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High-Pass And Low-Pass Filters

What do those switches with the "bent line" icons do?

by Phil O'Keefe

You've probably seen them in a few different places - those switches with the "bent line" graphics on microphones, mixing boards, mic preamps and EQ plugins - but what are those switches for, and when should you use them? Well that's exactly what we're going to cover in this article.

What we're talking about here are filters, which are a type of equalizer. These are designed to change the frequency response of the signal passing through them.

A switch or button for a high-pass filter (often abbreviated as HPF) will usually be indicated by a symbol such as one of the ones shown in Figure 1.

Figure 1: HPF graphic icons (with different slopes)

The symbol itself is a graphic representation of what happens to the signal when it passes through the filter. As with a frequency plot graphic, such as the one shown in Figure 2, the low frequencies are always on the left side, while the high frequencies are on the right.

Figure 2: A typical frequency plot or graph, showing alternative response curves with the HPF and LPF engaged

With this in mind, you can get an idea of what's happening. A high-pass filter is rolling off the lows, while allowing the highs to pass. High-pass and low-pass filters can be confusing for some people due to their names, but it's really pretty simple. Another way of looking at a high-pass filter is as a low frequency roll-off, while low-pass filters do the opposite and are essentially rolling off the high frequencies, allowing the lows to pass through unimpeded.

A low-pass filter indicator has the rolloff on the other side of the line (at the high end of the audible frequency spectrum), and is indicated with a symbol such as one shown in Figure 3.

Figure 3: Low-pass filter graphic icon

The Cutoff And The Slope

Some microphones, such as the Schoeps CMIT 5 U shown in Figure 4, have multiple settings for high-pass filtering. While the actual frequency response itself may not be shown, the graphics on the switch will often give you a general idea of what the different settings do, or the two settings may have a cutoff frequency indicated.

Figure 4: The Schoeps CMIT 5 U has two different HPF settings

A high-pass or low-pass filter has two main settings, although they're not always user-adjustable. The first is the cutoff frequency. This is the point where the filter begins to take effect, and is measured at the -3dB point. The second is the slope, which is measured in decibels (dB) per octave. Engaging a high-pass filter with a 6dB per octave slope at 80Hz will result in the signal an octave lower (at 40Hz) being attenuated by 6dB, and will drop by another 6dB (for a total of 12dB of attenuation) at two octaves below 80Hz, which is 20Hz.

A 6dB per octave slope is considered fairly gradual; steeper slopes are also commonly encountered, including 12dB and 18dB per octave. These roll off the signal below the cutoff frequency even more dramatically than a 6dB per octave high-pass filter would.

When To Use Them

High-pass filters on microphones are often engaged when a midrange or higher pitched sound source is being recorded. This can help reduce "room rumble" and the sound of semi-distant machinery (washing machines a few rooms away, sounds from distant traffic, air conditioning noise, etc.) from being recorded, resulting in a cleaner track. However, you should proceed cautiously when recording more full-range sound sources and sounds with a low frequency emphasis; instruments such as upright bass, electric bass amps and kick drums are rarely recorded with a high-pass filter.  

Low-pass filters have the opposite effect, and they are used in the opposite situation - whenever you want to deemphasize the high frequencies. These filters are much less commonly seen; most mixing boards and microphones lack a low-pass filter (the Lauten Audio Series Black LA-320 being a notable exception), and not a lot of hardware equalizers are equipped with them either. When might you use one? One example would be when miking a kick drum and trying to reduce bleed from the snare and cymbals. You also might decide to use a low-pass filter to tame a overly bright sounding vocalist or instrument.

How To Use Them

That't simple - push the button, click the switch, or click on the virtual button and listen! Experimentation is the key with low-pass and high-pass filters. Do you like what you are hearing? Only you can decide if it's right for the track and situation you find yourself working with, but high-pass filters are commonly engaged when recording vocals, guitars and other similar types of sound sources without significant low frequency content, while low-pass filters are more commonly used to mellow out a bright or strident sound source, or to help isolate a lower frequency sound and reduce "bleed", or pickup of nearby sounds with considerable high frequency content. -HC-

Do you have questions about high-pass and low-pass filters, or comments about this article? Then please join the discussion in this thread in Phil's Studio Trenches forum right here on Harmony Central!



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.  

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