Impedance or How Many Ohms Do You Have?

By John Strohbeen • Ohm Speakers • Tuesday, July 5, 2016


All passive speakers (relying on outboard amplification) have an impedance rating, given in ohms, provided by the manufacturer. This number helps you match the speakers to amplifiers. Amplifiers have ratings in watts when driving a load with a specified impedance, also conveniently listed in ohms. Is it better to get a speaker with bigger ohm rating (like 100 watts into 8 ohms instead of 100 watts into 4 ohms)? Maybe – maybe not.

If you already own an amplifier and/or speakers, it is important to check the compatibility of the amplifier with the speakers. If you own an amplifier that lists only a power rating into 8 ohms – and YES, the 8 ohms for the speakers is very important – you might damage the amplifier with lower impedance speakers. Often a manufacturer who only lists a power rating into 8 ohms will also print a warning about using lower impedance speakers. However, it is not important if you have (or are considering) an amplifier with a higher power rating into 4 ohms than into 8 ohms (typical of better amplifiers). The 4 ohm speakers can play louder before this amplifier clips if the speakers have the same efficiency as an 8 ohm speaker.

AC/DC – What Is Impedance?

In an electrical circuit with current that just flows in one direction (Direct Current or DC), the impedance is merely the resistance of the circuit and is calculated by the dividing the voltage at the source by the current in the circuit. (This is Ohm’s Law R=V/A.) Old fashioned flashlights are good examples of DC circuits. An electrical circuit that alternates its flow of current is called Alternating Current (AC). Music is the mechanical equivalent of Alternating Current with air flow as the current. Therefore, speakers only use AC. Explosions like cannons are the rare example of nearly DC sound. That is why speakers have a hard time accurately reproducing them.

Every sound wave has two parts: a compressed part, and a rarefied part. These parts repeat for every cycle of the wave, and are analogous to the positive and negative parts, respectively, of an AC signal.

In speakers, extra components (capacitors and inductors) used in most designs can have a dramatic effect on the flow depending on the frequency of the alternations. A capacitor can only accept a certain amount of electricity until it stops the current (unless you then increase the voltage at the source). If the capacitor has a small value, it will only allow very high frequencies through without reduction. The larger the value of capacitor, the lower this “cutoff” frequency becomes. So, capacitors in speakers act like filters. Inductors work the other way: small inductors only block the very high frequencies and everything else is passed without interference. A very big inductor only allows the very low frequencies to pass.

The impedance is changed by extra effects that the capacitors and inductors have, including their internal resistance and the leading or lagging of current and voltage in the devices. This makes things even more confusing and the variation from the lowest impedance point to the highest impedance point over the operating frequency range is often ten times or more. This is not EE 101, so we will leave those refinements to another blog.

The impedance specified by the speaker manufacturer is often near the lowest point, since amplifiers in general are happier with higher impedances. I do remember a review of an expensive, major brand speaker when the reviewer added the comment that he felt 1.9 ohms was a very low minimum impedance point for an 8 ohm rated speaker and warned readers to match them with an amplifier having high current capacity. Over the years, Ohm has made 4, 6 and 8 ohm speakers. Their very lowest impedance point was never less than 70% of the rating.

This is the graph of the impedance response of a single driver. As you can see, it varies with frequency, which is why the rated impedance of a speaker can vary greatly from its measured DC resistance, even though they’re represented the same in Ohm’s Law.

Does Impedance Affect the Quality of Sound?

The impedance curve of the speaker and the impedance curve of the speaker wire can have audible effects on the speakers’ sound quality. In general, you need thicker wire to minimize these effects with speakers having wide variations in their impedance curve. Impedance is more a matter of matching speakers to amplifiers - not choosing a better or a worse impedance.

In general, today, most speakers can handle a range of amplifier power. Likewise, most modern amplifiers can accommodate a variety of speakers and their varying nominal impedances. BUT – and there’s almost always a but – there are cases where a mismatch can spell disaster. And it’s often spelled as E-X-P-E-N-S-I-V-E to fix. So, pay attention and ask questions when you match speakers and amp.

See you on the 19th

‘Til then, Enjoy & Good Listening!

John

Article originally appeared on http://ohmspeaker.com
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