High Ohm speakers
I don't know much about the ins and outs of spreakers, but I get the impression that the ones designed for 8 ohms or higher, 16-ohms for example, seem to be very efficient and appeal to the SET tub amp crowd. These are the amps that cost megabuck yet are rated say 10W ! I suppose that's a sort of basic tradeoff: couple a high efficiency, high impedance speaker to a low power amp and visa versa.
I don't know much about the ins and outs of spreakers, but I get the impression that the ones designed for 8 ohms or higher, 16-ohms for example, seem to be very efficient and appeal to the SET tub amp crowd. These are the amps that cost megabuck yet are rated say 10W ! I suppose that's a sort of basic tradeoff: couple a high efficiency, high impedance speaker to a low power amp and visa versa.
Tubes do like the higher impedence, and I had a pair of Goodmans that were rated at 12 ohms. Also though 8 ohms is the commonly used impedence I was told that they are actually 7. It seems 8 just sounded better between 4 and 16. A practise used if you have extremely efficiant speakers and don't need the power is to put a SE pentode into triode and if you still had power to spare hook your 16 up to the 4. Why the 16 on the 4 is something I would like to know the reason for. Less distortion?
ohms is a resistance, so if the circuit has more resistance in it, it produces less volume, because it demands more power for the same amount of volume. Performance car stereos have 40 ohm speakers because the higher the ohms the more detailed the sound and the more power available the system has. The reason for 4 ohms in car stereos is that it takes less power, and cars run on very underpowered systems.
To me this is an on going topic. There is no difference from using 1 ohms, 2 ohms, 4 ohms, 6 ohms, 8 ohms, 12 ohms, 16 ohms, etc. You won't get higher SPL from a 4 ohm versus an 8 ohm. SPL relates to Qes, Vas, and Fs. Also the dampening factor doesn't do anything even if you get a one ohm speaker driver. The Re spec states the real ohm of the speaker driver.
In the car audio world. There is cabin gain and distortion. Cabin gain makes a very, very steep loudspeaker (similar like your small book shelf speakers) sound as large as huge home audio speakers. Distortion adds more harmonics in to the audio. This added effect creates more volume or loudness. Car audio systems are more efficient when they are powering 1 ohms to 4 ohms. Using either of those loads doesn't give an indicator of loudness. Mostly the designer have to select speaker drivers that have the highest SPL per watt RMS instead of how many ohms the speaker driver has if they want a very loud audio system.
BTW, I haven't done car audio and never will.
In the car audio world. There is cabin gain and distortion. Cabin gain makes a very, very steep loudspeaker (similar like your small book shelf speakers) sound as large as huge home audio speakers. Distortion adds more harmonics in to the audio. This added effect creates more volume or loudness. Car audio systems are more efficient when they are powering 1 ohms to 4 ohms. Using either of those loads doesn't give an indicator of loudness. Mostly the designer have to select speaker drivers that have the highest SPL per watt RMS instead of how many ohms the speaker driver has if they want a very loud audio system.
BTW, I haven't done car audio and never will.
Quote Electro: "The Re spec states the real ohm of the speaker driver.".
Yes and no. Because the speaker is always moving, it never quite hits the DC resistance of it's voice coil.
In bass reflex systems, with a big, open port, the impedance can come to within an ohm or so of the DC resistance.
David Weems recommends that you take the DC resistance and multiply by 1.25 to get the real minimum impedance that you are actually going to get.
Of course, throughout most of it's range, the speaker will be operating somewhat above it's rated impedance. So these ratings are traditionally really for minimum impedance.
Lately, speaker manufacturers have even begun to cheat on that. They take the impedance that a speaker operates at throughout a given range, and rate it at that. This will always be a higher rating than the old way. Then they include another spec called "minimum impedance". It is a nice way to make it seem that your speaker is drawing less current than it really will at certain frequencies. It seems to me that your amp has to be able to supply the current called for at the minimum impedance. What are you supposed to do-make a notch filter that blocks the low impedance frequencies so your amp doesn't blow?
I have seen speakers with 5 ohms DC resistance get rated at 8 ohms. I have also seem speakers with 7 ohms DC resistance get rated at 8 ohms. Clearly, these two are not equivalent-but they are rated the same.
So I would basically agree with Electro. Go by the DC resistance. The only difference is that I would take the DC resistance, multiply by 1.25 to get the real minimum impedance.
Yes and no. Because the speaker is always moving, it never quite hits the DC resistance of it's voice coil.
In bass reflex systems, with a big, open port, the impedance can come to within an ohm or so of the DC resistance.
David Weems recommends that you take the DC resistance and multiply by 1.25 to get the real minimum impedance that you are actually going to get.
Of course, throughout most of it's range, the speaker will be operating somewhat above it's rated impedance. So these ratings are traditionally really for minimum impedance.
Lately, speaker manufacturers have even begun to cheat on that. They take the impedance that a speaker operates at throughout a given range, and rate it at that. This will always be a higher rating than the old way. Then they include another spec called "minimum impedance". It is a nice way to make it seem that your speaker is drawing less current than it really will at certain frequencies. It seems to me that your amp has to be able to supply the current called for at the minimum impedance. What are you supposed to do-make a notch filter that blocks the low impedance frequencies so your amp doesn't blow?
I have seen speakers with 5 ohms DC resistance get rated at 8 ohms. I have also seem speakers with 7 ohms DC resistance get rated at 8 ohms. Clearly, these two are not equivalent-but they are rated the same.
So I would basically agree with Electro. Go by the DC resistance. The only difference is that I would take the DC resistance, multiply by 1.25 to get the real minimum impedance.
Surprised no one has mentioned...
The resulting drop in impedance (usually from eight to four ohms) when using a speaker systems having dual woofers in parallel comes in very handy, in my opinion, especially when using 7" or smaller woofers. Such woofers tend to be at least 3 dB lower in sensitivity as compared to your average tweeter. When you put the woofers in parallel, you cut the impedance in half, and efficiency goes up by 3 dB, while sensitivity goes up by 6 dB. This makes a great way to match up your woofer section to the more efficient tweeter, while only needing half as much power for the woofers. All the speakers in my home theater setup are dual woofer configurations, with the woofers in parallel. This greatly reduces the power needed, especially since I have seven speakers. The trick is, as has been mentioned, finding an amplifier/receiver that will drive four ohms without a problem. Fortunately, there's a pretty good selection of older used amplifiers on eBay which do the trick just fine, and many receivers will handle it as well. When you consider a 3 dB increase in efficiency, the increase in current is not as great as the halving of impedance would suggest. For example, using 50 watts as an example, an eight ohm speaker will draw 2.5 amps. Now, a speaker 3 dB more efficient will only take 25 watts for the same SPL, and if four ohms, will draw 3.5 amps, much less than the 5.0 amps you might normally assume using Ohm's law with all else being equal.
Anyhow, my point is, so long as I continue to build speakers with dual woofers (in fact I'm building a new pair right now), I'll make them four ohms at least for the woofer section. Why use twice as much power when I don't need to?
Assuming the amp/receiver is rated to drive four ohms to begin with, and driven within it's limits (as you'd expect from any equipment), there's no reason to expect a shorter life span than if using only eight ohm speakers. At least, this is for solid state gear, I can't speak to tubes.
Aaron Gilbert
The resulting drop in impedance (usually from eight to four ohms) when using a speaker systems having dual woofers in parallel comes in very handy, in my opinion, especially when using 7" or smaller woofers. Such woofers tend to be at least 3 dB lower in sensitivity as compared to your average tweeter. When you put the woofers in parallel, you cut the impedance in half, and efficiency goes up by 3 dB, while sensitivity goes up by 6 dB. This makes a great way to match up your woofer section to the more efficient tweeter, while only needing half as much power for the woofers. All the speakers in my home theater setup are dual woofer configurations, with the woofers in parallel. This greatly reduces the power needed, especially since I have seven speakers. The trick is, as has been mentioned, finding an amplifier/receiver that will drive four ohms without a problem. Fortunately, there's a pretty good selection of older used amplifiers on eBay which do the trick just fine, and many receivers will handle it as well. When you consider a 3 dB increase in efficiency, the increase in current is not as great as the halving of impedance would suggest. For example, using 50 watts as an example, an eight ohm speaker will draw 2.5 amps. Now, a speaker 3 dB more efficient will only take 25 watts for the same SPL, and if four ohms, will draw 3.5 amps, much less than the 5.0 amps you might normally assume using Ohm's law with all else being equal.
Anyhow, my point is, so long as I continue to build speakers with dual woofers (in fact I'm building a new pair right now), I'll make them four ohms at least for the woofer section. Why use twice as much power when I don't need to?
Assuming the amp/receiver is rated to drive four ohms to begin with, and driven within it's limits (as you'd expect from any equipment), there's no reason to expect a shorter life span than if using only eight ohm speakers. At least, this is for solid state gear, I can't speak to tubes.
Aaron Gilbert
Re: Re: Surprised no one has mentioned...
In the bass range, doubling drivers (so long as they are mounted very close together on the same baffle) does cause an almost 3dB increase in efficiency upto a practical limit where mechanical / electrical losses cause you to hit the wall of significantly dimished returns.
real said:
In the bass range, doubling drivers (so long as they are mounted very close together on the same baffle) does cause an almost 3dB increase in efficiency upto a practical limit where mechanical / electrical losses cause you to hit the wall of significantly dimished returns.
Im not 100% sure if this had been mentioned but surely the loudspeakers themselves are a limit to look at.
The debate with current and voltage etc is, as had been said, a juggling act between safety, distortion and practicality. So we arrive at an 8ohm load which seems to be a good happy place between 4 and 16 ohms. The simple fact is its harder/more expensive, to design/build an amplifier that will be completely stable in lower impedances. You need a bigger transformer, for more current etc.
But if you look at the hifi drivers themselves they have their limits too. It requires a certain amount of air shifted at a certain frequency to produce a given SPL. Regardless of how this driver is being driven, 8 ohm, 4ohm, 1kohm or 0.001ohm.
But the majority of speakers produced will reach their limits easily on a 100 watts 8 ohm amp. So why bother going to 4 ohms where you get more oomph, lower impedance = more watts on an up to date amplifier.
If you can reach the loudspeakers limits on 8ohms then thats much better then trying to do it on 4 ohms, for all the reasons mentioned before, higher damping factor, less distortion etc.
You could then argue well lets use 16 ohms instead for even lower distortion and higher damping factor, well in that case you need to increase the voltage of the amplifier. But this has major disadvantages, the voltages required to produce a large number or watts into 16 ohm speakers is very high, too high for the components we use now to handle easily. Thats why these monster amps from krell are bridged, basically a 600 watt krell is two smaller amps sandwiched together. Bridging is like a 4ohm load, so it lowers the voltage requirement for a given wattage, but increases the current requirements. However this is needed because we cannot go above certain voltage limits.
To me anyway 8 ohms is a happy balance of the two. We can get reasonably powerful amplifiers which dont present any problems when it comes to the extremes of current and voltage. And 99% of the time a 100 watts is enough to drive your speakers into their thermal or mechanical limits, with expected program material. Not sine waves, music.
The obvious exception is subs or huge speakers, but im talking about the norm, standard two ways with a tweeter and 5-8" midbass.
The debate with current and voltage etc is, as had been said, a juggling act between safety, distortion and practicality. So we arrive at an 8ohm load which seems to be a good happy place between 4 and 16 ohms. The simple fact is its harder/more expensive, to design/build an amplifier that will be completely stable in lower impedances. You need a bigger transformer, for more current etc.
But if you look at the hifi drivers themselves they have their limits too. It requires a certain amount of air shifted at a certain frequency to produce a given SPL. Regardless of how this driver is being driven, 8 ohm, 4ohm, 1kohm or 0.001ohm.
But the majority of speakers produced will reach their limits easily on a 100 watts 8 ohm amp. So why bother going to 4 ohms where you get more oomph, lower impedance = more watts on an up to date amplifier.
If you can reach the loudspeakers limits on 8ohms then thats much better then trying to do it on 4 ohms, for all the reasons mentioned before, higher damping factor, less distortion etc.
You could then argue well lets use 16 ohms instead for even lower distortion and higher damping factor, well in that case you need to increase the voltage of the amplifier. But this has major disadvantages, the voltages required to produce a large number or watts into 16 ohm speakers is very high, too high for the components we use now to handle easily. Thats why these monster amps from krell are bridged, basically a 600 watt krell is two smaller amps sandwiched together. Bridging is like a 4ohm load, so it lowers the voltage requirement for a given wattage, but increases the current requirements. However this is needed because we cannot go above certain voltage limits.
To me anyway 8 ohms is a happy balance of the two. We can get reasonably powerful amplifiers which dont present any problems when it comes to the extremes of current and voltage. And 99% of the time a 100 watts is enough to drive your speakers into their thermal or mechanical limits, with expected program material. Not sine waves, music.
The obvious exception is subs or huge speakers, but im talking about the norm, standard two ways with a tweeter and 5-8" midbass.
Re: Surprised no one has mentioned...
Aaron, I agree that having two woofers will get you 3dB more output power which will match your tweeter, but:
Why would efficiency go up at all? The individual drivers would still have the same efficiency, how could connecting them in parallel double efficiency? It just means that for the same input voltage, output power (sound power) will double - with two of the same woofer connected to your amp, each will consume the same amount of power as the single one previously would, so you are doubling your input power (from the amp). The doubling in output power comes from the doubling in input power, so it is not more efficient!
After all, if paralleling drivers increased efficiency by 3dB for each doubling of the amount of speakers, if we connected up an arbitrary number of speakers, we could have an efficiency > 100% - not possible!
Would sensitivity change? It depends, as some manufacturers specify sensitivity with 1W input or 2.83V (approx 1W into 8 ohm) input. If specified as 1W input, sensitivity should not change, but if specified as 2.83V in, it would go up by 3dB (doubling in output power). Please correct me if I'm wrong.
Regards,
Chris.
Aaron, I agree that having two woofers will get you 3dB more output power which will match your tweeter, but:
aaroncgi said:
Why would efficiency go up at all? The individual drivers would still have the same efficiency, how could connecting them in parallel double efficiency? It just means that for the same input voltage, output power (sound power) will double - with two of the same woofer connected to your amp, each will consume the same amount of power as the single one previously would, so you are doubling your input power (from the amp). The doubling in output power comes from the doubling in input power, so it is not more efficient!
After all, if paralleling drivers increased efficiency by 3dB for each doubling of the amount of speakers, if we connected up an arbitrary number of speakers, we could have an efficiency > 100% - not possible!
Would sensitivity change? It depends, as some manufacturers specify sensitivity with 1W input or 2.83V (approx 1W into 8 ohm) input. If specified as 1W input, sensitivity should not change, but if specified as 2.83V in, it would go up by 3dB (doubling in output power). Please correct me if I'm wrong.
Regards,
Chris.
I think the main reason that car speakers are 4 ohms is that for a long time they were powered straight off the battery with no DC-DC converter. That meant in order to get decent volume from the low voltage, you needed the lower impedence.
The biggest reason you see such low impendences in the car audio scene was that until a few years ago most SPL competition organizations had classes based on amplifier power. However, they classified people based on an amplifier's total power into 4 ohms at 12V.
So, to increase their performance, companies developed "cheater" amps. These would put out 20W x 2 into 4 ohms at 12V, but would do 1kW+ bridged into a 1/2 ohm load at 16V.
Some switched over to rating by cone area. All of a sudden, you get single subs capable of handing 3000W.
The biggest reason you see such low impendences in the car audio scene was that until a few years ago most SPL competition organizations had classes based on amplifier power. However, they classified people based on an amplifier's total power into 4 ohms at 12V.
So, to increase their performance, companies developed "cheater" amps. These would put out 20W x 2 into 4 ohms at 12V, but would do 1kW+ bridged into a 1/2 ohm load at 16V.
Some switched over to rating by cone area. All of a sudden, you get single subs capable of handing 3000W.
Chris, if you double the amount of cone area using the same power, you get a ~3db increase in SPL. If you double the amount of power with the same cone area, you get ~3db increase in SPL.
Theoretically, if you connected 4 8 ohm drivers in series parallel so that you had an 8 ohm load, you'd have 6db more output from the same amount of power. Therefore, more efficient.
The price you pay is that you have to buy 3 more drivers and have the extra box space to put them in.
Theoretically, if you connected 4 8 ohm drivers in series parallel so that you had an 8 ohm load, you'd have 6db more output from the same amount of power. Therefore, more efficient.
The price you pay is that you have to buy 3 more drivers and have the extra box space to put them in.
I have problems to understand that me too, can you explain further?
4 drivers in series-parallel that you power with 1W total, so 0.25W per driver are equal in my head to a single driver with 1W of power.
Or a single driver rated at 87 dB will give 93 dB SPL with 4W but 4 drivers with 4W for all of them will give the same SPL, each driver get 1W so 87 dB SPL per driver, then two times 87 dB is equal to 90 dB, then finally two times 90 dB is equal to the same 93 dB SPL.
Help please?
4 drivers in series-parallel that you power with 1W total, so 0.25W per driver are equal in my head to a single driver with 1W of power.
Or a single driver rated at 87 dB will give 93 dB SPL with 4W but 4 drivers with 4W for all of them will give the same SPL, each driver get 1W so 87 dB SPL per driver, then two times 87 dB is equal to 90 dB, then finally two times 90 dB is equal to the same 93 dB SPL.
Help please?
Simon:
Four eight ohm drivers in series-parallel connection, each 87 dB SPL for 1 watt, if spaced together like this:
00
00
will produce 93 dB per watt. The impedance for the set of four will also be 8 ohms.
You are quadrupling the area which moves the air, that leads to the increase in efficiency.
There will be less increase in efficiency if you space them like this:
0
0
0
0
Four eight ohm drivers in series-parallel connection, each 87 dB SPL for 1 watt, if spaced together like this:
00
00
will produce 93 dB per watt. The impedance for the set of four will also be 8 ohms.
You are quadrupling the area which moves the air, that leads to the increase in efficiency.
There will be less increase in efficiency if you space them like this:
0
0
0
0
Mr Teal said:
This is exactly why car audio subs usually have an impedance of 4ohms or less. I had a Zapco "competition series" C100 amplifier. It was rated for 2x25 watts rms in 4 ohms at 12v. With a strong alternator you could feed the amp 14v under load, which is good for a 36% increase, or 2x34 wrms. Each channel was stable down to 1 ohm, and the amp could be bridged into 2 ohms, which nets you 272 watts.
But when you enter the competition, you officially have a 50 watt system. With your cheater amp you get a 7db advantage over the guy who really does have a 50 watt system.
I think Orion made the ultimate cheater amp, by rating it for only 1 watt rms. In reality it could do well over 1000 watts rms, but competition rules stated that the manufacturers ratings had to be used - even if they were completely off.
Dan
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