Home Technical Info Why loudspeakers should all be rated at 1W/1m, Technically why 8ohm drivers can be superior to 4ohm
Why loudspeakers should all be rated at 1W/1m, Technically why 8ohm drivers can be superior to 4ohm
Why loudspeakers should all be rated at 1W/1m, Technically why 8ohm drivers can be superior to 4ohm PDF Print E-mail
Written by ///M5   
Monday, 14 July 2008 22:15
As most of you know, I do measurements for a living. So I have a few pet peeves about how things are done in this industry. Nothing is standardized and the accuracy of everything is questionable and typically quite subjective. In this thread, I would like to address something I would like to see standardized and that is the sensitivity of loudspeakers. There are a lot of you car audio nuts that are afraid to run home audio drivers in your car due to the fact they are 8ohms, but you really shouldn’t be in this thread I will explain why.

First off a big thanks to Nguyen for making this obvious that we need a thread like this on SSA.

Okay here are some facts:
1) Most of the time, speakers efficiencies are rated at 2.83V/m. (Personally I really wish they would rate them at 1W/1m)
2) 2.83v on a 4ohm speaker is 2 watts; 2.83v on an 8ohm speaker is 1 watt.
3) Going from 1 watt to 2 watts is a doubling of power. When you double your power you gain a theoretical 3dB in SPL.
4) Efficiency in a speaker is defined by the equation:
Efficiency = (B^2 * L^2) / (R * Sd^2 * Mms^2)
where B is magnetic field strength, L is length of the coil wire, R is resistance, Sd is surface area, and Mms is mass.
5) Your amplifier is always “happier” running at a higher impedance. You will get less power into higher impedances which will mean that your amp will run more efficiently and create less heat. The bonus is not only is it easier on your electronics, but on your charging system as well.

Now let’s dissect the facts a bit:
To prove fact two, we can look at the definition of power:
Power = Voltage^2 / Resistance therefore
P(8ohm) = 2.83v*2.83v / 8ohm = 1 watt
P(4ohm) = 2.83v*2.83v / 4ohm = 2 watts

Now we can ask what does this mean:
It means that most car audio manufacturers are rating their 4ohm speakers at 2x the power in their efficiency spec as are comparable 8ohm home audio drivers. We can then turn this around and adjust the efficiencies to level the playing field and make this more equal. In order to do this since the doubling of power adds 3dB all we have to do is subtract 3dB of efficiency from the 4ohm driver. Here is an example.

Speaker A is a 4ohm driver with an efficiency of 89dB rated at 2.83V/m
Speaker B is an 8 ohm driver with an efficiency of 87dB rated at 2.83V/m
Since we proved that at 2.83V/m Speaker A is getting 2 watts, to normalize this to a 1W/1m sensitivity rating we can subtract 3dB’s. In this case, we will restate the sensitivities based on 1W/1m:
Speaker A has an efficiency of 86dB at 1W/1m
Speaker B has an efficiency of 87dB at 1W/1m
All of a sudden the tables are turned and the previously less efficient speaker is in reality more efficient.

Now that we have looked at what is wrong with the rating system and provided information so that you aren’t so worried about using an 8ohm driver we can also look at the fundamental reasons why 8ohm drivers are more efficient than their 4ohm counterparts.
First we can take any generic speaker design and build two exact drivers other than one receives an 8ohm coil and the other a 4ohm coil. If we use the same exact wire, it is easy to figure out the relative length of the coil. L will be twice as long for an 8 ohm coil as it is for a 4 ohm coil. If we look at our definition of Efficiency from fact #4 we can see that as we increase the length of the coil, L, our efficiency will go up. So for identical drivers, just by having a higher impedance driver it will be more efficient.

As for fact 5, I put it in this discussion as it is important in the situation. You will be getting less power to your drivers, but as long as the make up for it in sensitivity it will be at the same volume, but with your amp putting out less power and therefore making less heat and overall being in a “happier” state.

In Summary:
On to the real reason for this thread, there are a ton of uninformed people that come onto audio sites looking for a 2ohm driver or some other lower impedance driver because that is what their amp is specified to do the power that they would like to run at. Basically I think this is hogwash. What you need to do is normalize the sensitivities of the driver and look at the power you have and see what will work the best. And while doing this do realize that intrinsically 8ohm drivers are more efficient than their 4ohm counterparts.

In the case where both speakers are rated at 2.83V/m
-The 4 ohm speaker is getting 2 watts, and the 8 ohm speaker only 1 watt
-With 2 watts versus 1 watt there is a theoretical 3dB gain, so to normalize we need to subtract 3dB from the 4ohm driver
-The amplifier however is only going to be outputting about half of the power at 8 ohms so to be fair we need to subtract 3dB from the 8ohm speaker.
=>This means that if the speakers are both rated at 2.83V/m and they have the same sensitivities, the output will be the same for both.
-Speaker A (4ohm) – 3dB (to reduce to 1watt) = Speaker B (8ohm) – 3dB (amplifier power reduction).
So at 2.83V/m as long as the sensitivities are the same the output will be the same.

In the case where both speakers are rated at 1W/1m this is much easier and why I would prefer if all speakers were rated this way.
-In this case the 8ohm speaker will need to be 3dB more efficient than the 4ohm speaker to have the same output as the only correction we can do here is that the amplifier will be putting out approximately half of the power which equates to a 3dB loss.
 

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