A Historical Perspective of the Loudspeaker Impedance Specification
Audio specifications fall into two broad categories: Those that are nice to know, from an enthusiast or intellectual standpoint and those that are genuinely helpful in assisting you to use, install or set up your equipment. Some specifications have a foot in each camp, so to speak, and serve both a practical and theoretical purpose.
Amplifier Power Output
Usually expressed in watts, this specification is probably the most often sought-out and compared number for amplifiers. Assuming the manufacturer has rated their equipment accurately and honestly, the wattage spec is useful in determining whether that particular piece of equipment is a good match for your speakers in terms of having sufficient power to drive it to SPL levels you find satisfactory in your listening room and also to decide whether your speakers and this amplifier are a proper match from a power output/power-handling perspective.
If you have an acoustically-dead listening space, 22 x 35 feet with 9-foot ceilings and inefficient tower speakers (85dB 1w/1m), you could quite reasonably assume that a 30 watt-per-channel integrated amplifier won’t give you lifelike SPLs at your listening position when playing Berlioz’ Symphonie Fantastique.
Likewise, two 500-watt monoblocks are going to be just a tad too much power for a set of 6 ½-inch audiophile 2-way speakers on stands in an 11 x 15-foot room, playing Time Out by the Dave Brubeck Quartet at moderate SPLs.
In both cases, the power specification provides tangible, usable information for the end user that helps that user make a better choice of equipment that will satisfy their needs.
Note: For more information on how much power you need, read: When to Upgrade From a Receiver to a Separate Amplifier and take our qualifying quiz.
However, power output also has “its foot in the other camp,” as it were: Sometimes the specification is interesting and fascinating but provides no actual benefit. When we look at spec sheets and compare product A to product B, the fact that A has 110 WPC and B has 125 WPC gives us no valuable information at all. It’s a rooting point, a rallying cry, a reason to cheer on our favorite brand for having “more watts for the same price” as them or minimally lower (but still inaudible) distortion.
There are many specifications that simply provide theoretical/abstract information, purely for the reader to get a sense of the equipment. But the specs themselves are not actionable, per se. Signal-to-noise ratio, for example. What are you going to do with that information? If A has an S/N of 91dB and B has an S/N of 84dB, that’s interesting. But you can’t change it, it doesn’t really affect anything about your setup, it has no actual bearing on equipment compatibility. S/N is strictly a “nice to know” specification.
At the other end of the usefulness spectrum are things like dimensions, weight, etc. If your shelf is only 15” deep and the receiver you’re looking at is 17” deep, well, you’ve got a problem, don’t you? If that bookshelf speaker weighs 25 pounds but the shelf is only rated for 20, you better know that.
What if you’re an installer and the in-wall speakers the client says he wants are 4” deep, but you’re installing them in a 3 ½” studded wall (plus the ½” thickness of the sheetrock)? Well, you’ve got 4” maximum to work with, and no room for error. If the wall or the speaker is off by even a 16th of an inch, boy, there are big problems. If a better speaker exists that’s only 3 ½” deep, that’d be a better choice. That depth specification can be all-important in real life, quite unlike a theoretical S/N spec.
Ok, so we’ve established that there are two kinds of specs: real, tangible, useful specs, theoretical “nice to know” specs and those that span both categories.
Speaker impedance is certainly a “spans both categories” kind of spec.
A speaker’s impedance is generally rated at 4 or 8 ohms, but as readers of this site are well aware, that is just a broad generalization of a speaker’s behavior. The impedance that a speaker presents to an amplifier varies widely depending on frequency. In a sealed speaker, the impedance generally spikes up to its maximum right at the frequency that’s known as “system resonance” (the point in the bass below which that speaker begins the 12dB/octave rolloff characteristic of sealed systems). See figure 1.
Figure 1—Sealed speaker impedance curve
In a ported speaker, there are generally two impedance peaks, corresponding to the system’s “tuning” frequency (the port’s tuning frequency) and the speaker’s woofer/box resonance. See figure 2.
Figure 2—Ported speaker impedance curve
In any case, you can see that a speaker’s impedance varies over a wide range of values. An “8-ohm” speaker may well have an impedance that ranges from a high of over 40 ohms at system resonance to a low of 4 or 5 ohms. The 8-ohm rating is purely subjective, an overall “average” of what the impedance curve looks like. Although there is a formal, scientific procedure that loudspeaker manufacturers are supposed to follow when stating impedance, (“The IEC method of specifying nominal loudspeaker impedance is set such that minimum impedance must not fall below 80% of nominal, so for an 8-ohm speaker this would be 6.4 ohms minimum, and for 4 ohms would be 3.2 ohms.”), this is almost never done, for reasons that will be clear as you read on.
Editorial Note about Speaker Impedance/Phase Amplifier Loading by Gene DellaSala
It's important to note that most modern day receivers and amplifiers have current limiting and thermal protection circuitry to make sure you can’t blow up them from a low impedance load as easily as a few decades ago. However, you should take care in choosing a receiver that will drive your speakers without tripping these circuits. Minimum impedance is not the only factor determining the difficulty of the load, phase angle of the load certainly factors in. You can have a relatively low minimum impedance speaker play without any issue on an amplifier that has minimal phase angle whereas the same amp may have more of an issue with a speaker with a higher minimum impedance but a higher phase angle.
The phase angle determines how much the current will lead or lag the voltage waveform in a reactive circuit.
Note: For a more technical discussion read: Loudspeaker Sensitivity & Impedance Explained and Understanding Ohm's Law, Impedance and Electrical Phase
Historical Context Background
The stereo store explosion: After the introduction of two-channel stereo in 1958, “stereo stores” sprang up like weeds all over the landscape. Independent stores, regional chains, and national chains were everywhere, as were electronic ‘grocery’ stores like Allied Radio (which morphed into Radio Shack), Olsen Electronics and Lafayette radio.
All of these stores had a general display area and special “sound rooms” where speakers could be compared to each other. This was called the “A-B” comparison. The salesperson would play two speakers and instantaneously switch from one to the other. For the customer, it was an effective and dramatic way to listen to speakers and make a purchase decision.
The better stores had what was called an “efficiency compensator.” This was a special board through which all the speakers on the sound wall were connected that allowed the store salesperson to adjust the relative loudness level of the two speakers under comparison so that they played at the same loudness. As is well known, the human ear tends to equate “louder” with “better.” The efficiency compensator was a good way to eliminate varying speaker sensitivities from the equation of evaluating their relative sound quality.
This was pretty much the norm through much of the 1980’s. Then stores began to realize that efficiency compensators were expensive, they increased the complexity of wiring up a soundroom and that perhaps the new breed of Baby Boomer and Gen-X-er customers weren’t that concerned after all.
Therefore, by the 1990’s, soundroom compensators had pretty much disappeared. The “louder is better” aspect of A-B speaker comparisons now loomed larger than ever, so speaker manufacturers kicked into action. They realized that if their speakers had a lower impedance (R), the speaker would draw more current (I) out of an amplifier (more “watts,” since P = I^2*R) and therefore play louder in the soundroom comparison. So the speaker companies began to produce speakers with ever-lower impedances.
Editorial Note on Power by Gene DellaSala
An ideal amplifier would act like a perfect voltage source and would double power with halving load impedance. So, all things being equal, an 8 ohm speaker drawing 1 watt from an amplifier would now draw 2 watts from that amplifier if it suddenly became a 4 ohm speaker which could result in up to a 3dB output sensitivity advantage as a result. And this happens while NOT changing the volume control setting on the receiver thanks to Ohm's Law!
The 1974 FTC Power Ratings Mandate: As stereo grew in popularity by leaps and bounds through the 1960’s, the electronics manufacturers began to inflate and exaggerate their amplifier power ratings in a blatant attempt to win the attention of prospective new customers. Things got so bad (30 watt-per-channel RMS amplifiers were being advertised as having “240 watts of total system musical peak power!”) that by 1974, the Federal Government had to step in with amplifier rating guidelines to ensure that the manufacturers rated their equipment honestly.
One of the new guidelines was a 1/3-power “pre-conditioning” requirement, which stated that amplifiers had to be run at 1/3-power at 1000Hz for an hour before the power output and distortion measurements could be made. The rationale, presumably, was that an amp that was properly “warmed up” would give more accurately-representative results than an amp that was measured “cold.”
The problem was this: the 1/3-power test made a class AB amplifier (which virtually all the amps were at that time) run very hot—especially at 4 ohms. Manufacturers found that they had to either modify existing equipment to increase the heatsinking or downgrade their power ratings so they’d run cooler at a lower power level.
For new equipment, the answer was simple: Simply don’t rate the amplifier into a 4-ohm load, since a 4-ohm load required too much expensive, heavy heatsinking and heavy-duty output transistors. Things were very competitive in the stereo biz and every dollar counted. If a manufacturer could save $10-20 by using less of that costly die-cast aluminum heatsinking and cheaper, less robust output devices, that could easily translate into a retail price that was $50-100 lower. In a cutthroat market, there’s a world of difference between a receiver priced at $299 when your biggest marketplace rival is $369.00
In the 1970’s, amps and receivers could handle 4-ohm loads. There were some really great lower-priced units that were pretty gutsy. The Sherwood S-7100A could deliver 20-25 watts per channel into 4 ohms all day long and the Pioneer SX 424 and 525 were also quite comfortable with 4-ohm speakers. A very popular 4-ohm “budget” speaker in those days was the Smaller Advent Loudspeaker. “Small Advents,” as they were called, were deliberately designed to be 4-ohm speakers. They wanted the Smaller Advent to have essentially the same excellent bass response and deep extension (-3dB in the mid-40’s Hz) as the Large Advent. In order to achieve this, they needed a woofer with more mass (they mass-loaded the center of the woofer, under the dustcap), for a lower resonance to offset the smaller volume of air in its compact cabinet. It worked just fine, but at a severe penalty in sensitivity—the Small Advent needed a lot of power to drive to reasonably-loud levels. To offset that, Advent made it a 4-ohm speaker, so it would draw more power out of its companion receiver and “seem” just as loud for any given volume control setting as a bigger 8-ohm speaker.
Sherwood S-7100A receiver, 1972
This was clever engineering and marketing on Advent’s part, made possible only because modestly-priced receivers and integrated amplifiers existed at that time that could handle 4-ohm loads. There were thousands of Small Advents and Sherwood receivers happily singing away in dorm rooms all over the country in the 1970’s.
Smaller Advent Loudspeaker, 1972 (Note mass-loaded dustcap.)
A Perfect Bad Storm
Fast-forward to the late 1980’s/early 1990’s—now we have a problem. Because of relentless price competition, receiver manufacturers couldn't really afford to make low-priced units that could drive 4-ohm loads. But speaker manufacturers no longer had retail soundroom efficiency compensators on their side to bail them out of a lower-efficiency disadvantage. So speaker manufacturers start “fudging” their impedance ratings, at exactly the same time that receiver manufacturers begin to print—right on the rear panel of their equipment—“Caution! Use only 8-ohm speakers or higher!”
If a speaker was actually rated in print as 4 ohms, it was a sales death warrant. No store would carry that brand because it wouldn't be compatible with their receivers.
But a high-impedance speaker would “lose” a soundroom A-B comparison because it wouldn't sound as loud.
This is what happened, far more often than not, and far more often than any speaker manufacturer would ever admit: Speakers that would have been rated as 4-ohm speakers in 1973 were falsely called 8-ohm speakers in 1992. It’s as phony and dishonest as can be. I remember a very highly-regarded 3-way tower from a very reputable manufacturer used dual 8-ohm 7-inch woofers in parallel. That’s a 4-ohm woofer section. The woofers crossed over to a 4-ohm 5 ¼-inch midrange. In the frequency range where the woofers overlapped with the midrange—around 500Hz, smack dab in the middle of the midrange—the speaker’s impedance dropped to about 2.7 ohms. That speaker fried many a mid-priced receiver, but the company never said there was a problem. After all, it was rated as being an “8-ohm” speaker, so it was safe to use with 8-ohm-only receivers, right?
I could go on with many, many more examples of well-known speaker companies—really well-known names—having all kinds of trouble with their speakers causing mid-price electronics to blow up from trying to drive a too-low impedance.
The impedance issue was not a problem for higher-end electronics. Good separates and even the so-called better receivers will handle 4-ohm loads. Many truly high-end power amps will cruise down to 2 ohms with no sweat.
Bottom Line On Speaker Impedance
But caveat emptor—Let the buyer beware! That speaker impedance specification is potentially a very important specification. You see all kinds of creative speaker impedance ratings these days. Here’s a good one: “Compatible with 4-8 ohm outputs.” What the heck does that mean? That doesn’t tell you the speaker’s impedance at all; it just vaguely implies that it will work ok if the amplifier has an output at 4 ohms. In other words, this rating tells you absolutely nothing of value, and it’s an obvious dodge by the speaker manufacturer.
The speaker’s impedance specification should be an extremely valuable data point in determining a speaker’s suitability for use with a particular amplifier. But unfortunately, as we’ve seen, the verity of the spec itself can’t always be trusted, not by a longshot. Having an actual graphical representation of loudspeaker impedance or at least knowing where the impedance minimums occur (and max phase angle), would be very useful. It's rare to get this kind of information, but maybe as educated consumers, we should be demanding it from our favorite loudspeaker manufacturers. What do you think? Please share your comments in the related forum thread below.