What Is Speaker Sensitivity?

Understanding One of the Most Important But Confusing Speaker Specs

Sensitivity drawing
Speaker Sensitivity. Brent Butterworth

What's the one speaker specification that's always worth looking at? Sensitivity. Sensitivity tells you how much volume you'll get from a speaker with a given amount of power. It affects not only your choice of speaker, but also your choice of amplifier. Sensitivity has been having an especially large effect on things like Bluetooth speakers, soundbars and subwoofers -- even though sensitivity isn't specified in those products.

What Sensitivity Means

If you understand the way sensitivity is measured, it becomes self-explanatory. Here's basically how it's done: Place a measurement microphone (or a sound pressure level meter) exactly 1 meter away from the front of the speaker. Connect an amp to the speaker and play a signal, setting the level so the amplifier delivers 1 watt of power to the speaker. Generally, a 2.83-volt signal is used, because that's 1 watt into 8 ohms, which is the most common average impedance for speakers.

Now observe the level in decibels (dB) measured at the microphone or the SPL meter. That's the sensitivity of the speaker.

The higher the sensitivity of a speaker, the louder it will play with a certain amount of wattage. For example, some speakers have sensitivity of just 81 dB or so. That means with 1 watt, they'll deliver just a moderate listening level. But the louder you go, the more physics works against you.

That's because every extra 3 dB you turn the volume up requires double the power. Want 84 dB? You'll need 2 watts. Want to hit some nice, loud 102 dB peaks in your home theater? You'll need 128 watts.

A sensitivity measurement of 88 dB can be considered roughly average. Below 84 dB is pretty poor sensitivity.

A measurement of 92 dB or higher is generally considered very good.

(Want to understand more about the basics of audio match? It's actually pretty simple -- just read "5 Simple Equations That Can Turn You Into an Audio Tech Whiz.")

Are Efficiency and Sensitivity the Same?

Yes and no. You'll often see the terms sensitivity and efficiency used interchangeably in audio. That's OK -- everybody knows what you mean when you say a speaker has "89 dB efficiency." Technically, they're different, but they're really just different expressions of the same thing.

Efficiency is the amount of power that goes into a speaker that is actually converted into sound. It's usually less than 1%, which tells you most of the power that goes into a speaker is being converted into heat, not sound. Sensitivity specs can be converted to efficiency and vice-versa; here's a handy online calculator to help you.

How Sensitivity Measurements Can Vary

Unfortunately, almost no speaker manufacturers tell you how they measure sensitivity, besides telling you what you already know: that the measurement was done at 1 watt/1 meter. Unfortunately, sensitivity measurements are done in a variety of ways.

You can measure sensitivity with pink noise, which is OK but because pink noise fluctuates in level, it's not very precise unless you have a meter that can average the level over several seconds.

It also doesn't permit much in the way of limiting the measurement to a particular band of audio. For example, a speaker that has its bass boosted by +10 dB will have a higher sensitivity rating, but it's basically "cheating" by throwing in all that unwanted bass. You can, however, apply weighting curves to your SPL meter to filter out the frequency extremes -- such as A-weighting, which focuses on sounds between about 500 Hz and 10 kHz.

I prefer to measure sensitivity by taking my on-axis frequency response measurement of the speaker (which I always perform at 2.83 volts), then averaging all the response data points between 300 Hz and 3,000 Hz. This gives me repeatable results that are accurate to within about 0.1 dB.

Then there's the question of whether the measurement was done anechoically on in-room. An anechoic measurement takes only the sound emitted by the speaker, and does not figure in reflections from other objects. This is the technique I use, mainly because it's repeatable and precise. An in-room measurement, however, gives you a more "real world" picture of the sound levels you'll get from a speaker. Typically, an in-room measurement gives you an extra 3 dB or so. Sadly, most manufacturers don't tell you if their sensitivity measurements are anechoic or in-room. The best case is when they give you both -- then you know they're not obfuscating.

What Does This Have to Do With Soundbars and Bluetooth Speakers?

Ever notice that internally powered speakers, such subwoofers, soundbars and Bluetooth speakers, never list their sensitivity? I think that's because it's a closed system, and what matters isn't the sensitivity -- or even the power rating of the amplifiers -- but the total volume you get from the unit.

It would be kind of nice to see sensitivity ratings given for the speaker drivers used in these products, though. The manufacturers rarely hesitate to specify the power of internal amplifiers, always touting impressive numbers such as 300 watts for an inexpensive soundbar or 1,000 watts for a home-theater-in-a-box system.

But the power ratings are nearly meaningless for three reasons:

1) The manufacturer almost never tells you how the power is measured (maximum distortion level, load impedance, etc.), or if the unit's power supply can actually deliver that much juice.
2) The amplifier power rating doesn't tell you how loud the unit will play unless you also know the sensitivity of the speaker drivers.
3) Even if the amp does put out that much power, you don't know that the speaker drivers -- which in soundbars and Bluetooth speakers are almost always very inexpensive -- can handle the power.

Let's say a soundbar rated at 250 watts is putting out more like 30 watts per channel in actual use. If the soundbar uses very cheap drivers that have 82 dB sensitivity, then theoretically, it can put out about 97 dB. That's a pretty satisfying level for gaming and watching action movies. Except those drivers might handle only 10 watts, which will limit you to about 92 dB. That's not really loud enough for more than casual TV watching.

If the soundbar has drivers rated at 90 dB sensitivity, then you need only 8 watts to push them to 99 dB -- and 8 watts of power is a lot less likely to push the driver past its limits.

The logical conclusion to reach here is that internally amplified products such as soundbars, Bluetooth speakers and subwoofers should be rated by the total volume they can deliver, not by their wattage. An SPL rating on a soundbar, Bluetooth speaker or subwoofer means something because it gives you a real-world idea of what volume levels a product can achieve. A wattage rating does not, so it has little meaning in products like this.

Here's another example. Hsu Research's VTF-15H subwoofer has a 350-watt amp, and puts out an average of 123.2 dB SPL between 40 and 63 Hz. Sunfire's Atmos subwoofer -- a much smaller design that is far less efficient -- has a 1,400-watt amp, but averages only 108.4 dB SPL between 40 and 63 Hz. Clearly, wattage doesn't tell the story here. It doesn't even come close.

As of 2014, there's no industry standard for an SPL rating for active products, although there are reasonable practices. One way to do it is to turn the product up to the maximum level it can achieve before distortion becomes objectionable (many, if not most, soundbars and Bluetooth speakers can run at full volume without objectionable distortion), then measure the output at 1 meter using a -10 dB pink noise signal. Of course, deciding what level of distortion is objectionable is subjective; the manufacturer could use actual distortion measurements, taken at the speaker driver, instead.

Obviously, there's a need for an industry panel to create practices and standards for measuring output of active audio products. This is what happened with the CEA-2010 standard for subwoofers, and because of that standard, we can now get a very good idea of how loud a subwoofer will actually play.

Is Sensitivity Always Good?

Why don't manufacturers make every speaker as sensitive as possible? Because they typically have to make compromises to achieve that sensitivity. For example, the cone in a woofer could be lightened to improve sensitivity, but that would likely make the cone more flexible, which would increase distortion. Also, when speaker engineers eliminate unwanted peaks in a speaker's response, they also reduce sensitivity.

But all things considered, having a speaker with better sensitivity is usually a better choice.

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