Whenever we find ourselves tracking or especially mixing, a number of different filters can be used to shape and enhance the audio. Among these, the high pass filter (HPF) is one of the most essential and useful tools for manipulating audio frequencies.
So, I decided to provide a comprehensive overview of high pass filters, explaining what they are, how they work, and how they can be effectively used to improve the quality of your mixes in my in-depth look at What Are Audio High Pass Filters & How to Use Them.
Let’s start with…
What is a High Pass Filter?
A high pass filter is an audio processing tool that allows frequencies above a certain cutoff point to pass through while attenuating or eliminating frequencies below that point. This cutoff frequency is also known as the -3dB point, where the filter’s output power is reduced by half (-3dB) compared to its original level. High pass filters are often denoted by their slope, measured in decibels per octave (dB/oct).
How do High Pass Filters Work?
They function by exploiting the principles of analog or digital signal processing. Analog HPFs typically consist of passive components like capacitors and resistors, while digital or software filters use algorithms to manipulate the audio signal.
In analog filters, the cutoff frequency determines the point at which the filter starts attenuating frequencies. Frequencies below the cutoff are passed through with little or no change, while frequencies above the cutoff point experience increased attenuation as they move farther away from it.
The rate of attenuation is determined by the filter’s slope, expressed in dB/oct. A steeper slope indicates a more rapid decline in amplitude as the frequencies move away from the cutoff.
Digital or Software HPFs
Digital or software-based high pass filters, on the other hand, use algorithms to process the audio signal. They operate by converting the analog signal into a digital representation and then manipulating it through various mathematical operations. Digital filters offer precise control over the cutoff frequency and slope, allowing for greater flexibility in audio processing.
How can you use High Pass Filters?
Eliminating unwanted low-frequency noise
High pass filters are excellent tools for removing rumble, hum, and other low-frequency noise that can degrade audio quality. By setting the cutoff frequency slightly above the lowest desired frequency, the filter can effectively remove unwanted noise while preserving the important audio content.
In certain audio recordings or live sound situations, excessive low-frequency content can muddy the mix or make it difficult to discern specific elements. By selectively attenuating low frequencies using a high pass filter, you can create more space for other instruments and improve overall clarity.
Preventing speaker damage
Speakers and amplifiers have limits to the low frequencies they can reproduce without distortion or damage. Applying a high pass filter to the audio signal going into a speaker system can prevent low-frequency signals from reaching the speakers, thus protecting them from potential harm.
Balancing the audio spectrum
In a well-balanced mix, different instruments and vocal elements should occupy their respective frequency ranges without overlapping too much. By utilizing high pass filters on individual tracks, such as bass guitar, kick drum, or keyboard pads, you can remove excess low frequencies that may interfere with other instruments, allowing each element to occupy its intended space in the mix.
Tips for the Effective Use of High Pass Filters
Set the appropriate cutoff frequency
Choosing the correct cutoff frequency is crucial. If set too high, you risk removing desirable low-frequency content, while setting it too low may result in an ineffective filter. Listen carefully and adjust the cutoff frequency until you achieve the desired balance and clarity in the audio.
Consider the slope
The slope of the high pass filter determines the rate of attenuation beyond the cutoff frequency. Use steeper slopes when eliminating unwanted low-frequency noise, but be mindful that extreme slopes can introduce phase shifting or alter the overall character of the audio.
Always use HPFs in moderation
While high pass filters are powerful tools, it’s important to exercise restraint and avoid excessive filtering. Overuse of high pass filters can lead to an unnatural or thin sound, particularly when applied indiscriminately across multiple tracks. Use your ears and make adjustments judiciously.
Use automation and EQ adjustments
High pass filters can be automated or adjusted dynamically during a mix during different sections of a song or to address specific audio issues. This allows for precise control over the low-frequency content, providing a more refined and tailored audio experience.
What are the best Analog High Pass Filters you can Buy?
When it comes to analog high pass filters, there are several notable options that have gained popularity among audio engineers and enthusiasts. These offer unique characteristics and sonic qualities that can enhance the audio processing chain. So, here are a few of the best analog HPFs that I personally recommend, starting with the…
Moog MF-101 Lowpass Filter
Although primarily known as a low pass filter, the Moog MF-101 can also function as a high pass filter. It is loved by engineers for its warm and vintage sound, and it offers flexible control over cutoff frequency, resonance, and drive. The Moog MF-101 delivers rich and musical filtering capabilities, making it a popular choice for both studio as well as live sound applications.
Universal Audio (UA) 1176LN
Although it is best as one of the most legendary audio compressors in recording history, the UA 1176LN also includes a high pass filter in its signal path. This filter allows engineers to eliminate unwanted low-frequency content while benefitting from the renowned compression characteristics of the unit. With its classic analog sound and versatile capabilities, this has become a staple in many professional studios.
This is a classic analog EQ module that includes a high pass filter among its other excellent features. Known for its precise and musical sound, the API 550A offers a selectable slope for the high pass filter. This allows users to tailor the filtering effect to their specific needs. This filter is highly regarded for its transparency and ability to enhance the clarity and definition of audio sources.
SSL E-Series Channel Strip (or E-series Console if you have the $$$)
This is modeled after the iconic SSL 4000 console, and includes its high pass filter that has become a favorite among engineers. This filter provides steep roll-off characteristics and is known for its ability to effectively remove low-frequency rumble and interference while maintaining the overall sonic integrity of the audio signal. The SSL E-Series Channel Strip offers a versatile and powerful analog processing solution for both mixing and tracking.
Ultimately, the choice of the best analog HPF depends on personal preferences, the desired sonic characteristics, the specific requirements of the audio project, and your budget. Therefore, it’s important to experiment and audition the different filters, if possible, to find the one that best suits your needs and complements your audio processing chain.
My personal favorite is the Universal Audio (UA) 1176LN if it is available, my studio only has two, so for a mixdown, they will be busy, but if I need a quality HPF while tracking, they would be my first choice. After that, I would choose the API550, especially for low frequency instruments which need to be controlled.
What are the Best Software High Pass Filters you can buy?
When it comes to software high-pass filters, there are several outstanding options available that provide precise control and excellent sound quality. They are also significantly less expensive than their hardware counterparts and can usually be trailed for a certain amount of time for free.
These software plugins offer a range of features and flexibility, making them popular among audio professionals and enthusiasts. So, here are a few of the best software high pass filters currently on teh market, starting with the excellent…
This is a highly regarded EQ plugin that includes a very powerful high-pass filter. It offers a visually intuitive interface with precise control over the cutoff frequency, slope, and resonance. The Pro-Q’s transparent sound and versatile features make it a go-to choice for many engineers in a number of varied audio production scenarios.
This is a comprehensive mixing suite that includes a high pass filter as part of its EQ module. Neutron provides an array of intelligent features, such as track analysis and adaptive EQ, allowing for efficient and effective filtering. With its advanced algorithms and user-friendly interface, Neutron is widely used in both mixing and mastering workflows.
Waves SSL G-Equalizer
The Waves SSL G-Equalizer plugin is modeled after the legendary SSL 4000 console, whose channel strip I included in the best analog HPF section, and also includes a high pass filter among its features. Renowned for its warm and musical sound, the SSL G-Equalizer offers precise control over the cutoff frequency and slope, allowing for detailed shaping of the audio spectrum.
This plugin is a favorite among engineers who are seeking a vintage analog sound in a digital environment.
Universal Audio UAD API 550A
Again modeled after the classic analog hardware that I mentioned in the analog HPF section, the UAD API 550A EQ plugin includes a high pass filter that provides precise control over low-frequency attenuation. The plugin emulates the characteristic API sound of the hardware unit, which is known for its punch and clarity, making it a popular choice for many audio professionals.
These best software HPFs offer exceptional sound quality and flexibility in shaping audio. As to, which you should choose, that will depend on personal preference, workflow requirements, and the specific characteristics of the audio project. Therefore, experiment with the different plugins by taking advantage of the free trials to assess their features to find the one that best suits your needs and enhances your audio production workflow.
In terms of a personal choice, I always go for the FabFilter Pro-Q first because I find it the most effective for most audio tasks and teh easiest to use after you get used to teh layout. After that, I would go for the SSL Channel or the API.
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What Are Audio HIGH PASS FILTERS & How to Use Them – Final Thoughts
HPFs are indispensable tools for audio engineers and producers seeking to shape and refine sound. By understanding the principles behind high pass filters and their applications, you can effectively eliminate unwanted low-frequency noise, enhance clarity, protect speakers, and balance the audio spectrum.
However, always remember to use high pass filters judiciously, setting the cutoff frequency and slope appropriately, to achieve the desired results without compromising the integrity of the audio. With skillful utilization, high pass filters can significantly improve the quality and impact of all your audio productions.
Happy recording and mixing!