Reactive Impedance (Box Rise)

Introduction

Reactive impedance, often called "box rise," refers to how a speaker's impedance (or resistance) changes as it plays sound. This is especially important because it affects what the amplifier "sees" and directly impacts the power delivered to the speaker.

How It Works

When a speaker is in use, its voice coil doesn’t just act as a simple resistor. Instead, the coil's properties and the speaker's enclosure cause the impedance to rise as the frequency or load conditions change. Here's why:

• Inductance of the Voice Coil: The voice coil acts like an inductor, creating resistance that increases with frequency. This makes the speaker harder to drive at higher frequencies.
• Mechanical and Acoustic Effects: The stiffness of the suspension and air pressure in the enclosure (the "box") also add resistance, especially around the speaker's resonant frequency.

What the Amplifier "Sees"

At certain frequencies, especially low bass notes, the impedance can increase significantly. For example, if your speaker has a rated impedance of 1 ohm, the amplifier might see 2, 4, or even 8 ohms due to box rise. As impedance rises, the amplifier delivers less power, which can affect overall output and performance.

Why It Matters

• Power Delivery: Higher impedance means the amplifier delivers less power to the speaker.
• System Design: Understanding box rise helps match your amplifier and speaker for optimal performance.
• Tuning and Testing: Knowing how impedance changes lets you fine-tune your system for better efficiency and output.

Conclusion

In short, box rise is a natural result of how speakers and enclosures work together, and it’s crucial for understanding how your amplifier and speaker interact in real-world conditions.