Suitable Materials for Sub Box

[wb131988]

I have now completed my system, thanks to you peepz, and it is truely awesome. However this is only true when I sit inside the car. But when I open the boot, why do I not hear the bass from the subs no where near as much?? I only want to know this as when I go to a car show I want my system to be heard. Also, does the box have to be completely sealed with thick MDF as I have an oval shaped piece of 4mm thick clear plastic inserted into the front of my sub box which seems to be causing this rattling noise..... Thanks

[Google]

[wb131988]

This is obviously more of a practical question than technical, but any help much be much appreciated.

[Hoedizzy]

This is a question for Mr. Sandness...

[sandness]

[1] You get louder, better bass in your car, doors shut, due to cabin gain. It is typically 12dB/octave in the average car. This emphasizes your low end. Being outside the car with the trunk open, you do no have the cabin surrounding you and lose the gain it supplied. What you hear will be closer to the speaker's anechoic response.

[2] The material from which an enclosure is to be built from needs to have characteristics such that it either raises or lowers the box's resonance frequency, such that it is out of the passband. Basically, there are two wasy to do this- Make a very stiff, rigid box with a resonance frequency above the speaker's passband, or build a solid, heavy box who's mass lowers the resonace frequency below the passband. The idea, either way, is to prevent resonances that give lead to poor sound quality and cancellation. The stiff, rigid box could use about any good wood (MDF, marine grade plywood, etd) along with plenty of bracing, fiberglass 3/8" thick or more (depending on the shape), among other materials. The mass loaded heavy enclosure would use MDF, HDF, or plexi of significant thickness to prevent flexing (Note: if your 4mm plexi is bigger than, say, a 3" circle, it needs to be much thicker - more along the lines of 3/4" to 1" for a significant size. Otherwise, it flexes and creates cancellations).
[3] As far as sealing goes, do it. Air leaks can not only be annoying, but also can make the sub sound as if it were over-excurting. Again, these leaks lower the sound quality.

Happy bassing

[sandness]

Here's an graphical example of cabin gain on a frequency response plot



Likewise, in home audio and such applications, you have room gain. Technically speaking, anechoic response is in free space- which simply never happens in real life applications, thus doesn't show us exactly what our ears will hear. In half space (single open flat plane) you get some reinforcement of the sound as it is "reflected" off the plane. In quarter space (2 perpendicular planes, like a wall and a floor) you get more reinforcement as waves are reflected and now only have to pressurize 1/4 or the space. Likewise, 1/8th space (3 perpendicular planes, like the corner of a room) provides even more reinforcement. These various loading techniques help give you the most bang for the buck in a room. The cabin gain is basically a derivation of such methods wherein you have even less volume to work with while providing many surfaces nearby to "contain" the pressure.

[fussnfeathers]

In laymans terms..........lmao

Car subs are designed with a specific "box size" in mind.....namely your car. When you open the trunk, you're increasing the "box size" about a million times, and you won't hear as much.

Air leaks will cause a different problem, with basically the same result.......you want the box to respond (or not respond, depending on the type) to the sub's air movement. Take a sealed box. The whole reason those sound the way they do is they don't "breathe".....the air inside stays inside, and responds accordingly. If there's a broken seal anywhere in the box, the air inside doesn't respond the way it should, it simply tries to push out of the box.....so all of your frequency response goes directly to that spot, like water in a leaky cup.

Not to mention the annoying whistling sound a leaky box makes.....enough to make your teeth grow hair.
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Big enough to scare you

[ToggleHead]

^^^ you boys got this one
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[wb131988]

it doesnt seem to make that annoying whistling sound u talk about, but I'll seal it good n proper anyway. So i either need some real thick plexiglass or MDF for the oval shape on my box- its the size of a 6x9 sandness lol
Cheerz for the information and detail, so instead of opening the boot I could open a front door so that the car 'amplifies' the bass from the sub box to front, and then outside at a car show. I tested this and it seems to work much bassier.

[wb131988]

Sandness your a real pro considerin youve only posted about 100!

[ToggleHead]

^^^^ doesnt mean he learned everything from HERE silly.....=]

Yes the speakers themselves have a box, but the SOUND's "box" is your car, which is why smaller cars tend to get more sound out of less equipment.....open your front doors, and the speakers are still using most of that box..... open the HATCH, and Earth becomes your box....try filling THAT...=]
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[fussnfeathers]

[wb131988]

So if I use the 3/4" MDF to fill the oval and seal it good n proper, will it sound better e.g. bassier?

[ToggleHead]

mostlikely wont get a result the human ear could determine...but as long as its nice and air tight...you will be happy
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[sandness]

[Sally]

[wb131988]

The whole box itself is made of about 3/4" thick MDF and fully sealed with sealant inside..... if you remember, before i had a 6x9 and a 12" JBL also in the box which i decided to take out cos of your recommendations- these are the two wholes that i need to fil with the same thickness MDF and seal securely. The box itself is very strong.

[wb131988]

So when I open the boot and turn the volume up, is the bass supposed to sound distorted??

[c o n v e x7]

another good way of stopping vibrations etc is to use sound deadening.
.increase SPL (how loud your stereo plays)
..increase dynamic range of your stereo system
...decrease resonance and coloration from speakers

also a type of padding (large cotton wall type stuff) goes behind the sub, making it a nice fit and stopping alot of uneeded vibration behind the sub in the enclosure.