What Is Impedance in Audio

Questions about the impedance in audio of the speakers and its influence on the operation of hi-fi amps and home-cinema are very numerous every week. Here are some practical ideas and tips to better understand the implementation of your audio system.

What is impedance?

Above all, remember that impedance is the resistance of an electric circuit to the alternating current that passes through it. This resistance is expressed in Ohms. The electrical circuit of a speaker and its speaker thus has impedance to the audio signal from the amplifier. When this resistance/impedance is too low, it is mechanically requested to the amplifier more current, which has the effect of soliciting more power of the latter. If the power supply is running at excessive speed, somehow gets out of hand, malfunctions can occur: increased distortion mainly and, at the extreme, transmission to the speakers of DC signals, with the effect of damage the speaker coils and their membranes. The amplifier may also be damaged. Let’s be clear, this only happens at very high volume, playing with the limits of the amplifier.

The nominal impedance of an enclosure is that measured at 1 kHz unless otherwise indicated. The low impedance is the lowest value of the impedance curve, measured from low to high, from 20 Hz to 20 kHz. Some manufacturers indicate this low impedance (Focal systematically), even specify that their speaker has an impedance of 4-8 Ohms, a low impedance of 4 Ohms and a nominal impedance of 8 Ohms.

Quick Impedance Video Overview

What is nominal impedance and low impedance?

The nominal impedance of an enclosure is that measured at 1 kHz unless otherwise indicated. The low impedance is the lowest value of the impedance curve, measured from low to high, from 20 Hz to 20 kHz. Some manufacturers indicate this low impedance (Focal systematically), even specify that their speaker has an impedance of 4-8 Ohms, a low impedance of 4 Ohms and a nominal impedance of 8 Ohms.

Why does impedance vary?

The impedance of a loudspeaker depends both on the mechanical properties of its membrane, its electronic filter, and its load. Indeed, the volume of air in which are plunged the speakers of bass/medium – it is called the load – influences the impedance curve of the speaker. Depending on whether the volume of air charging the loudspeaker is tuned (open to the outside) or not, the impedance is more or less disturbed. A bass-reflex charge thus causes an impedance peak at the resonance frequency of the vent, the excursion of the membrane being mechanically limited; it opposes a strong resistance to the amplifier. At the edge of this peak – at the frequencies immediately below and above – it is not uncommon to note a depression of the impedance,

In addition, the tweeter of a speaker can also be responsible for low impedance in the low register, precisely if it is not attenuated with stiff filtering allowing only the high frequencies. This transducer is not made to produce bass; its impedance is naturally very low under 1 kHz. To protect the power supply of the amplifier, 2nd, 3rd or 4th order filtering is used, with attenuation slopes of 12 dB, 18 dB or 24 dB per octave. A filtered tweeter with a slope of 6 dB / octave may be a problem for some amplifiers with weak power supply.

How to recognize an amplifier with a solid power supply?

An amplifier announced as compatible with speakers of 4 ohms or 2 ohms is necessarily equipped with very good power. A good index is given by comparing the advertised power for a load of 8 ohms and 4 ohms, from 20 Hz to 20 kHz. If the power announced in 4 Ohms is twice that announced in 8 Ohms, the power is really very solid. If it only increases by 30%, the diet can be considered average. Note that the switching power supplies of class D amplifiers are often adapted to difficult speakers (so very low impedance).

Is an 8 Ohm speaker still easy to power?

No, because its low impedance can be 4 times lower. Take the example of the Focal Aria 948, whose impedance at 1 kHz (nominal impedance) is 8 ohms and has a low impedance of 2.5 ohms. This speaker requires, if it has listened to a high level, a quality amplifier. An “8 Ohm” loudspeaker can have a very rough impedance curve, with multiple “impedance gaps” and be difficult to power. On the contrary, an enclosure of 4 Ohms, if its impedance fluctuates only very little, will be paradoxically easier to feed.

Are home theater amps suitable for low impedance speakers?

Yes, and more generally than hi-fi models … when a subwoofer is used and autocalibration has been used to adjust the setup. Indeed, the autocalibration determines the low cutoff frequency of the main speakers, central and surround, in order to entrust the restitution of the low frequencies to the box. As a result, the speakers rarely play at 100 Hz, which prevents the home theater amplifier from being used in the most demanding frequency band in terms of power and current. Note: Home theater amplifiers are all equipped with a protection circuit that cuts off the amplifier in the event of a malfunction of the power supply. If this happens often in your installation, it is that the impedance of one of your speaker pairs is too weak. Playing on the frequency of connection with the box can solve the problem (it must be increased). In the absence of a subwoofer, increasing the same frequency is possible on the center and surround speakers without affecting the overall balance, the main speakers providing 90% of the low frequencies of the sound message. If on the other hand, it is the main speakers that pose a problem of impedance and that you do not have caisson, it will be necessary to consider to replace the amplifier by a model more “muscular”. increasing this same frequency is possible on the center and surround speakers without prejudice to the overall balance, the main speakers providing 90% of the low frequencies of the sound message. If on the other hand, it is the main speakers that pose a problem of impedance and that you do not have caisson, it will be necessary to consider to replace the amplifier by a model more “muscular”. increasing this same frequency is possible on the center and surround speakers without prejudice to the overall balance, the main speakers providing 90% of the low frequencies of the sound message. If on the other hand, it is the main speakers that pose a problem of impedance and that you do not have caisson, it will be necessary to consider to replace the amplifier by a model more “muscular”.

What is the impedance switch on the back of some hi-fi amplifiers?

Some stereo hi-fi amplifiers have an impedance selector, with High and Low settings. This is the case when two pairs of terminal blocks (A and B) are present and the amplifier allows their simultaneous operation. Indeed, the two pairs of speakers are connected in parallel, which has the effect of dividing their nominal impedance by two. For example, two pairs of 6-ohm speakers do not have an overall impedance of 12 ohms, but 3 ohms, which can be problematic for high-volume power supply. Also, the impedance selector allows, when set to Low, electronically limit the amount of current delivered by the power supply. Depending on the model, feed efficiency may drop by 50%.

Can We Still Enjoy Dynamic Listening with Two Pairs of Low Impedance Speakers?

Yes, just leave the impedance selector on High. Obviously, we must be careful not to push the volume to an unreasonable level so as not to disturb the power supply too much. However, the distortion is audible before any incident.

Can You Connect 4 Ohm Speakers to an Amplifier for 8 Ohm Speakers?

Yes, it is perfectly possible and the speakers will produce music quite normally. In practice, the amplifier does not detect the impedance of the speaker: it works, a point that’s all. If the impedance of the speakers is 4 Ohms at 1 kHz, it may be higher in the frequency band between 40 and 100 Hz, for example, where the amplifier is most stressed. In the above case, the low impedance is equal to that of most “8 Ohm” speakers.

Why Do Some Home Theater Amplifiers Not Support The Same Impedance for Each Speaker?

It is not uncommon for some input or mid-range amplifiers to support different impedances on the front (left and right) and center and surround channels. When low impedance speakers are connected to the left and right front channels, the manufacturer requires the connection of higher impedance speakers for the auxiliary channels. Indeed, the power supply will be very solicited by the left and right voices – which will reproduce all the low frequencies – and it would not be that the central and surround speakers of low impedance also (4 Ohms for example) “shoot “To excess on the diet. The result would be a setting in the protection of the amp and the momentary stop of the cinema session.

I Read Everything … But It’s Really Too Complicated

Let’s summarize: if you listen to your music and movies at a moderate level – you do not interfere with children who sleep or your neighbors – do not worry about this notion of impedance. Just get a good speaker and continue enjoying your music.