Description
The Shure SM58 and SM57 are classified as cardioid dynamic microphones. These are microphones that use a moving coil in a magnetic field to create an acoustic pickup pattern that resemble a cardioid. This is a curve created by a circle encircling around a fixed one of the same size, with a point on the perimeter of the latter serving as the curves starting and ending point. The resulting curve creates an apple-shaped pattern deemed suitable for people who wish to isolate the vocals from background noise. Thus, the Shure SM58 and SM57 are sometimes referred to as unidirectional microphones.

Frequency Response
Each microphone has a frequency response, which is a range of frequencies that the device can accommodate for accuracy of sound. Thus, the wider the range, the more accurate the sound is. The frequency response is usually measured in hertz. The Shure SM58 has a frequency response range of 50 to 15,000 Hz. That of the Shure SM57 is slightly wider, at 40 to 15,000 Hz.

Sensitivity and Impedance
The Shure SM58 and SM57s sensitivity refers to the measure of response of the instrument to an incoming signal by producing an output signal. This is usually measured in dBV, a unit of measurement representing the relationship of decibels with voltage. For the SM58, the sensitivity is 54.5 dBV per Pa, or Pascal -- the unit of pressure used for measuring force. The SM57 is less sensitive, at 56 dBV/Pa. The microphones impedance is the measurement of their opposition to electric current flow in ohms. The impedance of both mics is 150 ohms. Shure, however, notes that the actual rating is 300 ohms for the SM58 and 310 ohms for the SM57.

Physical Specifications
The Shure SM58 is a larger microphone than its SM57 sibling. It measures 6 3/8 inches in length and 2 inches in width, with a weight of 10.5 ounces. By comparison, the 10 oz. SM57 is 6 3/16 inches long and 1 1/4 inches wide.