Perhaps you’ve noticed some microphones come with switchable options: On/off switches; high-pass filters and other equalization boosts/cuts; polar pattern switches; and attenuation pads. Let’s focus on attenuation pads and how they work inside a microphone.
So what is a microphone attenuation pad? A “PAD” stands for Passive Attenuation Device. The circuitry in active mics may overload if the incoming signal from the capsule is too strong, causing audio signal distortion. Pads reduce signal levels before the active amplification process in order to avoid overloading the microphone circuitry.
That’s the short answer. But let’s get into more detail about how passive attenuation devices work and how to use them in practice!
What Is A Microphone Attenuation Pad And What Does It Do?
A PAD (Passive Attenuation Device) is exactly as its name suggests. It is a passive device designed to attenuate an audio signal. The device is a special type of passive electrical circuit made up entirely of resistors.
The passivity of a pad means it requires no power to function. Passive circuitry is also exempt from distortion up until the point where the electrical components become damaged. Audio signals will never reach the point of damaging the pad circuitry. And so passive pads will pass any audio signal without that audio signal becoming distorted!
Attenuation means decreasing the level of an audio signal. An attenuation pad is easily conceptualized as the opposite of an amplifier. The function of a pad is to output the same signal as its input, only at a lower level, preserving the signal-to-noise ratio as best as possible.
A microphone attenuation pad reduces the level of the microphone audio signal coming from the microphone capsule before the signal reaches the active circuitry.
Variable (switchable) attenuation pads allow for the following:
- Disengaging the pad increases mic sensitivity and improves the signal-to-noise ratio.
- Engaging the pad increases the maximum sound pressure level a microphone can handle before distorting.
To learn more about microphone sensitivity, signal-to-noise ratio, and maximum sound pressure level, check out the following My New Microphone articles:
• What Is Microphone Sensitivity And Why Does It Matter?
• What Is A Good Signal-To-Noise Ratio For A Microphone?
• What Does Maximum Sound Pressure Level Actually Mean?
This option proves to be quite valuable among active microphones. We’ll discuss this further in the following sections of this article!
Signal Flow And The Circuitry Of Pads
In terms of electrical circuitry, a passive attenuation device is an input/output resistive network designed to attenuate a signal to a suitable level for the connected load. It decreases the signal strength between input and output without altering other characteristics of the signal.
The microphone capsule acts as a transducer, changing acoustic energy (sound vibration) into electrical energy (audio signal). The output AC voltage of the capsule is directly proportional to the sound pressure level the capsule is subjected to.
If variable pads are part of an active microphone’s design, they are put in-line between the microphone capsule and the active circuitry.
Pads are needed when the voltage from the capsule would otherwise overload the active circuitry.
The Circuitry Of The Passive Attenuation Device
An attenuation pad is a two-port passive resistance network designed to attenuate the output signal. Pads attenuate audio signals without distortion and equally at all frequencies with no phase shift. Attenuator resistors should be made of pure non-inductive material and are not wire wound in order to avoid nonlinear frequency attenuation.
A typical pad reduces gain with a resistive voltage divider circuit. Once again, attenuators are basically the opposite of amplifiers.
This pad voltage divider circuit reduces the voltage running through it. Most often, the reduction is expressed in negative decibels.
Microphone attenuation pads are typically predetermined switches. The toggle switches available on the body of the microphone actually change the flow of electricity within the pad circuit of the microphone!
Why Do Active Microphones Benefit From Pads?
Active microphones benefit from pads due to the overload protection pads provide to active circuitry. When audio signal AC voltages high enough in level are sent through active circuitry, distortion may ensue. Pads reduce this AC voltage to greatly reduce this risk of distortion.
Before we go any further, let’s answer an important question…
Why would microphones require active circuitry?
All the condenser microphones I know of are active. Some modern ribbon microphones are also active!
Active circuitry is designed to change the electrical audio signal in 2 main ways:
- Amplify the audio signal AC voltage to a usable mic level.
- Decrease the AC voltage’s impedance to a workable level.
The electrical signal generated by a microphone capsule is often too weak and has extremely high impedance. Condenser capsules often suffer from this situation and always have active electronics. Ribbon microphones are also sometimes supplemented with active electronics to improve their audio signal levels.
The audio signal generated by the microphone capsule is minuscule compared to the mic level required for proper signal audio signal flow. For this reason, active amplifiers are designed to immediately boost the audio signal to workable levels before the microphone outputs its signal.
This amplification needs to happen immediately due to the extreme impedance in the audio signal. High impedance doesn’t allow the signal to travel very far without succumbing to severe degradation. This is why we can’t rely on microphone preamps to boost the entirety of a microphone’s audio signal. By the time the signal would reach the preamp through lengths of cable, the high-end frequencies would be lost.
Active circuitry is also designed to greatly decrease the impedance of the AC voltage audio signal. This part of the active circuitry is known as the impedance converter. This is a good thing for the reasons stated above.
So microphones that require active circuitry need it to produce a low impedance mic level signal from a weak, high impedance signal provided from their capsules.
For more information on microphone impedance, check out my articles Microphone Impedance: What Is It And Why Is It Important? and What Is A Good Microphone Output Impedance Rating?
Now, where were we? Oh yes, overloading the active circuitry!
Overloading Active Circuitry
Too much voltage from the capsule, when amplified by the active circuitry, will actually overload the microphone circuitry. Overloading happens when the amount of current running through the wires of the mic exceeds the amount of current those wires can handle.
The Passive circuitry of “passive microphones” rarely overloads. Overloading is extremely unlikely from non-amplified audio signal voltage. Passive microphones often use step-up transformers to boost their signals at their output. Transformers are passive devices that work on the principle of electromagnetism.
When a strong enough AC voltage is passed through a transformer, saturation (gentle “warm” compression) may occur. Conversely, the distortion that happens in active circuitry is not so “gentle” or “warm.”
Overloading the active circuitry will result in distortion of the audio signal. This distortion ranges from barely noticeable on audio transients to full-out clipping of the audio signal.
To learn more about mic clipping, check out my article What Is A Microphone Clip? (Physical And Electrical).
The good news is in most practical situations, there will not be any permanent damage to any part of the microphone.
The AC voltage output of a microphone capsule is directly proportional to the sound pressure level the microphone capsule is subjected to. For this reason, active microphones nearly all come with a maximum SPL (sound pressure level) rating. Typically the max SPL is rated as the point where 0.5% total harmonic distortion happens through the active circuitry and at the microphone output.
For more information on active microphones, check out my article Do Microphones Need Power To Function Properly?
The Reason For An Attenuation Pad
Let’s do a quick recap here:
- Mic capsules convert acoustic energy (sound) into weak electrical energy (audio signal/AC voltage).
- Active microphones boost this signal by means of active circuitry (amplifier and impedance converter).
- Audio signals may be too much for the active circuitry to handle, causing distortion.
Attenuation pads solve this problem by reducing the amount of signal being sent into the active amplifying circuitry. The pad is part of the microphone design between the capsule and the active circuitry.
This seems counterproductive: reducing the signal so that we may then amplify it. However, this is the standard manner of attenuation pads being implemented in active microphones.
For more info on mic signals, check out my article What Is A Microphone Audio Signal, Electrically Speaking?
The Variable Attenuation Pad In Action: AKG C 414
Let’s use the AKG C 414 XLII condenser microphone as an example.
AKG is featured in My New Microphone’s Top 11 Best Microphone Brands You Should Know And Use.
The C 414 features 4 “preattenuation pad” switches: 0 dB, −6 dB, −12 dB, and −18 dB.
The pad selections coincide directly with the C 414’s maximum sound pressure level ratings:
At 0 dB pad, the max SPL is 140 dB SPL (200 Pascal)
At −6 dB pad, the max SPL is 146 dB SPL (400 Pascal)
At −12 dB pad, the max SPL is 152 dB SPL (800 Pascal)
At −18 dB pad, the max SPL is 158 dB SPL (1600 Pascal)
For every 6 dB of additional attenuation, the audio signal passing through to the active circuitry is reduced by 6 dB.
It’s important to note that the diaphragm and capsule of this condenser mic handle all of these max SPL ratings with ease. The active circuitry is the limiting factor for overloading and distortion.
In-Line Microphone Pads
Not all microphones have built-in attenuation switches. If one of these “padless” mics requires attenuation, there are a few options.
First, many preamps have pads at their inputs. If the mic signal is very strong at the preamp input, engaging a pad can help smooth out the levels and allow the preamp gain to be more effective.
Second, there are in-line passive attenuation devices that can be connected between the microphone and preamp in order to bring down the mic signal before it reaches the preamp input.
One such example of an in-line pad is the Shure A15AS (link to check the price on Amazon), which provides 3 pad options (-15 dB, -20 dB, and -25 dB). The A15AS also passes phantom power for use with those active mics that require phantom power.
For more on microphones and phantom power, check out my article Do Microphones Need Phantom Power To Work Properly?
It is critical to note that preamp and in-line pads come after the microphone output. This means that they will not affect the microphone sensitivity, signal-to-noise ratio, or maximum sound pressure level ratings.
The Balancing Game
So why not just design microphones to have higher max SPL values and do away with the variable attenuation options? The answer has to do with preserving as much signal-to-noise ratio as possible while avoiding unnecessary signal distortion.
Active microphone circuitry is inherently noisy. As long as the circuitry is powered up, it will create a subtle amount of noise. This is referred to as microphone self-noise and is even given as a specification on active microphone datasheets.
For more information on microphone self-noise, check out my article What Is Microphone Self-Noise? (Equivalent Noise Level).
Self-noise is given in A-weighed decibels (dBA). dBA is referenced against the threshold of human hearing and is based on the way humans hear different frequencies along the audible spectrum.
The self-noise of a microphone is fixed. Therefore, the signal-to-noise ratio is dependent on the amount of audio signal that is amplified by the active circuitry (the microphone sensitivity).
If the signal is attenuated, the signal-to-noise ratio suffers. Therefore the application of attenuation pads should be avoided if the sound pressure level is not above the microphone’s max SPL rating.
If the sound pressure level is above the max SPL rating of the microphone (say the microphone is placed in front of a kick drum), then a pad should be engaged. In this case, the signal is strong enough to provide a strong signal-to-noise ratio while the attenuator pad protects the signal from distortion through the active circuitry!
So the variable attenuation pad options are a good thing. Though it is up to the audio technician to choose the proper variation appropriate to the microphone and sound source in question.
What does the attenuation pad switch on my preamp/mixer do? The attenuation pad switch on a preamp/mixer effectively reduces the level of the input audio signal before that signal reaches the amplifier stage of the preamp/mixer. Pads are engaged to avoid overloading the active amplifiers within mixers and preamps, allowing for a clean audio signal chain.
Should I utilize a pad on my microphone or on my preamp/mixer input?Pad the microphone before padding the preamp/mixer input. This will reduce both mic distortion and original preamp input level. If the mic is not distorting, padding the preamp is often better since padding the mic worsens the signal-to-noise ratio of the signal. In extreme cases, pad both.
For information on all the microphone switches and specifications, check out my article Full List Of Microphone Specifications (How To Read A Spec Sheet).