Speaker Sensitivity and Amplifier Power

Speaker Sensitivity

Speaker sensitivity is a measure of how loud a speaker will play at a given input power or at a given input voltage. Sensitivity is normally measured with a 1 watt power input or a 2.83 Vrms voltage input. The acoustic output is normally measured in an anechoic space at a distance of 1 meter from the speaker. At 8-Ohms, 1 watt and 2.83 Vrms are equivalent. The following chart shows typical sensitivity ranges:

Note that some hi-fi speakers use horn-loaded drivers and have sensitivities near 100 dB. One classic example is the Klipsch Heresy. It was introduced in 1957 and had a sensitivity of about 96 dB, 1W @ 1 m. It is still made today and the current version is rated at 99 dB, 2.83 V at 1 m. The Heresy was designed to be a hi-fi speaker but, because of its sensitivity, it became a popular sound reinforcement speaker in the 1960's and 1970's.

Some Benchmark AHB2 customers have horn-loaded speakers with sensitivities as high as 109 dB, 1 W at 1 m (Avantgarde Trio). Other AHB2 customers have low-sensitivity 85 dB speakers like the Benchmark SMS1. When it comes to power requirements and noise requirements, there are vast differences between driving 85 dB speakers and 109 dB horns. Nevertheless, both groups of customers can use the AHB2, because the AHB2 has been specifically designed to operate at both extremes.

Every 3 dB Requires a Doubling in Power

With any speaker, we need to double the power for every 3 dB increase in loudness. If a speaker requires 1 W to produce an acoustic output of 85 dB, it will require 2 W to produce 88 dB, 4 W to produce 91 dB, 8 W to produce 94 dB, and 16 W to produce 97 dB, which approximately matches the 1-watt output of the original Klipsch Heresy. In 1957, power amplifiers were small and high sensitivity was essential. With a 96 dB sensitivity, the Heresy could play loud with just 10 or 20 watts, but would burn up somewhere between 25 to 100 watts.

Low-Sensitivity Applications

Today, with improved power handling capabilities, we have the option of selecting low-sensitivity speakers paired with high-powered amplifiers. If we wish to build a small enclosure with extended low-frequency response, we can do this at the expense of sensitivity. This is the approach taken with the Benchmark SMS1. The 85 dB SMS1 has slightly more low-frequency extension than the 99 dB Heresy, but in a much smaller package. Given sufficient amplifier power, good power handling and reasonable loudness requirements, a low-sensitivity design can offer very high performance in a compact package. The smaller size offers vastly improved stereo imaging in nearfield and midfield applications. One additional advantage of nearfield speaker placement is that this tends to reduce the acoustic contribution of the room. For these reasons, most studio monitors are now designed to be used in a nearfield configuration. Many home applications are also well served by a nearfield to midfield speaker placement.

High-Sensitivity Applications

When a large space must be driven to significant sound pressure levels, there is no substitute for high sensitivity. But, high-sensitivity woofers require large cabinets and this usually increases the spacing between drivers. When driver spacing increases, the speakers must be placed farther from the listener. If these speakers are placed too close to the listener, the drivers will not integrate into a precise stereo image. When this happens, the locations of instruments and voices are blurred within the stereo image. For these reasons, physically large speakers may not be practical in many home environments. If a large space is available, room treatments should be a high priority because of the room effects that come with increased listening distances.

Amplifier Configuration

The Benchmark AHB2 has a switch-selected bridged mono mode that provides a 2 x increase in output voltage and almost a 4 x increase in output power. This fully-balanced monoblock mode is well-suited for low-sensitivity speakers in a medium to large listening space. Note that the monoblock mode is not needed with the Benchmark SMS1 if they are used in their intended nearfield or midfield configuration (less than 10 feet from the listener).

In bridged mono, the AHB2 can deliver 380 W into 8 Ohms and more than 480 watts into lower impedances. The bridged mono mode can easily handle the difficult regions of a speaker's impedance curve and can be used with speakers having a 4-Ohm nominal impedance.

When is 1 W Equivalent to 316 W?

If we do the math and compare extreme examples, we will find that a low-sensitivity 85 dB speaker requires at least 316 W to produce a sound pressure level of 110 dB. In contrast, a very high sensitivity 110 dB horn can deliver the same acoustic output using just 1 W.

If the 110 dB horn was driven with 316 W it would produce a painfully loud 135 dB output. The 85 dB speaker would go up in smoke long before reaching anything close to this acoustic output. This is why high efficiency is necessary in high-output sound-reinforcement systems.

In most hi-fi applications we are not forced to use high-efficiency designs. If space is limited, a compact low-efficiency design may provide better results.

Power Compression

At high output levels, the acoustic output is always somewhat lower (1 to 2 dB) than we would predict. In extreme cases, power compression can reach 3 to 6 dB.

Voice coil heating is the primary cause of power compression. When a speaker is driven hard, the voice coil heats up and the resistance increases. As the resistance increases, the voltage sensitivity decreases and the acoustic output is reduced.

Some manufacturers of high-powered systems include specifications for power compression. These specifications can be used to adjust the sensitivity calculations at high power levels.

In our example above, the 85 dB speaker would require slightly more than the calculated 316 Watts to produce 110 dB. The reason for this is power compression.

Power compression usually produces distortion. It can also cause an imbalance between high-frequency and low-frequency drivers in a multi-way system. If you want to accurately produce high output levels at low distortion you will need high-sensitivity drivers.

Noise Considerations

In a hi-fi application, high-sensitivity speakers can expose the output noise produced by the power amplifier. It is not uncommon to hear audible amplifier noise when using high-sensitivity speakers in a home environment. In many cases this forces users to select low-powered amplifiers with very low output noise voltages. Even when low-powered amplifiers are selected, the amplifier noise is usually still audible. While low-power amplifiers offer a partial solution to the noise problem, they can severely limit the peak output of the system.

The Benchmark AHB2 is the only high-powered amplifier that is designed to be noiseless when driving very high-efficiency speakers. The A-weighted signal to noise ratio is 132 dB in stereo mode (135 dB in mono mode). This is 17 to 30 dB quieter than the best competing amplifiers. More importantly, the A-weighted output noise of the AHB2 is -112 dB relative to 2.83 volts. Since speaker voltage sensitivity is measured at 2.83 volts, we can use the sensitivity to calculate the acoustic noise produced by the amplifier-speaker combination. If we select the worst case, an extremely sensitive 110 dB speaker, the acoustic noise will be -2 dB SPL at 1 meter from the speaker. This means that the acoustic noise is 2 dB lower than the threshold of normal hearing. This indicates that the noise produced by the AHB2 should be inaudible in this worst-case example.

In contrast, most high-quality amplifiers produce a noise voltage that is 17 to 30 dB higher than that of the AHB2. When these competing amplifiers are connected to 110 dB speakers the acoustic noise will be 15 dB to 28 dB above the threshold of hearing! The AHB2 may be the only power amplifier that is capable of noise-free operation when driving extremely sensitive speakers.

First-Watt Considerations

With any speaker, most musical details are produced with just fractions of a watt. For this reason, the amplifier perfomance is very critical at power levels below 1 watt. Small amounts of crossover distortion produced by push-pull transitions in the output stage may translate into audible defects. This is especially true when driving high-sensitivity speakers. High-sensitivity speakers can reveal the shortcomings of traditional class-AB amplifiers. It is not surprising that many people do not like the way high-sensitivity speakers sound when driven from a class-AB amplifier.

The AHB2 uses a patented feed-forward error correction system to eliminate crossover distortion. This system provides 1st-watt performance that exceeds that of a class-A amplifier.

When needed, the AHB2 can deliver the power required to drive low-sensitivity speakers to reasonably loud levels. This combination makes the AHB2 well suited to meet the needs of both high-sensitivity and low-sensitivity speakers.

Peak Sound Pressure Level

Peak sound pressure level can be calculated if we know the speaker sensitivity and the amplifier output power or output voltage. The actual output will be slightly lower due to power compression in the speakers.

In stereo mode, the AHB2 can deliver 100 W into 8 Ohms. This is 20 dB above 1 W and 20 dB above 2.83 V. This means that the AHB2 can drive speakers to a level that is 20 dB higher than the output that they produce at 2.83 V. With a high-sensitivity 110 dB speaker, the amplifier could deliver a sound pressure level of up to 130 dB (ignoring power compression).

The power output of the AHB2 can be boosted by almost a factor of 4 by operating it as a fully-balanced monoblock amplifier (switch selectable). In mono mode, the AHB2 can deliver 380 W into 8 Ohms. This is 25.8 dB higher than the level at which speaker sensitivity is measured. This means that a low-sensitivity 8-Ohm 85 dB speaker would deliver up to about 110.8 dB when driven bridged mono (ignoring power compression).

If we return to the example of the Klipsch Heresy, the AHB2 can drive the modern 99 dB Heresy III to a peak sound pressure level of 119 dB in stereo mode or 124.8 dB in bridged mono. The original Heresy was rated at 25 W and could not have withstood the 380 W bridged-mono output of the AHB2.

At a sensitivity of 99 dB, the amplifier noise produced by the AHB2 will be about 13 dB below the threshold of hearing at a distance of 1 meter from the Heresy III speaker.

Power Sensitivity (dB, 1 W @ 1 m)

If a speaker produces a sound pressure level of 90 dB with 1 W at a distance of 1 meter, measured in an anechoic space, we specify this as a sensitivity of 90 dB, 1 W @ 1 m. This notation tells us how loud the speaker is when it is driven with 1 W of amplifier power.

This notation is a measurement of the speaker's power sensitivity and it is usually accurate at just a few frequencies because speakers never have flat impedance curves. Speaker impedance always varies with frequency and this means that the power sensitivity also varies with frequency. The power sensitivity may vary by more than 10 dB over the frequency range of the speaker. For this reason, the power sensitivity is only valid at the frequencies where the impedance is exactly at the rated nominal impedance of the speaker. The solution is to take speaker impedance out of the specification. This can be done by measuring sensitivity at a fixed voltage (2.83 V) instead of at a fixed power (1 W).

Speakers Respond to Voltage

Speakers are designed to produce a relatively flat frequency response when driven from a fixed input voltage. For this reason it is better to express speaker sensitivity in terms of input voltage instead of input power. A voltage sensitivity specification is useful over most of the speaker's rated frequency range while a power sensitivity specification is only accurate at frequencies where the speaker's input impedance is exactly the same as the nominal impedance of the speaker. It is also easier to measure sensitivity using a fixed voltage instead of a fixed power. For these reasons, most speaker manufacturers now publish voltage sensitivity specifications.

Voltage Sensitivity (dB, 2.83 Vrms @ 1m)

If we do the math, power = (voltage^2)/resistance, it takes 2.83 Volts to deliver 1 watt into an 8-ohm load. If we have an 8-ohm speaker that produces 90 dB, 1 watt at 1 meter, we can express this as 90 dB, 2.83 Vrms at 1 m (assuming the impedance is 8 Ohms at the test frequency). Rather than making impedance and power measurements, we can simply drive the speaker with 2.83 Vrms and measure the output sound level. The results will be similar over the entire bandwidth of the speaker. The voltage sensitivity gives us an easy way to predict the sound pressure level at any frequency within the normal operating range of the speaker. We just need to know the voltage that is being supplied by the power amplifier.

One major advantage of voltage sensitivity specifications is that we can make meaningful comparisons between speakers with different impedances.

Power Sensitivity Specifications can be Misleading

Power sensitivity specifications can make it difficult to compare the sensitivities of speakers that have different impedances. This is shown in the following two examples:

Example 1:

In this example all 3 speakers have identical power sensitivity ratings (90 dB, 1 W @ 1 m). If we only looked at the power sensitivity ratings we might get the impression that all three speakers would play with the same loudness on our system.

But, if we look at the voltage sensitivity ratings we can see that speaker 1 will play 3 dB louder than speaker 2 and 6 dB louder than speaker 3 when driven by 2.83 Vrms. At this voltage, speaker 1 will consume 2 W, speaker 2 will consume 1 W, and speaker 3 will consume 1/2 W.

Driving Low-Impedance Loads

At high levels, most power amplifiers will have difficulty delivering twice the power into the 4-Ohm speaker, and the 3 dB advantage over the 8-Ohm speaker will be reduced by 1 or 2 dB. The reason for this is that most power amplifiers have unregulated power supply rails and these rails tend to sag when driving low impedances. At high power levels into 4-Ohms, traditional amplifiers may clip 1 or 2 dB lower than when driving an 8-Ohm load.

In contrast, the Benchmark AHB2 power amplifier has tightly regulated power supplies and this allows it to provide an almost perfect doubling of power when the impedance is reduced from 8 Ohms to 4 Ohms. The AHB2 will deliver 100 W into 8 Ohms and 190 W into 4 Ohms. If you do the math, 190 W is only 0.2 dB lower than 200 W. At the clip point of the AHB2 amplifier, speaker 1 should be 2.8 dB louder than speaker 2 (assuming that we are not running into the mechanical limitations of the speakers).

Example 2:

A quick examination of the power sensitivity numbers for speakers 4, 5 and 6 would make it look like speaker 6 would play the loudest. But, when we compare the voltage sensitivity, we can see that all three speakers will play with the same loudness when driven by 2.83 Vrms.

Note that speaker 4 will be the hardest to drive at high levels. Traditional power amplifiers may have their outputs reduced by 1 to 2 dB when driving speaker 4 at the amplifier clip point. In contrast, the AHB2 will drive all three speakers to approximately the same level.

Converting Power Sensitivity to Voltage Sensitivity

A speaker manufacturer may provide the power sensitivity instead of the voltage sensitivity. If this is the case, the power sensitivity will need to be converted to a voltage sensitivity if we wish to make meaningful comparisons to other loudspeakers.

If the nominal impedance is 8 ohms, 1 W is equivalent to 2.83 Vrms. But, if the nominal impedance is not 8 Ohms, you can use the following chart to convert from power sensitivity to voltage sensitivity (assuming the power sensitivity was measured, or adjusted to the nominal impedance):

Amplifier Output Voltage

If we specify the amplifier output in terms of voltage instead of power, it is easier to calculate the acoustic output of the combined amplifier-speaker system. The following chart for the AHB2 shows the maximum RMS output voltage of a sinusoidal signal.  It also shows the output noise voltage.

All voltages are expressed in dB relative to 2.83 V. This scale-factor makes it easy to add these numbers to the speaker voltage sensitivity.

The maximum output voltage can be added directly to the speaker voltage sensitivity in order to determine the peak loudness of the amplifier-speaker system. Select the row that corresponds to the nominal impedance of your speaker.

Likewise, the noise voltage can be added to the speaker voltage sensitivity to determine the audibility of the amplifier noise at the output of the speaker. If the result is a negative number, the acoustic noise is lower than the threshold of hearing.

AHB2 Output Voltage and Output Power

Speaker Efficiency

Speaker efficiency is not the same thing as speaker sensitivity. Loudspeakers are actually very inefficient electrical to acoustic transducers. 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 remaining power produces heat.

High-sensitivity speakers may convert as much as 20% of the electrical power into acoustic power. Nevertheless, this leaves 80% that is wasted as heat. There is nothing efficient about speakers.

Remember we specify the "sensitivity" of speakers and not the "efficiency" of speakers.