Loudspeaker Sensitivity and Efficiency Explained

Sometimes you'll come across people (or journalists) discussing loudspeakers, and they'll throw around terms like "sensitivity" and "efficiency", sometimes with some numbers and units flung in. But what does this actually mean, and how much does it practically matter?

When people discuss the efficiency or sensitivity of a speaker, they're really trying to come up with a way to describe how loud it will go, for their amplifier. More technically, what they're really asking is how to reason about the relationship between acoustic power output and electrical power input.

If we deliver the same amount of electrical power into a very sensitive speaker, that speaker will move a lot of air, and our ears will detect that air pressure change, and it will sound "loud". If we have a very inefficient speaker, the same amount of power applied will result in less air being moved, and thus our ears will perceive that as comparatively quieter. You'll sometimes hear speakers being described as relatively "easy" or "hard" to "drive", or as an "easy" or "hard" load. We're really just trying to find a way to compare two speakers in terms of how loud they will sound for a given amount of input power.

Note that I used sensitivity and efficiency interchangeably - but they're not the same thing. They're related, and you can derive one from the other. Efficiency of a speaker is stated as what percentage of the electrical input power is converted to acoustic output power. Speakers are actually incredibly inefficient. 99% or more of the power is converted to heat in the drive unit (this is why if you "over-drive" your speakers, they will basically melt and stop working). The number most people talk about is the sensitivity, and it's this that we're going to focus on.


Some Example Specifications

You'll see the sensitivity figures of a speaker in their specification - here are a few examples from manufacturers we deal with:

- ATC SCM7 - 84dB @ 1W @ 1m
- Fyne Audio F701 - 91dB 2.83V @ 1m
- Dali Menuet SE - 86db
- Rega Kyte - 89dB

What do these numbers mean, and how do we compare them?

Well, in the simplest possible terms, the bigger the number, the more sensitive (and thus efficient) the speaker. So the most efficient speaker on the list is the F701, and the least efficient is the SCM7.

That might be all you need to know and all you need to take away, but it's probably worth digging just a little bit deeper.

Here are some things you might want to know:

- How is this measured?
- What does a difference of 5-6dB actually mean?
- What is the practical application for me?


How is this measured?

Unfortunately there isn't an iron-clad standard and manufacturers sometimes play fast and loose with the numbers and the tests, but the principle works like this:

If we want to establish the efficiency or sensitivity of a speaker, we drive the speaker under test with a known voltage at a known frequency, and measure the sound pressure with a calibrated microphone placed 1 meter away from the speaker.

In practice there are a lot of other variables - is the environment like a real room or was the measurement taken in an anechoic chamber? Was the microphone pointed at the most sensitive drive unit? Was it pointed at the centre of the speaker cabinet? Was it a 1KHz frequency, or was it a range of frequencies, or just some generated noise? What was the characteristic impedance of the speaker under test? Was it 4 ohms? 6 ohms? 8 ohms? How far away was the microphone? How was it calibrated?

These variables mean that we're not really guaranteed to be able to be completely confident in our ability to compare the figures quoted by manufacturers. Of course the simplest and most pragmatic approach is to visit a trusted dealer, and compare the speakers side-by-side, and see for yourself, but it's useful to understand a bit more from first principles.

You'll notice that in our specification list the ATC value says @1W @1M. Where did the 1W come from? Well, it's not "watts" that drive a speaker, its "volts". Watts are a measure of work done - or how hard the amplifier had to work.

There's a simple relationship between power, voltage and impedance - the power expended is the voltage squared, divided by the impedance. We obviously need to know two of these to work out the third.  

In the case of ATC, we've only been given the power.In the case of Dali and Fyne we've only been given the voltage.  In the case of the Rega we haven't been given anything!

The impedance is easy to find, though, as that will be specified by the manufacturer. Impedance is a measure of how much resistance the speaker offers to an alternating current. The bigger the number, the more resistance the speaker offers. The ATCs and the Fynes have an impedance of 8 ohms, the Dalis 4 ohms, while the Regas have 6 ohms.

If we plumb those numbers into the formula, we'll find that the ATCs were driven with 2.83V. This matches what Fyne says - Fyne are also 8 ohm speakers, so driving that load with 2.83V will result in 1W of power being expended.

What about the Dalis and the Regas? Well the Dalis have an impedance of 4 ohms - half that of of the ATCs. A speaker with a lower impedance offers less resistance to the current being provided by the amplifier, which means more current will be drawn, and the amplifier will have to work harder - in fact twice as hard.

The 2.83V into the 4 ohm load will actually have resulted in 2 watts of power, not one. This means that, perhaps counter-intuitively, a speaker with a *lower* impedance is actually *harder* to drive - the amplifier needs to work harder, and deliver more current, compared to a speaker with a higher impedance.

Now, in practice it's a moot point as to whether this matters. The whole idea behind using voltage rather than power in the sensitivity spec is that we're saying that if the two speakers are driven with the same amount of voltage, regardless of the impedance, the sound pressure will be the same.

So in simple terms, we can compare the manufacturers figures meaningfully and get a sense of relative sensitivity and efficiency.

 

What does a difference of 6dB actually mean?

It's all very well to know that the F701s are 6dB more efficient than the SCM7s, but it's hard to conceptualise whether that's a big deal or not.

If I told you that one cable cost £85 and another cost £91 you'd probably be thinking - what's £6 between friends?

However if I told you one colourless liquid had a pH of 8 and another had a pH of 2, hopefully you'd avoid sticking your fingers into the second one... sea water has a pH of 8, Hydrochloric Acid has a pH of 2.... that's a big deal if you want to go for a swim.

The way the dB scale for sound pressure level works is this: every time you add 3dB, the sound pressure doubles. So something with a sound pressure of 85dB is half as loud, in terms of how a human ear experiences it, as something with a sound pressure of 88dB.

So, in practice, 6dB makes quite a big difference - if one set of speakers is 6dB less sensitive than another, you'd need to double the amount of power twice to achieve the same volume on the less efficient speakers.


How is this relevant to me?

The main situation in which you might care about this is when trying to decide if your amplifier has enough power to drive your speakers to the volume you want.

To be clear, practically speaking, the answer is almost always "yes", and again, the best way to find out is by booking a demo with a trusted dealer, and finding out for yourself, but there are some consideration to take into account.

It stands to reason that a more efficient speaker needs less power to deliver the same volume as a less efficient one, but now we can start to quantify it.

Take the ATCs - the spec tells is that 1 watt of power resulted in 84dB of SPL, at 1 meter. Now, you're probably not going to be listening to your speakers 1 meter away.

The rule of thumb here is that as you double the distance, the SPL goes down by 6dB.

So in an average room, you might be listening at a distance of 4m - that's 1 doubled up to 2, 2 doubled up to 4. So we lose 12dB, and our 1W of power will deliver 72dB of sound pressure. For reference, that's just louder than regular conversation.

Now, to add 3db of volume, you need to double the input power. I'd say normal listening levels are in the mid to high 80s. If you like to listen very loud, maybe over 90.

Let's take the loud listener as an example. 90db is 18dB more than 72. Since for every 3 dB of volume, we need to double the power, and 3 divides into 18 6 times, we need to double the input power 6 times to deliver a volume of 90dB at 4 meters.

- 1W delivers 72dB
- 2W watts delivers 75dB
- 4W delivers 78dB
- 8W delivers 81dB
- 16W delivers 84dB
- 32W delivers 87dB
- 64W delivers 90dB

So, if you want to listen to your music really loud, you need an amplifier capable of delivering 64 watts to listen to the ATC SCM7s at that level, if you're sitting 4 meters away. For normal listening volumes, 16-32W is probably enough.

Now let's compare the Fyne 701s. According to the manufacturer, they're 6db more sensitive than the SCM7s - so at 4 meters away, they're delivering 78dB instead of 72 db. For reference, that is equivalent to some chamber music in mid flow, in an auditorium.

Let's see what happens here

- 1W delivers 78dB
- 2W delivers 81dB
- 4W delivers 84dB
- 8W delivers 87dB
- 16W delivers 90dB

So we only need an amplifier which can deliver 16W of power to achieve loud listening volume, and only 4-8W for normal listening levels.

So if you were to have a single-ended Class A valve amp with only 12W of power, the Fyne (or another speaker with sensitivity of 90+ dB) would be an excellent match. The ATCs, by contrast, might be a bit of an ask.

What about the Dalis? Well, this is where impedance starts to come into consideration. I said earlier that it doesn't really matter that the Dalis were measured with 2.83V into a 4 ohm load, while the ATCs were measured into an 8 ohm load. The sensitivity is still the sensitivity, because it's defined by the voltage applied, not the power expended.

However, remember that the Dalis needed twice as much power as the ATCs, because they draw twice as much current, so the amplifier works twice as hard. This means when we're working out the power needed to deliver listening volume we need to start at 2W not 1W - or rather we're going to need twice as much power to deliver similar maximum volume levels.

So while it is true that the Dalis are more effecient than the ATCS - 86dB vs 84dB - remember that to deliver maximum volume we need a much more powerful amplifier.

Subrtracting the 12db for the 4 meter distance - we're starting out at 74dB, but we need 2 watts not 1 to generate that level of sound pressure.

- 2W delivers 74dB
- 4W delivers 77dB
- 8W delivers 80dB
- 16W deliver 83dB
- 32W delivers 86dB
- 64W delivers 89dB
- 128W delivers 92dB

So to achieve a sensible listening volume at 4 metres you'd need at least 32W - not too different from the ATCs, but to get over 90dB, you'd need 128W. This isn't a problem if you have a very powerful amplifier, but if you're using a smaller amplifier, and you want to listen at very high volumes, you might want to take the impedance of the speaker into account.

Let's try and summarise this into some byte-sized recommendations:

- Unless you have a low powered amplifier and want to listen at very high volumes, you can more or less compare the sensitivities of speakers regardless of impedance
- Larger speakers are more sensitive, and easier to drive - if you have a low powered amplifier, or a large room, larger speakers will require less power to generate the same volume
- Speakers around 90dB will work absolutely fine with low-powered valve amplifiers, but less efficient speakers will need more power, unless you are in a very small room, or listen at low volumes
- Fynes are generally our most efficient speakers and work very well with all amplification
- ATCs are a bit harder to drive, but with their 8 ohm impedance are perfectly fine for most solid state amplifiers, and the more powerful valve amplifiers
- Dalis are a but easier to drive than ATCs but if used with lower powered amplifiers may run out of steam at higher volumes


I hope you found this interesting, and now have a bit more of an understanding of loudspeaker sensitivity and efficiency, and how to use that information when matching your speakers and your amp.

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