Speaker impedance vs sensitivity

[buck91]

Forgive the basic question. I am familiar with most speaker specifications but a little confused about impedance. I understand the concept but as far as I have ever been concerned it was more so to ensure the speakers wouldn't cause a problem with your amplifier. All other things being equal, what effect does impedance have on a speakers performance? For example say I have two speakers, both at 180w, 93db, 45hz-21khz range but one is 2.8ohms and one is 4ohms nominal. What does this mean to me?

 

[sloinker]

Impedance will vary over the frequency applied to the speaker. If impedance gets low enough it may cause damage to the amplifier. Speaker performance is an individuals opinion. Some folks like tight focused midrange and seek out speakers that accomplish this. Others may want to move a lot of air with giant woofers. Some folks want realistic or loud sound levels but want to use a puny tube amp to drive the speakers, think Klipschorn's.

 

[sloinker]

BTW, sensitivity is measured with 1 watt RMS into the speaker and the sound pressure level measured in decibels determines sensitivity. The higher the sensitivity number in decibels the more efficient the speaker and the smaller the amplifier needed to make realistic volume levels.

 

[The_Nuke]

You want to make sure the speaker's impedance is greater than or equal to the lowest impedance your amplifying source can handle. 'Equal to' is better than 'greater than' for speaker impedance because the higher the impedance, the fewer watts the amplifying source can push through the speaker, all other things being equal.

So for your example, the answer depends upon the amplifying sources lowest stable impedance. If it can handle 2 ohm speakers, the 2.8 ohm speakers are your best bet (but it will handle the 4 ohms just fine). If your amplifying source has 4 ohm as its lowest stable impedance, the 2.8 ohm speakers are out of the question. Pushing your amplifier down to 2.8 ohm could overheat and damage it.

One other note, if your amplifier is an actual amp, and it is a dual channel amp capable of 2 ohm operation per channel, BUT the speaker is a subwoofer, you MUST go with the 4 ohm sub. The reason being that you will be bridging 2 channels to power the sub, and when you bridge two channels on an amp, the lowest impedance it can do will now be double its impedance per channel. So 2 ohm stable per channel amp becomes 4 ohm stable when bridged. Bridging two 2 ohm channels and trying to power a 2 ohm sub could overheat and damage the amp.

Clear as mud?

 

[MolaKule]

In AC theory, Impedance matching is needed in order to transfer the maximum power from the amplifier's output stage to the speaker.

Ri is the Internal Impedance of the amplifer, RL here is the speaker "load" impedance.

BTW, impedance actually has two parts or terms, a real part we call resistance, and an imaginary part that expresses whether the load has capacitance or inductance.

But most people only use the Resistance term in matching simple devices.

As others have stated, speaker sensitivity and impedance are two different things.

 

[SubieRubyRoo]

On the sensitivity side, for car door speakers, higher sensitivity is generally better, regardless of impedance, because you will get more output per watt of input. For subwoofers, a lower sensitivity is actually better than higher IF you are trying to cram the sub into a significantly smaller box than optimal- a lower sensitivity sub will actually play deeper and louder in a given tiny box than a high sensitivity sub. Once you reach the optimum size box, the higher sensitivity takes over as you would expect.

Impedance, as others have stated, needs to be picked based on what your amplifier is rated for. That being said, if you are looking at Polk, JBL, or Infinity, they have designed most of their speakers at 2.8 ohms based on the average car's additional wiring resistance to present a "true" 4 ohm load to the amplifier. If you're using the factory wiring behind the radio, you should be fine. If you are rewiring the car from the amp to the speakers with 12ga copper wiring, you should make sure your amp can handle it because the reduced resistance of larger wire will present a load much closer to the rated 2.8 ohms.

 

[Dave9]

Quote
In AC theory, Impedance matching is needed in order to transfer the maximum power from the amplifier's output stage to the speaker.
You do not impedance match power amps and speakers. The amp is going to have the lowest impedance possible (that the design and materials quality allows), typically under 1 ohm.

Speaker impedance must not be too low for the amp, including consideration of a bridged 2 channel sub, but on the other hand if you go with a higher impedance speaker you limit the amp's possible output power to it because it runs out of voltage swing. For example a 200W amp at 4 ohms, is not 200W at 8 ohms. I mean it "could" be but odds are they did not design for both more voltage and more current than one or the other needs, or else they would have just rated it a higher wattage.

Unless the amp is class D with isolated grounds, the lower the speaker impedance the worse your channel separation will be, because with shared ground you're going to have ground current flowing through one speaker from the signal on the other one and vice versa.

Sensitivity is more about the intended use and material quality, though generally a low impedance speaker is built for bass and has more mass to move (all else equal) so will have a lower sensitivity, and be less likely damaged by cranking the amp volume up.

Ultimately the other variables in the speaker design are what you are choosing and just matching impedance to the amp to the extent that you don't want it much higher than the lowest impedance the amp can handle so you have the most output power available from it. It also means you need lower gauge speaker wires for the higher current that lower impedance speakers use.

 

[NO2]

Impedance should not be much of a problem in auto speakers, which tend to have a small range overall. I would select on quality of sound first, and then add amplification as necessary. Most head units cannot drive a subwoofer or speakers to very loud levels. You will also need a crossover, usually integrated into the head and/or amps to prevent frequencies outside of the range of the speaker to cause distortion or damage the speaker.

 

[RooflessVW]

Your setup (amp and number of speakers) determines speaker impedance.

For example, not a lot of mono amps are 1 ohm stable, so you would want to drive two 4 ohm speakers in parallel for a 2 ohm load. You could also drive 2x 2 ohm dual-VC subs as a 2 ohm load, with the speakers wired in parallel but the voice coils in series, or the vice versa.

Keep in mind that the lower the impedance of your speakers per channel, the hotter your amp will run.

As an example, in my car I planned to run 10 speakers but I only wanted to use two amps. I'm driving 2x 4 ohm subwoofers in parallel with a 2 ohm stable amp, and I'm driving 8x 2.3 ohm speakers wired in series as 4x 4.6 ohm channels.

 

[sloinker]

Car audio, yawn. I'm old.

 

[ARCOgraphite]

Originally Posted by buck91
Forgive the basic question. I am familiar with most speaker specifications but a little confused about impedance. I understand the concept but as far as I have ever been concerned it was more so to ensure the speakers wouldn't cause a problem with your amplifier. All other things being equal, what effect does impedance have on a speakers performance? For example say I have two speakers, both at 180w, 93db, 45hz-21khz range but one is 2.8ohms and one is 4ohms nominal. What does this mean to me?



Looks like you are talking about Home audio.

Higher impedance low phase angles are easier to drive
Its more at what the amplifiers low impedance capabilities are. If its current limited then You would like want a voice coil type speaker
system with Nominal 8 ohms with possibly dip to 6 ohms above the resonant frequency in the Bass region.

The Amp will limit your speaker choices.

If you have a Jeff Roland Model 7 or a Bedini or a Electron Kinetics Eagle 2A then you are free to move into low impedance Magnepans or 3-way Theil or Apogee Diva.

I would Disregard the "power doubling" into lower impedances as being of beneficial consequence in HOME AUDIO. But it is true big transformers in amps can double power into low impedance. E.G: 100W rms 8 ohms, 200W rms 4 ohms, 400W rms 2ohms.

 

[buck91]

Thanks for the great replies so far! I'm looking at Avery basic set up. Currently running some so-so aftermarket 6x8s (kickers) off my factory headunit. They are ok but the sound is a little tinny and there are some obvious holes in the frequency response. Looking at some other middle of the road options like jbl and infinity...

 

[ARCOgraphite]

To get the most volume You want 4 ohm midrange drivers for the car. Tweeter impedance doesnt matter as little power goes there

You are current rich and voltage limited in a car scenario

so for bass with a 2 ohm stable amp you want low impedance woofers.

A lot of cheap car speakers are just that - cheap car speakers,

Look for quality say 6" two way box home speakers on CL or Eeebay
and use those components. in your car.

I would say Kicker aint all that.

Focal, Boston Acoustics, b&o

Dont kill you ears - youll regret it later.

 

[meep]

One thing not mentioned, which is the original post title, is that cutting impedance in half results (generically) in 3db of output gain, so more output is demonstrated while there's not change in efficiency, but rather at the expense of more power consumed due to the decreased resistance presented in the driver. THEN begin the discussion on whether or not the amp will accept the lower impedance. Rumor has it, for instance, that some honda HUs don't take to the 2.8 offerings well.