Shares of digital audio processor specialist Cirrus Logic (NASDAQ: CRUS) fell 26.1% in the first half of 2018, according to data from S&P Global Market Intelligence. The company was exposed to weak Apple (NASDAQ: AAPL) iPhone sales, and the Cupertino account represents about 80% of Cirrus' quarterly sales.The close relationship with Apple is hugely positive for Cirrus and its shareholders when times are good for its largest customer.