.
The discrepancy between speaking and effective lengths in the relatively simple case of the organ flue pipe can be explained in terms of an End correction at each open end. In the more complex cases of woodwind instruments many other factors influence the effective length, including the size and positioning of open and closed finger-holes, irregularities in the bore, the mechanical properties of reeds, and the degree of lip cover in flutes. Some authors define speaking length to include end corrections; it is then synonymous with effective length.
N.H. Fletcher and T.D. Rossing: The Physics of Musical Instruments (New York, 1991, 2/1998)
C.J. Nederveen: Acoustical Aspects of Woodwind Instruments (Amsterdam, 1969/R)
MURRAY CAMPBELL