At lunch yesterday a friend asked a simple question I could not answer: Why are speakers usually designed and specified at some common impedance such as 2. 4, 8, or 16 ohms, and where did the standard come from?
Embarrassed, I searched for this without success. Explanations of speaker impedance quickly devolved into water pipe analogies and "What is an ohm?"
I understand impedance and I know that speaker impedance is not constant with frequency and that the nominal value is just an approximation. What I
can't seem to find is where the common nominal values (standards?) come
from. Some committee? Commercial practicality? Implied from required
speaker dynamics?
Any light you can shed would be appreciated.
Embarrassed, I searched for this without success. Explanations of speaker impedance quickly devolved into water pipe analogies and "What is an ohm?"
I understand impedance and I know that speaker impedance is not constant with frequency and that the nominal value is just an approximation. What I
can't seem to find is where the common nominal values (standards?) come
from. Some committee? Commercial practicality? Implied from required
speaker dynamics?
Any light you can shed would be appreciated.
Practicality is certainly one big factor.
The lower the impedance, the less the driving voltage source needed to generate a given power. A 1k speaker for example would not draw much current (and consequently not make much noise) when fed directly from (say) an amplifier running on 9 volts. Make that speaker 3 ohms and its a different story.
Where the historical preferred values came from I have no idea, unlike resistors which I think are logarithmically based to account for tolerance and overlap in the range.
The lower the impedance, the less the driving voltage source needed to generate a given power. A 1k speaker for example would not draw much current (and consequently not make much noise) when fed directly from (say) an amplifier running on 9 volts. Make that speaker 3 ohms and its a different story.
Where the historical preferred values came from I have no idea, unlike resistors which I think are logarithmically based to account for tolerance and overlap in the range.
Impedance in Ohm is the AC equivalent of Resistance with DC. An amplifier needs a certain minimum load to function correctly, otherwise it is overloaded, heats up and dies.
So to match speakers and amps you see the Ohm's specified.
Never use speakers with a lower ohm value, then the amp is specified for.
A 8 ohm speaker connected to a 100W 4Ohm amp, will get about half the power as it's impedance (resistance) is twice as high.
2x 8 ohm speakers in parallel will give 4 ohm, in series 16 ohm etc.
"What is the difference between resistance and impedance?
"Note that impedance and reactance are both given in units of 'ohms' just like resistance. Capacitance is measured in Farads and inductance in Henries, and these relate to impedance, but they are not measures of impedance. As I said, the impedance of a capacitor is inversely proportional to its capacitance and the impedance of an inductor directly proportional to its inductance.
This may sound a little abstract. Impedance really is an abstraction of things that are far more complicated (things like time constants and rise times) that electrical engineers have to constantly consider. The idea of 'impedance' allows for many of these things to be wrapped up into one subject so that they are easier to communicate.
The short answer is -- impedance includes reactance, and reactance includes effects which vary with frequency due to inductance and capacitance. "
So to match speakers and amps you see the Ohm's specified.
Never use speakers with a lower ohm value, then the amp is specified for.
A 8 ohm speaker connected to a 100W 4Ohm amp, will get about half the power as it's impedance (resistance) is twice as high.
2x 8 ohm speakers in parallel will give 4 ohm, in series 16 ohm etc.
"What is the difference between resistance and impedance?
"Note that impedance and reactance are both given in units of 'ohms' just like resistance. Capacitance is measured in Farads and inductance in Henries, and these relate to impedance, but they are not measures of impedance. As I said, the impedance of a capacitor is inversely proportional to its capacitance and the impedance of an inductor directly proportional to its inductance.
This may sound a little abstract. Impedance really is an abstraction of things that are far more complicated (things like time constants and rise times) that electrical engineers have to constantly consider. The idea of 'impedance' allows for many of these things to be wrapped up into one subject so that they are easier to communicate.
The short answer is -- impedance includes reactance, and reactance includes effects which vary with frequency due to inductance and capacitance. "
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I guess the engineering community used 1 / 2 / 4 / 8 / 16 / 32 / etc. because it makes for a simple log-2 series.
It wouldn't be very practical to use a linear series, because a 13 Ohm speaker really isn't very different from a 14 Ohm speaker.
Antother (related) question: what is the "official" definition of an 4/8/16 Ohm speaker? What is the minimum impedance allowed? Is this the minimum from DC to daylight, or only at frequencies higher than the bass impedance peak(s)?
I am asking because I keep reading different "definitions", but I can't tell which is right.
It wouldn't be very practical to use a linear series, because a 13 Ohm speaker really isn't very different from a 14 Ohm speaker.
Antother (related) question: what is the "official" definition of an 4/8/16 Ohm speaker? What is the minimum impedance allowed? Is this the minimum from DC to daylight, or only at frequencies higher than the bass impedance peak(s)?
I am asking because I keep reading different "definitions", but I can't tell which is right.
Back when capacitors were still called condensers, the majority of the loudspeakers in the UK were labelled either 3 ohm or 15 ohm.
An 8 ohm rated speaker will have a measured impedance of 8 ohm at a frequency of 400Hz.
i.e. 400Hz is the reference frequency at which speaker impedances are measured.
Really? Can you back this up with some reference? Also, most speakers will not measure at 2/4/8/16/etc. Ohm at 400 Hz, so how would these be classified in terms of "nominal impedance"?
Andrew Jones said that official definition for 4ohm speaker is that it shouldn't drop more than 20% from that value.
So, for a speaker to be considered a 4ohm, minimum impedance shouldn't go lower than 3.2 ohms from 20-20000Hz
So, for a speaker to be considered a 4ohm, minimum impedance shouldn't go lower than 3.2 ohms from 20-20000Hz
If the question is 'why such low impedances?' (and really, old tube amplifiers would have been much easier to make for driving high impedances!), the answer would be because it would take many turns of voice coil wire to reach into high impedances. More turns would make it harder to make a coil to fit into the narrow magnetic gap needed, and would increase the likelihood of one of the turns peeling loose and rubbing in there. A single or double layer coil makes things easier, and leaves you single or double digit impedance ranges.
I can't back up the 400Hz suggestion with a solid reference, but the statement "8 Ω is the standard Z Impedance for speakers, measured at 400Hz" is mentioned here:
Speakers: Impedance (Z)
I have also come across the following information:
"IEC 60268-5 provides the international standard definitions and requirements for loudspeakers and states that the nominal impedance of a loudspeaker is determined such that the minimum impedance is never less than 80% of the nominal impedance."
Measuring Speaker Impedance – Digilent Inc. Blog
I look forward to any additional/alternative information that members may have to offer. 🙂
Then there's this document:
https://www.isce.org.uk/wp-content/uploads/EngineeringNote-11.3-loudspeakers-impedance-and-power.pdf
Which states:
"The (IEC) standard goes on to say that the minimum impedance within the rated frequency range shall not be less than 0.8 of the rated impedance, and if it is lower than that outside the rated frequency range, including d.c., the manufacturer should say so. Since it usually islower at d.c., the d.c. resistance should be specified, but it often isn't. To determine the rated impedance, the manufacturer should measure the minimum impedance of a number of samples, find the average and set the rated impedance at 1.25 (= 1/0.8) times that average value. The value can't be predicted very accurately (without a sophisticated simulation program) because the electrical impedance depends to some extent on the mechanical and acoustic characteristics of the drive unit, or of all the drive units and crossover networks in a multi-driver loudspeaker system."
https://www.isce.org.uk/wp-content/uploads/EngineeringNote-11.3-loudspeakers-impedance-and-power.pdf
Which states:
"The (IEC) standard goes on to say that the minimum impedance within the rated frequency range shall not be less than 0.8 of the rated impedance, and if it is lower than that outside the rated frequency range, including d.c., the manufacturer should say so. Since it usually islower at d.c., the d.c. resistance should be specified, but it often isn't. To determine the rated impedance, the manufacturer should measure the minimum impedance of a number of samples, find the average and set the rated impedance at 1.25 (= 1/0.8) times that average value. The value can't be predicted very accurately (without a sophisticated simulation program) because the electrical impedance depends to some extent on the mechanical and acoustic characteristics of the drive unit, or of all the drive units and crossover networks in a multi-driver loudspeaker system."
Higher Z nice for tubes as OPT can have smaller turns ratio. Low Z for cars as power supply voltage is limited. That was especially important with inexpensive factory head units back in the day.
Look at any moving coil speaker impedance curve.
You´ll see it peaks at resonance, then lowers to a wide minimum, then it starts rising again.
Typical minimum is between 250Hz and 450Hz, depending on speaker.
The above definition "nominal impedance 20% above minimum" happens around 400Hz in many speakers, so it´s not a bad generic answer.
Tweeters? : same thing, but resonance is much higher, think a kHz or two (or higher) so the minimum considered is the one above resonant peak.
As to why 4/8/16 ohm "preferred" impedance?
I commercially make speakers from scratch,and can tell you that 8 ohm voice coils are easy to wind, and double/half that no big trouble.
Way higher than that, wire becomes uncomfortably thin; way lower, voice coil is too heavy and sound is muddy, efficiency also suffers.
So for practical reasons, 4/8/16 became most popular.
Pure Darwinism.
You´ll see it peaks at resonance, then lowers to a wide minimum, then it starts rising again.
Typical minimum is between 250Hz and 450Hz, depending on speaker.
The above definition "nominal impedance 20% above minimum" happens around 400Hz in many speakers, so it´s not a bad generic answer.
Tweeters? : same thing, but resonance is much higher, think a kHz or two (or higher) so the minimum considered is the one above resonant peak.
As to why 4/8/16 ohm "preferred" impedance?
I commercially make speakers from scratch,and can tell you that 8 ohm voice coils are easy to wind, and double/half that no big trouble.
Way higher than that, wire becomes uncomfortably thin; way lower, voice coil is too heavy and sound is muddy, efficiency also suffers.
So for practical reasons, 4/8/16 became most popular.
Pure Darwinism.
Yes , it´s a cheesy Marketing trick.
And a Cat and Mouse game.
Since minimum impedance is about 20% less than nominal, in practice an 8 ohm capable amp *should* be able to drive about 6 ohms resistive load,and in fact most (all?)n do.
So far so good, no big deal.
Enter Marketing Dept: as soon as they learnt that bthe 100W amplifier (8 ohm) puts out some 125W into 6 ohm test loas, they started showing that rating on brochures.
Not much later, basically 8 ohm speakers started being advertised as 6 ohm and tbhe circle was completed.
In my book, a 6 ohm rated speaker needs a 4 ohm capable amp, just to play it safe, in case that one drops extra 20% at some point.
Not surprisingly, some "exotics" claim 4 ohm but drop to scary 2 ohm at some frequencies. Oh well.
And a Cat and Mouse game.
Since minimum impedance is about 20% less than nominal, in practice an 8 ohm capable amp *should* be able to drive about 6 ohms resistive load,and in fact most (all?)n do.
So far so good, no big deal.
Enter Marketing Dept: as soon as they learnt that bthe 100W amplifier (8 ohm) puts out some 125W into 6 ohm test loas, they started showing that rating on brochures.
Not much later, basically 8 ohm speakers started being advertised as 6 ohm and tbhe circle was completed.
In my book, a 6 ohm rated speaker needs a 4 ohm capable amp, just to play it safe, in case that one drops extra 20% at some point.
Not surprisingly, some "exotics" claim 4 ohm but drop to scary 2 ohm at some frequencies. Oh well.
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