We audiophiles just need to have the best sounding audio system. Custom making one appeals to us, but if we don’t know what terms such as 4 ohm or 8 ohm that are mentioned on the speakers mean, then surely there will be a problem. Therefore, in the interest of all those who aspire to listen to the best sound, in this Techspirited article, we shall be providing a quick basic sound engineering lesson on the meaning and relevance of speaker impedance.
Keep in Mind
Speaker impedance is a variable quantity that changes in accordance to the signal frequencies that are fed into it. Since music normally has a lot of frequencies that are simultaneously present, manufacturers end up printing what is known as the ‘nominal’ impedance on the speakers. It usually represents the lowest values to which the speaker’s impedance will dip down to.
In today’s world, it’s all about inter-compatibility. And I am not just referring to the one that must exist between you and your other half. I am also talking about the one that needs to exist between your amplifiers and your speakers! It has a name too―impedance matching.
There exists a plethora of options for speakers and amplifiers in the market at present. But not all are compatible with each other. Hence, if one desires to build a customized sound/music system, understanding the speaker impedance rating and how it relates to the amplifier configuration is a must. But don’t worry. Even though it may sound intimidating, the concept of speaker impedance is actually pretty easy to grasp. So without further ado, let’s go ahead and find out what it is.
The term impedance, represents that property of an electric circuit which results in an opposition to the current flow through it when voltage is applied. In a DC circuit, impedance is the same as resistance, which is the opposition to current flow, resulting from the basic shape, size, and composition of the conductor.
In an AC circuit, due to the constantly varying nature of the signal, two more factors responsible for impeding the current, namely, inductance and capacitance, get added. Their combined effect along with the resistance of that circuit is known as the effective impedance of the circuit.
To better understand the concept of impedance, let’s take the analogy of a dam. A dam is build across a river or a stream to store water. When all its gates are closed, a dam acts as a barrier that withstands the pressure of the water and opposes the flow of the river/stream. The dam can be thought of as the impedance, while the pressure of water upon it as the voltage in a circuit. The flow of water in the river can be imagined to be the current in that circuit.
When all of its gates are closed, the dam represents a very high impedance, which acts against the pressure from the flowing river and almost completely stops the flow of water. Thus, a very high impedance in a circuit will allow very little or no current to flow through it.
However, when one or more gates of the dam are opened, water begins to flow throw them. This state represents a lower impedance applied in the circuit, which allows a certain amount of current to flow through it.
Thus, clearly, an inverse relationship exists between the current and the impedance. Higher the effective impedance of a circuit, lower will be the amount of current flowing through it and vice versa. This relationship was understood long back by George Simon Ohm, who was able to described it mathematically. It came to be known as the Ohm’s Law, and is represented as:
Impedance = Voltage ÷ Current
The signal that drives a speaker is the electric equivalent of the actual audio signal produced by the transducer (microphone). It varies with time, and therefore, displays properties similar to those of an AC signal. Hence, the opposition to the flow of a signal introduced by a speaker is measured as the speaker’s impedance. A speaker having low impedance will let more signal flow through it easily, while another one having higher impedance will restrict the signal more.
From the Ohm’s law, we get the speaker impedance formula as below.
Speaker impedance = (Output voltage of Amplifier) ÷ (Current flowing through it)
The unit of impedance is known as ‘ohm’.
A speaker having 4 ohm impedance will offer much lower opposition to the audio signal coming from the amplifier as compared to an 8 ohm speaker.
Take the example of an 8 ohm speaker connected to an amplifier producing 16 volts(V) AC. Using the Ohm’s Law, we can determine that the current flowing through it would be (16 ÷ 8) = 2 amperes(A). If the amplifier output is increased to 24 volts, then the current flowing through it will be (24 ÷ 8) = 3 A.
Now if, a 4 ohm speaker is connected to the 16 V amplifier, the current flowing through the circuit would be (16 ÷ 4) = 4 A. If the amplifier output is increased to 24 V, then the current flowing through it would be (24 ÷ 4) = 6 A.
Clearly, thus, a low impedance speaker will allow more signal to pass through it as compared to a high impedance speaker. So the question arises, in a high impedance vs. low impedance speaker comparison, for example, 8 ohm vs. 4 ohm speaker, which one would be better? Well, really, the answer depends on your amplifier’s capabilities.
Most audio amplifiers have an output impedance of 8 ohm, so connecting a 4 ohm speaker to them might damage the amplifiers. If your amplifier is capable of driving a 4 ohm speaker, it might result in greater sound pressure at the output which may result in better sound quality as compared to an 8 ohm speaker connected to it. However a pair of 4 ohm speakers require more power as well as thicker wires in comparison. They also produce more heat. This might not make them an ideal choice for everyday applications.
Speakers are driven by audio amplifiers, which raise the level of the original audio signal enough to overcome the speaker’s impedance. However, any speaker cannot just randomly be connected to any amplifier. For optimum audio output, certain voltage and output parameters need to match between the amplifier and the speaker. This can be explained by the maximum power transfer theorem.
The maximum power transfer theorem in electricity states that maximum transfer of power from a source (amplifier) to a load (speaker) takes place when the output impedance of the source is equal to the input impedance of the load. The following are the consequences of impedance mismatch between the amplifier and the speaker.
a. If the speaker impedance is too high, then the amplifier won’t be able to drive it fully, resulting in low volume.
b. If the speaker’s impedance is lower than the output impedance of the amplifier, then it will allow excess current to flow, and the amplifier may be damaged.
c. When using tube amplifiers, if speakers having impedance value that is too high is connected, then the output tubes or the output transformer within the amplifier may get damaged.
d. In solid state amplifiers, if the impedance value of the speaker is too low, then the amplifier tends to overheat.
e. Connecting too many speakers to a single amplifier too may cause a mismatch of impedance that can result in the amplifier getting damaged.
Thus, it is paramount that the amplifier’s output impedance be matched, i.e., made equal to the impedance of the speakers, for maximum power (volume) to be transferred and the best audio output possible to be attained.
In the remaining part of this Techspirited article, we shall look at some examples showing the different speaker configurations that can be used to connect to the audio amplifiers, and how you can match the output impedance of the amplifier to that of the speaker.
Each of the following configurations is represented via a speaker impedance wiring diagram. For their analysis, we will need the following two concepts of electricity.
1. Series Combination of Impedance.
When impedances in a circuit are connected in series, then their equivalent resistance is the sum of all of them.
Zeq = Z1 + Z2 +Z3 +………+ Zn
2. Parallel Combination of Impedance.
When impedances in a circuit are connected in parallel, then their equivalent resistance is the sum of their inverse.
| Zeq | = | 1 | + | 1 | + | 1 | + | . | . | . | + | 1 |
| z1 | z2 | z3 | zn |
You may even use the following calculator to calculate speaker impedance.
Example 1
As can be seen in the above example, we have a 100W amplifier having 8 ohm output impedance and an 8 ohm speaker connected to it for the sake of impedance matching. As there is only one speaker in this configuration, the speaker will experience the entire 100W from the amplifier. Therefore, in this case, it is recommended that an 8 ohm speaker rated at least 100W be connected.
Example 2
In this example, we have a 100W amplifier having 8 ohm output, matched with two 4 ohm speakers. As can be seen in the diagram, the two 4 ohm speakers are in series, and therefore from the above series formula for impedances, we find that the total impedance in the above circuit will be Zeq= (4 + 4) = 8 ohm. Thus, impedance matching is achieved.
The power output from the amplifier will be equally split among the two speakers. Thus each speaker experiences at least 50W of power from the amplifier. Hence, in such a case, it is recommended that two speakers having 50W or 100W each should be used.
Example 3
In this example, two 16 ohm speakers are connected in a parallel configuration to the 100W amplifier having 8 ohm output impedance. From the above formula for parallel combination of impedances, we get Zeq= [(1÷16 )+ (1÷16)] = 8 ohm. Thus impedance matching is achieved.
The power in this case too will be divided equally among the two speakers. Hence, here, it is recommended that two 16 ohm speakers each rated at 50W or 100W should be used.
Example 4
In this example, four speakers having 8 ohm impedance each, are connected in a series and parallel plus combination to a 100W amplifier with 8 ohm output impedance.
Notice that the first two and the last two speakers are connected in parallel with each other. Hence, referring to Example 3, we can work out that both parallel combinations will have an effective impedance of 4 ohm each.
Also, notice that these two parallel combinations are in series with each other. Therefore, referring to the analysis used in Example 2, we get the effective impedance of the entire circuit to be as 8 ohm. Thus, impedance matching is achieved.
Since four speakers are used, power will be equally divided among them, i.e., each speaker will receive at least 25W of power from the amplifier. Therefore, it is recommended that, in this case, 4 speakers rated at 25W or 50W be used.
In conclusion, a speaker’s impedance is an important parameter that dictates the amount of current that will pass through it. It is essential that you match your speakers to your amplifiers or receivers to avoid overworking or damaging the amplifier and for getting the best audio output from it.