What is sympathetic resonance (and does your library have it)?

Instrument emulation drives a major part of the sampling and plugin world. We want to have “in-the-box” what we used to only have outside the box. Part of what makes a product sound “real” is fully understanding how the original instrument or device works. On the piano, there are dozens of factors to take into account for why each piano sounds the way it does. 

One that I think is often misunderstood is sympathetic resonance. I see a large number of sample library developers who claim to have ‘sympathetic resonance’ as a feature, but with a rare exception, most simply get it wrong.

This doesn’t mean their libraries don’t sound good. But it does mean they have either misunderstood what sympathetic resonance really is, or they just want to use the term and hope you don’t know what sympathetic resonance is.

What Is Sympathetic Resonance?

So let’s get right to it. A piano is an extremely complex instrument, with many factors going into not only how the sound is crafted, but how it reacts to your playing. There are no less than 8,000 or more individual parts on a typical grand piano. But let’s simplify the core of what happens when you press a note.

Every single note on a piano has between one and three high-carbon steel (and sometimes copper) strings, and these strings resonate when struck, creating the tone you hear. The striking mechanism is called a hammer, which is a felt-covered piece of wood.

But there is one other important piece, called the “damper”. This is a felt pad that rests against the string, and when a note is struck, this pad lifts away from the string right before the hammer strikes it.

As long as you continue to hold the key down, the damper remains disengaged, and the note rings out. In addition, when you hold down the sustain pedal, it simultaneously lifts all dampers across all strings at once. So any notes hit while the pedal is down will continue to ring out until either the pedal is lifted, or the key is released, whichever comes last.

So that brings us to sympathetic resonance itself. Part of what makes a piano sound the way it does is that as each string resonates, the soundwaves from it also resonate everything else inside the piano. This includes the wood frame and walls, the cast iron soundboard that holds all the strings, etc. But this also includes any other strings that are currently “open”, or not being dampened.

Notes that are not dampened will get ‘moved’ and vibrate by soundwaves from other notes being played. Even if the original sound of the other notes has faded away, the strings for that note will continue to resonate until dampened, as they are affected by the soundwaves bouncing around the inside of the piano. This is sympathetic resonance.

You can get a similar effect if you press the sustain pedal with one foot, and stomp on the floor with the other. You may hear ALL the strings resonate as they aren’t being dampened!

Sympathetic Resonance in Virtual Pianos

So now that we know what sympathetic resonance is, how does that affect the pianos we play “in the box”. You will see a lot of piano libraries claim they contain sympathetic resonance, but do they really?

Most of the time: NO. They do not.

With the rare exception, these libraries only pretend to simulate sympathetic resonance. The overwhelming majority of piano libraries out there (and I’ve tried almost all of them) will include some resonance samples that get triggered while notes are played or the sustain pedal is pressed, but they aren’t reactive in any way. In fact, they are usually resonance samples from the same note. So if you press a middle C, the resonance you are getting is also from that same middle C. They are just additional subtle vibrations from the same note.

Sympathetic resonance is when strings from one note are excited and vibrate because the soundwaves from other notes are hitting them. This can get quite complex because every open string will have resonance that is created from every other string being played. And this is why most libraries do not include anything that resembles real resonance. The number of voices required for such a thing can quickly become ridiculous, especially when the sustain pedal has lifted all dampers. This would eat your CPU for breakfast in many cases.

So as a compromise, many libraries just add these much simpler additional resonances that play. They are static and not reactive to other notes. But they can still add something good to the sound. Then other libraries will use additional non-sample emulations using either synthetic sounds or impulse responses to get a sound that is closer to real sympathetic resonance.

Does this mean hope is lost for virtual pianos? Of course not. It’s possible that most people will never even hear the difference, both as a player and a listener. But true sympathetic resonance is part of the glue that holds a natural piano sound together.

So that brings me to…Pianoteq.

Over the years, Pianoteq by Modartt has been one of my favorite virtual instruments. It is a true physical modeling emulation of how a real piano works. I won’t get into all the details, but Pianoteq brings you endless abilities to craft your own piano. And while many will argue that the sound isn’t as believable as a good sample library (though again, to a normal listener, they will never know- Pianoteq still sounds VERY good), those same people will tell you that no library PLAYS as well as Pianoteq.

One of the reasons it is so fluid and fun to play is because it emulates all these nuances of how a real piano reacts without having to worry about static samples. And that includes what I believe is the most accurate sympathetic resonance emulation of any plugin or library on the market. Even if I don’t use Pianoteq on everything, if I want to enjoy playing, it’s one of the first I bring up.

I say all this just as an honorable mention because Pianoteq is one of the few, if not only, plugins that properly emulates sympathetic resonance. But what about your library?

How do I know if my library has real sympathetic resonance?

Chances are, the piano library you are playing does not have realistic sympathetic resonance. However, there are a couple out there that do at least attempt it in the correct way. But if you want to test, especially if you are reading this in the future where processing power is even greater, here is the easiest way to do so.

1. Hold down a series of notes very gently, such as a chord, and keep them pressed until they are no longer making any sound. Continue holding down these notes for the duration of the test. This is important. Don’t release these keys.
2. Once your held notes have stopped ringing, and without the sustain pedal down, use your other hand to quickly strike a different chord somewhere else on the keyboard in a staccato manner. Just a quick hit and let go. It’s important that this is a different chord than the first step, preferably a chord with no notes in common. Remember to continue holding down your original chord through the whole process.
3. You should hear some additional tones continuing to ring out even after you have struck the notes in step two. These tones won’t last very long, but you should hear them. If you don’t hear anything ringing out after you strike the second chord, then there is no sympathetic resonance happening. If you did hear these tones, go to the next step.
4. Listen carefully to the resonance tones you heard. Strike this second chord again as many times as you like, as long as the notes from the first step remain pressed. Do these tones resonate at the same pitches as the notes being struck? In other words, do the ringing notes sound like an extension of the second chord you hit? In theory, they should more resemble the notes from the first chord in step one that you are still pressing down, because sympathetic resonance would be vibrating those strings.

This is a quick and simple process to know if your library is emulating true sympathetic resonance. Most libraries don’t. But this physical aspect of a piano’s sound is extremely helpful for sounding realistic. Sympathetic resonance is one of many glues that hold the sound together.

If you have a library that you believe properly emulates sympathetic resonance, let us know in the comments! Or if you have a question about a particular library that claims to have it, we can test it for you.

I hope this was helpful! Until next time…