Microphone Miscellanea Part 1

Microphone Miscellanea Part 1: Impedance, Levels and SPL

Can't somebody keep the math to a minimum and just tell me what I need to know to connect and use this thing?

by Phil O'Keefe

Okay, so you probably know what a microphone is (a transducer that converts acoustical energy into electrical energy) and you know which end is the business end and how polar patterns work, ( if not, be sure to check out this polar pattern article here on HC ), but what about all those numbers that manufacturers throw at you in ads and spec sheets about things like impedance, SPL and phantom power? What does it all mean?

I can hear some of you now - "hey, I'm a musician, not an engineer - can you just tell me what I need to know to connect and use this thing properly?" If that's you, you've come to the right place. In this 3-part article, we'll cover the basics to get you up and running. Ready? Let's start with impedance.  

IMPEDANCE 

Impedance is the resistance of an electronic component or device to the flow of alternating current. It's measured in ohms, which is sometimes spelled out, and other times is represented with Ω, the Greek symbol for omega. You'll also sometimes see it represented with the letter Z, as in Low-Z or High-Z, which is shorthand for low impedance or high impedance, respectively.

Note that some microphones have low impedance (50-600 ohm) outputs, while others have high impedance (above 10,000 ohm, or 10 kohm) outputs. The two are not interchangeable without using impedance matching transformers, and without boring you with the math, the basic idea is to use low or high impedance microphones only with inputs designed to accommodate specific microphone types.

An example of a high impedance input is a guitar amp's 1/4" input. It's designed to work properly with the high impedance output of an electric guitar. You can also plug a high impedance mic straight into a guitar amp, and several amp models from the '40s and '50s actually had a 1/4" "mic input" for this purpose. The most commonly used application for high impedance mic and guitar amp pairs today is probably for use with blues harp / harmonica. Many computer audio interfaces also feature a high impedance input for use in "direct recording" high impedance sources like guitar and bass, but if yours lacks one, high impedance sources can also be plugged into a low impedance input with the use of a "direct box," which will properly match things up. You can think of a high-impedance input as presenting the least amount of "friction" to a guitar signal.

Some microphone such as the Shure 545 SD (Figure 1) offer a switchable output impedance, and can work as a low or high impedance mic. Check your manual - if your mic offers this, make sure set the output to low impedance - unless you plan on plugging it into a high impedance input, such as the input on your guitar amp.

Figure 1: The Shure 545SD has an internal switch that lets you set it for low or high impedance operation

For recording and live sound, nearly all modern, professional microphones suitable for recording and live sound are low impedance models. They can drive longer cable runs without the risk of picking up interference (like hum and noise), don't suffer the high frequency signal loss typical of a high impedance mic connected with a long cable, are typically of higher quality than high-impedance models, and are compatible with the types of computer audio interfaces, mixing boards (live and studio), and the external recording-oriented microphone preamplifiers in common use. 

Note that the input impedance of those types of devices is generally higher than the output of a low impedance microphone (usually somewhere in the 1,000 - 2,000 ohm range, or 5 to 10 times higher than the microphone's output impedance). This is actually a correct match-up that results in optimal performance for the pair. To avoid signal loss, you should always plug into an input with the same, or a higher, input impedance than the microphone's output impedance. (The concept of matching impedances is a leftover from the days of telephone technology.) Also note that the input impedance can influence a mic's sound, so some preamps let you switch among different input impedances so you can choose what you like best.

MICROPHONE LEVELS 

Different types of audio signals operate at different levels, and unless you use a device to convert them, those levels are neither interchangeable nor compatible. With microphones, it's important to understand the difference between microphone- and line-level signals.

Line level signals typically connect audio devices together. For example, the output of a mixing board will usually be a line-level signal, and the input of a PA amplifier is designed to accept that type of a line level signal and then amplify it and send a (speaker level) signal out to the loudspeakers.  There are two different "line levels."

• The level used with most consumer audio equipment is based on a -10 dBV reference and connects home entertainment devices like DVD players, TVs, stereo components and the like. These typically use unbalanced RCA coaxial connectors and occasionally, 1/8" connectors. 
• The +4 dBu level standard is used with professional audio equipment. Associated connectors are usually either balanced 1/4" TRS connectors (Figure 2) or balanced XLR connectors. Balanced cables use two "hot" wires instead of the single hot wire used in unbalanced lines, and are wired in such a way as to reject hum and noise interference, thus allowing  longer cable runs than unbalanced cables. This is why balanced lines are more common with professional equipment. 

Figure 2: A pair of balanced 1/4" TRS connectors

-10 dBV and +4 dBu line levels use a different reference level (0.316V RMS for -10 dBV line level and 1.228V RMS for professional line level). You likely won't hurt anything by interchanging gear with these different levels, although a +4 dBu signal will usually overload a -10 dBV input and will probably cause distortion, while a -10 dBV signal feeding a +4 dBu input will result in a weak and noisy signal.

Except in very rare cases, the signal level coming out of your microphone (which is usually in the -60dBV / 0.001V to -40dBV / 0.010V range) is much lower than line level and will not interface with line level equipment properly until it is amplified sufficiently and brought up to line level. This is what a microphone preamplifier (mic preamp or just "preamp" for short) does. You'll find preamps built into mixing boards, audio interfaces and even as stand-alone units designed for recording purposes. They all share the same raison d'être - to take the mic level signal that comes out of your microphone and boost it up to a line level output signal that is suitable for use by other devices in your system.  

SOUND PRESSURE LEVEL (SPL)

There are lots of other areas where "levels" come into play in audio engineering, like the level of a signal you record to your DAW, how sensitive a device (like a microphone) is and its output signal level, but when it comes to microphones one of the most important considerations is how loud of a sound it can record without distorting. This will usually reference "SPL" - an abbreviation that stands for Sound Pressure Level.

Sound pressure level is expressed in decibels. Decibels describe a ratio, so they need a reference level in order to mean anything. In the case of SPL, the reference level is usually based on the threshold of human hearing, or 20 micropascals (0.000020 Pa). Remember - decibels are not a linear scale, but a logarithmic scale -  doubling the number doesn't represent a doubled value, but a multiplied one. We use log scales to make it easier to deal with the huge range of numbers that we'd otherwise have to write out when dealing with the difference between the softest and loudest sound levels the human ear can perceive.

Microphones can also deal with a wide range of sound pressure levels, and you'll often see a specification for their maximum SPL. This is the loudest sound level they can capture without audible distortion, and is usually measured at the point where distortion reaches 0.5%. A microphone with a maximum SPL specification of 120 dB will distort significantly earlier than a microphone that can handle 130 dB SPL. In actual use either one would probably serve you well, but it depends on how loud of a sound source you want to record.

Most sound sources in a home studio environment will be below 130 dB SPL, although some, such as a very loud guitar amp, drum kit being played with power and enthusiasm, or a trumpet blast at close range may hit that level...as may the police siren when the cops come to investigate a noise complaint! As a comparison, a jet engine can generate sound pressure levels in the 140 dB range, even when measured from fifty meters away. Some other typical SPL figures for other common sound sources include 40 dB SPL (background noise level in a quiet library), 50 dB SPL (a quiet residence), 60 dB SPL (average human speech from 1 meter away), 70 dB SPL (a vacuum cleaner at 1 meter) and 110 dB SPL (a chain saw; also at 1 meter), and 110-115 dB SPL for an orchestra or loud rock concert. Note that while babies screaming on airplanes may seem louder than that...they aren't. Really. -HC-

Have comments or questions about this article? Then be sure to join the discussion in this thread right here on Harmony Central in the Studio Trenches forum!

You can find part 2 of this article at this link  ...and part 3 at this link.

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Phil O'Keefe is a multi-instrumentalist, recording engineer / producer and the Senior Editor of Harmony Central. He has engineered, produced and performed on countless recording sessions in a diverse range of styles, with artists such as Alien Ant Farm, Jules Day, Voodoo Glow Skulls, John McGill, Michael Knott and Alexa's Wish. He is a former featured monthly columnist for EQ magazine, and his articles and product reviews have also appeared in Keyboard, Electronic Musician and Guitar Player magazines.