Some microphones come with switchable options including various polar patterns, attenuation pads, and, as the title of this article suggests, high-pass filters! The microphone high-pass filter (HPF) proves to be a useful option for many microphone applications. Let’s talk about HPFs and how they relate to microphones.
So what is a microphone high-pass filter and why use one? A high-pass filter effectively cuts out the frequency response of a mic below a certain set point, allowing only the frequencies above this point to “pass” through as the audio signal. High-pass filters remove unwanted and excess low-end energy that otherwise degrades the audio signal.
Let’s discuss microphone high-pass filters in more detail in the rest of this article!
What Is A Microphone High-Pass Filter And Why Use One?
A high-pass filter (HPF) is an electronic filter that passes signal frequencies above a certain cutoff point while attenuating signal frequencies below the cutoff point. An HPF, in the case of a microphone, is part of an electronic circuit. Audio signals are AC voltages and can be passed through these electronic high-pass filters. HPFs alter a microphone’s frequency response by “rolling off” frequencies below a certain cutoff point.
High-pass filters are also commonly referred to as “low-cut filters” or “bass-cut filters.” Any of these three names describe the purpose of the filter quite well: to filter out bass frequencies and allow high frequencies to pass.
The truth is, not many instruments contain low-end energy. It’s common advice to high-pass filter all the microphones or channels that aren’t specifically reproducing low-frequency sound sources. This is typically done in the mixing console or digital audio workstation but can be done before if a microphone has an HPF switch!
We need a good low-frequency response when recording instruments like the kick drum, bass guitar, tuba, piano, organ, or others that have an extended low range or a low-frequency fundamental. However, it’s advisable to high-pass filter the instruments that do not contain low-end information.
To learn more about microphone frequency response, check out my article Complete Guide To Microphone Frequency Response (With Mic Examples).
So Why Should We High-Pass Filter Microphone Signals?
As mentioned, it’s often advised to high-pass microphone signals that do not contain essential low-end information. Often this is done in a channel strip in either a digital audio workstation (DAW) or mixing console, although some microphones come with built-in HPF switches!
Here is a short list of reasons why high-pass filters are applied to microphone audio:
- To rid of low-end rumble and noise in the signal
- To help reduce the proximity effect within directional microphones
- To help reduce plosives in directional microphones
- To remove unnecessary low-end so that a microphone signal may better fit within a mix
High-Pass Filters To Rid Of Low-End Rumble And Noise
HPFs are often applied to simply rid of low-end rumble and noise in a microphone signal. Just because a microphone’s frequency response makes it capable of capturing low-frequencies, doesn’t necessarily mean we should be capturing low-frequencies.
There aren’t many sound sources that produce purposeful sound under 50 Hz. Most low-end sound energy is in the form of ambient noise and rumble. Common sources of this low-end noise include:
- Electrical mains hum (50-60 Hz).
- Air conditioners.
- Trucks passing by or idling near the mic.
- The Earth itself.
Even in soundproof audio booths, these external low-end sources can find their way to the microphone. Typically these sounds reach the mic through vibrating solids rather than air.
Pro tip: Use a microphone shock-mount to help further reduce a microphone’s susceptibility to low-end rumble!
To learn more about microphone shock mounts, check out my articles What Is A Microphone Shock Mount And Why Is It Important? and Best Microphone Shock Mounts.
This low-end energy is often unnecessary information in the audio signal and actually takes up some energy and headroom of the signal. By high-passing and removing low-end noise from an audio signal, we’re left with a cleaner signal with more headroom! It’s a win-win.
Caution must be taken not to filter out too many low-frequencies. Once we start filtering out the real audible information of an instrument, we run the risk of “thinning” the instrument out, making it sound unnatural and weak.
For more information on reducing noise in microphone signals, check out my article 15 Ways To Effectively Reduce Microphone Noise.
High-Pass Filters To Reduce Proximity Effect
Directional microphones (that is, all mics that aren’t omnidirectional) exhibit proximity effect. Bi-direction “figure-8” polar pattern microphones have the most proximity effect.
The proximity effect is a boost in bass-frequencies as a directional microphone is moved closer to its sound source.
Sometimes the proximity effect is used to our advantage (think about the gravitas of voiceover or radio voices). However, many times it’s a negative consequence of directional microphones (think of a gross bass boost on an electric guitar within a mix).
Keeping with the electric guitar example, if we are to close-mike a guitar amplifier cabinet, chances are we’d want to high-pass the microphone signal to combat the proximity effect. Unnecessarily boosting the low-end frequencies will make the guitar sound unnatural and will interfere with other elements of the mix, which we’ll get to in a moment.
For more information on microphone proximity effect, check out my article What Is Microphone Proximity Effect And What Causes It?
High-Pass Filters To Reduce Plosives
Plosives are those “p-pops” or “b-pops” we get in the microphone. They’re caused by gusts of air that create a massive pressure change at the microphone diaphragm. These gusts of air are commonly produced by people’s mouths when they speak or sing words with ‘P’ or ‘B.’
Plosives contain a lot of low-end information, and so a high-pass filter may help lessen the intensity of the sound of plosives. Note that HPFs aren’t a good strategy for dealing with the cause of plosives. They’re simply a solution to help deal with the symptoms. Often times even with a high-pass filter, some of the plosive energy with still be heard clearly in the signal. This isn’t good.
Pro tip: Use a microphone pop-filter and position the microphone slightly off-axis to help further reduce a microphone’s susceptibility to plosives!
To learn more about plosives and how to protect your microphone from plosives, check out my article Top 10 Tips For Eliminating Microphone Pops And Plosives.
High-Pass Filters And Mixing
I saved the best for last here. High-pass filters are best used in the context of an audio mix. Even if a microphone signal contains musical information in the low-end, it may be advantageous to cut out these low-end frequencies to allow other instruments to occupy that “frequency real estate.”
Let’s return to our example of close-miking the electric guitar amp once again. On top of high-passing the microphone signal to reduce the proximity effect and low-end rumble, we may reason that the guitar amp sounds better within the mix when it’s high-passed.
The fundamental (lowest) frequency of the low E (lowest pitched) string on an electric guitar is 82 Hz. But in a typical rock band setup, there would also be bass guitar and a drum kit.
- Bass guitar E string has a fundamental of 41 Hz but has its first harmonic at 82 Hz
- The kick drum often benefits from an EQ boost around 80 Hz
So what are we to do? In just 3 instruments, we have a lot of competition at roughly 80 Hz. Bass guitar and kick drums should typically occupy most of the low-end real estate in this particular mix setup. Having the extra element of the electrical guitar could lead to a build-up of low-frequencies and a “muddy” mix. So, even though the electric guitar has information in the bass-frequencies, it may be favourable to actually cut those frequencies out of the sound to make way for the kick and bass guitar!
Often times these decisions are made at the mix level rather than the recording stage. However, I wanted to provide this information as a generality when discussing microphone high-pass filters. High-pass filters are arguably the single most important processing device in mixing audio and certainly the most important in terms of audio equalization.
Pro tip: High-pass the channels in your mix that do not contain low-end information!
So all this begs the question: should I high-pass at the microphone itself or later on in the channel strips of my mixing console or DAW? Let’s discuss this answer further:
To High-Pass At The Microphone Or At The Mixing Console?
Well, most times we don’t have a choice since not many microphones have a switchable high-pass filter. However, if we are using a microphone that has HPF option(s), should we ever use them instead of the HPFs supplied by mixers?
The answer, of course, depends on what we’re recording and exactly how we want to treat the signal.
Analog mixers often have setpoint high-pass filters. The cutoff frequencies are typically either 60, 80, or 100 Hz with a steep roll-off to effectively cut out frequencies below the set point. In this case, there are no adjustments: the HPF is either engaged or disengaged at the set cutoff frequency.
Digital mixers and digital audio workstations often have parametric EQs. With these systems, we can make all kinds of adjustments to the high-pass filter: the cutoff frequency, the roll-off slope, and even the amount of boost at the cutoff frequency before the roll-off. Digital parametric EQs provide maximal flexibility with setting our high-pass filter.
Whether we’re using a built-in microphone high pass filter or an HPF in a console, the filtering happens after the microphone capsule output. This means that high-pass filters do not affect the way the microphone capsule changes sound to an audio signal. It does mean, however, that a microphone’s built-in HPF is affecting the audio signal before it’s sent through cable and amplified, whereas a console’s HPF affects an already amplified signal.
So if we have a digital mixing system, is it ever worth using the HPF in the microphone itself? The answer is still yes, there are still benefits to a switchable HPF on a microphone.
Here is a list of benefits to having a built-in high-pass filter in a microphone:
- The signal is “high-passed” before amplification.
- The high-pass filter is designed specifically for the microphone.
The Signal Is “High-Passed” Before Amplification
One benefit of a built-in microphone HPF is the filtering happens before the microphone preamplifier.
Mic preamps apply gain to incoming microphone signals. Although some preamps colour the sound (they increase the amplitude of some frequencies differently than others), generally speaking, the gain is applied to the entire audio signal. In other words, the preamp can’t tell the difference between “noise” and “signal.” It simply boosts everything as one AC voltage audio signal.
If the low-end energy of the microphone signal isn’t needed, why subject the preamp to the extra noise? This fits in well with the rule of thumb “get it right at the source.”
If we’re going to get rid of the low end anyway, it’s typically best to do so as soon as possible. This allows all further processing to be done on the signal we want rather than a relatively noisy version that requires processing.
Caution should be taken if a microphone’s HPF cuts out some of the sound source’s natural frequencies. Cutting too much low-end from a source will cause it to sound unnatural and thin. Doing so at the microphone level will make it difficult to fix in the mix.
For more information on microphone preamplification and gain, check out my article What Is Microphone Gain And How Does It Affect Mic Signals?
The High-Pass Filter Is Designed Specifically For The Microphone
Microphone manufacturers dedicate a lot of time to making their mics “just right.” If a manufacturer decides to put a switchable high-pass filter on their microphone, it definitely has a purpose and is designed to work with the microphone. The HPFs are tailored, so to speak, to the specific microphone’s frequency response. HPFs aim to provide an equally practical frequency response for the user.
There are 2 main reasons a microphone would have a switchable high-pass filter:
- To remove low-end rumble from the output signal
- To counteract the proximity effect
To understand the purpose of a microphone’s HPF, take a look at the frequency response diagram on the mic’s specification sheet. Here are some generalizations:
- If the HPF has a lower cutoff point (roughly 100 Hz or less) and a steep roll-off slope (roughly -12 dB/octave or more), then its purpose is mostly to remove low-end rumble while maintaining a decent bass-frequency response.
- If the HPF has a higher cutoff point (more than 100 Hz) and a gentle roll-off slope (-6 dB/octave or less), then its purpose is to counteract the proximity effect.
- HPFs often accomplish both these purposes, to some extent.
An example of a microphone with HPFs for removing low-end rumble while maintaining bass responsiveness is the popular AKG C 414.
The C 414 has 3 HPF switches:
- HPF @ 40 Hz with −12 dB/octave roll-off
- HPF @ 80 Hz with −12 dB/octave roll-off
- HPF @ 160 Hz with −6 dB/octave roll-off
A cutoff frequency of 80 Hz with −12 dB/octave is a common setting for high-pass filters because of the infamous 50-60 Hz hum that often finds its way into audio signals.
The C 414 high-pass filters effectively remove low-end rumble and noise while maintaining the integrity of the signal’s frequencies above the cutoff.
An example of a microphone HPF for reducing proximity effect is in the legendary Neumann U87. Its cutoff frequency is around the 1,000 Hz mark! However, the frequency roll-off happens at a gentle -3 dB/octave.
Obviously, this HPF will affect the bottom-end of any sound source, but the purpose of this HPF is to combat the proximity effect. With the HPF engaged, a close sound source will sound quite natural instead of having a strong bass boost. The proximity effect doesn’t go away, but it is accounted for by the HPF.
Take a look at the frequency response graphs for a better understanding of the HPF purpose. But most importantly, listen with a critical ear and make up your mind as to whether or not the HPF makes the signal sound better or worse. The subjectivity of sound is what makes it so awesome!
There are also in-line HPF options. Shure makes an in-line filter called the Shure A15HP (link to check the price on Amazon). It is a balanced XLR in-line filter that reduces low frequencies below 100 Hz. We can put this external filter directly after any XLR microphone to effectively act as an HPF before preamplifier gain. The A15HP also passes phantom power, so it can be used with active microphones!
How Do High-Pass Filters Work?
So how do high-pass filters work? I’m no electrician and so I had to ask myself that question as well.
Although different microphones have different designs and high-pass filters, HPFs are all based on relatively simple circuitry. Microphone audio signals are AC electrical signals (typically measured in millivolts) and so they pass through electrical circuitry within the microphone.
High-pass filters, like the microphones they’re designed for, can be either active or passive. Active HPFs require power for the op-amp in their design while passive HPFs do not require power to function properly.
Passive HPFs are typically made from resistor-capacitor circuits (RC circuits). The input audio signal (voltage) across the series combination of a capacitor and resistor is effectively high-passed, and the output audio signal is filtered!
The cutoff frequency of the filter is determined by the following equation:
fc = 1/(2πRC)
- fc is the cutoff frequency (in hertz)
- R is the resistance of the resistor (in ohms)
- C is the capacitance of the capacitor (in farads)
Note that if we were to swap the positions of the capacitor and resistor in the RC circuit, we’d have a low-pass filter!
Active HPFs are slightly more complex RC circuits with active op-amps integrated into the circuitry.
These circuitry parts are integrated into microphone design with switches. These switches are activated by toggle switches on the body of the microphone. When the switch is engaged, the audio signal is sent through the high-pass filter RC circuitry before the output. If the switch is disengaged, the audio signal doesn’t pass through the HPF RC circuit at all!
Do microphones have low-pass filter switches? Microphone manufacturers do not add low-pass filters to their mics. Dynamic mics have naturally “filtered” high-frequencies. Condenser mics, which have extended high-end, don’t either since high-frequencies don’t affect signal gain nearly as much as low-frequencies. If required, low-pass post mic!
What switchable options do microphone manufacturers include in their mics?
- Passive Attenuation Devices (attenuation pads)
- Polar Pattern Switches (in multi-pattern large-diaphragm condensers)
- High-pass filters
- Presence boosts
- On/Off switches
For more information on passive attenuation devices, check out my article What Is A Microphone Attenuation Pad And What Does It Do?
For information on all the microphone switches and specifications, check out my article How To Read A Microphone Specification Sheet Or Data Sheet.