Why sub boxes are important

[xtremerevolution]

Last night I was talking to skitchin online about his old sub and sub box in light of the recent sale on partsexpress that I posted in another thread. I decided to run a sub box model for him using the box he built and his specific sub, and I figured the results would be beneficial to anyone who has a remote interest in car audio. I'll go over the 4 biggest factors I look at when designing a sub box, as well as a thorough analysis of his sub box in respect to those factors and how it sounds.

 

His sub is here:

http://www.rockfordfosgate.com/products/product_details.asp?item_id=91839&locale=en_US

 

The box he built is here, in the middle of the page:

http://www.rockfordfosgate.com/scripts/rightnow.cfg/php.exe/enduser/std_adp.php?p_faqid=469&p_prod_lvl1=19&p_prod_lvl2=26&p_cat_lvl1=5&p_cat_lvl2=&p_search_text=469&p_search_type=2%3Afaqs.faq_id

 

Box is 2.25 cubic feet tuned to 40hz and was run off of a Rockford Fosgate 500W amp.

 

 

Frequency Response

The first thing I look at is frequency response. You'll need to pay attention to the black line. For the best sound quality, you generally want as flat of a frequency response as possible, which you can then tune with your head unit to better serve your preferences. To put things into perspective, the lowest note on a bass guitar comes in at 41hz. Kick drums have harmonics that hit in the low 100s. Most rap, dubstep, and techno have synthetic bass that plays in the mid and low 30s, with some material digging into the high 20s. Rarely will you have any material that goes far below 30hz with the exception of organ music, which gets down to 18hz. Keep in mind, those are "as low as" they will go. Most sub-bass content is in a usable range of 29-125hz, which we try to make as flat as possible. Above that point, your front speakers should be taking over as the bass starts to become more directional, and below that point, content either doesn't exist, or the ear can't hear it (threshold is 20hz).

 

The following chart is a modeling of his frequency response with his car's boundary loading and cabin pressurization gain factored in. Basically, how this sub will sound in his car. If you took an RTA system to this sound system, you'll find that it measures very close to this. Now that we know where the notes play, we need to talk about volume. You'll see a distinct peak here at 42hz, which is close to the 40hz tuning frequency of the port on this box. To put volume into perspective, a 10db increase is perceived by the human ear as a 2x increase in loudness. Consequently a 1db increase is a 10% increase in perceived loudness.

 

So, how does this box look? Terrible. Its what audio guys call a "one note wonder." It plays a 42hz tone 18db higher than it plays a 90hz+ tone. Its made to be nothing but loud. This will make the bass sound boomy, and because it is so much louder than the other notes the sub needs to play, it will not blend well with front speakers and it will overpower and drown out the other bass, midbass, and some mid frequencies. 18db is a huge difference.

 

 

Excursion

The second factor is excursion control. The dotted line is the xmax of this particular sub, and the box is modeled with a 500W RMS power input. Xmax, in layman's terms, is the electrical excursion limit of a subwoofer. The purpose of a sub is to move air, and to move air with a sub, it needs excursion. Once you increase excursion past the rated xmax (cranking the volume), you start to increase distortion at an exponential rate. Now, while bass distortion is difficult to notice, it does play an effect on how well the subwoofer blends. To describe this, when you're playing within the xmax range, you hear the bass in its fullness. It is as transparent as it can be for that particular sub, and it sounds like its coming from everywhere. Once distortion starts creeping in at higher volumes, you start hearing the subwoofer as well as the bass it produces, and you can tell that its coming from behind you. You can pinpoint the source. The harmonic distortion created is definitely noticeable when you've heard the difference between a sub that has it and one that doesn't. In this regard, this box did a good job. However, a high pass (subsonic) filter would need to be set on the amp to prevent the sub from reaching xlim (maximum mechanical excursion) and bottoming out, as the excursion rises rapidly in frequencies below the tuning frequency.

 

 

Vent Air Velocity

With ported boxes, one needs to worry about the speed of the air moving through the port. The port is designed as a ratio of the volume of the port to the volume of the rest of the box. The volume of the port is not counted as box volume. A general rule when designing a box is to keep port velocity below 34m/s at its highest peak. Any higher than that and the air becomes turbulent. This causes chuffing (aka port noise) and de-tuning. De-tuning as a result of port noise causes a loss in output, which is counterproductive. This is one thing that this box does well, as the slot port design is equivalent to a 7.25" round port in port area. However, this is also quite excessive, as it caused the box to be much too large, and did not take good advantage of the port area being used up. A better box would have had a slightly smaller port and kept the same length, thus lowering the tuning frequency and making the box sound less boomy as described above. For port noise purposes, this is good, if not too good.

 

 

Group Delay

The last factor I look at (but certainly not the least important) is group delay. To explain how this works, one needs a fundamental understanding of how subwoofer boxes work. The primary purpose of a subwoofer box is to separate the backwaves of a sub from the frontwaves. The reason is because these bass frequencies are omni-directional and they "wrap" around the subwoofer. If the front waves meet the back ones, they cancel each other out and no output is created. Anyone who has tested a subwoofer in free air (not in a box) will notice that the sub produces no bass even if the cone is moving violently. This is why. A sealed box is the simplest way to accomplish this. A ported box is designed with a vent that serves to increase output. At a certain tuning frequency, the air inside the port resonates and creates significantly higher output, often 6-10db higher in that range than a sealed box would. Now, whenever we have output in addition to that created by the subwoofer's cone, we call that augmented output. All augmented output has a delay. Between the time when the sub creates the note, to when the air in the box moves, to when the air in the port moves and creates a resonance exists a consequential delay between when the sub creates the tone and the port creates that same tone at a greatly increased output. This is why many people have claimed that a sealed box will sound "tighter" than a ported box, although a well designed ported box will be designed in such a way as to minimize group delay or keep it in a range where it cannot be heard or distinguished. In this particular case, a sealed box would sound much tighter and cleaner.

 

Back to this specific box, a sealed box typically has a group delay of 5ms or less. Here, we have a peak group delay of ~28ms. To put this into perspective, a 50ms delay is audibly perceived as an echo. The rule of thumb when designing a box is that in order for group delay to be nearly inaudible or at "acceptable" levels, the product (multiplication) of the group delay and the frequency at which it occurs needs to be at or less than 400. In our particular case, we have 28ms of group delay at 42hz, which comes out to 1176. This group delay will definitely be audible and will cause this sub to sound loose and uncontrolled. Think of it this way. What happens if you have to make a transition from a 40hz tone to a 100hz tone very quickly? With a low group delay sub, you'll hear one, then the other. With a high group delay sub like this one, you wouldn't be able to tell them apart because even though the amplifier is playing a 100hz tone 25ms after the 40hz tone, you'll hear them at the same time due to how the box is built. It just won't sound as clear.

 

 

 

I hope that the above has demonstrated the need for properly designed sub boxes, and also demonstrated how poorly manufacturer specified boxes can be designed. Keep in mind, the above was a box designed by Rockford Fosgate, so one would assume they knew what they were doing. For teenage kids who care about nothing than being heard from blocks away, this would do it, but for anyone who has passed that phase and wants some quality output, this is a horribly designed box.

 

That being said, I am available to design professional subwoofer boxes for anyone interested and I do it for free, so let me know if you're interested. Feel free to ask any questions if I wasn't being clear on any of this.

Edited December 27, 2011 by xtremerevolution

[skitchin]

I've owned my sub for years and I've certainly noticed all of the negative points you mention in this post. Especially the one note wonder - certain frequencies just rattle your whole inner ear, where other ranges just seem to fall on their face. It looks like a couple other guys here have picked up that T3 from PartsExpress, I'd personally love if you could design me something. Just looking for something simple and easy to put together. I don't need the pass-thru slot for anything, I've always just opened it to let air and bass flow.

[xtremerevolution]

I've owned my sub for years and I've certainly noticed all of the negative points you mention in this post. Especially the one note wonder - certain frequencies just rattle your whole inner ear, where other ranges just seem to fall on their face. It looks like a couple other guys here have picked up that T3 from PartsExpress, I'd personally love if you could design me something. Just looking for something simple and easy to put together. I don't need the pass-thru slot for anything, I've always just opened it to let air and bass flow.

 

What music do you mostly listen to? How many of these T3 subs are you looking at getting? Still planning to use the same amp? That slot is the port btw.

 

Sent from my HTC Awesome using Tapatalk

[skitchin]

What music do you mostly listen to? How many of these T3 subs are you looking at getting? Still planning to use the same amp? That slot is the port btw.

 

Sent from my HTC Awesome using Tapatalk

 

I'd say primarily hard rock + metal, secondary would be dubstep + rap, with basically everything but country in-between.

 

Basically, if AC/DC sounds good, I'd be a happy boy .

[video=youtube;RukUetw0hAM]http://www.youtube.com/watch?v=RukUetw0hAM

Edited December 26, 2011 by skitchin

[xtremerevolution]

So you want something that plays linear from around high 30s on up, especially in to the 100s. Shouldn't be too hard. I'll work on something tonight. I have a feeling more more people will be able to use the same box model. I'll make an SQ and SPL box for those who might still want nothing but raw bass.

[Garrett Powered]

I remember bumpin that tune on my 18" sub my blazer had in the early 90's thinking it was the hardest hitting anything on that system. don't need that much excursion for that song to pound pretty hard with an 18" sub. or most other butt-rocker stuff for that matter.

[xtremerevolution]

I remember bumpin that tune on my 18" sub my blazer had in the early 90's thinking it was the hardest hitting anything on that system. don't need that much excursion for that song to pound pretty hard with an 18" sub. or most other butt-rocker stuff for that matter.

 

True that. Cone area is king. Unfortunately, fitting an 18" sub in a w-body is another issue entirely unless you eliminate your back seats and go IB.

[Garrett Powered]

I was thinking about that also. maybe f-glass in a reverse mounted one using the wheelwell space and above it a little. cone facing the trunk latch. magnet facing towards the front of the car. an 18" could be glassed in easier than trying to fit that much wood in your trunk though. and the next woofer I want is a TC sounds 18 neodymium anyway so thats why haha.

[xtremerevolution]

I was thinking about that also. maybe f-glass in a reverse mounted one using the wheelwell space and above it a little. cone facing the trunk latch. magnet facing towards the front of the car. an 18" could be glassed in easier than trying to fit that much wood in your trunk though. and the next woofer I want is a TC sounds 18 neodymium anyway so thats why haha.

 

Not a bad idea, but you need to remember that you'll still need the airspace either way. That, or a parametric EQ. If the airspace is too small, you'll raise your qtc and lose your bottom end. That said, I would absolutely love to see it done.

 

Sent from my HTC Awesome using Tapatalk

[skitchin]

For comparison purposes, what would the Frequency response model look like w/ your recommendations?

[xtremerevolution]

For comparison purposes, what would the Frequency response model look like w/ your recommendations?

 

I'll pull up another model on my laptop tomorrow. Which volume did you want the model for?

[skitchin]

T3 sub in a 1.25 cu ft sealed and pushed by the same 500w RF amp.

[xtremerevolution]

Models attached for 1.25 and 1.75 cubic feet (gross) boxes. I had recommended the 1.25 cubic feet for you based on listening preferences, but both would work. The 1.75 cubic foot box has slightly more bottom end dig to it.