The reduction of acoustic microphone size using microelectromechanical systems (MEMS) technology enables increased spatial and temporal resolution. Whether the small size can be effectively utilized depends on the signal‐to‐noise ratio and minimum detectable signal (MDS) that are a function of the structural geometry, material properties, and transduction method. The optimal MDS depends on both electronic and thermomechanical noise sources. Fundamental noise sources may be divided into frequency independent thermal noise and frequency dependent excess noise dominating at low frequencies. There have been some questions regarding the dominance of electrical or mechanical sources of the excess noise in piezoresistive microphones [A. Zuckerwar et al., J. Acoust. Soc. Am. 113, p. 3179–3187 (2003)]. Noise power spectra have been measured for various piezoresistive microphones. We present results on the bias dependence of the excess noise that indicate that the primary source of excess noise is electrical. The relative contributions of mechanical and electrical noise sources will be discussed.