Mitigation of low-frequency vibrations has long been a major challenge. One promising research direction points to architected materials–widely referred to as acoustic or elastic metamaterials. They can exhibit a phononic band gap, ie, a range of frequencies in which no vibration can propagate. While many recent studies attempted to demonstrate low-frequency band gaps, there is no consensus on which frequency ranges should be calledlow''orultra-low’’. The exact meaning of low frequency varies from a fraction of one Hz, to several Hz, and up to many kHz. The word``low’‘is a relative concept that depends on application-specific scenarios. To facilitate a generally meaningful discussion and a fair comparison among different systems and configurations, we advocate a universal and dimensionless frequency for all vibro-elastic metamaterials: f= a/λ, where a denotes the size of a metamaterial unit, and λ is the …