Full Range driver with good bass for large enclosure

[Don Hills]

Hello FRDers

I'm soliciting recommendations for a fullrange (or near fullrange, with a tweeter) driver that can provide useful bass in a sealed enclosure, even if the enclosure has to be large. This implies a driver in the 6.5 to 8 inch size range. Having accurate Thiele-Small parameters available is a must.

What I plan to do is to work out what differences there are between two different layouts.

Layout 1: Put the driver in a small enclosure, which will restrict its bass, then add a small sealed box woofer to extend the overall bass response to a low enough frequency (40 Hz or so).

Layout 2: Put the driver in a small enclosure, with the woofer added in "isobaric" configuration behind it. The intent is that the woofer will be sized and driven to largely relieve the backpressure on the fullrange driver so it thinks it is in a much larger box at low frequencies.

Layout 1 has been done before, but I don't recall seeing Layout 2 discussed. Either no-one has done it, or it has been considered but rejected for some good reason.

This is a 3 phase project:

Phase 1:
Model the two configurations to determine approximate dimensions etc.

Phase 2: If it looks like layout 2 might work, build a prototype using a cheap driver - I have several drivers similar to the Philips 9710.

Phase 3: If the results are promising, repeat the process with some higher quality (expensive) drivers.

There are many things to consider, such as the effectively decreasing rear volume size for the fullrange driver as the frequency increases.

[Cal Weldon]

I am wondering if you have to use the same drivers when running isobaric so the "mightier" driver doesn't "hurt" the full range unit. I wondered about that a bit ago but never asked anyone. I think the distortion caused by the extra travel might not be worth it but I'm no expert so...

[18Hurts]

I'm not an expert in isobaric although I built a dual 15" isobaric sub

Normally, you have to use identical drivers for that configuration--the push-pull lowers distortion if they were the same. Using different drivers would mostly likely increase distortion. If you hooked a proper woofer behind a full range, as soon as the FR hit Xmax, the woofer would continue to push which would destroy the full range.

From what I understand, isobaric is generally used below 100Hz since the air trapped between the drivers gets very nonlinear over 100Hz. The one woofer behind the front mounted woofer trick adds much more air to the mix so it gets worse. I use the "clam shell" configuration with the faces of the woofers bolted together to limit the amount of air space between them--works like a champ crossed over at 80Hz.

Layout #1 makes sense, model the full range in a sealed box until you get the roll off you want along with the size you require. A small sealed woofer as a "helper woofer" (DIY lingo is FAST) Downside is the size increases to have the size to hold the woofer driver.

You might be in luck though--the Mark Audio Alpair 12 has 9mm of Xmax or a "stroke" of 18mm with a 6.5 inch cone. It is NOT a woofer as the very thin aluminum cone can stress fracture if you push it too much. Mark has decided to make a paper cone version for us more "spirited" folks that tend to drive full ranges to the deeper portion of the bass band.

The details are sketchy at this point, but it is the exact speaker I want for a computer desktop system. I don't think I'd ever see the paper cone stroke the full 18mm running a 10 watt chip amp but it would be nice to have all the durability I can get. The Alpair 12 with paper cone will be out by March, 2012 so something to watch.

It might need some "assistance" up high--think over 10KHz but adding a small tweeter with a single cap (and a switch to turn it off if desired) not a big deal.

[planet10]

Originally Posted by Don Hills

I'm soliciting recommendations for a fullrange (or near fullrange, with a tweeter) driver that can provide useful bass in a sealed enclosure, even if the enclosure has to be large. This implies a driver in the 6.5 to 8 inch size range. Having accurate Thiele-Small parameters available is a must.

What do you consider accurate T/S parameters? I've never seen a set. That would be a full set of curves for the parameters of interest.

FRs that will give bass to 40 Hz in a sealed box are VERY rare.

Layout 1: Put the driver in a small enclosure, which will restrict its bass, then add a small sealed box woofer to extend the overall bass response to a low enough frequency (40 Hz or so).

This is the way to do it. A FAST. Lets you use a smaller FR so that you don't have the short comings of the larger drivers in the mids & top. To get max performance i usually do the mid-tweet in an aperiodic midTL, not sealed.

Layout 2: Put the driver in a small enclosure, with the woofer added in "isobaric" configuration behind it. The intent is that the woofer will be sized and driven to largely relieve the backpressure on the fullrange driver so it thinks it is in a much larger box at low frequencies.

This limits the FR to frequencies below the frequency equivalent to 1/2 wavelength of the depth of the isobarik chamber (althou driving the rear driver with a separate filtered response could go a ways to help in that regard. Won't get you the bass you want thou.

...I don't recall seeing Layout 2 discussed... but rejected for some good reason.

dave

[Don Hills]

Thanks, folks.
I'm well aware of the classic isobaric setup. I wouldn't be trying to "overdrive" the fullrange driver beyond its natural excursion limits. What I want to try is to take a full-range driver that performs adequately in the bass when used in a large sealed enclosure or large TL, then put it in a small box but make it think it's in a large box. In other words, use the "rear" woofer to increase the box volume seen by the FRD at low frequencies.

Another way of looking at it is that in a "classic" isobaric, the pressure change measured between the drivers is half of that in the enclosure. I intend to use a rear driver and drive so that the rear driver does all of the work at bass frequencies, therefore resulting in zero pressure change between the drivers. The "front" driver thinks it's in a very large box or infinite baffle.

Part of the design process will be to work out how much "help" a given FRD will need to make it think it's in a large box, and at what frequency to shut down the "helper" (think of it as a ".5" arrangement.)

I realise that the classic arrangement (FRD with separate helper woofer) would be easier and could be made to perform better, but that misses the point. It's much easier to fly to the top of a mountain in a helicopter, but some people still prefer to do it the "hard" way. When asked why, they usually shrug and say "Because it's there."

[Don Hills]

Originally Posted by planet10

What do you consider accurate T/S parameters? I've never seen a set. That would be a full set of curves for the parameters of interest.

The usual LF parameters would be adequate, because that's the area I would be working with. Thanks to 18hertz for the Alpair 12 suggestion as a design model, I found the T/S specs, although it has a remarkably low Vas. A driver with a higher Vas would make my point better. Maybe I should use my Altec 420a drivers, they are near fullrange, Fs around 22 Hz and Vas 800 litres...

Originally Posted by planet10

FRs that will give bass to 40 Hz in a sealed box are VERY rare.

Well, however low it goes, then. The point is to try for the characteristics of a FRD designed for a large IB or a very large enclosure, while in a much smaller box.

Originally Posted by planet10

This limits the FR to frequencies below the frequency equivalent to 1/2 wavelength of the depth of the isobarik chamber (althou driving the rear driver with a separate filtered response could go a ways to help in that regard. Won't get you the bass you want thou.

That's one of the questions to be answered, from prior experience or by experiment - how much will the relatively small volume behind the FRD affect its performance above the frequency where the rear driver stops "helping" (is low passed)? Midrange drivers are commonly placed in relatively small enclosures. The space is usually filled with damping material. Can the space in my proposed unit be filled with sufficient damping material (fibreglass etc) to be effective at mid/high frequencies without severely affecting the assistance provided by the rear driver?

[FullRangeMan]

Originally Posted by Don Hills

Hello FRDers
This implies a driver in the 6.5 to 8 inch size range. Having accurate Thiele-Small parameters available is a must.

Hard to find precision TS parameters, because they can vary to 20 per cent according the temperature and humidity.
Also there is no standard for TS measurements, each company do the TS at its manner.

IF you want big cone FR, the Nirvanas Super15CF and Super15Alnico seems suited to you. The recommended box are the 13.6 BassReflex model, as seen on the site.
Audio Nirvana Full-Range Speakers For Sale. High Efficiency, DIY, Single Driver, Monitor Speakers with no crossover.
Good Luck

[Don Hills]

Thanks for the Audio Nirvana pointer. Their 8 inch range looks to have parameters in the range I'm looking for. I can see that I erred in asking for "accurate" T/S parameters - what I meant to say was that I wanted something better than the grossly inaccurate values supplied by some manufacturers. I should have realised that manufacturers who make high quality / performance / price drivers are likely to take a similar amount of care in measuring the parameters.

Now to fire up AkAbak and build some accurate models for Phase 1...

[planet10]

Originally Posted by Don Hills

I can see that I erred in asking for "accurate" T/S parameters - what I meant to say was that I wanted something better than the grossly inaccurate values supplied by some manufacturers. I should have realised that manufacturers who make high quality / performance / price drivers are likely to take a similar amount of care in measuring the parameters.

It has been shown that those factory parameters may not be as inaccurate as you think. The factories use standardized kit (LMS/LEAP) to masure their drivers, and what we use is typically measuring on a different part of the T/S curve. Althou wildly different, both sets of numbers can be accurate. Have a look at the FE127 data below. My measures (using Fuzzmeasure) & Mark's (using LMS) on the same drivers, both equally valid.

You cannot get into the trap on thinking of T/S as being scalars.

As well as changing with "weather" even fairly tightly QCed drivers, can range +/- 20% within a batch.

dave

[FullRangeMan]

Originally Posted by Don Hills

Thanks for the Audio Nirvana pointer. Their 8 inch range looks to have parameters in the range I'm looking for. I can see that I erred in asking for "accurate" T/S parameters - what I meant to say was that I wanted something better than the grossly inaccurate values supplied by some manufacturers. I should have realised that manufacturers who make high quality / performance / price drivers are likely to take a similar amount of care in measuring the parameters.

Now to fire up AkAbak and build some accurate models for Phase 1...

OK, If you can afford the hi price stay with the Super12Alnico or 15Alnico, cause the Alnico sound signature are very pleased to the humam brain, it is a kind of even hamonic, as the tube sound hamonics.