A spring-mass system with \(m=0.5 \mathrm{~kg}\) and \(k=10,000 \mathrm{~N} / \mathrm{m}\), with negligible damping, is used as a vibration pickup. When mounted on a structure vibrating with an amplitude of \(4 \mathrm{~mm}\), the total displacement of the mass of the pickup is observed to be \(12 \mathrm{~mm}\). Find the frequency of the vibrating structure.