Downward firing Sub

[mart34]

Hello,

I have recently designed a subwoofer. It consists of a 15" driver, 500Wrms panel amp and a 2.5 cu ft enclosure.
My intention was to have the box positioned so that the driver would fire downwards.
But due to the weight of the driver, I am worried that when working at its hardest the driver might blow itself out of the bottom of the enclosure.
Is this possible?

The driver is held to the box with 8 screws and they are all screwed tightly into the mdf enclosure. Should I use Nuts & Bolts?

Any help will be appreciated

Thankyou

 

[simon5]

First, you need to check if your driver is suited for downfiring applications.

Screws should be enough to hold it in place, nuts and bolts won't hurt but it's harder for maintenance.

 

[richie00boy]

Are you aware of t-nuts? I think they will be your solution. If your box is made from MDF this isn't the best material to screw into, although as long as you haven't put in & taken out the screws too many times you will probably be OK.

 

[mart34]

I have never heard of T-Nuts. Can I get these from any DIY store, or do I have to buy them from a specialist store?
Also, how do I know if my driver is suited to downward firing?

Thanks for the useful replies.

 

[richie00boy]

T-nuts are like a big washer with prongs sticking out one side and a threaded part. Basically you put one where your bolt hole is and knock it onto the wood in place of your normal nut. Very useful when you can't get into the box to hold a nut. You can get them from B&Q and many other places.

Some people complain that they fall out when removing the bolt, but I have never ever had a problem with them.

As for the woofer and downward firing, on some speakers the cone sags too much when downward firing. Basically just hold your cone vertically then tilt it towards the floor. If the cone does not move then you are OK.

 

[simon5]

With this document, you are able to check if your driver is suited for downfiring or not.

http://www.adireaudio.com/Files/TechPapers/DriverOrientation.pdf

 

[Vikash]

T-nuts are your first port of call, especially with large/heavy drivers IMO. If you can source brass inserts in small quanitities then even better: http://www.vikash.info/audio/mtm_floorstander/build_day08.asp

 

[mart34]

I calculated that gravity will make my driver sag by 0.62mm. My driver has an xmax of 13.5mm. So, the sag of the driver as a percentage to the xmax is 4.8%. It is suggested that, drivers are suitable for downward firing if this percentage is less than 5%. So, I guess this just about cuts it.

What are the main negative effects of this sag?

Do the positive effects of downward firing out-weight the negative effects of sag?

Thankyou for the informative responses.

 

[Bill Fitzpatrick]

What are the main negative effects of this sag?

Do the positive effects of downward firing out-weight the negative effects of sag?

In the short term, less excursion capabilities in one direction. In the long term, who can say. I've seen downward facing woofers sag to the max over a couple of years. This was in the late 70s' though - maybe materials have changed enough to prevent this these days.

I didn't know there were any positive effects of downward firing subs.

If you're concerned about 8 screws not being enough, drill 8 extra holes.

 

[Keld]

I didn't know there were any positive effects of downward firing subs.

Only thing that comes to mind is driver protection.

 

[simon5]

Some BL curves are better in one-way than the other.

The Shiva, for example, have a higher Xmax when downfiring.
Probably caused by something like this:
Xmax 16.6 mm backward and 17,6 mm forward.
So they rate it at 16.6 mm Xmax, because it's effectively 16.6 mm if front firing.
If downfiring, the driver sit 0.5 mm down, so 17,1 mm Xmax both ways now.

In early Shiva versions, you can see the BL curve on the DUMAX sheets at +19 mm forward and -14 mm backward, so I'm probably right.