Down firing subwoofer

[punkrokr1701]

I'm buildinbg a downfiring subwoofer and i was wondering is it better to just have the driver firing straight into my carpet ( which is about 3/8" carpet) or to install a 3/4" piece of birch as a base for the entire enclosure?

[infinia]

Hi
Not quite sure if I understand your question correctly. I would think the opening/s would be at least equal or 2-3 times greater than the Sd of the bass driver. Most likely some optimum distance above the floor depending Freq tuning and location of the sub in relation to walls or corner.
edit> FYI Have you read up on pitfalls of using drivers not designed for downfiring applications?

[punkrokr1701]

yeh im sorry the question is a little foggy, im just wondering if the subwoofer will do better as far as sound radiation firing straight into the carpet or firing into a hard surface b/c of how the lower sound waves reflect.. The sub will be about 2 inches above the ground in a corner in a 8ft^3 tuned @ 34Hz with about a 600 watts rms input, the driver will be an MJ-18. And yes i have done the research on drivers that won't work for downfiring subwoofer there's formula that determines that sag of the driver and if the sag is more than 5% if the Xmax it wouldnt work but this driver will do great down firing.

[infinia]

OK I get a better picture. I would go without a wood base for your sub. The carpet floor shouldn't change the bass so much as the higher frequencies, which all things else being equal will be a good thing. Should help your low pass filter effect there.

[HipoFutura]

When I built my subs I went with a base plate. You can see in the photo where I have the two subs stack on top of each other with one flipped up side down. The base allows for reflection, where as the carpet will act as insulation and absorb sound. The base plate provides some flexibility - like stacking them. Don

[AndrewT]

Hi,
have you put some real numbers into that sag formula?

I seem to recall that the two formulae end up only using Fs, but no-one came back to challenge my conclusion.
http://www.diyaudio.com/forums/showt...377#post965377

If the Xmax only depends on Fs and you aim for a driver with a low Fs then it must also have a VERY long Xmax. Go for a higher Fs and it is no longer a sub, but more of a normal bass frequency speaker.

[infinia]

Andrew
I don't follow your logic. Doesn't the Xmax depend on the motor structure not on Fs. I think this is a suspension thing whereby the cone/spider sag eats into a portion of the drivers X range or linearity range.

edit>Re: another false logic from your post.
Just because no one challanges your position doesn't make it true.

[AndrewT]

Hi Infinia,
thanks for the response.
Your the first and only.

You're right that because no one challeges then it may simply be they can't be bothered or it's not important enough or on cursory glance cannot see the flaw in the argument.

Can you see what was wrong with my interpretation in my original posting?

....the Xmax only depends on Fs......

should have said

......the Xmax:sag ratio, only depends on Fs (and if you aim for a driver with a low Fs then it must also have a VERY long Xmax).....

[infinia]

Hi
Andrew you made a mistake in your math. Cms is squared not cancelled.

[AndrewT]

Hi Infinia,
I hope you're right because my observation does not seem correct.

I went back and checked again, could you do likewise?