Loudspeakers are rather insensitive and inefficient transducers. Knowing and comprehending their sensitivity and efficiency ratings is key to understanding how speakers work as a whole.
What is speaker sensitivity and efficiency? Sensitivity and efficiency are similar specifications/measurements of how well a speaker converts amplifier power (electrical energy) into acoustic (mechanical wave) energy. Put differently, they state the loudness of a speaker at a given amp power in decibel values and percentages, respectively.
In this article, we’ll discuss speaker sensitivity and efficiency in greater detail, covering the definitions; measurement protocols, and how to make sense of the specifications to better choose and use speakers in the real world.
Table Of Contents
- Loudspeaker Sensitivity Vs. Efficiency
- What Is Speaker Sensitivity?
- Passive Vs. Active Speakers
- How Is Speaker Sensitivity Measured?
- How To Use Speaker Sensitivity Ratings
- Sensitivity, Power & Distance
- The Relationship Between Speaker Sensitivity & Impedance
- What Is A Good Speaker Sensitivity Rating?
- Is It Possible To Increase A Speaker’s Sensitivity?
- What Is Speaker Efficiency?
- How Is Speaker Efficiency Measured?
- How To Use Speaker Efficiency Ratings
- What Is A Good Speaker Efficiency Rating?
- A Note On Speaker Burn Out
- Related Questions
Loudspeaker Sensitivity Vs. Efficiency
As mentioned in the beginning paragraphs, sensitivity and efficiency are related but they are not actually the same thing. That being said, they are often used interchangeably. Let’s start our discussion, then, by stating the differences.
Speakers are transducers. Transducers are devices that convert one form of energy into another form of energy.
In the case of speakers, the conversion is between an inputted audio signal (electrical energy) and an outputted sound wave (mechanical wave energy).
For more information on how speakers work as transducers, check out my article How Do Speakers & Headphones Work As Transducers?
Speaker sensitivity and efficiency are both measurements of a speaker’s acoustic output (mechanical wave energy) at a given amplifier input power (electrical energy).
However, the ways in which these specifications are presented are different.
Speaker sensitivity is given as a sound pressure level (in decibels) per unit of input power at a given measurement distance.
Sensitivity specifications are generally given as decibels of sound pressure level per 1 watt of power at 1 measurement distance of 1 meter.
Speaker sensitivity: X dBSPL / 1 W @ 1 m
Speaker efficiency, on the other hand, is presented as a percentage. It is the ratio between the amplifier output power and the speaker’s acoustic output power.
Efficiency specifications are defined as the speaker’s sound power output divided by the electric power input. Both sound power and electric power are measured in watts.
Speaker efficiency: Pac/Pe • 100%
Note that the radiated sound power output is the cause and the sound pressure is the effect. The sound power is unaffected by the diffusion of sound in the medium whereas the sound pressure level will continually decrease as the sound wave propagates through the medium.
These are the technical specifications for sensitivity and efficiency. We’ll get into each in more detail.
However, when most people use the terms “sensitivity” and “efficiency”, they are referring to the same thing: sensitivity (as defined above).
With this primer, let’s get into the rest of the article.
What Is Speaker Sensitivity?
Once again, the simple definition of speaker sensitivity is the capability of the speaker to convert electric power from an amplifier into sound.
An even more basic definition is that sensitivity refers to how loud a speaker will be at a given input signal level.
As we’ve discussed, sensitivity is generally given as a dB SPL / 1 W @ 1 meter rating.
The sensitivity rating of the 8 Ω, 1600 W Pyle Pro PADH212 (link to check the price on Amazon) is given as 98 dB (1 W, 1 m).
Other manufacturers give sensitivity ratings relative to the voltage of an amplified signal rather than the power of the amplifier. This voltage is typically 2.83 Vrms for 8 Ω speakers.
The sensitivity rating of the Klipsch R-41M (link to check the price on Amazon) is given as 90 dB @ 2.83V/1M.
Klipsch is featured in the following My New Microphone articles:
• Top Best Home Speaker Brands You Should Know And Use
• Top Best Subwoofer Brands (Car, PA, Home & Studio)
• Top Best Loudspeaker Brands (Overall) On The Market Today
I’ll discuss the relationship between sensitivity and impedance in the section titled The Relationship Between Speaker Sensitivity & Impedance.
And yet other manufacturers publish sensitivity ratings with only a dB value, leaving it up to the buyer to figure out what this spec means and how it was measured.
The sensitivity rating of the 8 Ω, 1200 W QSC E110 (link to check the price on Amazon) is simply given as “95 dB”.
QSC is featured in the following My New Microphone articles:
• Top Best Subwoofer Brands (Car, PA, Home & Studio)
• Top Best PA Loudspeaker Brands You Should Know And Use
• Top Best Loudspeaker Brands (Overall) On The Market Today
It’s also important to note that it’s only the passive speakers that come with sensitivity ratings because they connect to separate amplifiers. Active speakers do not have sensitivity ratings since they are designed with their own dedicated built-in amps.
We’ll discuss passive and active speakers shortly in the section Passive-Vs.-Active-Speakers.
For now, let’s quickly run through a few definitions to better understand the following variables:
If you already know the definitions of the above factors, please skip ahead to the following section So What Is Speaker Sensitivity?
Decibels Sound Pressure Level (dB SPL)
What is dB SPL? dB SPL (decibels sound pressure level) is a measurement of the sound pressure level at a given point within a medium in terms of decibels.
This definition needs more.
Sound pressure level is the local pressure deviation from the ambient atmospheric pressure that is caused by a sound wave.
SPL, like other pressure measurements, can be measured in Pascals The Pascal is an SI unit. 1 Pascal is equal to one newton per square metre. Standard atmosphere (atm) is defined as 101,325 Pa.
101,325 Pascal (Pa) is equal to 14.70 pound-force per square inch (psi).
Remember that sound pressure is the deviation from ambient pressure within a medium and not the actual pressure at a given point in space!
It’s also critical to note that the sound pressure level of a sound wave decreases as a sound wave propagates through the medium. This is because energy is lost due to to friction (heat) within the medium.
This decrease in SPL can be explained by the inverse-square law that states the sound pressure level will be quartered for every doubling of distance from the sound source.
This quartering of the pressure (a quartering of the amount of Pascal) can also be stated as a drop of ~6 dB.
The decibel (one-tenth of a bel) is a relative unit of measurement that expressed the ratio of one value to another on a logarithmic scale.
When applied to sound pressure level, dB SPL expresses the ratio of the measured SPL field quantity to the threshold of human hearing (being 0 dB SPL). In Pascals, the threshold of human hearing is universally accepted as 2.0 • 10-5 Pa or 0.00002 Pa
To convert between Pascals and dB SPL, we may use the following formula:
SPL = 20 log10 (P / Pref) dB
The reference Pressure (Pref) is the threshold of human hearing 2.0 • 10-5 Pa.
So if we were to convert a sound pressure level of 1 Pascal into decibels sound pressure level, we would calculate the following.
SPL = 20 log10 (1 / 2.0 • 10-5) dB = 94 dB SPL
Here is a table to relate dB SPL and Pascal values:
|dB SPL||Pascal||Sound Source Example|
|0 dB SPL||0.00002 Pa||Threshold of hearing|
|10 dB SPL||0.000063 Pa||Leaves rustling in the distance|
|20 dB SPL||0.0002 Pa||Background of a soundproof studio|
|30 dB SPL||0.00063 Pa||Quiet bedroom at night|
|40 dB SPL||0.002 Pa||Quiet library|
|50 dB SPL||0.0063 Pa||Average household with no talking|
|60 dB SPL||0.02 Pa||Normal conversational level (1 meter distance)|
|70 dB SPL||0.063 Pa||Vacuum cleaner (1 meter distance)|
|80 dB SPL||0.2 Pa||Average city traffic|
|90 dB SPL||0.63 Pa||Transport truck (10 meters)|
|100 dB SPL||2 Pa||Jackhammer|
|110 dB SPL||6.3 Pa||Threshold of discomfort|
|120 dB SPL||20 Pa||Ambulance siren|
|130 dB SPL||63 Pa||Jet engine taking off|
|140 dB SPL||200 Pa||Threshold of pain|
What is a watt? The watt is a measurement of power (in this case, electric power). Power represents the rate of energy transfer (in this case, the electrical energy from the amplifier to the speaker).
In SI units, 1 watt is defined as 1 joule per second.
There are generally two ways in which wattage is expressed:
- Root mean square (rms)
Peak wattage refers to power output at the peaks of the audio signal from the amplifier.
RMS wattage refers to the more continuous power output of the amplifier.
The rms value of a pure sine wave is calculated as 1/√2 of the peak amplitude of the sine wave.
When it comes to speaker power handling, manufacturers will often give both Wpeak and Wrms values. However, with sensitivity, Wrms is used.
What is a volt? The volt is the SI unit for electrical potential. It is measured between two points in a circuit. In the case of speakers, the two points are at the output of the amp and the input of the speaker.
1 volt can be calculated in several ways:
There is a potential difference of 1 volt between two points where 1 amp of current dissipates 1 watt of power:
V = P / I
There is a potential difference of 1 volt between two points where 1 amp of current flows through 1 ohm of resistance:
V = IR
There is a potential difference of 1 volt between two points where 1 joule of energy is used to produce 1 coulomb of electrical charge:
V = J / C
Audio signals are often measured in volts (and millivolts) or decibels relative to volts.
In the case of voltage measured in decibels:
- dBu: decibels voltage relative to a voltage of 0.775 volts
- dBV: decibels voltage relative to a voltage of 1 volt.
However, at speaker level, it’s also common to see volts to measure the voltage of the speaker level signal.
Voltage, like power, can be measured as peak or rms.
Peak voltage refers to the voltage at the peaks of the amplified audio signal.
RMS voltage refers to the continuous/average voltage of the amplified audio signal.
Once again, the rms value of a pure sine wave is calculated as 1/√2 of the peak amplitude of the sine wave.
When it comes to speaker sensitivity, we’re concerned with Vrms.
Note that, because different speakers have different impedances, voltage is considered to be a better value than power for understanding speaker sensitivity. More on this in the section The Relationship Between Speaker Sensitivity & Impedance.
What is a metre? The metre is the base unit of distance in the International System of Units. It is defined as the length of the path travelled by monochromatic light in a vacuum in 1/299,792,458 of a second.
1 metre (m) is equal to 1000 millimetres (mm); 3.281 feet (ft), or 39.37 inches (in).
So What Is Speaker Sensitivity?
So speaker sensitivity tells us how loud a speaker will be (in dB SPL) at a listening distance of 1 metre when an audio signal with 1 watt of power (or an equivalent voltage) is applied to the speaker.
Passive Vs. Active Speakers
If a speaker has a sensitivity rating, it is most certainly a passive speaker. Conversely, if the speaker does not include a sensitivity rating, there’s a good chance that it’s active (unless it’s passive and the manufacturer just didn’t bother including a rating).
Why is this?
Well, remember that the sensitivity rating is a function of the power or voltage applied to the speaker. This voltage (and power) must be at speaker level, as opposed to line level, to properly drive the speaker drivers at listenable levels.
Therefore, all speakers require amplifiers.
Passive speakers rely on external amplifiers and receive speaker level signals at their inputs.
Active speakers (and powered speakers) receive line level (or mic or instrument level) at their inputs and have built-in amplifiers to boost the signal to speaker level.
So each speaker has its own sensitivity but it’s the passive speaker, as an entire unit, that takes speaker level at its input.
The Edifier S3000Pro (link to check the price on Amazon) is an example of an active speaker and does not have a sensitivity rating. In addition to having an active design, this speaker also has Bluetooth and USB connectivity.
The Mackie Thump 12A (link to check the price on Amazon) is a popular example of a powered loudspeaker which also has a built-in amplifier and, therefore, no sensitivity rating.
Therefore, the sensitivity rating, which applies to the input power/voltage of the speaker (the output power/voltage of the amplifiers) only applies to passive speakers. This is, once again, because passive speakers rely on external amplifiers rather than internal amplifiers to bring the audio signal up to speaker level.
For a detailed article on all the differences between passive and active speakers, check out my article What Are The Differences Between Passive & Active Speakers?
How Is Speaker Sensitivity Measured?
The idea of how speaker sensitivity is measured in not difficult to grasp. However, in practice, it proves technically difficult to perform the measurement.
As we’ve discussed, sensitivity refers to the acoustic output (measured in dB SPL) at a distance of 1 metre when and a given applied power from an amplifier (typically 1 watt) or a given voltage (often 2.83 Vrms).
So understanding how sensitivity is measured isn’t so difficult:
- Set up the speaker and amplifier.
- Send the appropriate power or voltage from the amp to the speaker.
- Measure the sound pressure level at a metre from the speaker.
But there’s more to it than that.
In order to produce a speaker sensitivity rating that allows for comparison, a manufacturer should be measured in an anechoic chamber (a non-reflective, soundproof room with no inherent noise).
This environment is often referred to as “free-space” or “full-space.”
Behringer defines the sensitivity rating of its 8 Ω, 800 W B212XL (link to check the price on Amazon) as 95 dB (Full-Space).
Behringer is featured in the following My New Microphone articles:
•Top Best DAW Control Surface Brands In The World
• Top Best Eurorack Module Synth Brands In The World
• Top Best MIDI Controller Brands In The World
• Top Best Mixing Board/Console Brands For Home Studios
• Top Best Synthesizer Brands In The World
The amplifier sends 1 watt (or a designated voltage) to the speaker at one or more specified frequencies. This is often a 1 kHz tone (as is the case with typical microphone measurements such as sensitivity and maximum SPL).
Sometimes the test tone is different (300, 400, 500, 600 Hz, etc.) or an average of different tone measurements. This may lead to a more accurate sensitivity rating but it ultimately makes comparing different speakers difficult due to the variation in measurement protocol.
Other times, manufacturers may cheat a bit and use a tone that gives the highest sensitivity rating. Others may use pink noise.
Through all the variability, the 1 kHz tone is standard.
The sound level meter (measurement microphone) is mounted on an unobtrusive boom to avoid reflection. It is positioned in 1 metre in front of the speaker on-axis (pointed in front of and toward the centre of the high-frequency driver).
How To Use Speaker Sensitivity Ratings
So we see that that sensitivity rating is useful but very specific in the way that it is measured.
It tells us the sensitivity in a specific listening location and at a specific tone in an impractical listening environment.
However, this is not how we listen to speakers.
Rather, we listen to our speakers in non-ideal reflective rooms where we move around and alter our relative location with the speakers. We also listen to music and audio that spans across the entire range of human hearing (20 Hz – 20,000 Hz) rather than to single-frequency test tones.
So then why should we be concerned with a passive speaker’s sensitivity rating anyway?
There are several reasons why a speaker sensitivity rating is useful:
- To compare speakers
- To know how loud a speaker will be
- To understand how powerful an amplifier the speaker will need
Sensitivity ratings allow us to effectively compare the relative sensitivity and loudness of speakers.
For example, we could have 3 different speakers with 3 different sensitivity ratings:
- Speaker A: 84 dB SPL (1w/1m)
- Speaker B: 87 dB SPL (1w/1m)
- Speaker C: 90 dB SPL (1w/1m)
In the above example, we infer that if each speaker was connected to the same amplifier playing the same audio at the same level, Speaker C would be the loudest, followed by B and then A.
3 dB may seem like a small difference but if we break it down, it’s actually a doubling of the amp power.
In other words, an amplifier driving Speaker A would be required to output twice as much power than if it was driving Speaker B and 4 times as much power than if it was driving Speaker C.
That is not to say that any of the speakers will sound better than their counterparts. Sensitivity has little to do with sound quality. It simply has. todo with output per input.
Also, again, there are variations in the way manufacturers test for the sensitivity ratings of their speakers (acoustic environment; on-axis speaker; using 1 watt vs. voltage; the frequency/frequencies of the test tone(s), etc.). However, sensitivity ratings should give us a fair idea of how loud. a speaker will be compared to another.
As discussed above, the sensitivity rating will tell us how loud a speaker will be. For a trained ear, this specification can be read and the user will understand how loud the speaker will be when paired to a given amplifier.
Knowing the sensitivity of a passive speaker will help us to choose an appropraite amplifier.
Speakers with higher sensitivities do not require as much power from their amplifiers. Conversely, speakers with lower sensitivities will need more power from their amplifiers.
Sensitivity, Power & Distance
In this section, we’ll further our understanding of the relationship between speaker sensitivity; amplifier power, and sound pressure level as a function of distance.
First, let’s dive back into the confusing discussion of decibels.
Decibels: Power Ratio Vs. Amplitude Ratio
To understand the use of decibels in speaker sensitivity, we must understand the difference between a power quantity and a root-power quantity.
Power quantity: power itself or a quantity directly proportional to power (acoustic intensity, for example).
Root-power quantity: a quantity with which its square is proportional to power in linear systems (voltage, current, sound pressure level, for example).
This can be shown in the equation: (P / Pref) = (R / Rref)2
So then, a doubling of power can be shown as 3 dB increase. This 3 dB increase, however, would only cause the voltage (and sound pressure level at a given point in the medium) to be multiplied by √2.
Similarly, a 6 dB increase in power would quadruple the power but only double the voltage and sound pressure level.
In these exercises, let’s use out Speaker A from the section Comparing Speakers. Speaker A has a sensitivity rating of 84 dB SPL (1w/1m).
So, if our amplifier was to provide Speaker A with 1 watt of power, we’d hear 84 dB SPL at a distance of 1 metre from the speaker.
Note that the sound pressure level would likely vary slightly depending on the frequency content of the audio signal vs the test tone. It would also vary due to dynamics in a typical audio signal. However, the 1 Wrms means the average would be 84 dB SPL.
Applying 3 dB of gain at the amplifier would boost the amplifier’s output power to 2 watts.
Because the sound pressure level in the sensitivity rating is dependent on power, a 3 dB increase in amp gain will produce a 3 dB increase in SPL at 1 metre. More specifically, Speaker A would produce 87 dB SPL at 1 metre.
Let’s say we apply 20 dB of gain and bring the power of the amp up to 100 watts. This would result in Speaker A producing 104 (84 + 20) dB SPL at 1 metre.
Increasing Power & Distance
But what if we were listening from further away than 1 metre? Let’s discuss listening to the aforementioned Speaker A with 2 watts (+3 dB gain) and 100 watts (+20 dB gain) at distances of 2 metres, 4 metres and 8 metres away.
Remember that sound pressure is a root-power quantity and that the inverse-square law states that the sound pressure levels drops by half its value (-6 dB) for every doubling of distance.
So then, Speaker A, when 2 watts is applied, produces sound with an SPL of:
- 87 dB SPL at 1 metre
- 81 dB SPL at 2 metres
- 75 dB SPL at 4 metres
- 69 dB SPL at 8 metres
At the risk of making things even more complicated, our perception of loudness makes it so that a 10 dB power increase will cause a perceptual doubling of loudness at mid/high frequencies while only 6 dB is required at lower frequencies. This is due to psychoacoustics which is an infinitely complicated subject. If this confuses you (like it does me), then please disregard this information in terms of speaker sensitivity.
The Sensitivity, Power & Distance Relationship In A Diagram
Let’s have a look at an illustration to help better understand the relationship between amplifier power, speaker sensitivity, distance and sound pressure level.
In the illustration below, we’ll use the examples of Speaker A (sensitivity rating of 84 dB SPL @ 1W/1m) and Speaker C (sensitivity rating of 90 dB SPL @ 1W/1m):
The Relationship Between Speaker Sensitivity & Impedance
Speaker impedance is the sum of the DC resistance and AC reactance comprising inductance and capacitance. It is a frequency-dependent value that is measured in ohms (Ω).
So what is the relationship between sensitivity and impedance?
Well, speaker drivers, which are the transducers components responsible for producing sound, are ultimately driven by the electrical current that flows through their conductors.
So although we measure the power of the power amplifier and speaker sensitivity as a result of that power, it’s not actually the power itself that drives the speaker drivers.
I hope that makes sense.
For an in-depth read on speaker drivers, check out my article What Are Speaker Drivers? (How All Driver Types Work).
So, now comes impedance.
Impedance can be thought of as “AC resistance” and can be simplified (albeit over-simplified) by thinking of it as resistance.
Speaker impedance varies greatly from model to model and even varies significantly across the frequency response of the speaker. However, we will generally classify speakers into nominal impedance values.
These nominal impedance values include 4 Ω; 6 Ω; 8 Ω; 12 Ω; 16 Ω and others.
The 8 Ω speaker is quite common and is why we’ll sometimes see a speaker’s sensitivity rating given as a sound pressure level per 2.83 volts at a distance of 1 metre.
This is because 1 watt of power into an 8-ohm speaker will cause 2.83 volts (rms). This, again, is over-simplified but is commonly used. Let’s have a look at the math:
Ohm’s law states that V = I • R or that voltage is a product of current and resistance.
Another formula states P = I • V or that power is a product of current and voltage.
So then, if we rearrange the first equation to be I = V / R and substitute the I in the second formula, we have:
P = V2 / R
So if we have an 8 Ω speaker and an amp supplying 1 watt of power, we get a voltage of √8 or 2.83 volts.
Note that a speaker with 4 Ω and an amp supplying 1 watt would have a voltage of √4 or 2 volts.
A speaker with 16 Ω and an amp supplying 1 watt would have a voltage of √16 or 4 volts.
If we go back to V = I R, we see that a greater voltage will cause more current to flow though a speaker with a fixed R (we’ll call that impedance though it’s technically incorrect).
This means that the 4 Ω speaker will harder to drive (require more power) than the 8 Ω speaker, which will be harder to drive than the 16 Ω speaker.
This seems counterintuitive and there’s loads (pun intended) more to know about impedance than this.
Back to the relationship between impedance and sensitivity.
As stated, a speaker with 8 Ω will require (or at least draw) less power from the amp to drive its driver than a 4 Ω speaker would.
In fact, if we rearrange our above formula to be V2 = P • R, and we remember the following:
- The flow of alternating electrical current is responsible for driver movement and sound production from a speaker.
- I = V / R so the current is dependent on voltage.
Then we can infer that all else remaining the same, a 4 Ω speaker will require twice the power of an 8 Ω to drive the same amount of current.
It also means that a 4 Ω speaker will have an additional 3 dB of sensitivity over an 8 Ω speaker.
This is because if the power is to remain the same (as is the case with the 1w/1m sensitivity rating, then a speaker with half the impedance would theoretically have √2 times more voltage. This √2 times increase is the same as a 3 dB increase with root-power quantities such as voltage.
What Is A Good Speaker Sensitivity Rating?
Sensitivity is only one of the many factors that make up the performance profle of a speaker.
The truth is that moving-coil speaker drivers (which make up the vast majority of speaker drivers) are notoriously insensitive and inefficient.
That being said, a good sensitivity rating for a passive moving-coil speaker could be in the 84 dB SPL (1w/1m) to 90 dB SPL (1w/1m) range. Sensitivity ratings above 90 dB SPL (1w/1m) are considered excellent.
Low-sensitivity speakers less power to drive them; generate less heat, and their components generally have longer lifespans.
Is It Possible To Increase A Speaker’s Sensitivity?
There’s no easy way to increase the sensitivity rating of a speaker without replacing its drivers or the components that make up its drivers.
What Is Speaker Efficiency?
As was previously stated, there is a difference between speaker sensitivity and speaker efficiency even if non-technical people use the terms interchangeably.
Speaker efficiency refers to the ratio between the amplifier’s electric output power (at the speaker input) and the speaker’s sound/acoustic output power.
More specifically, the efficiency specification is given as a percentage calculated as the speaker’s sound power output divided by the electric power at its input multiplied by 100%.
Speaker efficiency (ŋ)= Pac/Pe • 100%
We know that electric power is the rate, per unit time, at which electrical energy is transferred by an electric circuit. Electric power is measured in watts.
Sound power (also known as acoustic power) is the rate at which sound energy is emitted, reflected, transmitted or received, per unit time. It is also measured in watts.
Sound power, unlike sound pressure level, is related only to the source (speaker) and is independent of the location or medium characteristics. It is also unlike SPL in the fact that it cannot be directly measured with a microphone/meter and must be calculated instead.
When sound propagates through a medium acoustic sound power is transferred.
So the acoustic/sound power produced by the speaker is related to the speaker (and the audio signal) rather than the environment/medium the speaker finds itself in.
The speaker efficiency, then, tells the amount of power a speaker will produce when a given electrical power is presented to the speaker by an amplifier.
Again, the efficiency rating of a speaker, when it is given (which is rare), is a percentage.
Moving-coil speaker drivers/transducers are notoriously inefficient. More of the electrical power is lost as heat and very little is actually converted into sound power. Efficiency ratings between 0.1% to 4.0% are common.
There is a way to convert efficiency to sensitivity and vice versa. Note that the efficiency must be in decimal form (rather than in percentage) and the sensitivity must be given as a dB SPL per 1 watt at 1 metre.
Sensitivity in dB = 112 + 10 log (efficiency)
Efficiency = 10(Sensitivity in dB – 112)/10
Remember that efficiency is the percentage of acoustic energy radiated in all directions from a speaker whereas sensitivity is the level of sound pressure directly in front of the speaker (on-axis).
How Is Speaker Efficiency Measured?
Most manufacturers do not bother with calculating or publishing efficiency ratings. The efficiency of speakers is low and so sensitivity ratings look much better.
However, we can calculate efficiency with the following equations:
Sensitivity in dB = 112 + 10 log (efficiency)
Efficiency = 10(Sensitivity in dB – 112)/10
Again, the efficiency must be in decimal form (rather than in percentage) and the sensitivity must be given as a dB SPL per 1 watt at 1 metre.
So then, the following table shows the related values of various speaker efficiency and sensitivity ratings:
|Efficiency (decimal)||Efficiency (percentage)||Sensitivity (1w/1m)|
|0.200||20%||105 dB SPL|
|0.100||10%||102 dB SPL|
|0.050||5%||99 dB SPL|
|0.020||2%||96 dB SPL|
|0.010||1%||93 dB SPL|
|0.005||0.5%||90 dB SPL|
|0.002||0.2%||87 dB SPL|
|0.001||0.1%||84 dB SPL|
Things get infinitely more complicated when sensitivity is measured according to a voltage. This is because the test tone(s) being used with encounter various load impedances from the speaker (remember that impedance changes with frequency).
So let’s say we use the standard 2.83 volts, which is the product of 1 watt into an 8-ohm speaker.
At the test tone, the impedance of the speaker may be much higher than 8 ohms, in which case less power will be drawn and the sensitivity (relative to 1 watt) will increase.
How To Use Speaker Efficiency Ratings
Speaker efficiency rating, if we can get them at all, help us to better compare speakers in an apples-to-apples manner since the specification is more standardized.
Other than that, it essentially tells us the same thing sensitivity ratings do: how loud the speaker’s output will be at a given input.
What Is A Good Speaker Efficiency Rating?
When discussing good sensitivity ratings, I suggested that sensitivities between 84 dB SPL (1w/1m) and 90 dB SPL (1w/1m) are good and that sensitivity ratings above 90 dB SPL (1w/1m) are considered excellent.
Translated into efficiency, we have:
- Efficiency ratings between 0.1% and 0.5% are good.
- Efficiency ratings above 0.5% are great.
A Note On Speaker Burn-Out
It bears repeating that speakers are actually quite inefficient.
Most of the electrical energy sent to moving-coil speaker drivers is lost as heat. As mentioned above, if only 0.5% of the electric power is converted to acoustic power, then the speaker would be relatively efficient.
The main reason for this is not actually to do with poor speaker design (engineers would have figured that out by now). Rather it is due to the difficulty of achieving proper impedance matching between the acoustic impedance of the drive unit and that of the air into which it is radiating.
Speakers, therefore, must be designed to dissipate the heat produced in their coils and drivers. The production of heat can have devastating effects if not properly dissipated that will lead to the burning and/or melting of the voice coil and to speaker burn-out.
So, sending too much electric power to a speaker will cause too much heat within the driver (due to the poor efficiency). This heat will burn-out the speaker.
For more information on speaker burn-out, check out my article Loudspeaker Blow-Out: Why It Happens & How To Avoid/Fix It.
How many watts is a good speaker? The best wattage (power handling rating) of a speaker depends on the power output of the amplifier that is driving the speaker. It’s best to match “big speakers” with “big amps” and “small speakers” with “small amps”. Mismatching speakers and amps can lead to poor signal output, distortion, and even blow-out.
Does more watts mean more bass? Because low-end bass frequencies require more power to be produced by a speaker, a speaker with higher power handling (measured in watts) would likely be more capable of producing more bass. Of course, there are other factors (size, frequency response, enclosure design) that also play a role in the bass production of a speaker.