MICROPHONE GAIN

The signal from a microphone must be amplified before it can be recorded, broadcast, used in sound reinforcement or interfaced with other audio equipment. This page addresses the question of how much amplification is needed. Below we will walk through a typical gain calculation.

First let's start with a refresher course on decibels (dB, dBV and dBu):

Understanding dB

dB Calculation

We will begin with the following assumptions:

1. We need to amplify the mic's signal to a level of +4 dBu. This represents 0 dB on a line-level system.

2. We are using an RCA 77-D, a mic used in radio and early television ;

3. We want to record speech/dialog at a distance of 24" with the mic mounted on a boom.

Acousticians have determined that conversational speech at a distance of three feet has a typical loudness of approximately 63 dB SPL. Sound Pressure Level (abbreviated SPL) refers to the acoustical loudness of sound waves through air and is not to be confused with electrical dB.

Using the following equation:

difference, dB = 20 * log (distance 1 / distance 2)

we can calculate the intensity of sound waves at other distances. At a distance of one foot, the acoustical intensity of speech is 72 dB SPL. At one inch the intensity is 94 dB SPL.

The Pascal (Pa) is a unit which is used to measure pressure. Conveniently, 94 dB SPL — the acoustical sound pressure generated by speech at a distance of one inch — equals 1 Pa (Pascal). This greatly simplifies our calculations.

Microphone sensitivity is often stated by manufacturers as dBV at one Pascal of sound pressure. This refers to the amount of electrical energy generated by a mic at one Pa (Pascal). The sensitivity of the Shure SM7B is given as -59 dBV/Pa. Keeping in mind that one Pascal is the sound pressure of speech at a distance of one inch, an electrical output of -63 dBV will be obtained from an RCA 77-D when picking up speech at one inch.

Using the above equation we find that moving the mic further away, from one inch to three inches, results in an output level of approximately -68.5 dBV. To simplify our calculations we can use the rule that each doubling of distance between mic and talker requires an additional 6 dB of gain to achieve the same electrical output. Doubling the distance from three inches to six inches gives us an output of -74.5 dBV. Doubling the distance again to 12 inches results in -80.5 dBV, and another doubling to 24 inches results in an output level of -86.5 dBV. Thus, to achieve our desired level of +4dBu we will need 86.5 dB of gain when picking up conversational speech at a distance of 24 inches as the table below illustrates:

Dist-ance	Speech (acoustical loudness, SPL)	Mic Output Level at 1 Pa (RCA 77-D)	Gain Required for +4 dBu
1"	94 dB SPL	-55 dBV	65 dB
3"	84 dB SPL	-64.5 dBV	74.5 dB
6"	78 dB SPL	-70.5 dBV	80.5 dB
12"	72 dB SPL	-76.5 dBV	86.5 dB
24"	66 dB SPL	-82.5 dBV	92.5 dB

Some mic mixers and consoles amplify mics by as much as 80 dB. Many prosumer preamps provide only 60 dB of gain, resulting in low audio levels when used at a distance. This specification should be checked prior to purchasing a mic preamp, mixer or console.

To relate the above to digital audio, many prosumer audio interfaces popular with professional and amateur musicians accept an unbalanced input of -10 dBV. This typically gives a digital amplitude of 0 dBFS.