Quick Links
Sensitivity and impedance are both important factors for headphones, but they can be tough to understand. We'll take a look at both and show you why they're important and how to use that knowledge while headphone shopping.
Why Headphone Impedance and Sensitivity Matter
If you're like most people, you probably just want to plug your headphones in and start listening. With a standard set of wired earbuds, this is perfectly doable, but as your headphones get larger and more expensive, things get complicated rather quickly.
Larger headphones, especially those aimed at audio professionals or audiophiles, often require a headphone amp. If you're tempted to just buy a headphone amp and call it a day, unfortunately, it's not that easy.
To start, your headphones may not need a headphone amp at all. You also may be surprised to find out that, depending on your headphones and the amp you choose, a headphone amp may even make your headphones sound dramatically worse. So, what are you supposed to do when it comes to buying headphones?
This is where sensitivity and impedance come into play. We'll start with sensitivity, as this one is a little simpler.
Headphone Sensitivity Explained
Both impedance and sensitivity affect sound quality and volume. Sensitivity is far less likely to have a noticeable impact on sound quality by itself, but it does have a major impact on volume, which can in turn impact sound quality.
Talking simply and taking no other factors into consideration, the more sensitive the headphones, the louder they'll be when plugged into a given source. Of course, things aren't really this simple because impedance and other factors also impact volume. Still, this is the easiest way to think about it.
Sensitivity is a measurement of the volume of a given speaker or set of headphones at a specific power rating. Usually, for headphones, this is a frequency like 1 kHz at 1 milliwatt (mW) of power.
Comparing headphone sensitivity can be tricky, as it's expressed by manufacturers in a few different ways. Some companies like Sennheiser go by sound pressure level (SPL) and will show a figure like 103 dB SPL 1 V for the Sennheiser HD650, for example. Sony, on the other hand, shows a figure of 106 dB/W/m for its MDR7506 headphones.
Even this doesn't take the dynamics of the music you listen to into account. Modern pop music doesn't have much in the way of dynamic range, but classical recordings, for example, can have a dynamic range in the neighborhood of 20 dB.
As the volume of the source material increases, so does the power output of the amplifier, which can increase total harmonic distortion (THD). To combat this, your amplifier needs to be loud enough to handle this power without distortion, so this means you'll need a more powerful amplifier.
Headphone Impedance Explained
If you're used to dealing with impedance in speakers, you may think that this is fairly simple. Just make sure the impedances match, right?
The problem is that headphones use a much wider range of impedances than speakers. While speakers generally range from two ohms to eight ohms, headphones can range from 16 ohms to well into the hundreds, depending on the type of driver.
While impedance does matter for volume, it has a far greater impact on sound quality than sensitivity. Plus, impedance matching, while it's still important, isn't as clear-cut as it is with speakers.
If youw want to play it safe, aiming to match impedance perfectly will work. This will always get you the ideal volume, but it may not always sound as good as the headphones possibly can. This is part of, but definitely not all of, the reason you'll see headphone fanatics testing matches between different headphones and amplifiers.
How Impedance Affects the Sound of Headphones
Typically, you can lump headphones and earbuds into one of two categories: low-impedance and high-impedance. There isn't any agreed-upon definition for what is high or low impedance, but we'll set the cutoff at 50 ohms.
Low-impedance headphones---under 50 ohms---are generally intended for more casual use. This doesn't mean they're lower quality or can't sound great, just that you can plug them into your phone or computer, and they'll sound good.
High-impedance headphones won't work great with your mobile devices. They might be too quiet, or you could notice that the bass is less defined. Instead, you'll want to power this with the headphone jack on your home stereo or with a dedicated headphone amp.
Here we're calling anything over 50 ohms high-impedance, but this isn't really true. Typically, headphones between 50 and 100 ohms can play well with either your laptop or your headphone amp, and it's increasingly common to find headphones in this range.
If you have a headphone amp for a set of high impedance headphones, don't assume that it will make every headphone sound better. Plugging in your low-impedance headphones could send too much signal voltage into them. At the best, you'll get distorted, unpleasant sound, while at the worst, you could blow the drivers in the headphones.
Knowing Your Headphones' Impedance
How can you know what impedance a given set of headphones is? The manufacturer will specify this either on the box or on their website, but you can also get some clues based on the type of headphones. Note that this is a moot point in wireless headphones, as they use their own built-in amplifiers.
Portable headphones like earbuds or in-ear monitors use either very small dynamic drivers or balanced armature drivers, which are increasingly common. At this size, the dynamic drivers will be low-impedance, and balanced armature drivers are always both small and low-impedance. This means they'll sound great anywhere, even if you plug them into your phone without a headphone jack.
Over-ear headphones use larger dynamic drivers, and these are often higher impedance. These are also often less sensitive, so a headphone amp will often benefit over-ear headphones, especially if their drivers are on the larger side.
Planar magnetic headphones are becoming increasingly popular due to their often amazing sound quality. These are frequently easy to drive in general, meaning you can get great results without a headphone amplifier.
If you're not sure what to make of all this and just want a recommendation for a great set of headphones, be sure to check out our list of the best headphones you can buy.