Hi Ally, just stopping by your build thread to say hi. good work on the vehicle.
I did want to comment regarding the discussion of efficiency above, the ratings of both 1w/1m efficiency and 2.83V sensitivity are actually calculated numbers based on driver parameters. They are basically accurate over any area where the response of the driver is not affected by the enclosure or baffle. It is the job of the system designer to then design the system to get the desired efficiency over the needed range. There are some limitations the designer has to work with though.
In sealed enclosures, down at lower frequencies the system is controlled by the enclosure and the efficiency of the woofer has almost no affect on the output of the system. We can use the SBP15 in 10cf sealed (basic trunk IB) as a means of comparison. The following shows the woofer with efficiency adjusted from 85-93dB by just altering the moving mass of the driver. You can see that once you get to 30hz, there is essentially no difference in output as it is completely controlled by the woofer.
No matter what woofer you put in, or how efficient it is at first, the limitation is the enclosure. This becomes more apparent in midrange/midbass drivers where you have smaller volumes to work with. The dominance of the enclosure is pushed up higher in frequency. Often times I see people making tiny sealed midbass enclosures and expecting miracles from the driver. At this point the only way to compensate for this is with EQ and added power. There is nothing wrong with using EQ to help bring up the low end in a sealed system. It is used in subwoofers all the time in everything from pro, hifi, recording and car markets and can be very effective. The Evolution Acoustics MM7 uses this approach with 4 sealed SBP15's and it is very successful. Just be aware that the efficiency of the driver alone doesn't control the low end output.
MMSeven | Evolution Acoustics
The other way to gain back efficiency is with a vented enclosure. Many people think of vented enclosures as boomy, slow, or not as tight as sealed enclosures, especially for midbass. In reality a properly designed vented enclosure can often be much lower distortion at any given SPL than a sealed enclosure. With low Q drivers that roll off very high, using a vented box will allow you to get the same input with as much as 1/6 the power around the tuning frequency. This keeps thermal distortions down, eddy currents low, and leads to much lower overall distortion at any given SPL. Vented enclosures also keep driver excursion way down at and around the tuning frequency. This keeps the driver operating in a more linear range, again lowering distortion.
The only issue you should be aware of is group delay, which is the derivative of phase. In a subwoofer application, tuning at 30hz and under there is not much to worry about. The wavelength of the frequencies is long enough that any group delay isn't a real audible factor. In midbass/midrange boxes it can become more of an issue. The best idea then is to tune lower. If you look at the group delay curve, by 1/2 octave above the tuning frequency the group delay comes down to a much more reasonable level, almost the same as a sealed enclosure. The mistake people often make is tuning a midbass enclosure right at the bottom of the range where they want to cross over. Ideally you want to tune 1/3 to 1/2 octave below your crossover point. You give up a slight amount of efficiency by doing this, but pushing the peak in group delay lower is IMO worth this slight loss. In this case you can see the difference between using the TD6M in a sealed enclosure of .5cf and a vented enclosure of .5cf.
If someone was to use a sealed enclosure, the efficiency is down by 9dB at 80hz. In comparison with the vented enclosure the response is less than 3dB down at the same point. You can see how much it is the design of the system and not the driver itself that determines the end efficiency. Also this 6dB difference means 4x less amplifier power to get the same SPL level. The difference between 1W and 4W may not seem significant, but the difference between 50W and 200W, and the difference in distortion and compression definitely is. There is some increase in group delay but crossing over at 80hz there will be little effect as the 5ms group delay is far shorter than the wavelength at 80hz.
That all said, there are some woofers that do not use realistic numbers for efficiency ratings at all. Many car companies used to give ratings WITH cabin gain included. Others will give an efficiency rating but not state whether it is a 1w/1m or 2.83V rating. I have seen dual 1ohm coil drivers with claimed efficiency of 98dB but in reality that is a 2.83V rating at 1/2ohm and the driver is more realistically 83dB 1w/1m. It is best to always put the parameters into a modelling program and let it calculate the efficiency for you. Others still will give efficiency based on measured curves, typically at the highest point of a breakup. Eminence does this for example measuring at a given frequency where the cone directivity is increasing the on axis output. This does not give a comparable number though because at other frequencies there is no way to get this actual efficiency no matter how the system is designed. In general though, the parameters of the driver will give you a very close estimate of real world efficiency for any point where the enclosure is not altering this efficiency.