2-ways, tweeters and chamber isolation.


2012-03-25 2:08 am

This is something that often comes up with my speaker designs, and I was hoping someone could explain it to me.

The short version of the question is: Can I get a tweeter or tweeter like speaker to play down to 200 hz without building a separate chamber for it in the speaker box. If I can't, what cutoff can I do so?

I am designing a speaker and for various reasons, I want it to be a simple as possible. Its a mobile boombox, and I am modeling it after popular 2.1 computer speakers. But I want the whole subwoofer and two satellites to be in one enclosure.

I can give the specifics of the project if desired, but really, I want to hear about surface mount, or enclosed full ranges that will be unaffected by being mounted in the wall of a woofers box.

Thanks in advance for your thoughts, I really appreciate it.

You can get deep bass from small drivers, in earphones that is. For any given sound level the needed volume that the driver has to move increase by 8 times for every octave deeper it gets. So to play loud at 5 kHz a 5 cm2 dome does not move visably while at 50 Hz a 500cm2 cone moves so much that you can see it move.

You can get away with say a Tangband 3" driver if you have the speakers close and accept that max output and bass below say 100 Hz is pretty much not there.

If you mount the small boxes in the corners (3 surfaces) of the room or at least in the wall junctions (2 surfaces) you will get more output the the direction of the sound source may or may not work.

I understand the appeal of having a unit box of loudspeaker that you can mount as you want unaffected by were is at.

First any reflective surface will affect how the loudspeaker work by reflections or by contstraining radiation angles.

Secondly all loudspeakers has a resonance determined my the mass of the moving parts and the compliance of the system, just like tying an apple to a rubber band, the bounce rate of the apple will be determined on the size (mass) of the apple and the stiffness(compliance of the rubber band suspening it.

The suspension of the loudspeaker consists of two parts the mechanical suspension of the driver itself and the "springyness" of the air in the box enclosing it.

If the driver air spring is much softer than the mechnical suspension then the loudspeaker is dominated by the driver characteristics. If the air spring is stiffer than the mechnical suspension the box determine the characteristics of the system.

To compate apples to apples the mechanical compliance is converted to Vas. That is a airvolume that would give the driver the same compliance as the mechanical suspension.

So to get that "enclosed full ranges " you have to enclose those fullranges in boxes much larger than the Vas of the driver. For small drivers you might get away with 50L but in many cases you have to go to 500 liters and upwards.


2012-03-25 2:08 am

Thanks for your comment. I am trying to avoid the construction of the cavities, so its the sealed back mids that are of interest to me. As I don't have 4 liters to spare.

Dr. Boar,

I feel like my question was unclear. I am seeking to build a 2.1 system, with some sort of sealed back tweeter, so I can have one very simple subwoofer box, with two surface mounted, high range speaker, without building interior chambers. I am wondering where I can do that crossover point, as I so enjoy the sound of a subwoofer without the need to play the mids.
There are more than a few open backed 3" range "full range" drivers that work very well indeed when HP filtered in the 150-300Hz range. Fostex FF85WK is the first that comes to mind - I like this driver a lot, and don't find it needs a lot of help up top, so think of it as a really wide band mid/tweeter.

Other great candidates include the Alpair6M, Fountek FR88, and no doubt several TBs.

Just isolate them with a length of 4" ID ABS plumbing pipe, stuffed with your favorite flavor of fibrous fill. If you want to get creative, and yield better results, taper a separate enclosure opening out the back, and stuff for a quasi -aperiodic / TL. Mount the small driver at the top of your enclosure and a single slanted panel can do the trick - a la the "Tyson" of a couple of years back.

edit: PS, why exactly the resistance to separate interior chambers? few "sub" woofer drivers will have decent enough performance extending high enough into the upper mids to integrate well with surface mounted back "high range" drivers, few of those of which will extend low enough

also, your description of "2.1" in post #4 implies that you're looking to get a reasonable semblance of stereo separation from a single enclosure that is presumed to be no larger than the selected bass driver requires. - I'm not so sure that'll happen
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Great points! Its a boombox, and I am attempting a very simple design (almost as a challenge) and to be blunt, I'm focusing on the smallest box for the sub, and wanting to have the most room inside possible. My benchmark is a thrift shopped Boston Acoustics Speakers 2.1 BA745. I am unconcerned with the stereo separation... but I love the deep bass response. I am willing to build out a chamber, I was just checking to see if some sealed back options were out there, simply to save space and time.

And I might be wrong on this, but the low bass always seems to sound better when its a dedicated subwoofer. That little boston speaker system has clear sub 100hz sound, and I have a hard time thinking it would sound as good if that little 4" speaker was being asked to play up to 2,000 hz, like a normal bookshelf speaker, no?