High Pass Vs Low Pass Filter – What’s the difference?

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What are high and low pass filters, and why should you care? This short article explains all you need to know in simple language. To understand high pass vs low pass filter, you need to recognize how they shape the tonality of a signal. The aim is to tweak and mix the audio to create the desired result. A few mics have filter switches built-in, so it’s important to know when to use them and why.

Why We Need Pass Filters

High and low pass filters have a specific function. It is to influence the energy above or below whatever the target frequency is. Sound engineers usually refer to this as the cutoff frequency in pass filters. High and low pass filters don’t cut or boost the frequency content by tweaking gain. Instead, they remove the undesirable content so that it doesn’t exist on the recording.

The High-Pass Filter (HPF)

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Some people refer to the HPF as a bass-cutter, which explains its function better. It lets you reduce or cut any deep rumble in a voice, instrument, or space that would otherwise spoil the audio. The frequency content outside the cutoff point attenuates smoothly in a downward direction. Thus, the high pass filter lets the highs pass while blocking the lows.

Here are the 3 benefits of an HPF for microphones:

  1. To rid the signal of noise and low-end rumble
  2. To lessen the proximity effect
  3. To reduce plosives

#1 The Problem with Low-End Rumble

Microphones that capture low frequencies have their place, but it’s not always desirable. A lot of low-end rumble picked up by mics is the result of ambient sound or background noise. Much of this interference is under 50Hz. Examples are air conditioners, electrical hum, nearby appliances, passing traffic, and ground vibrations. Shock mounts to help lessen the problem, as do high-pass filters.

#2 The Problem with Proximity Effect

Some microphones produce more bass the closer it is to the sound source. That boost can become exaggerated and spoil the sound quality. It’s called the proximity effect as the low-frequency response kicks in the nearer the mic is to the person or instrument. Some radio talk show presenters benefit from this as it fattens up their voice. A high-pass filter can reduce or cut it for situations that don’t.

#3 The Problem with Plosives

Plosives are those annoying windy or popping noises that occur when someone talks or sings into a mic. They are the result of breath-heavy consonants, i.e., Pa, Ta, Ka, Ba, Da, and Ga. That’s why most artists place a pop filter between them and the microphones. Some mics are more sensitive to plosive sounds than others. Both high and low pass filters can further improve plosive issues.

The Low-Pass Filter (LPF)

The LPF works opposite to an HPF. That is, it lets the lows pass while reducing or blocking unwanted higher frequencies. It can be useful for mics that are overly harsh or bright. No microphones come with a built-in low-pass filter, but a few do have high-pass filter switches.

Pass Filter Controls

Pass filters typically have only two controls, i.e., Frequency and Q. The frequency control is what sets the specified cutoff point of the filter. The Q control (also ‘slope’ in pass filters), influences the steepness of the cutoff slant. The Q is measured in decibels (dB) per octave. It determines how aggressively the equalization or EQ begins to remove the above or below frequency cutoff points.

The Q settings on both high pass and low pass filters tend to start at around 60dB per octave. It’s a gentle slope that users can adjust to more aggressive settings as required. EQing needs a critical ear, and knowing what pass filters do and how they work is vital for improving skills.

Understand the Names

You are sure to come across the other names for these filters as well, i.e., High Cut and Low Cut. A High Cut Filter is a term used to describe a Low Pass Filter, and Low Cut is for a High Pass Filter. It sounds confusing, but it’s not when you consider what each one does.

  • High cut filters cut the highs and let the lows pass, hence Low Pass
  • Low cut filters cut the lows and let the highs pass, hence High Pass

HPF and LPF Used Simultaneously

High and low pass filters can work simultaneously. The term used to describe this is Band Pass Filter (BPF). That’s because only a precise ‘band’ of frequencies get to pass through. A Band Pass Filter is usually in the form of a plug-in rather than a feature on a mixing console.