What is speaker sensitivity?

By Adam | Internet
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If you’ve ever looked at some of the technical specifications on a set of speakers, you will no doubt have come across Speaker Sensitivity.

Speaker Sensitivity is one of the most important specifications of a speaker, yet it is also one of the least understood and is often confused with speaker efficiency.

So let’s clear up the confusion and look into more detail about what speaker sensitivity to help you understand those tech specs.

Speaker sensitivity is a measure of how loud a speaker will be for a given amount of power.  In other words, the higher the sensitivity of the speaker, the higher the volume will be for a certain amount of power delivered by an amplifier.  It’s important to note that speaker sensitivity is not about sound quality but it is an important specification that must be considered prior to purchasing speakers.

So speaker sensitivity is basically the speaker’s ability to efficiently use the energy output of an amplifier to create sound.  

Speaker sensitivity is measured in decibels (dB), but how is it actually measured?

Well, that’s what we’re going to explore first…

How speaker sensitivity is measured

Measuring speaker sensitivity is generally done by manufacturers and consultants in an Anechoic chamber (acoustically non-reflective) but it is possible to do it yourself at home.

To measure speaker sensitivity, you first need to get yourself a measurement microphone or Sound Pressure Level (SPL), which you purchase by following the link to Amazon.

Now you start by placing your SPL exactly one meter away from the front of your speaker.

Next, you should connect your amplifier to the speaker and play a signal through your amplifier; but you need to adjust the level of the amplifier such that it delivers just a single Watt of power through the speaker.

Now, when you look at the readout on your SPL meter at this point, what you’re looking at is the sensitivity of your speaker measured in decibels (dB). Decibels are a measure of loudness so it’s a good number to look for especially if you like to listen to loud music!

(Amazon, #CommissionsEarned)

So, as an example, this Klipsch 5.2 Walnut System (Amazon, #CommissionsEarned) has a sensitivity of 98dB.  Typically, you would see something like:

Sensitivity: 98dB @ 2.83V/ 1m

What this means is that the speaker produces a sound level of 98 dB when the speaker is driven with 1 Watt of power when measured from a distance of 1 meter.

Voltage Sensitivity

So far we have looked at power sensitivity.  However, a speaker’s sensitivity is not accurate for all frequencies because the impedance curve of a speaker is never flat.  

This is because speaker impedance always varies with frequency and therefore power impedance varies with frequency.

In fact, the power sensitivity may vary by more than 10 dB over the frequency range of a given speaker.

So it is much more useful to take impedance out of the equation and express speaker sensitivity in terms of input voltage instead of input power.

Voltage sensitivity is useful over the speaker’s full frequency range whilst power sensitivity is only accurate for certain frequencies; where the speaker’s input impedance is exactly the same as the nominal impedance of the speaker.

Not only is voltage sensitivity a much more useful specification, it is a lot easier to measure using a fixed voltage rather than a fixed amount of power.

That is why manufacturers often express speaker sensitivity in terms of input voltage rather than input power, hence the 98dB @ 2.83V/ 1m rating above.

So, let’s look at an example.

Given that power = (voltage^2)/resistance, then if we have a standard 8-ohm speaker that produces a sound level of  90 dB with an input power of 1 Watt measured at a distance of 1 meter, the we can say that the speaker is rated at 90 dB, 2.83 V(RMS) at 1m.

It’s important to note actually that this does not hold for 4 ohm speaker. If 2.83 volts from the amplifier drives a 4 ohm speaker, it produces 2 watts of power. So for a 4 ohm speaker, 2.83v/1m equals 2w/1m.

So it’s a good idea that when you’re reading the speaker sensitivity specification, if you see 2.83 Volts as the reference, check the impedance.

If the speaker impedance is less than 8 ohms, then it is being driven by more than 1 watt of power.

So, rather than measuring impedance and power, we can drive a speaker with 2.83 V(RMS), which is much easier to maintain and then we can measure the amount of sound coming out of the speaker.

This gives us a reading which will be valid for the entire bandwidth of the speaker.  As such we can use voltage sensitivity to measure the sound pressure level at any frequency within the normal operating range of the speaker.

All we need to know is the voltage that is being supplied by the amplifier into the speaker.

Given our new way to measure sensitivity, we can now use voltage sensitivity specifications to make meaningful comparisons between speakers with different impedance values.

What do the numbers mean in a practical sense?

Basically the higher the sensitivity rating of a speaker, the louder the speaker will be for a certain amount of input power.

Home audio speakers typically range from about 80 dB up all the way up to about 100 dB in terms of sensitivity.

As such, 84 dB speakers are less efficient and require more power from an amplifier to get the same volume as more efficient 95 dB speakers for example.

It might not seem that there is much difference in the numbers, but decibels are logarithmic in nature meaning the difference between values is larger than you think.

It’s important to note how much input power is needed to increase speaker volume.  

You have to double the input power to produce a 3 dB increase in sound output (assuming the speaker is not reaching its limits).

You can see from the table below just how loud a speaker will play with a specific starting power:

Power in Watts Volume in dB
1 85
2 88
4 91
8 94
16 97
32 100
64 103
128 106
256 109
512 112

Its apparent from the table above how it starts to take a lot of power to make a speaker play very loud.

Lets have an example, say you got some new speakers rated at 90 dB and your old speakers were rated at 87 dB, the increase in volume is equivalent to doubling the power from your amplifier (although that is not the same as being twice as loud, 10 dB is about twice as loud).

If you’re wondering how loud 87 dB or 88 dB actually is then if you take silence as being 0 dB, then 10 dB is normal breathing.  

Your alarm clock is about 80 dB and an ambulance siren is about 120 dB.

Here are a few more common sounds (with their decibel ratings) that will help you understand these ratings further.

A speaker with a sensitivity rating of over 90 dB is considered excellent but it does not indicate quality or lack of quality.

Rather it is an indicator of how loud the speaker will be for a given power input.

Is Speaker Sensitivity and Efficiency the Same?

Often speaker sensitivity and efficiency are used interchangeably and although they are related, they are not exactly the same.

Speaker efficiency is the amount of power that goes into a speaker that is actually converted into sound.  

Technically, this is calculated as the ratio between your amplifier input power and the acoustic output power.

If a loudspeaker was perfectly efficient, it would convert 1 W of electrical power into 1 W of acoustic power.

Low-sensitivity speakers convert less than 0.5% of the electrical power into acoustic power. The rest of the power is lost as heat.

Some high-sensitivity speakers may convert as much as 20% of the electrical power into acoustic power.  

But even in these speakers, 80% is wasted as heat.

So it’s pretty safe to say that there is no such thing as an efficient speaker.

That’s why it’s important to specify the “sensitivity” of speakers and not the “efficiency” of speakers.

Obviously you want speakers which are as efficient as possible, because efficient speakers require less power to drive them and they also generate less heat and the components tend to last longer in them too.

How important is speaker sensitivity?

Speaker sensitivity isn’t the be all and end all of specifications, but it does directly relate to how loud a speaker gets.

If you keep distance and power coming into the speaker from an amplifier the same, a lower sensitivity speaker would sound quieter than a higher sensitivity speaker (assuming the room and setup is the same).

A higher sensitivity speaker doesn’t necessarily mean that the speaker has better quality sound than a lower sensitivity speaker, but it does mean that the amount of power you have to feed into the speaker is a lot less for the same amount of volume.  

So this could save you from having to buy a larger amplifier.

You may wonder why manufacturers don’t produce speakers that are as sensitive as possible.

It’s typically because compromises need to be made in order to achieve certain levels of sensitivity.

For example, the cone in a woofer/driver could be lightened to improve sensitivity.

But this likely results in a more flexible cone, which would increase overall distortion.

And when speaker engineers go about eliminating unwanted peaks in a speaker’s response, they usually have to reduce sensitivity.

So it’s aspects like these that manufacturers have to balance out.

But with all things considered, choosing a speaker with a higher sensitivity rating is usually a better choice.

You may end up paying a little bit more, but it will be worth it in the end.