What Does Speaker Efficiency Mean?

When you are planning to buy new speakers, you must look into the key specifications of the available options. This will help you in making the correct choice as per your unique requirements. One such technicality is the ‘speaker efficiency’. In this segment you’ll get to know about it. Before we move further into the topic, it is important to note that speaker efficiency and speaker sensitivity are often used interchangeably.

While sensitivity is measured in decibels (db), efficiency is measured in percentage terms. However, you might come across instances where efficiency of speakers is expressed in decibels. For instance, someone might refer to the efficiency of a particular speaker as 86 db. That’s because, generally both are used as interchangeable concepts although technically they are not. Let’s get on with the article to know the difference between them in great detail.

What does speaker efficiency mean?

Speaker efficiency is the amount of power that goes into a speaker (input) that is actually converted into sound (output). It the percentage of electrical energy that ends up as sound energy. The remainder of the electrical energy (majority) turns to heat inside the speaker’s voice coil. Usually, the efficiency value is less than one percent meaning that most of the power sent to a speaker ends up as heat.

Efficiency is the ratio of acoustical output power to the electrical input power, measured in watts (expressed in percentage). The measurement is taken at a distance of 1 metre from the loudspeaker.

What is speaker sensitivity?

The sensitivity rating of a speaker system is one of the important specifications to look out for. Sensitivity tells us the amount of volume we can get from a speaker with a given amount of power.

Sensitivity is the sound pressure exerted directly in front of a speaker (dB) with respect to the amount of power input (say 1 watt) at a given distance (usually 1 meter).

The higher the sensitivity of a speaker, the louder it can play with a specified amount of wattage. For instance, consider a speaker with sensitivity of around 81 dB or so. This means at one watt of power, they’ll deliver just an average listening level. If you need 84 db, you’ll need two watts. Why so? This is because every additional 3 dB of volume requires double the power.

Sensitivity ratings around 88 dB is the moderate range. Sensitivity below 84 dB can be considered as poor sensitivity.  92 dB or higher sensitivity is very good and should be sought after. As such, it is always paramount to properly decide the purpose of the audio speakers while looking for sensitivity rating.

Difference between Sensitivity and Efficiency

So let’s properly understand this. Are Sensitivity and Efficiency the same thing? Is it fair to use them interchangeably? Let’s see.

The answer to the concerned question can be both yes as well as no. As already mentioned, you’ll see the terms efficiency and sensitivity often used interchangeably in audio. To a certain extent that’s fine (although technically they’re not). Most people have a hard time understanding what it means when it is said that a speaker has 92 dB efficiency.

Technically speaking, efficiency and sensitivity are different things but they describe the same process. Sensitivity ratings can be used to find out efficiency specifications and vice-versa. Thus, generally there is a tendency to use both the concepts interchangeably, as you can derive one from the other.

Let’s take an example. Let’s assume you want to convert sensitivity to efficiency, assuming an 8-ohm speaker and 2.83V applied. You will find that a loudspeaker with a sensitivity of 92dB is 1% efficient. And for each 3dB increase in sensitivity, the efficiency doubles. So 95dB sensitivity is 2% efficient and so on.

However, sometimes that can lead to miscalculations. For instance, if a speaker cabinet is designed in such a manner that it concentrates sound in a tight pattern in front of the speaker. In such a case, it’s sensitivity could be quite high compared to efficiency.

Different ways to measure sensitivity

Measuring sensitivity with pink noise: You can calculate sensitivity using pink noise. But pink noise is not very precise as it fluctuates a lot. However, you can take care of the fluctuation issue if you have a meter that calculates averages over several seconds. Another issue is that pink noise doesn’t permit much in the way of limiting measurement to specific band of audio. For example, if a speaker has its bass boosted by +10 dB, it will exhibit a higher sensitivity rating. But that’s basically cheating. The sensitivity ratings showed improvement because of the extra bass. One could apply weighting curves such as A-weighting. It will focus on sounds from 500 Hz to 10 kHz to an SPL meter filtering out the frequency extremes. But that’s whole lot of extra work on your hands.

On axis frequency response measurements:  You can also measure sensitivity by taking on-axis frequency response measurements of speakers at a specific voltage. Here you need to find the average of all the response data points between 300 Hz and 3,000 Hz. This approach is really efficient at delivering repeatable results with high accuracy down to about 0.1 db.

Location of sensitivity measurement: Another critical question is whether sensitivity measurements were done in an anechoic manner or in-room. Anechoic measurement considers the sound emitted by the speaker alone. It ignores the reflections from other objects.

In-room measurements give us a truer picture of the sound levels emitted by a speaker. However, it isn’t without its own problems. In-room measurements generally give you an extra 3 dB or so. Most manufacturers don’t inform whether their sensitivity measurements are anechoic or in-room. Thus, the best thing to do is to see for yourself.

How to increase power?

An amplifier is an electronic device that amplifies the magnitude of voltage/current/power of an input signal. It takes in a weak electrical signal/waveform (input) and reproduces a similar stronger waveform at the output using an external power source.

Low-efficiency speakers convert less than 0.5% of the electrical power into acoustic power while a major portion of the power is lost as heat. Some high-efficiency speakers may convert up to 20% of the electrical power into acoustic power. Nevertheless, in these speakers as well, 80% of power is wasted as heat.

So we see that there is no such thing as an efficient speaker.

Thus, the need of power amplifier arises in such situations. An audio power amplifier (or power amp) is an electronic amplifier used to amplify low-power electronic audio signals. There are various types of amplifiers depending on the changes it makes to the input signal. Thus, amplifiers can be broadly classified into Current, Voltage and Power amplifiers. Let’s understand more about power amplifiers.

Meaning of a Power Amplifier

It is an electronic amplifier used to increase the magnitude of power of a given input signal. The power of the input signal is magnified to a level high enough to drive loads of output devices such as speakers.

The input signal to a power amplifier needs to qualify a certain threshold. Here instead of directly passing the raw audio/RF signal to the power amplifier, it is first pre-amplified using current/voltage amplifiers. It is then sent as input to the power amp after making the necessary modifications.

Types of Power Amplifiers

Power amplifiers can be divided into three types depending on the type of output device that is connected:

Audio Power Amplifiers

This type of power amplifiers increases the magnitude of power of a weaker audio Signal. The amplifiers of speaker driving circuitries in televisions, mobile phones etc. come under this type. The output range of an audio power amplifier varies from a few milliwatts to thousands of watts depending on the types of device.

Radio Frequency Power Amplifiers

These are used when wireless transmissions require modulated waves to be sent over long distances. These signals are transmitted using antennas. The magnitude of power signals fed to the antenna determines the range of transmission.

These power amplifiers are used in wireless transmissions like FM broadcasting. Radio Frequency Power amplifiers are used to increase the magnitude of power of modulated waves for reaching the required transmission distance.

DC Power Amplifiers

DC Power Amplifiers are used in electronic control systems that require high power signals to drive motors or actuators. They amplify the power of PWM (Pulse Width Modulated) signals. Power is increased in the input taken from microcontroller systems. The amplified signal is the fed to DC motors or Actuators.

Power Amplifier Classes

There are multiple ways of organizing a power amplifier circuit. The operation and output features of various types of circuit configurations differs from each other. To distinguish the features and specific uses of various power amplifier circuits, different Power Amplifier Classes are used. Here letter symbols are assigned to different Amplifier classes to identify the method of operation.

They are broadly classified in two categories. First, power amplifiers used to amplify ‘analog signals’ come under A, B, AB or C category. Second, power amplifiers used to amplify Pulse Width Modulated (PWM) digital signals come under D, E, F etc.

The most common power amplifiers are the ones used in audio amplifier circuits that come under classes A, B, AB or C. So let’s take a brief look at them.

Class A Power Amplifier

Analog waveforms consist of positive highs and negative lows. The entire input waveform is used in the amplification process in this class of amp.

A single transistor amplifies both the positive and negative halves of the waveform. This makes Class A amplifiers the most commonly used type of power amplifiers (because of their simple design). They are still popular, even though there are amplifiers with better designs.

In this class, the active element (transistor in this case) is in use all the time. This applies even when there is no input signal. This leads to a lot of heat generation. This in turn reduces the efficiency of Class A amplifiers to 25% in normal configuration.

Class B Power Amplifier

Class B power amplifiers are designed to reduce the inefficiency and heating problems present in the Class A amplifiers. Instead of using a single transistor, this class of amplifiers use two complementary transistors. Thus class B can reach a theoretical efficiency of about 75%. These amplifiers are used in battery operated devices such as FM radios and transistor radios.

There exists a small distortion at the crossover region due to superposition of two halves of the waveform. To deal with this signal distortion, class AB amplifiers are used.

Class AB Power Amplifier

They are a combination of class A and class B amplifiers. Class AB amplifiers are designed to reduce the less efficiency problem of Class A amps. These amplifiers also deal with the problems of Class B amplifiers like the distortion of signal at crossover region.

Class C Power Amplifier

Class C amplifiers allows for greater efficiencies but sacrifices the quality of amplification. They are used in high frequency oscillators. They are also used in amplification of Radio Frequency signals.

Other Power Amplifier Classes

Power amplifier classes such as D, E, F, G etc. are used to amplify PWM modulated digital signals. They come under the category of switching (on/off) power amplifiers. They turn the output either constantly ON or constantly OFF.

Because of their simple functioning, power amplifiers falling under the above mentioned classes can reach theoretical efficiencies of up to 90-100 percent.

Conclusion

Efficiency is the percentage of electrical energy that is converted to sound energy. It is closely related to sensitivity of speakers, but both aren’t the same.

Efficiency and sensitivity, although related, are different data. Both indicates how well the speaker can convert electrical energy to sound. We can know the efficiency if we know the sensitivity and vice versa. But the conversion might not be exactly correct or exactly proportional to the corresponding values as there are other externalities. Thus it is important to distinguish the two.