Soundcheck Diaries

Last Thursday, I had the distinct pleasure of mixing one of the most beautiful shows I’ve worked on yet. The evening featured Tony Guerrero on trumpet, accompanied by an exquisite string quartet—two violins, viola, and cello—along with piano, bass, and drums.

Tony is an accomplished trumpeter, composer, arranger, and producer whose career has spanned nearly four decades, with credits alongside artists including Arturo Sandoval, Freddie Hubbard, Paul McCartney, Brian Wilson, and many others. Nights like this remind me how incredibly blessed I am to work with phenomenal, world-class musicians and to have a small part in bringing their music to an audience.

The show sounded like magic.

The strings seemed to sail effortlessly through the air, weaving themselves around Tony’s beautiful, melodic trumpet lines. The quartet was rich and full, with a warmth that seemed to melt into the music. There are moments when you’re mixing and everything comes together in a way that makes you stop thinking technically for a second and simply listen.

But the strings didn’t start out sounding that way through the sound system.

Each instrument in the quartet was close-miked with a DPA 4099 instrument microphone. Acoustically, the strings sounded gorgeous. Through the microphones, however, I was hearing more of the bow against the strings than I wanted. There was an exaggerated high-frequency texture—a scratchiness and edge that made me feel as though I could practically hear the rosin flying off the bows.

My goal wasn’t to change the character of these beautiful instruments. In fact, it was exactly the opposite.

I wanted the reinforced strings coming through the PA to sound as much as possible like the strings I was hearing acoustically in the room.

That meant gently removing what I didn’t need while preserving everything that made the quartet sound beautiful in the first place.

And for that, my tool of choice for the evening was the low-pass filter.

Before we talk about what a low-pass filter does, take a moment to listen.

In this clip from the show, listen closely to the string section. Notice the warmth and fullness of the instruments, and how smoothly they sit beneath Tony’s trumpet without the bow noise becoming a distraction.

This is the sound I was working toward—and the low-pass filter played an important role in getting me there. For the best listening experience—and to really hear the difference—use headphones or closed-back earbuds.

Letting the Instrument Through

So what exactly is a low-pass filter?

A low-pass filter, or LPF, allows the frequencies below a selected point to pass through while gradually reducing the frequencies above it. In other words, it lets the lows pass and rolls off the highs.

If you’ve used a high-pass filter HPF before, think of the LPF as its counterpart. A high-pass filter removes unwanted information from the bottom of the frequency spectrum; a low-pass filter does the same thing from the top.

The point where you set the filter is called the cutoff frequency. Frequencies above that point don’t simply disappear—they are progressively attenuated according to the slope of the filter. The farther above the cutoff you go, the greater the reduction.

Where Do I Find the LPF?

On a digital mixing console, the low-pass filter is typically found within the EQ section of the input channel. Depending on the manufacturer, it may be labeled LPF, Low Pass, or sometimes High Cut. Different consoles place the control in different locations, but the concept is the same.

And this is another reason I always encourage engineers to learn the concept, not just the console. Once you understand what an LPF does, you can find it and use it whether you’re standing behind a Yamaha, Allen & Heath, Behringer, Midas, or another digital console.

Making Reinforced Strings Sound Like Real Strings

For this show, I wasn’t using the low-pass filter because there was something wrong with the string section. Quite the opposite. Standing near the stage and listening to them acoustically, they sounded beautiful.

My job was to make sure that beauty survived the trip through the microphones and PA.

Close-miking with the DPA 4099s gave me the isolation and control I needed for a live mix, but it also put the microphones very close to the physical interaction between the bow and string. That meant some of the upper-frequency bow noise and scratchiness was more pronounced through the sound system than it was when I simply listened to the instruments acoustically.

So I used the LPF to gently tame that excessive top end.

I didn’t choose a cutoff frequency because a chart told me where a violin, viola, or cello was supposed to be filtered. I listened.

I started with the filter high and gradually brought it down while listening to the instrument. As the LPF moved lower, that exaggerated bow texture began to soften and the strings started sounding smoother and more natural. The trick was knowing when to stop. Bring the filter down too far and you begin taking away the openness and detail that make strings beautiful in the first place.

The goal wasn’t to hear the filter working.

The goal was to stop hearing the microphone and start hearing the instrument.

Below are some EQ screenshots from my actual mix. You’ll be able to see where I placed the low-pass filters on the individual string channels and how those settings became part of shaping the finished string sound you heard in the video.

Let the Music Be the Guide

A low-pass filter is a simple tool, but in the right moment, it can make a remarkable difference. That evening, my goal wasn’t to make the strings sound processed—it was to make the microphones and sound system disappear and allow the natural beauty of the instruments to come through. That’s ultimately what good live sound is about. We have an incredible collection of tools at our fingertips, but knowing when and why to use them begins with listening. The music should always tell us what it needs.