"纯量波动方程式" can be translated into English as "scalar wave equation".
Here, "纯量" (chúnliàng) means "scalar" in English, referring to a quantity that has only magnitude and no direction, while "波动方程式" (bōdòng fāngchéngshì) translates to "wave equation," which is a second-order partial differential equation describing the propagation of various types of waves, such as sound waves, light waves, or water waves.