downward firing sub questions

[john trials]

I'd like to get some more information regarding downward firing subs. I'm amazed that there isn't more information in Speaker Building 201 and/or The Loudspeaker Design Cookbook (or the web).

I'd like to get some information so I can build the sub, but I'd also like to learn more about this...I'd like to UNDERSTAND the sub, not just build it. Any web links to informational pages would be great...the more physics involved, the better.

I plan on using Zaph's archived sub design: the RSS315HF-4 12" subwoofer, and a 2 cu ft volume cube.

Questions:
1) I mainly want to have the driver downward firing to hide the driver for aesthetics...am I going to adversely affect the sound by changing a side-firing sub into a downward-firing sub?

2) What is the minimum recommended clearance between the subwoofer driver and the floor (and WHY...what would happen if the clearance was only an inch or so...I'm not going to do this, I'm just curious)? The only answer I could find on the web was driver diameter/4 (or 3" of clearance in this case), but no explanation as to why.

3) Is it bad to have the sub firing at carpet, or would a hard surface be better? Most downward firing subs are sonotubes, which have a rigid baseplate that the sub is aimed at. Is there an acoustic advantage to the hard surface, or is it just for stability of the legs?

[1Michael]

1. Many of us have our drivers firing down and works quite well. Yes there is the issue of 'sagging' of the driver that can adversely affect the performance, but like I said many of us do it anyway...

2. For a 12'' I would give it at least 3'', for my 15" sonosub I went with 4". Why because the sound waves need a little breathing room. Having the driver almost touch the floor just does not sound right to my ears, not to mention the fact that having that powerful sound energy directly reflecting back into the driver may not be good for it.

3. Does not matter. My sonosub has a wood surface, my 18" sub fires onto carpet. It makes no audible difference to my golden ears

PS, try not to make it a cube shape if at all possible :T

[jkrutke]

Not to shoot down bugger's response, but I'll give it a shot with my opinionated answers to the questions:

1)Not initially, only later when sag sets in. And sag WILL set in. Don't even pay any attention to the sag equation you see around, there's more to it than that. That equation only works if a driver is never used. As a driver is used, particularly at high level or high excursion, the suspension deforms at a molecular level, and in a different way than what most people would consider "breaking in a driver". It basically breaks in against gravity and slowly offsets more and more outward.

The world is full of people who have done downfiring subs, and they are inevitably quick to defend their choice of sub configurations. Just be aware that I've measured suspension offsets on several long term downfiring subs. Some that were only supposed to have .5mm of sag according to that stupid equation were nearly half wave rectified with 100% sag. Xmax can end up being +0 / -full coil length. How soon it gets there only depends on how hard the driver is used.

2)the other questions almost seem irrelevant now, but the minimum distance to the floor is the one that limits restriction. At some point output becomes reduced and the T/S parameters start to drastically change too. It's roughly 2" for most subs up to 12".

3)At carpet is better than at hardwood or linoleum for example. This is the one true benefit of downfiring subs: the carpet absorbs high order harmonics and makes the sub far less locatable. Or to say it a different way, you can cross over a downfiring sub higher if you wanted.

[evilskillit]

Interesting post by Zaph. I've often guessed that was probably the case and have vowed to never make a down firing sub just on general principal. I suppose one thing to consider tho is how long it takes the driver to get to that bad of a state. I have seen plenty of factory made downfiring subs that seem to be working ok after a few years.

Also unless you go super expensive most sub drivers are less than $200. As long as it lasted for a few years it might have been worth it due to the increased visual appeal, decreased likelyhood of something damaging the cone and the decreased audible high order noise.

However if you don't tend to have the money to fix things at later dates or just like your stuff to last forever like I do, I would imagine downfiring is probably a bad idea.

[john trials]

Thanks for the great information, guys. This will be my first sub, and I'm not expecting to go near Xmax. My room is small and my house is usually occupied with other people, so volume is kept on the low side.

On the other hand, I can imagine loving the sub and letting it really crank, so I should probably plan for that! Maybe down-firing is not the way to go. Like I said, I'm thinking of down-firing for aesthetics only.