Broadband achromatic acoustic focusing remains challenging because most metalenses provide only single frequency phase compensation and lack the approximately constant group delay that is required to suppress focal drift. Here, we show that achromatic focusing can be achieved when the unwrapped transmitted phase of each unit cell in the metalens varies linearly with frequency. Based on this criterion, we design a compact acoustic metalens with deep subwavelength side-branch cavity units. An analytical model is developed to describe the broadband transmission characteristics of the proposed units and to enable predictable linear phase control. Within the investigated parameter space, all unit cells exhibit inherently linear phase responses with tunable phase slopes and frequency-dependent transmission amplitudes. Leveraging these blocks, we realize achromatic focusing by mapping the required phase profile at each frequency onto the corresponding unit cells. Finite element simulations demonstrate a nearly stationary focal length over the frequency range from 3 to 8 kHz. Experimental measurements reproduce the main broadband focusing tendency, although larger deviations are observed at low frequency. These results support the proposed design criterion for broadband achromatic acoustic focusing. The adaptability of the analytical framework is further demonstrated through the design of a second metalens with a different focal length.