Nominal & Load Impedance

October 26, 2013 in AV Design Tips, The Basics by Sam Davisson

Why we love speakers

AKA Impedance Part II

Ok, before I jump into this I’ll apologize to anyone who is offended by the cartoon. It’s a bit of an inside joke with a good friend of mine and I couldn’t resist the urge

Nominal impedance in electrical and audio engineering refers to the approximate designed impedance of an electrical circuit or device.

Actual impedance varies with frequency changes. It is possible for that variance to be quite large. Despite that, it is normal to speak of nominal impedance as if it were a constant resistance with no reactive components. Which is often far from the truth. Nominal impedance is referring to a specific point on the frequency response of the circuit being considered.

Loudspeaker impedance’s are kept relatively low so that the required audio power can be transmitted without the use of high voltages. The most common nominal impedance for loudspeakers is 8 Ω. Also used are 4 Ω and 16 Ω.

The once common 16 Ω is now mostly reserved for high frequency compression drivers since the high frequency end of the audio spectrum does not usually require much power to reproduce the sound.

Diagram showing the variation in impedance of a typical mid-range loudspeaker. Nominal impedance is determined at the lowest point after resonance.

Diagram showing the variation in impedance of a typical mid-range loudspeaker. Nominal impedance is determined at the lowest point after resonance.

The impedance of a loudspeaker is not constant across all frequencies. In a typical loudspeaker the impedance will rise with increasing frequency from its dc value (see diagram) until it reaches a point of mechanical resonance. Following resonance, the impedance falls to a minimum and then begins to rise again. Speakers are usually designed to operate at frequencies above their resonance, and for this reason it is the usual practice to define nominal impedance at this minimum.

The ratio of the peak resonant frequency to the nominal impedance can be as much as 4:1. It is, however, possible for the low frequency impedance to actually be lower than the nominal impedance. A given audio amplifier may not be capable of driving this low frequency impedance even though it is capable of driving the nominal impedance, a problem that can be solved with the use of crossover filters.

Note that is also may be possible to solve the problem above by underrating the amplifier. This is not a solution I would recommend but it is a possibility

Speaker Basics

Getting very basic here, our use of the word SPEAKER is the shortened form of the word LOUDSPEAKER and it refers to a device that converts electrical signals into sound waves that we can hear. A loudspeaker has several parts:

  • Cabinet – It should be noted that the cabinet performs a much greater function than to simply house the components that make up the loudspeaker. The discussion of which is well beyond the scope of this post. But if you truly want to understand loudspeakers and how they function to produce audiophile quality sound it is a subject worth delving into.
  • Driver (aka speaker) – Converts electrical energy into sound waves (more on this to come)
  • Crossover Network – As we will discuss, speakers are designed to reproduce specific ranges of frequencies. The crossover network filters the frequencies sending only the frequencies that can be used to a specific driver. Passive crossovers will typically be installed inside of the cabinet. Speakers meant for high end audio use will either have switches allowing for the passive network to be bypassed or have the passive crossover removed altogether. In these instances an external crossover, typically built within the systems digital signal processor (DSP), will be used.

Speakers (drivers) are constructed much in the same manner as a musical instrument in the fine tolerances and attention to detail make all the difference to the sound quality. Large speakers are suited for low frequencies, small speakers – high frequencies. Each speaker can only function efficiently and with linearity within 3 octaves of it’s design. Theoretically a single speaker would have to change diameter from 1" – 24′ to maintain a similar level and dispersion over the complete frequency spectrum.

Speaker cross section

The majority of drivers consist of paper or plastic molded into a cone shape, loosely suspended in a frame so as to easily move back and forth to vibrate the air. Glued to the back of the cone is a coil of wire aka voice coil suspended within a strong magnetic field. Passing electricity through the wire causes a magnetic field around the wire which attracts or repels which in turn causes the cone to move back and forth. The larger the magnet and voice coil the greater the power and efficiency of the driver. Because of the tight tolerances no two speakers are truly identical.

Load Impedance

For the load impedance discussion I’ll use the Ω symbol for resistance. Z is the symbol for impedance. As we have discussed impedance could be stated as resistance that varies with frequency.

Load impedance is the load speakers represent to the amplifier. The maximum power rating of an amp is always in reference to the load impedance. For example if the amplifier is rated for 100W (watts) of power into a 4Ω load it is only capable of producing 100 watts with a 4Ω load. If you connect an 8Ω load, the amplifier will only be capable of producing 50W of power. The higher the load impedance the cooler and more reliable the amplifier will be, and the lower the internal distortion. But it will produce less power.

Conversely, if you present a 2Ω load the amplifier will attempt to deliver 200W of power which could quite possibly damage the amplifier. If you don’t see smoke and flames the amplifier will run much hotter and less reliably.

Calculating Speaker Impedance with Multiple Speakers

Parallel, Series and Series Parallel Speakers

Calculating speaker impedance follows the same laws of electronics as for purely resistive circuits. For speakers in parallel divide the speakers nominal impedance by the number of speakers. Therefore, 2 8Ω speakers have a nominal impedance of 4Ω and 4 8Ω speakers have a nominal impedance of 2Ω.

For speakers connected in series, add the nominal impedance’s of the speakers. Therefore 2 8Ω speakers nominal impedance is 16Ω.

You should only connect speakers in a series-parallel configuration with an even number of speakers. First calculate the nominal impedance for the series speakers. Then calculate the 2 series pairs in parallel. In other words, 2 8Ω speakers in series is 16Ω. The 2 sets of series speakers in parallel would then represent a nominal impedance of 8Ω

Connecting speakers in series and series parallel may be essential in certain applications where many speakers are required to be connected to one amplifier. However connecting speakers in series causes the distortion of each speaker to be reflected into the others. But connecting speakers in series does not effect the reliability of the speakers or amplifier.

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