hi, just a quick question. as far as i have read, if you double the power, you get +3db? this doesn't seem to make sense, take this for an example: a sub with an efficiency rated as 87db @ 1w should give 90db @ 2w 93db @ 4w 96db @ 8w 99db @ 16w 102db @ 32w 105db @ 64w 108db @ 128w 111db @ 256w 114db @ 512w 117db @ 1024w 120db @ 2048w
so to get higher than 120db, it would need over 2000 watts of power. how do people reach 130's, 140's, etc? most subs are rated at about this sort of efficiency, but don't always have 2000w+ peak power.
right if u have a certin type of cabin it could add around 6dBs or more and a ported box could do around 12dBs. so if ur statistically at 120dB above and u gain 8dBs off the cabin and 12 of the ported box, ur at 140dBs and so on..
efficiency ratings and SPL aren't the same thing. you need to take into account the enclosure, the vehicle and so forth. also remember the +3dB = 2X the power or the same power to two subs instead of one is limited to within the mechanical abilities of the driver.
a little more info from one of my faqs on efficiency:
Speaker Sensitivity and Efficiency.
Speaker sensitivity is a specification provided by all manufacturers of high-quality speakers. The sensitivity rating has no relation to sound quality, as some of the very best speakers have low ratings. Sensitivity ratings simply tell you how much sound a speaker will produce for a given power input.
Sensitivity ratings are given in decibels per watt at one meter, or db/Wm. So, with an input of one watt (usually white noise), a speaker with a sensitivity of 90 db/Wm will produce 90 decibels of sound at a distance of one meter. A sensitivity of 90 is considered average, with ratings of 87 and below considered low sensitivity and above 93 considered high sensitivity. To increase the volume by 3 db, you must double the power. So, using the example above, to make 93 db you would need two watts, and to make 96 decibels, four watts.
Most of the time your system is cruising along producing only a few watts. You need extra power for loud bass passages, crescendos in classical music, and other highly dynamic passages. Your speakers may need more than 10 times the average power to re-create these dynamic passages accurately, and if you are playing loudly to begin with, you may need an awful lot of power if you have speakers with a low sensitivity rating.
So, when you are buying an amplifier, consider your speakers, your vehicle size and how loudly you want to play. If you have sensitive speakers, you probably will not need as much power -- even 20 clean watts would probably be enough. If your speakers are only moderately sensitive, your vehicle is large or exceptionally noisy at highway speeds and you want to play loudly, you will need more power in order to faithfully reproduce dynamic passages.
"Sensitivity," which is expressed in dB, should not be confused with "efficiency" that is expressed as a percentage of power out relative to power in. Efficiency data for loudspeakers suffers from many problems such as failure to consider variations in frequency response.
Speaker efficiency is the ability of the speaker to do work or use power. The more efficient the speaker; the less power is required for the speaker to produce sound. Voice coil design, type and size of the magnets, speaker cone design and material, speaker size, etc. all play a critical role in determining speaker efficiency. However, speaker size is a good general method for guessing efficiency.
Typical speaker efficiency (for physicists) is about 5%. Meaning that for 100% power input, you get about 5% acoustical work back.
Keep in mind that when considering subwoofers, or any speaker that will get more than ~100 watts RMS of power, these measurements are affected by other factors that make this specification less than useful when choosing between speakers.