What is impedance in Speakers?
The importance of impedance
When considering audio equipment, understanding impedance is crucial for anyone looking to optimise their sound system. Impedance, measured in ohms (Ω), is a key factor that influences the performance of speakers and amplifiers. It essentially represents the resistance to the flow of electrical current within a circuit.
When dealing with sound systems, particularly those involving passive speakers and amplifiers, matching impedance is vital to ensure efficient power transfer and to prevent potential damage to the equipment. This article delves into the concept of impedance in speakers, explores output impedance in amplifiers, and explains the importance of matching impedance between these components.
Speaker Impedance
Speaker impedance is a fundamental aspect of audio engineering that significantly affects the performance and compatibility of audio systems. In essence, speaker impedance refers to the opposition a speaker presents to the electrical current being supplied by an amplifier. In layman's terms, a speaker is basically an electric piston, and it takes a certain amount of force before it will move, which is a measurable amount. This measurement is its resistance to the incoming electricity, known as impedance.
A speaker is a moving object and its electrical parts swing with the amount of energy required depending on the frequencies (musical notes) it is reproducing. This means the impedance loading to the amplifier is not constant; it varies with the frequency of the audio signal. To simplify this, speakers and amplifiers have a ‘nominal’ stated impedance, with ratings of 4, 6, 8, or 16 ohms being the standard.
The impedance of a speaker influences both the power it receives from an amplifier and the quality of sound it produces. Lower-impedance speakers (e.g., 4 ohms) draw more power from an amplifier than higher-impedance speakers (e.g., 16 ohms). This means that using a lower-impedance speaker with an amplifier not designed to handle it could lead to overloading, potentially causing distortion or damage. Conversely, using a high-impedance speaker with an amplifier designed for lower impedance will result in insufficient power delivery, leading to extremely poor audio performance.
Understanding speaker impedance is crucial when designing or upgrading an audio system, especially when connecting several sets of speakers to the same amplifier. Ensuring that the impedance of the speakers matches the capabilities of the amplifier helps in achieving optimal sound quality and maintaining the longevity of the equipment.
Speaker output impedance in Amplifiers
An amplifier's speaker outputs will be marked with either a range of impedances or a specific acceptable value, which will generally be 4, 6, 8, or 16 Ohms. These are tappings from the amplifier's output stage and represent its minimum to maximum loading from any connected speakers. The range is there mostly for convenience, allowing you to connect a wide selection of speakers without issue.
Using a basic analogy, think of the amp as a person and the speaker as a weight. The person can lift from zero to say 30kg, so thats the range. At 15kg (50%), it's perfectly manageable but requires effort, so your body makes more power available to cope. At 30kg however, the load requires a lot of energy, which reduces the power level available due to fatigue.
This is why when you look at the specs of power amplifiers, the 4 Ohm output will have a higher power output rating than the 8 Ohm output. The lower the impedance, the more current can flow, and more current means the amplifier produces more power. It gets confusing because a lower impedance actually means a greater load on the amp, which is a bit counterintuitive.
A lower output impedance from an amplifier offers a better dampening factor, which is the amplifier's ability to control the speaker's motion. This results in tighter, more accurate bass response and overall sound clarity, which is why 16 Ohm speakers for example are often described as sounding thin or weak.
Going back to the weight lifting example, this is why the majority of hi-fi speakers are 8 Ohm rated, as for most amplifiers this is the midpoint of their acceptable working range, which is a happy medium of good power output without over-exertion. Larger PA amps aren't quite as fussy about a lower impedance load as they will include cooling fans and large heatsinks to remove the extra heat made when running harder.
Impedance matching Speakers to an Amplifier
Impedance matching between speakers and amplifiers is crucial for achieving the best audio performance and protecting your equipment. When the impedance of the speakers matches the output impedance of the amplifier, power transfer is maximised, and the system operates efficiently.
The process of impedance matching involves ensuring that the amplifier can handle the total impedance load presented by the speakers. This is particularly important in setups with multiple speakers. For instance, wiring multiple speakers in parallel lowers the overall impedance (two 8 Ohm speakers wired together on the same cable will look like a 4 Ohm load to the amp), which can potentially overload the amplifier if that figure drops below the amplifier's minimum impedance rating. On the other hand, wiring speakers in series increases the overall impedance, which can lead to insufficient power delivery from the amplifier.
Impedance matching is not just about preventing damage; it also impacts sound quality. Properly matched impedance results in a balanced and clear audio output, with minimal distortion, optimal frequency response, and better control of the speaker driver's movement. Therefore, when setting up an audio system, it is essential to pay close attention to the impedance ratings of both the speakers and the amplifier, ensuring they are compatible and matched for the best possible performance.
You can read further on the subject in our 'Guide to matching Speakers and Amplifiers'
Read More...
Understanding Speaker Impedance: A Deeper Dive
Lets take a look into some of the technical factors of speaker impedance and how it affects the performance, sound quality, and how different wiring will change the loading to the amplifier
Types of Impedance: Resistive and Reactive
What you need to remember with speakers and amplifiers is you are sending an AC signal from one to the other, but there is a DC element at play from the electro-mechanical parts. The speaker is a physical device, and its components present a certain amount of resistance to the incoming electrical signal before they do anything. This is the speaker's impedance rating (it's impeding or restricting the current flow coming into it). So logically, the lower this rating, the lower the restriction, and the higher the current flow.
Impedance in speaker systems is a mixture of resistive and reactive measurements. Resistive impedance, as the name suggests, is a set resistance to the flow of current caused by the speaker's components and remains constant, while reactive impedance on the other hand, varies with the frequency of the incoming audio signal reacting with those components.
The voice coil in a speaker has both reactive and resistive elements, with a fixed DC resistance reading from its wire, but also a reactive measurement from the induction of its coil and the magnetic field surrounding it, and of course the physical movement of the cone itself.
Nominal impedance, which is the rating you find on the back of a speaker, stated in Ohms, is the total of both the DC resistance of the voice coil wire, and the inductance caused by the AC signal (the music signal from the amp) on the magnetic field around the coil.
You can see this visually if you connect a multimeter to a speaker’s terminals. On an 8 Ohm rated speaker for example, you will see an impedance rating of around 6 Ohms when it's at rest. If you gently push on the speaker cone you will see that reading swing. It confuses people seeing a 6 when they expected 8, but you need to remember that a meter is reading only the fixed DC coil resistance and not the AC component.
The importance of understanding impedance in passive audio systems
In passive audio systems, where the speakers do not have built-in amplifiers, understanding impedance is particularly important. Passive speakers rely entirely on external amplifiers for power. The impedance of these speakers affects how much power they draw from the amplifier and how efficiently the system operates.
A mismatch in impedance can lead to several issues. If the speaker impedance is too low for the amplifier, the amplifier might overheat and shut down, or even get permanently damaged from trying to pass too much current. Conversely, if the speaker impedance is too high, the amplifier might not be able to deliver sufficient power, resulting in poor sound quality. Properly matching the impedance ensures that the system operates within safe limits, delivering optimal sound quality without risking damage to the components.
Effects of parallel and series wiring on impedance
When connecting multiple speakers to an amplifier, the method of wiring—parallel or series—affects the total impedance presented to the amplifier.
Parallel Wiring: In parallel wiring, the positive terminals of all speakers are connected together, and the negative terminals are also connected together. The overall impedance decreases as more speakers are added. (resistors in parallel are halved). For instance, two 8-ohm speakers wired in parallel result in a total impedance of 4 ohms. This setup can draw more current from the amplifier, potentially exceeding its capabilities if too many speakers are connected.
Series Wiring: In series wiring, the speakers are connected end-to-end, with the positive terminal of one speaker connected to the negative terminal of the next. The overall impedance increases with each additional speaker. (resistors in series are doubled). For example, two 8-ohm speakers wired in series result in a total impedance of 16 ohms. This setup can make it difficult for the amplifier to deliver enough power, leading to reduced volume and sound quality.
You can read more on this in the article ‘How many Speakers can be connected to a PA Amplifier’
Impedance Differences in Speakers and Amplifiers
High Impedance Speakers: High-impedance speakers (e.g., 16 ohms) draw less current from the amplifier, which can be beneficial in avoiding overloads and excessive heat buildup. However, they require higher voltage to achieve the same power output, which will limit their performance with most amplifiers, resulting in a thin-sounding performance.
Low Impedance Speakers: Low impedance speakers (e.g., 4 ohms) draw more current and can produce higher sound levels with the same power input. They are more efficient but require amplifiers capable of handling the increased current without overheating or distorting.
High Impedance Amplifiers: Amplifiers with high output impedance are commonly used in large distributed audio systems. They are known as 100-volt line amps and feature special step-up output transformers. They require the use of matching 100V line speakers which feature step-down transformers. They are less efficient in controlling speaker motion than standard low-impedance amplifiers which means slightly poorer sound quality, but have the huge advantage of being able to connect to multiples of speakers in a chain without any loading issues.
Low Impedance Amplifiers: The standard for hi-fi and power amplifiers. Modern amplifiers typically have low output impedance of 4 to 8 Ohm, offering better control over the speakers and improved sound quality. They are designed to handle a wide range of speaker types, providing greater flexibility in system design.
Practical implications of impedance in audio systems
Understanding impedance is not just a technical exercise; it has practical implications for anyone setting up or upgrading an audio system. Here are some key considerations:
Equipment Compatibility: Always check the impedance ratings of both your speakers and amplifier before connecting them. Ensure that the amplifier can handle the total impedance of the speaker setup to avoid damage and achieve optimal performance. Even with protection circuitry, an amplifier is a delicate piece of equipment and a large mismatch, a sub 4 Ohm load, or even no load, can destroy the output stage quickly, so make sure you know what you are doing.
Sound Quality: Proper impedance matching results in better sound quality. Misaligned impedance can lead to distortion, reduced clarity, and poor frequency response. Matching impedance ensures that the amplifier can control the speakers effectively, delivering clear and balanced audio. You should look at a speaker's impedance rating and its efficiency rating to determine that it’s a good match for your amp.
Safety and Longevity: Overloading an amplifier with a low impedance load can cause it to overheat and fail. Conversely, running a standard amplifier with a high impedance load can strain the speakers and reduce their lifespan. Proper impedance matching helps protect your equipment and extend its longevity.
Exclusions: Impedance is only an issue on passive audio equipment. Active speakers, active subwoofers, active studio monitors etc all have built-in amplification which has been matched carefully to the speaker driver requirements. With active equipment, you are only ever using line-level connections.
The other previously mentioned exception is the 100-volt line format used for commercial audio. These high voltage/high impedance systems get around the loading issues associated with multiple speaker connections by the use of step-up/step-down power transformers on the amp and on the speakers. This removes impedance calculations from the equation, allowing you to wire many speakers in a daisy chain (parallel) back to the amplifier without issue. The only downside is the output has a slightly lower sound quality than a standard system.
Final Thoughts
Impedance is a fundamental concept in audio engineering that plays a critical role in the performance and compatibility of speakers and amplifiers. Whether you're a casual listener or an audio enthusiast, understanding impedance can help you optimise your audio system, ensuring that it delivers the best possible sound quality while protecting your equipment from damage.
By considering the types of impedance, the effects of wiring configurations, and the implications of high and low impedance, you can make informed decisions when setting up or upgrading your audio system. Proper impedance matching between speakers and amplifiers is essential for achieving efficient power transfer, clear and balanced audio, and the longevity of your equipment.