Hi Adrian,

The 18 sounds like a beast.  I think the Fs is a bit too low for the imperial.  Maybe you could try a scaled UP version to get the horn frequency down to 30Hz if you wish to use this driver. It would need to be 1.3 times bigger.

Regards Philip

Yep

But I thought the round about number was a tuning of 35 and give or take from there,

But it dounds like fun ! Having an Imp is a dream come true. Well thats  what I think anyway.

All the others are, hmmmm, well,  just speakers

It will work great in your room, but you'll have to go outside and walk 150 feet from your house to hear the bass.    

Your neighbors will love it because your whole house will become a resonator.

About the Fs and the 18" driver ... I think enough people have successfully tried woofers with Fs below 40 to show that isn't an absolute issue.

Could anyone chime in here who has used an 18" driver or 15s with Fs below 40 hz to reassure future builders as to what problems, if any, can be expected? Dank, didn't you use 18s?

This is good information, Gaz. Thanks for posting.

So can you describe the moment of "unloading?" I'm most interested in the details of what happens when the amp is being pushed, but still not clipping. Amp clipping would sound so nasty at that SPL, I wouldn't want you to test it out now (or in the future).    Please just speak from the experiences you've had.

Thanks.

A speaker with Fs=30 and Qts=.35 will probably do ok in the standard Imperial.  Not great, but ok.  At $68 I don't think you will find a better choice.    All the 18's I've used have all been around 30 hz.  I've tried a number of Cerwin Vega 18's, an Eminence 18082, and a couple of Madison Executioner X18's.  My favorite is the Cerwin Vega XL18-4 with a Fs=28 and a Qts = .1675.  I keep mentioning the Qts because the number obtained by dividing Fs by Qts is relevant here, it relates to the speakers strength.  For a horn type enclosure you want this number to be greater than 100, the higher the better.  My Cerwin Vega favorite has a Fs/Qts of 28/.1675 = 167.  Your 8 ohm Mach 5 is 30.4/.35 = 87 which is marginal.  The high Qts scares me more than the low Fs.

I do think you will be missing out on something though if you build for a single 18.  There seems to be an interaction between the speakers that makes a combination of 2 different speakers sound better together than either one when paired with the same type.  Steve D. noticed this on the full range Imperial and recommends 2 different speakers because of it.  I have seen the same thing on my standard Imperials used as subs.  I'd at least be sure to build your Imperial so it could be easily modified in the future to add a second speaker if you ever decided to do it.  There was a lot of talk on this forum a while back about removable baffle boards which is how I did mine - and I've used that feature a couple of times.

DanK

I think decoupling behaviour was noticed when I was running the Imperials with a Luxman amp (LV-92, at a guess good for about 50 odd watts rms, correct me if Im wrong). By accident the volume was control was turned up high and the drivers were oscillating @ about 20Htz @ max amplitude. I cut the power straight away. I don't think this was actual decoupling, more like amp behaviour when driven to excess. I've never noticed the same problem when driven with my Playmaster - the 185 wpc beast. A good amp, as we all know, grips the speakers with a fist of steel. I can pass on the details of the Playmaster if you wish. For a guy whose previous soldering experience was a 10 component siren, this home built baby is a beaut.
Cheers
Gaz