Nominal impedance (Z) is the simplified, rounded value used to describe a speaker’s average impedance across its frequency range. Unlike DC resistance (Re), which is a fixed value, impedance varies with frequency, often rising at resonance and at higher frequencies due to voice coil inductance (Le).
Because impedance isn’t constant, manufacturers round it to standard values—typically 4Ω, 8Ω, or 16Ω—to make system design easier.
Why Standard Impedance Values?
The reason we commonly see 4Ω, 8Ω, and 16Ω speakers is simple: compatibility and ease of wiring. These values allow multiple drivers to be connected in series or parallel without creating unpredictable impedance loads that could damage amplifiers.
- 4Ω speakers are common in car audio and some PA systems, as they draw more power from an amplifier.
- 8Ω speakers are standard in home and PA systems, allowing for flexible wiring.
- 16Ω speakers are often used in guitar cabinets and some PA setups, where multiple drivers are wired together.
Most amplifiers are designed to handle specific impedance loads, so using standard values ensures predictable performance and prevents excessive current draw or amplifier overheating.
Nominal impedance is not a fixed number but a rounded-off guideline to help with speaker and amplifier matching. Understanding this helps ensure proper wiring, optimal power transfer, and amplifier protection in PA systems, home audio, and car audio setups. Your amplifier may specify a power output into 8 ohms, but your woofer may actually have an average of around 7-7.5 ohms at the frequencies you are using it, this could result in the power to your speaker being slightly higher than anticipated, so it’s important to remember these are GUIDELINE figures only.