better box design tips

Recently I received a private message about using test equipment, box modeling software and maximizing a subs potential. As I was answering the questions it dawned on me that this might be good information to post where others could view it. I wrote to the sender and got his ok on this, so here it is Hi Just a few quick questions, I’m a beginner at most of this stuff but I have a good grasp of it. The test equipment I have is the woofertester for measuring t/s parameters, BassBox Pro for design and a basic spl meter up to 130dB. I am hoping to get a copy of trueRta and a decent Mic too. Can I make a graph of the transfer function using a test cd and my spl mic? How would this affect my design if 1. I want SQ, 2. I want louder bass. In your article you suggest designing the box to peak around 35-40 Hz if I remember correctly. Is this the best region to boost the bass? Any other suggestions for test equipment or words of wisdom? Thanks Answer You are on the right track with the test equipment that you have. The woofer tester that you have is worth its weight in gold! To my knowledge they have discontinued that device due to lack of demand. I used the woofer tester II to retest the parameters on all of my projects up until just not to long ago. I recommend using the physical-mechanical parameters that you get from the manufacturer and then adjusting the electrical parameters in Bass Box pro using the information that you get from the woofer tester. Due to how speakers are mass produced the actual t/s parameters are prone to large deviations. This will cut that margin of error out of the equation. Have you discovered that the woofer tester can export its data directly to Bass Box Pro? Pretty cool! You can make a fairly accurate transfer function measurement using an SPL meter and a test CD. You can make an even better measurement if you know the frequency response of your SPL meter. If your meter is a radio Shack meter you are in luck. I have been told that the frequency response of that particular meter has been documented many times by many reputable sources. I suggest checking out speaker builder forums to find more leads on the frequency response of your particular meter. Car audio forums are not a good source for that type of information. The Radio Shack meter is also a good general purpose mic to use with True RTA. The way that all of this info changes your design is evident as soon as you look at how the “in car” frequency response looks after the transfer function has been accounted for compared to the generic 12db in car response that Bass Box Pro can simulate on its own. This information is entered in the “Acoustic properties box”. You will likely notice a peak it the cars transfer function around 50hz (depending on your car). This is due to the natural resonant frequency of your car. Below that point you will see yet another steady rise as frequency lowers. This is due to reflections with in the car. Just like your sub has a resonant frequency and your box has a resonant frequency so dose your car. If you are looking for high SPL then you should be considering vented enclosures and subs that work well in them (it should be noted that Bass Box Pro’s EBP indicator should be considered but is not accurate for car audio use). Take a close look at the cone displacement graph and the custom amplitude response graph at the same time. It is very important that you make sure that the sub will be able to control its self at all usable frequencies. A stellar frequency response is useless if the subs cone has to travel too far to make that happen. For ultimate SPL I use this formula. 1. Make F3 and Fb occur as close as possible to the same frequency. Due this with out the transfer function or generic in car response. 2. Play with box size to obtain the highest spl (largest peak on the custom amplitude graph). Do this with the transfer function on and being accounted for. F3 and Fb might need to be adjusted for best out put and lowest cone displacement ratio as box size changes. Play with it. 3. do your best to line up the dip in the cone displacement graph with the peak in the custom amplitude graph. This way the when the sub is moving the least the out put is the greatest. 4. Now change your box design to a “Vented with active high pass EQ filter” design. 5. Set FX (high pass filter) to the same as Fb and Qx (boost) to 1.93 6. Now after you have tweaked all of that and think that you are in the ball park, start playing with port area, keeping in mind that changes in port area will have a profound affect on cone displacement, SPL and group delay. Don’t make any assumptions on this step. Calculate various sizes and see what happens. As a follow up I like to plot the custom amplitude response of a sealed enclosure and a regular vented enclosure with out the known transfer function of your car and compare the three. The results will all be worlds apart. This way you can see the advantages or disadvantages of your design with perspective. I have seen SPL differences as much as 33db! When its all said and done if you are looking for something that is more sound quality it is easy to see that you can still use this information as a vary strong tool. By simply adding some equalization to lower the peaks in your subs response and possibly increasing box size and lowering Fb you can achieve very accurate response with huge output good group delay and lower distortion. As far as the best place to peak the bass response. Speaker selection largely dictates where you can obtain a boost. As you play with various speaker designs you will find that speakers have a sweet spot where they are the most efficient and boost is obtainable. For SPL boost at the vehicles resonant frequency (the peak around 50 hz) For sound quality boost below this point to get a flat but raised response that extends below 40hz (42 hz is where the kick drum and bass guitar produce the most energy). Personally I like a real kick in the butt. I design my boxes to have high output and good cone control through out the bass region and an Fb usually around 30hz and a 6db peak at that point. This way not only is the fundamental bass note strong but so are the harmonics and any low frequency reflections that may have been captured in the recording. It takes a speaker with a low Qts, and Fs, high power handling, a good bass Eq, and a lot of amp power to pull this off. This type of bass reproduction has a life of its own! Remember that a peak in the response can be caused by transfer function, box design, Eq or all of the above. Manipulate them to get what you want.

It should be noted that I tend to design high order enclosures that make the most of the subs efficiencies. Due to this fact I tend to favor subs that have a low resonant frequency and a Qts of less than .5. The guidelines that I mentioned above generally do not work well with subs that have a Qts above .5 or an Fs above 40hz. In my opinion any sub with parameters that fall out side of this range are for the most part poorly designed. High Qts values reflect a poor motor structure and high Fs is usually caused by a poorly planed sub over all. Also I always strive for good group delay (response time). I don’t believe in sacrificing sound quality for SPL. Good designs give you both.