I used a ReAmp to run a sine sweep from my computer through this DIY Marshall clone. Now I can make anything sound as if it was played through the amp’s speaker.

This month I bring you … magic!

Actually, I bring you an article on convolution reverb. But even though the technology has been around since the turn of the century, it still feels magic to me. Plus, we’ll conclude with a pretty decent trick: putting a Marshall amp inside an acoustic guitar.

Basics first: Last month we talked about recording amps with multiple mics to capture room ambience. But when working in dull-sounding bedrooms and basements, sometimes it’s best to simply close-mic your amp and add artificial ambience in the mix. Analog-era engineers moistened dry tracks by piping recordings into echo chambers, re-recording the sound, and combining the wet and dry versions. They also used mechanical reverbs, such as plates and springs.

Today’s software plug-ins accurately mimic those methods. But convolution reverb—also known as impulse response (IR) reverb—adds unique new wrinkles. IR reverb can make it sound as if your amp was recorded in any acoustic space, including ones that don’t exist in nature. You can also mimic the sounds of speakers, cabinets, mics, and other hardware.

When working in dull-sounding bedrooms and basements, sometimes it’s best to simply close-mic your amp and add artificial ambience in the mix.

Math and magic. Nowadays some DAWs come stocked with IR reverb plug-ins, such as Space Designer (included with Apple’s Logic Pro) and REVerence (part of Cubase). There are high-end third-party options with stunning libraries, such as Audio Ease’s Altiverb, as well as low-cost alternatives such as LiquidSonics Reverberate, and even freeware such as SIR’s SIR1 (PC only).

IR reverbs are a bit like CD players: While it’s an exaggeration to say they all sound exactly the same, the IR files you load matter more than the plug-in itself. With the pricier options, you’re paying for superior libraries. But you can also get great results with shareware plug-ins and freeware IR files (just Google “free impulse responses”). Most high-end libraries are copy-protected, yet there are plenty of great-sounding freebies there for the taking.

Even if you don’t own a dedicated IR reverb plug-in, you’ve probably played through them. Impulse responses are one way that hardware and software amp modelers mimic analog gear.

How do they work? I told you—magic! Okay, there’s some math involved too. (Look up “convolution” and “impulse response” on Wikipedia if you’re curious.) The basic idea: You play a recording of a benchmark signal (usually a sine-wave sweep or a starter pistol shot) in the space you want to clone, and then record the ambient sound into an IR-making tool (such as Impulse Response Utility, included with Apple’s Logic Pro). The software compares the dry test tone to the reverberant recording, and subsequently applies the same adjustments to any audio material. The results can be amazing.

Across space and time. Let’s check out some examples in escalating order of weirdness.

Clip 1a features the bone-dry distorted sound of a close-miked Marshall 18-watt clone and a ratty overdrive. For Clip 1b I’ve added the Altiverb plug-in within Logic Pro, selecting an IR captured at New York’s famed Clinton Studios from a distance of about five feet. The result sounds like it was recorded with a combination of close and far mics in Clinton’s tracking room. (Clinton closed in 2010, so you get time travel as well as magic.) Clip 1c is a wetter sound using an impulse from Hollywood’s Cello Studios, now called EastWest. Clip 1d is wetter still—and depicts what your amp would sound like inside the Great Pyramid at Giza, where the IR was captured.

Freaky, right? It gets freakier.