5. Understanding Specifications
In addition to some of the basic design characteristics weve discussed, there are some technical specifications that are particularly associated with microphones below, well look at the most important to understand:
Numbers alone will not tell you that much in this area, as not many microphones have such a limited frequency response that they do not cover the necessary range for live sound. More useful is a frequency response graph which displays the amplitude of response at any given frequency, generally only for sound directly into the front of the microphone, although there are sometimes additional graphs available for off axis response. Although you might expect an ideal microphone to have a straight line graph, it is variation in frequency response that is mainly responsible for giving different mics their distinctive characters. Microphones such as the Shure SM58 which are specifically designed for live singers, often have a slight dip at around 200-300Hz and a lift around 4-8kHz this serves to increase intelligibility and flatter the voice.
Sensitivity expresses the microphones ability to convert the movement of its diaphragm to electrical voltage. To accurately measure the sensitivity of a microphone, a manufacturer will place it in a reference sound field to measure the output voltage against a known sound pressure, and this is stated as its sensitivity, usually in mV/Pa where higher numbers are better, or sometimes in dBV, where confusingly figures are negative, and numbers closer to zero (i.e. less negative) are better. Sensitivity is generally not very important in live situations, as mics are typically placed very close to the sound source.
A microphone, like all electronic devices, generates noise. The amount though, is usually well below the noise level of other equipment and so isnt generally a factor with loud or moderately loud sound sources. However, microphone noise can be a problem with soft sound sources - manufacturers are aware of this problem and many mics are now advertised as having low noise. It is usually termed 'self noise, equivalent noise SPL or noise floor' in specifications and is the audible noise level the microphone produces when it is placed in isolation from external sound sources. Dynamic mics usually have very low self-noise as they dont have the electronic components of condensers. As with sensitivity, this specification is not terribly important in live sound, as the volume of sound arriving at the mic will generally dwarf any self noise.
Sound Pressure Level (SPL)
Sound Pressure Level is a reference standard that indicates how loud a sound is in deciBels or dB. The ability of a microphone to handle high SPLs without distortion is primarily determined by its construction. Dynamic mics are generally the most robust in this respect and can usually handle SPLs of 140dB or more. Ribbon microphones are the weakest and can actually be destroyed by too high a level. Although the diaphragm of most condenser mics generally wont distort except under the most severe SPLs, its built-in pre-amplifier can be overloaded to the point of distortion, so figures can vary widely depending on the quality of the mic. Many condensers do have an attenuation pad switch though, which reduces the level of the signal before the pre-amp to avoid overloading. When using the pad, keep in mind that the signal-to-noise ratio of the device is degraded by the amount of attenuation, so in normal SPL conditions its wise to remove the pad as its unlikely to be an issue with all but the very loudest of singers!