Rings of Equal Delay.
One option might be to vary the widths of the rings so that there are equal delay sections between them, in which case the on-axis pressure is

${p}_{\text{norm}}=kaD\left(0\right)=ka\sum _{n=0}^{N}{e}^{-jka\frac{n+1/2}{N+1}}\left(\frac{{a}_{n}^{2}-{a}_{n-1}^{2}}{{a}^{2}}\right)$

(15.21)

where the radius of the nth ring is

${a}_{n}=a\sqrt{1-{\left(1-\frac{n+1}{N+1}\right)}^{2}}\text{,}$

(15.22)

a
_{0} is the radius of the center disk and a
_{
−1}
=

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