We use a Fourier analysis method to study the spiral structure of the disc galaxies in the Frei et al. (1996) sample. We show that m=1 components are more often associated with asymmetries between the two arms of an m=2 spiral than with global lopsidedness of the disc. The power in the m=2 components is correlated with the arm class. The majority of galaxies with arm class 9 or 12 show clear signs of modulation in their spiral arms, suggesting the action of an amplification mechanism like the Swing amplification. Finally we report new detections of underlying large-scale bisymmetric components in flocculent spiral galaxies.