Nominal Speaker Impedance Explained

Impedance can be a tricky speaker spec to understand. Baked into that uncertainty may be the fear (especially for anyone over a certain age) that if you mess up the pairing of your speaker’s impedance with that of what your amplifier can handle, you’ll end up with blown-out speakers. We’ll get to why that’s highly unlikely in a moment.

First, some background info on electricity. Resistance is the broad term for the opposition to the flow of electrical current. Impedance is opposition specifically to alternating current (AC), which is usually the type of power speakers use; though there are some applications where square waves (which is similar to alternating DC) are used for alerts and alarms.

Think of impedance as the resistance the speaker presents to the current running into it from the amp. Impedance is measured in ohms (and written with 𝛀 as the unit). Impedance isn’t a constant number (more on that below too). The lower the impedance (so, say, 4 𝛀 versus 8 𝛀), the more current goes through, and the more power the speaker draws. 

Back in the day (think the 1960s and ’70s), most speakers were rated at 8 𝛀 or 16 𝛀 because the amps then couldn’t handle lower impedance. Impedance is linked to how loud a speaker gets at a particular voltage level or gain setting.

The good news is that computer technology and integrated circuits (ICs) have largely solved the problem. The ICs in today’s class-D amplifiers are really good at protecting themselves and (in the case of “smart amplifiers”) speakers too. So even if you follow Spinal Tap’s advice and turn it up to eleven on one of today’s amps, automatic gain controls will kick in and limit the gain or shut the system down altogether before any damage is done.

How Better Amps Help You

Thanks to today’s smarter amps, it’s much easier to find speakers that’ll work for your OEM application. Usually, from both a cost and timeliness perspective, the best speaker for a new system is the one that already exists. Therefore, if a manufacturer has a large catalog of speaker prototypes to select from, there isn’t much of a worry that you won’t be able to find an amp to work with the system. Off-the-shelf speakers will almost always save you money versus a custom design on an OEM audio project. 

Establishing Impedance Ratings 

The nominal impedance rating of a speaker (i.e., 8 𝛀) really is just a reference point because impedance changes constantly with frequency.

When a manufacturer charts impedance on a graph, they show a line that has a peak at some relatively low frequency (called the resonant frequency). The measured impedance then falls dramatically as the frequency rises, and the nominal impedance will typically be where that trough bottoms out. The measured impedance usually begins to rise again as frequency rises. The frequencies after the first peak is where most speakers’ usable range begins.

The nominal impedance is the minimum impedance a speaker will present to an amp while in standard use. 

The rise in impedance with frequency is due in part to the speaker’s voice coil which acts as an inductor and increases the impedance at higher frequencies. Some speakers are built with copper shorting rings (also called faraday rings) to minimize the inductance. In many uses for a speaker, this isn’t an issue you have to think about.

However, there is another type of speaker option that is good in rugged conditions like military settings or outdoor alarm systems. It’s a piezoelectric speaker. A piezo speaker doesn’t have a voice coil. Instead, AC hits a ceramic material in the piezo speaker and causes it to vibrate and generate sound waves. Because of their design, piezo speakers don’t handle low frequencies well. They act as a capacitor, and impedance drops as you increase frequency (this is why they’re good as alarm system speakers—plus, they’re very robust), but rises significantly below their resonant frequency, which is often 1 kHz or higher.

A Note on Battery-Operated Speakers

It feels like more of our lives depend on lithium batteries every day. When these batteries are powering amplifiers, you’ve got some extra issues to consider. Most all batteries inherently have a voltage limit, while newer LiPo (lithium-polymer) and Li-ion (lithium-ion) batteries have a huge current capacity. The way to get around the voltage limit of batteries is to go with speakers with lower impedance, thus allowing for more current to flow through and give you the power you need.

If you’re designing a new audio system, you probably shouldn’t lose sleep over impedance. But if you do have unique application questions and want sound advice, please reach out to our team. We’d be happy to talk.