Two numbers decide whether a speaker suits your amp. Sensitivity tells you how loud it plays for a given amount of power, and impedance tells you how hard it pulls on the amp to get there. Vintage speakers range wildly on both, from horn speakers that play loud on a few watts to sealed bookshelf boxes that want real power and 4-ohm loads that some receivers dislike. Here's how to read the numbers, and why old ratings don't always line up.
Sensitivity and the 3 dB rule
Sensitivity is usually given as the sound level a speaker makes from 1 watt, measured 1 meter away, often tested at 2.83 volts, which is 1 watt into 8 ohms. Most speakers are inefficient, turning only around one percent of the electrical power into sound, so small differences in sensitivity matter a lot.
The rule to remember is that doubling the power adds about 3 dB, and ten times the power adds 10 dB. Turn that around and a speaker rated 95 dB needs about a tenth of the power of an 85 dB speaker to play equally loud. Every 3 dB less sensitive means roughly twice the amp power for the same volume.
Horns need very little
Horns get a lot of sound out of a small amp. A horn can typically produce about 10 dB more sound power than a cone speaker from the same amp output, which is why horns pair well with low-power amps, often in the 5 to 25 watt range. The Klipschorn is rated around 105 dB, and Paul Klipsch demonstrated concert levels on as little as 1 watt per channel.
The Klipsch Heresy is rated at 96 dB, which puts it well into small-amp territory. Its midrange and tweeter are horn-loaded, and even a modest tube amp will drive it to loud levels in a normal room. That's why Heresys, Cornwalls and the Altec horn speakers make sense with low-power tube amps.
Sealed AR boxes want more
Acoustic Research built its name on the sealed acoustic-suspension box, and sealed speakers need more power for the same bass output than ported designs. AR's own figure for the AR-3a is 86 dB. AR recommended at least 25 watts RMS per channel and said the speaker would accept up to 100 watts per channel on normal program material. It recommended 20 watts for the AR-5 and AR-2ax.
Put the Heresy's 96 dB next to the AR-3a's 86 dB and the gap is 10 dB, which by the rule above means the AR wants roughly ten times the power to match it. The two figures come from different sources, so treat that as a rough guide. A 15-watt amp that's plenty for a Heresy will run out of room on an AR-3a.
The AR-11 is also rated 86 dB. AR said 25 watts into one speaker reaches 100 dB in a room of 3,000 cubic feet, and that it can be used with amps up to 150 watts per channel. Advent recommended 20 watts per channel for the same size room and called its speaker safe with amps up to 120 watts.
Old ratings don't line up
Older literature often used different measurements, and comparing them to modern 1 watt, 1 meter figures goes wrong fast. JBL's 1970s home brochures quoted how loud a speaker played at 15 feet from 1 watt. The L100 brochure says 1 watt produces 78 dB at 15 feet, and the L36 Decade's is 76 dB. JBL didn't give a 1 meter figure until about 1978, when the sheet for the JBL 4311 rated it at 91 dB. JBL also said the L100 works with as little as 10 watts RMS, with up to 150 watts RMS as the optimum.
Altec measured at 4 feet, with 1 watt of pink noise limited to 500 to 3,000 Hz, until about 1982. On that basis the Valencia 846B is rated 97.5 dB, the 878B Santiago 91 to 92.5 dB and the Model 19 99 dB. Those aren't 1 meter figures. When Altec switched to 1 meter in 1982, it rated the Model 19 at 100.5 dB. Use the old numbers to compare speakers within one brand, and be careful comparing across brands.
One Altec is a special case. The 2878B version of the Santiago has Altec's 770A electronic crossover and bi-amplifier built in, with 60 watts RMS for the bass and 30 for the highs, so check that the built-in amp works before you buy one.
Impedance and 4-ohm loads
Most speakers are rated at 8, 4 or 16 ohms. That nominal figure isn't the whole story, because real impedance moves with frequency and can dip below the rating. An amp that handles the nominal load may struggle with the dips. Most solid-state amps are designed for 4 to 8 ohms, and tube amps use taps on the output transformer to match the load.
Several speakers here are 4-ohm designs. The AR-3a and AR-11 are both rated at 4 ohms nominal, as are the Infinity RS III and RS 2.5. A restorer describes the EPI M250 as a stable 4-ohm load. The Altec Valencia 846A was offered as 8 or 16 ohms. Restorers often warn that many vintage receivers are happiest with 8-ohm speakers, and that running 4-ohm speakers, or two pairs at once on the A and B outputs, can trigger the protection circuit or overheat some of them. Check your receiver's rated minimum load before you buy a 4-ohm pair.
The Kappa 8's 2-ohm load
The Infinity Kappa 8 needs the most care. Infinity rated it at 4 ohms and 89 dB, but hifi-wiki notes that under the IEC standard its nominal impedance is 2 ohms. Its woofer network is designed to draw current for flat bass down to very low frequencies, which is part of why the load drops so far. Weak receivers can overheat or shut down. Pair it with a high-current amp that's rated for low impedances.
Clipping kills tweeters
An underpowered amp is a bigger risk than an overpowered one. When an amp runs out of power it clips, and clipping adds high-frequency harmonics that can overheat a tweeter. Voice coils fail from heat. The Restoring the AR-3a guide calls open tweeter voice coils a common problem on that speaker, most often caused by very high volume or a small amp driven into clipping. A bigger amp played at sensible levels is kinder to an old tweeter than a small one pushed hard.
A quick way to match
- Horn speakers. Klipsch Heritage and Altec horns run well on small tube amps. The Heresy's 96 dB rating shows why.
- Sealed AR boxes. Plan on at least AR's recommended 25 watts, and more headroom is better. Make sure the amp is rated for 4 ohms.
- Big Infinitys. Use a high-current amp rated for low impedances. The Kappa 8 needs one that's stable near 2 ohms.
- Old ratings. Check whether a figure is at 1 meter, 4 feet or 15 feet. Compare like with like.
Ask the seller what amp the pair was used with. See Acoustic Research, Infinity, or browse everything.