[thirdeye]

Hello,

I understand that putting two woofers in "Parallel" such as in an MTM should net you around a 6 db increase generally on the acoustic side of things in efficiency?

However, how do you choose based on this information a tweeter to match without it being too high and padding it down?

Example if you used an 88 db driver, and put another in parallel you end up with about 94 db or not?

**So if you have dual 88 db woofers in parallel should you shoot for a 91 db TWEETER, or a 94 db TWEETER as it seems more that you should pick a driver match only 3 db higher?**

Also, when specs. on woofers state they can be in a .5 cuFt cabinet for example with a 2" x 6" port and hit 60 hz does this simply mean you should double that volume to 1 cuFt, and also just add a second port of the same 2" x 6" length?

Does it have any performance advantages placing these 2 ports vertically or horizontally?

Thanks

[ptmconsulting]

I think it's pretty much understood that you will have to pad down any tweeter. Just pick one that gives you the bandwidth you need, and the kind of sound you like, and the front panel size you will need to keep your mid-bass drivers close enough, and just work it out in the crossover.

[Pete Schumacher]

Depends on the application of course, whether in-wall or out in the room.

Since most speakers are designed to be away from walls, an effect known as baffle diffraction will affect bass response dramatically.  What happens is that half the sound energy wraps around the enclosure, effectively reducing the forward sensitivity by up to 6dB.  To fully compensate for this attenuation, the pair of 88dB woofers in parallel will basically be 88dB at the lowest frequencies when teamed up, with their forward response rising 6dB by the time they get to 1Khz.  The crossover will provide the equalization to bring everything back into balance.  The room needs to be considered as well, and because a room will tend to reinforce bass levels somewhat, the full compensation of 6dB may be too much.  Usually between 3 and 6dB of equalization will bring the balance back for a stereo pair.

So for a pair of 88dB woofers, a 91dB tweeter should do the trick.  If the speakers are in-wall, you'll need a 93-94dB tweeter.

Your assumptions about how to construct the enclosure volume and additional port are correct.  Figure it for one woofer, and then double it up for a pair, twice the volume with an additional port.

[thirdeye]

Thanks Pete!

Exactly what I was looking for. Yeah I always sort of thought really you were going to look for a high frequency device only about 3 db higher than the original one woofer efficiency when doing a parallel woofer deal.

But then everybody talks about the + 6 db they actually create. I thought also that electrically they only create about a 3 db "Easier" load to the amp itself not really a 6 db, although maybe 6 db louder acoustically which means you need that tweeter to be down a bit more than a full 6 db higher.

I do understand as pointed out above that all tweeters may need a little work or knocked down with resistance, I just did not want to have use larger resistance or L-pad type values than necessary why not get closer to the natural driver match with a tweeter closer in output in the first place?

In any case I appreciate your explanation and it makes total sense. I wish I could say that I have full access to some good measuring equipment and I don't! But I really was just more curious about the basic on this so making decisions when scouring thru the 100's of options I can make more educated choices what might work a little nicer.

Thanks

[Duke]

I understand that putting two woofers in "Parallel" such as in an MTM should net you around a 6 db increase generally on the acoustic side of things in efficiency?

Actually, paralleling two woofers gives you a 6 dB increase in voltage sensitivity, but of this, only 3 dB is actually from an increase in efficiency.  Remember that you are also halving the impedance of the load, so that most amplifiers will put out twice the wattage into the new, halved-impedance load.   That's where the other 3 dB comes from. 

However, how do you choose based on this information a tweeter to match without it being too high and padding it down?

Example if you used an 88 db driver, and put another in parallel you end up with about 94 db or not?

**So if you have dual 88 db woofers in parallel should you shoot for a 91 db TWEETER, or a 94 db TWEETER as it seems more that you should pick a driver match only 3 db higher?**

This is where you have to read the specs closely.  Your twin woofer section is now  4 ohms and 91 dB efficient, but it has a 94 dB voltage sensitivity.   So if you're pairing it with an 8 ohm tweeter, you want it to be 94 dB efficient, because then it will also have a 94 dB voltage sensitivity (the convention is to specify voltage sensitivity at 2.83 volts, which is 1 watt into 8 ohms, but 2 watts into 4 ohms; unfortunately, many manufacturers say "efficiency" when they should be saying "voltage sensitity" for a 4-ohm diver because it looks more impressive on paper).  If you're pairing up with a 4 ohm tweeter, you want one that has a voltage sensitivity of 94 dB.  So to sum up, convert everything to voltage sensitivity, and that's what you want to match up. 

If your crossover frequency is in the baffle step region, and if you choose to compensate for the baffle step, you can follow the advice of Pete above. 

Also, when specs. on woofers state they can be in a .5 cuFt cabinet for example with a 2" x 6" port and hit 60 hz does this simply mean you should double that volume to 1 cuFt, and also just add a second port of the same 2" x 6" length?

Does it have any performance advantages placing these 2 ports vertically or horizontally?

Yes, if you double the internal volume, double up on the ports as you have described. 

I like to put the ports on the back of the speaker cabinet, usually at a different height than the woofer(s) so that, when I toe the cabinet in, my bass sources (woofer and port) are displaced relative to one another in all three dimensions.  This helps to smooth out the room interaction.

If this is a very high power application, put one port lower than the bottom woofer's magnet, and the other port higher than the top woofer's  magnet.  This will allow some "chimney effect" cooling of the hot woofer magnets.  I use this technique in my bass guitar cabinets. 

Best of luck with your project!

[thirdeye]

Hi Duke,

Yes I see where the confusion comes in now! I will have to start looking a little closer to those conversion #'s they throw out there.

Quick question on that "Chimney" effect? I do hope to get a fairly efficient and high power handling for sure as I would be looking to run these drivers in the MTM only down into the 50's or 60's maybe hoping to also stay away from X-max.

However, being the cabinets will likely have the binding posts at the bottom making it pretty difficult to get that port down there, can it be just above the bottom woofers magnet? And then take the second port close to the top of the cabinet just above that drivers magnet... More of an offset toward the top type of deal this way?

Also, I would like these drivers to sound at least "Full" and not be like a zero body monitor, in which case it is planned they will be supplemented by some nice servo subs in the future, however I still want them to stand alone well as a speaker.

But they do not have to go super deep, just have nice punch, and some bass to them without being driven too hard, so do you suggest getting into the 50's at least to be safe which will require porting?

Or do you think running them sealed which in fact all 4 drivers I am considering can run sealed as well in a dual config MTM with a 1 ft3 cabinet and will get into the 65 hz range?

And again the keys to the actual design I am looking at are:

* High power handling / High output

* High efficiency

* High quality sound

* Not required to play the deepest bass, but good enough to sound very smooth, and with good tone on their own when not used with subs or at least the "full effect" with decent impact.

Thanks again

[Duke]

Quick question on that "Chimney" effect?

Chimney effect is where hot air rises and exits through one opening (in this case a vent), thus drawing cool air in another opening (a lower vent).   If it's easy to accomplish this sort of arrangement you might as well, but if not, then don't worry about it.

However, being the cabinets will likely have the binding posts at the bottom making it pretty difficult to get that port down there, can it be just above the bottom woofers magnet? And then take the second port close to the top of the cabinet just above that drivers magnet... More of an offset toward the top type of deal this way?

That would be fine.

Also, I would like these drivers to sound at least "Full" and not be like a zero body monitor, in which case it is planned they will be supplemented by some nice servo subs in the future, however I still want them to stand alone well as a speaker.

But they do not have to go super deep, just have nice punch, and some bass to them without being driven too hard, so do you suggest getting into the 50's at least to be safe which will require porting?

Well designing a speaker is all about juggling tradeoffs, just take as many things into account as you can (including anticipated boundary reinforcement, or lack thereof).   And even then, it's nice having a bit of flexibility because the subjective low-end performance may not be exactly what you expected.  If you use two 2" internal diameter ports, you can plug one of them with an expandable plumber's test plug and have a lower tuning frequency available.  Or plug them both and have a low-Q sealed box, which might partner better with a subwoofer system.   I like to use Precision Ports, which are modular and can be shortened or lengthened (use tape to join the sections together), and work with plumber's test plugs (some less expensive ports may claim a certain diameter but it's an approximation and so test plugs may not work).

[thirdeye]

Thanks