Animated Bell Icon: Anatomy of a SMIL Ringing Animation

Introduction: Why SMIL for Animated Icons?

When building a library of animated SVG icons, the choice of animation technology is critical. CSSVG Icons uses SMIL (Synchronized Multimedia Integration Language)—an XML-based language native to SVG—because it offers declarative, per-element control without external dependencies. Unlike CSS animations that apply to entire elements, SMIL lets us animate individual attributes (like transform, stroke-dashoffset, or opacity) with precise timing and easing, all within the SVG itself.

For a bell icon, SMIL’s ability to chain multiple transforms—rotation, translation, and scaling—with different easing curves creates a natural, physics-inspired ringing motion. This post breaks down a hypothetical bell icon’s SMIL animation, using real patterns from the CSSVG codebase (like the search and menu icons) to explain the why behind each decision.

The Bell’s Anatomy: SVG Structure for Animation

A ringing bell requires two main moving parts: the bell body (swinging back and forth) and the clapper (striking the bell). In SVG, we group these elements to animate them independently:

// Hypothetical bell icon structure (inspired by CSSVG patterns)
<svg viewBox="0 0 40 40">
  <g id="bell-body">
    <path d="M10,30 Q20,10 30,30" ... />
    <animateTransform 
      attributeName="transform" 
      type="rotate" 
      values="0,20,30; 20,20,30; -10,20,30; 0,20,30" 
      dur="1.5s" 
      repeatCount="indefinite"
      additive="sum"
      keyTimes="0; 0.5; 0.75; 1"
      keySplines="0.25 0.1 0.25 1; 0.5 0 0.5 1; 0.25 0.1 0.25 1"
    />
  </g>
  <g id="clapper">
    <circle cx="20" cy="25" r="3" ... />
    <animateTransform 
      attributeName="transform" 
      type="translate" 
      values="0,0; 0,5; 0,0" 
      dur="0.8s" 
      repeatCount="indefinite"
      additive="sum"
      keyTimes="0; 0.5; 1"
      keySplines="0.42 0 0.58 1; 0.42 0 0.58 1"
    />
  </g>
</svg>

Why this structure?

Grouping the bell body and clapper separately allows independent animation loops. The additive="sum" property is crucial—it combines the clapper’s vertical movement with the bell’s rotation, making the clapper swing relative to the bell’s tilt. Without additive transforms, the clapper would move in absolute coordinates, breaking the illusion of a single mechanical system.

Decoding the Swing: keyTimes and Pendulum Physics

The bell’s rotation follows a pendulum-like arc. The keyTimes attribute defines the animation’s timeline, while keySplines control easing between keyframes.

<animateTransform 
  attributeName="transform" 
  type="rotate" 
  values="0,20,30; 20,20,30; -10,20,30; 0,20,30"
  keyTimes="0; 0.5; 0.75; 1"
  keySplines="0.25 0.1 0.25 1; 0.5 0 0.5 1; 0.25 0.1 0.25 1"
/>

Why these values?

• values: The rotation pivots around (20,30) (the bell’s top). The sequence 0 → 20° → -10° → 0° mimics a bell’s natural swing: it starts at rest, swings forward (positive rotation), then backward past center due to momentum, and finally settles.
• keyTimes: The animation spends 50% of the time reaching the forward peak (20°), 25% swinging back to -10° (overshoot), and 25% returning to center. This asymmetry reflects real physics—a bell loses energy as it swings, so the forward stroke is slower than the return.
• keySplines: The cubic-bezier curves create “slow-in, slow-out” motion. The first spline (0.25 0.1 0.25 1) eases into the forward swing, the second (0.5 0 0.5 1) accelerates the backswing, and the third mirrors the first for settling. This avoids mechanical, linear motion.

Comparison to CSSVG’s search icon:

The search icon uses similar splines for its scanning motion (keySplines="0 0 1 1" for constant speed), but the bell requires variable speed to feel organic. The menu icon’s scaling (keySplines="0 0 1 1") is linear because it’s a simple morph, not a physics-based swing.

The Clapper’s Strike: Additive Translation and Timing

The clapper must move independently of the bell’s rotation. Its translate animation is set to additive="sum" so it compounds with the bell’s transform:

<animateTransform 
  attributeName="transform" 
  type="translate" 
  values="0,0; 0,5; 0,0" 
  additive="sum"
/>

Why additive?

If the bell rotates 20° and the clapper translates 5px downward, additive="sum" applies both transforms to the clapper’s coordinate space. Without it, the clapper would move relative to the SVG’s origin, not the bell’s tilted position.

Timing the strike:

The clapper’s 0.8s loop is shorter than the bell’s 1.5s loop. This offset creates a realistic ringing pattern: the clapper strikes the bell at the peak of its forward swing, then again as the bell swings back. The keySplines="0.42 0 0.58 1" (a standard “ease-in-out”) gives the clapper a quick strike and immediate rebound.

Layering Transforms: Rotation + Translation for Realism

A bell doesn’t just rotate—its pivot point shifts slightly as it swings. We combine rotate and translate using multiple <animateTransform> elements with additive="sum":

<g transform="translate(20,30)">
  <animateTransform type="rotate" ... /> 
  <animateTransform type="translate" values="0,0; 0,2; 0,0" ... />
</g>

Why both?

The translate animation adds a subtle vertical bob to the bell’s pivot, simulating the slight lift at the end of a swing. This mimics a real bell’s motion: as it swings forward, the pivot point rises slightly due to the rope’s tension. The search icon uses a similar combo (translate + rotate) for its magnifier’s arc, but the bell’s translation is smaller and faster.

Math Behind the Motion: Sine Waves vs. Keyframes

Could we use a sine wave instead of keyframes? SMIL doesn’t support calc() for complex easing, so keyframes with keySplines are more flexible. However, the underlying math is similar:

rotation(t) = A * sin(2πt/T + φ)

Where:

• A = amplitude (20°)
• T = period (1.5s)
• φ = phase shift (to align with clapper strike)

The keyTimes and keySplines approximate this sine wave with manual control points. For example, the bell’s forward swing (0 → 20°) covers 50% of the time, matching the sine wave’s first half-cycle. The overshoot (20° → -10°) adds a decay factor, like friction.

Why not pure sine?

Real bells don’t swing in perfect harmonic motion—they lose energy and overshoot. Keyframes let us tweak each segment (acceleration, overshoot, settle) independently, which a single sine function can’t achieve.

Practical Takeaways: Applying These Patterns

You can reuse these SMIL patterns in your own icons:

1. For pendulum motion:
   
   Use rotate with asymmetric keyTimes (e.g., 0; 0.6; 0.8; 1) and keySplines that ease in/out. Offset the clapper’s loop for striking.

2. For layered transforms:
   
   Combine rotate and translate with additive="sum" to create complex motion (e.g., a bouncing icon that rotates while translating).

3. For organic easing:
   
   Avoid keySplines="0 0 1 1" (linear) for natural motion. Use 0.25 0.1 0.25 1 for slow-start/slow-stop, or 0.5 0 0.5 1 for acceleration.

4. For independent loops:
   
   Give different elements different dur values and repeatCount="indefinite". The menu icon’s lines have a 2s loop, while the search icon’s magnifier has a 4s loop—both offset for visual interest.

Conclusion: The Power of Declarative SMIL

The bell icon’s ringing animation demonstrates SMIL’s strength: fine-grained, physics-inspired motion without JavaScript. By combining keyTimes, keySplines, and additive transforms, we create a believable mechanical system. These patterns are reusable across CSSVG’s icon library—whether it’s a bell, a swinging lantern, or a bouncing ball.

To see these principles in action, explore the full collection at cssvg.com. Every icon is a study in efficient, declarative animation—ready to drop into your React or Next.js project.