Demonstrating Sound Cancellation with Speaker Phase Reversal

Sound Cancellation Experiment

Components and Materials

• Low‑voltage AC power supply
• Two identical woofer‑type speakers
• Two 220 Ω resistors (to limit power to each speaker)

For best results, use matching speakers enclosed in standard speaker cabinets. The resistors prevent over‑driving the drivers when the AC supply is connected.

Cross‑References

Lessons In Electric Circuits, Vol. 2, Chap. 1: Basic AC Theory

Learning Objectives

• Illustrate how phase shift can cause constructive or destructive interference between waves
• Highlight the role of speaker phasing in stereo audio systems

Schematic Diagram

Illustration

Instructions

1. Connect each speaker to the AC supply through a 220 Ω resistor. This resistor controls the power delivered to the driver.
2. Generate a low‑frequency tone at 60 Hz and verify that both speakers emit the sound.
3. Place the speakers one to two feet apart, facing each other, and listen to the 60 Hz tone.
4. Reverse the polarity (swap the positive and negative leads) on one speaker only. Compare the perceived volume.
5. Repeat the polarity swap back and forth, noting how the sound level changes.

Reversing one speaker’s connections inverts its phase. When the two waves are in phase, they reinforce each other and the tone is louder. When they are 180° out of phase, they cancel, producing a noticeable drop in volume. This principle applies to all wave phenomena—acoustic, electrical, water, light—and is the basis of active noise‑cancellation technology.

In a multi‑speaker stereo system, proper phasing ensures that sound waves add constructively rather than partially canceling, maximizing overall loudness. While AC systems lack a fixed “polarity,” the relative wiring sequence still affects performance.

Noise‑cancellation systems capture ambient noise, generate an inverted copy, and superimpose the two signals. For steady‑frequency noise, cancellation is straightforward. For broadband, random noise, advanced signal‑processing is required to produce the precise opposite waveform in real time.