Unknown to most enthusiasts, not all subwoofers are created equal. Subwoofers depending on size, placement, and packaging have different characteristics unique to that setup. For example larger subwoofers such as 15 inch or 18 inch subwoofers require larger enclosures for the subwoofer to operate properly. This is partly due to the fact that the larger mass of a bigger sub needs more air inside the box to help the subwoofer move and oscilliate and dampen its oscillations correctly.
The rear wall of the subwoofer box acts as a reflection panel that the subwoofer uses to reflect and compound bass waves to help the subwoofer itself reach a higher peak response (or a harder hit). At the same time the air inside the subwoofer enclosure acts as an air cushion to help dampen the subwoofers return after it excursion and control its oscillation so that it does not distort the sound and is able to track the audio wave accurately; similar to the suspension on your car preventing never ending oscillations and controlling body shake so that the suspension can track the road's surface.
A side effect of using a larger subwoofer is that a larger moving body in a larger enclosure is more naturally tuned with a lower resonant frequency. This is similar in nature to beating a larger bass drum vs playing the a smaller set of bongos. And so a different sized subwoofer, in a different sized enclosure, with a different set of ports or vents will have a different response to different audio.
These variations can be best understood and visualized through a spectrum analyzer that you can see used in SQ (sound quality) competition...
On a typical spectrum grahp, the loudness of the subwoofer , measured in dB, varies at different frequencies ranging from 15hz (the lowest possible human audible frequency) up to 200hz. These frequencies from 15 to 200hz are generally considered bass, where mid-bass drivers cover the lower midrange from 250hz up to 2000hz and midrange speakers shine in the range from 2000 to 8000hz where most human voice occurs, after that come the tweeters that take us from 8000 up to 16000hz or more.
Anyway, within that range of 'bass' frequencies a single vented subwoofer has a typical response curve with a peak response at around 40 to 50hz. This response is great for music like Rap or hip hop or even psychedelic trance and hard techno with deep and even subsonic bass being a characteristic of these genres of music. However if you were to play things like classic rock and some pop songs on this same system, you will find your bass lacking because those tracks more commonly use bass sounds that are in the higher bass range closer to mid bass or around the 125 to 200hz mark. So, an out of the box subwoofer (even if designed by a great company) may not hit all the frequencies you want it to hit properly depending on your style of music.
Once solution to this problem is to get a subwoofer system such as the infiniti basslink system which has an internal sound processor designed for live signal processing to improve the performance of the subwoofer in its enclosure no matter what type of music you throw at it.
However, for the diy enthusiast or for someone who already has a set of subwoofers in possibly a home made enclosure, then a parametric equalizer such as the Clarion EQS746 7-band equalizer is the way to go. The EQS 746 has 3 of its 7 bands in the bass and midbass audio range 50 Hz, 125 Hz, 345 Hz. This is something you have to make sure you check for when shopping for an Equalizer that can tune your bass frequencies because some Equalizers only have 1 slider for anything below 500 for example and those won't work for tuning the details of your bass subwoofer.
A quick way to know if you need such a product installed on your car is to think about this... "Do I constantly find myself playing with the gain knobs and bass boost on my system depending on what song or what genre of music I am listening to?"
If the answer to that question is Yes, then installing and tuning a parametric equalizer can give you a similar bass boost depending on the frequency range where you need the boost the most. This way when you listen to a song that has a different frequency bassline, that frequency will already be boosted and you won't have to mess with the settings on your audio system from song to song. This way you can spend much more time enjoying your music and less time fiddling with the settings.
Two important frequencies to work with here are the 50hz and 125hz frequency. Based on a typical vented subwoofer spectrum image I acquired, the graph shows that at 40 to 50hz we have a peak response of 83dB whereas at 125hz the response rolls off to around 66dB. What this means is that your bass at 50hz (when listening to a Rap track with deep bass) is about 17 times louder than your bass at 125hz (when you are listening to rock or classical music). This large difference in loudness is exactly why you find yourself having to fiddle with your gain knobs and exactly what we are trying to solve here.
With the EQS746, (or any similar multi-band EQ) you can lower the gain for the 50hz frequency, and increase the gain for 125hz and 345hz respectively. With some trial and error, and while using music tracks that are known to be good 'test tracks' for testing the complete audio range of your sound system, you can find a setting that gives you the most 'linear' response across the entire bass range. You resultant spectrum analysis should be as flat as possible between 25 and 200hz with the gain rolling off at frequencies lower than 25 hz or above 200hz. This is all performed with only the subwoofer playing that is; if you had the entire system running when doing these tests, your midrange and midbass speakers should continue this linear trend past 200hz and pick up where the subwoofer would drop off.
Once you've found this flat response setting, you can adjust the overall gain of your system to increase the whole bass range up with reference to the rest of the audio to make up for taking some gain out of our 50hz range and reducing the overall power of our bass hit. Once you've made that adjustment, you are done and you now have 'set it and forget it' awesome sounding system that can process anything that you throw at it.
Secrets of DIY Subwoofer Equalization
Secrets of DIY Subwoofer Equalization
7 Inch Car Monitor