Impedance Ratings: What do they mean?

The previous article “RMS vs PMPO Power Rating: Which one is realistic?” shed some light on amplifier power ratings and the truth behind them. Going with the same flow of enabling the audiophile reader with wisely putting together a great sounding system or buying a great pair of headphones, here is an article dedicated to the “Impedance Ratings” specified on almost all high fidelity audio gear. Let us explore what these ratings mean and what they signify in the real world and most importantly, get our understanding about them right.

Resistance & Impedance

It is a sine qua non to understand impedance from a technical standpoint, for the later portions of this article to make sense. The not-so-tech-savvy audiophile need not panic about reading the following explanation in this article. There has absolutely been no intention of making this article an electrical engineering class hand-out.

Going by the definition, Resistance/Impedance is the opposition offered by a conductor to the flow of electrical current through it. This is better explained by a simple analogy. Plug a garden hose into the tap and turn the tap on until water starts flowing out from the other end of the hose, with maximum force. (It is assumed that there is ‘no’ nozzle attached to the free end of the hose.) This flow of water is similar to the electrical current (denoted by the letter ‘I’) in the conductor. The pressure head of the water in the tank is similar to the voltage (denoted by the letter ‘V’) in the electrical circuit. It is the pressure head of water in the tank that caused the flow. Similarly, it is the voltage which drives current in a conductor. Now plug a finger tightly into the free end of the hose, so that the water flow completely stops. Now plugging a finger into the hose has resisted/impeded the flow of water. Now slightly open the free end of the hose so that very little water flows out. Now the resistance/impedance to the flow of water has decreased and hence some water started flowing out. Now open the free end of the hose even more and observe the flow increase. The principle is very much the same in electrical circuits. Notice that, though the flow of water was completely stopped at one point, the pressure head was still there. Similarly in electrical circuits, even though the voltage is maintained constant, if the impedance varies, the electrical current also varies.

This phenomenon was studied by a German physicist and mathematician, Georg Simon Ohm, who chalked out a mathematical relationship among Voltage, Current and Resistance/Impedance, which is nothing but the infamous “V = IR” (‘R’ denotes the Resistance), the “Ohm’s Law”. Resistance/Impedance is measured in a unit called “Ohm” in honour of Georg Simon Ohm.

Now that we got our basics right (hopefully), it’s time to know how ‘Resistance’ differs from ‘Impedance’ or if they are both the same. No. They are indeed not the same. ‘Resistance’(R) is defined with respect to Direct Current (DC) and DC Voltage. Whereas, ‘Impedance’ (denoted by the letter ‘Z’) is defined with respect to Alternating Current (AC) and AC voltage in circuits, where the current and voltage have constantly varying amplitudes and changing directions (positive to negative and again back to positive and so on). In the latter scenario, the ‘inductance’ and the ‘capacitance’ also play a role in impeding the flow of current through the conductor in addition to the conductor’s resistance. Hence, ‘Impedance’ is the net opposition offered to electrical current due to all the three parameters. The relationship of inductance and capacitance to impedance is however beyond the scope of this article.

Due to the above mentioned relationship, impedance varies with frequency. Since music/audio is all about random yet sweet-sounding, frequency variations, loudspeaker impedances go through serious variations which are often nightmarish to amplifiers and amplifier designers.

Input & Output Impedances

A device which acts as a voltage source has something called the ‘Output Impedance’. It is the net/effective impedance of the device’s internal circuitry as measured from the outside. It is the impedance measured across the device’s output terminals. On the other hand, a device which receives a voltage via its input terminals has an ‘Input Impedance’. It is the impedance seen by a device which is connected to the input terminals and provides voltage through them. Input impedance is also called the ‘Load Impedance’ and the output impedance is also called the ‘Source Impedance’.

For maximum power transfer between the source and the destination, the output impedance of the source and the input impedance of the destination must be equal. When they are equal, the system is said to have matched/balanced impedances. If the source and the destination are separated by a large distance, then the impedance of the cable connecting them, should also be the same as the load and source impedance values.

During long cable runs, the capacitance of the cable contributes to the total impedance. The output impedance of the source and the capacitance of the cable would together form a first-order low-pass filter, producing a 6dB/octave attenuation above a certain frequency. So, it would always be great to choose cables with the least possible capacitances and to keep cable runs as short as practically possible.

HiFi Audio Impedance Ratings

Most loudspeakers in typical HiFi setups have nominal impedances quoted as 4, 8, 15 or 16 ohms. Noticed the word ‘nominal’? That means the impedance values are not fixed or constant throughout the audio spectrum. These are values measured at a particular frequency with a constant frequency signal source. Loudspeakers with higher impedances mentioned above are typically for use with valve amplifiers. Loudspeakers are complex systems and the ones coupled with passive crossovers are a challenge for the amplifiers.

Care should be taken while matching amplifiers to loudspeakers that the output impedance of the amplifier and the input impedance of the loudspeakers match as accurately as possible, to ensure maximum power gets transferred. If a speaker with impedance lower than that of the amplifier is connected to the amplifier, it might load the amplifier down by drawing more current. This will eventually lead to over-heating and a possible failure. For amplifiers with a protection circuitry in place, the system might get automatically shut down in such scenarios. But it has been widely observed that such protection circuits add to a lot of distortion and that there is a significant reduction in the musical performance.

On the other hand, where the loudspeaker impedance is way higher than the amplifier output impedance, maximum rated power would not get transferred to the loudspeaker. There would not indeed be any damage though. There would be a reduction in the performance.

Great caution must be taken while using valve amplifiers. Never should they be run without a loudspeaker connected, since they can get damaged readily. This does not apply to solid state amplifiers though.

Personal Audio Impedance Ratings

Unlike consumer speakers, whose impedance ratings range within 4 to 16 Ohms, the headphones have a very wide impedance range. It is typically in the range of 16 Ohms to 600 Ohms. As a result, it is pretty daunting and challenging to pick a pair of headphones with just the right impedance value which can be a perfect fit for the audio sources we have. There are a lot of compatibility issues here.

When dealing with headphones, there is yet another equally important parameter to be considered i.e. the ‘Sensitivity’ (efficiency). For headphones, sensitivity is usually expressed in units of ‘dB/mW’. What to decipher from it? In simple terms, ‘sensitivity’ specifies how loud the headphones will play for a given power or voltage level.

Let’s list down a few guidelines for making the task of choosing the ‘right’ pair of headphones a breeze:

• If the source is a battery powered device like a portable music player or a phone or a laptop, it would be great to go for lower impedance headphones with impedance in the range of 16 – 32 Ohms and with a sensitivity of at least 100 dB/mW. The lower the impedance, the better they match up with battery powered devices.
• Most of you might be aware of the European maximum volume regulations. If the source is a phone complying with such regulations, it is super important to go for headphones with 16 Ohms impedance with a sensitivity of 100 dB/mW.
• If the source is being used, coupled with a dedicated headphone amplifier or a DAC, there is no better place than the amplifier/DAC manufacturer’s device specifications themselves, to go look for the ratings of matching headphones.

It is always wise to go for headphones with higher sensitivity ratings; at least 100 dB/mW. Make sure the headphone impedance is not lower than the source impedance since it can load the source down and cause a lot of distortion. If the source at hand has very low output impedance and enough output power, it would be good not to go for headphone amplifiers, since they might introduce other issues making things worse.

Wrap up

The task of matching impedances might look dirty and daunting. But the effort really pays off in the end in terms of the boost in audio quality and performance. One of the key advantages, an active speaker system offers, in this regard is worth mentioning. The end consumer is free from all the hassles of matching loudspeakers with amplifiers. All the dirty work will have already been handled and properly tested by an expert engineer during the manufacturing process.

This post is prepared by Vinay.