Speaker Testing CDs?

My main speakers (also non-girly acoustic suspension) have 12-inch woofers. The sub-woofer is a separate component, with the radiators on opposing sides.

Friend of mine had AR-LST's, the king of acoustic suspension speakers. They'd put out the SPLs for sure.

I've just always preferred the sound of bass-reflex. Chalk it up to growing up in a home with a Klipsch Cornwall.

Nothing wrong with bass-reflex - when properly done, which most aren't - but acoustic suspension generally produces a much tighter bass.

Depends on the WOOOOFer.

The WOOOOFer and the enclosuuuuure.

Suuuure. This gives me an idea though, ever since I had the surrounds replaced on my woooofers a couple years ago they've been flabbier than they usta be. Maybe I should try sealing the ports. I think maybe they were turned into acoustic suspension.

You say they sound "flabbier than they usta be." Does this mean they've always sounded flabby? Isn't that a standard characteristic of bass-reflex? (Sorry, as an acoustic suspension fan, I just couldn't help myself.)

It sure seems possible that an improper surround can cause the problem, although the specific reason might be difficult to diagnose. I'm sure you haven't changed the physical tuning parameters of the box, but changing the stiffness or range of the surrounds could change the cone excursion (maybe causing unloading or ringing) or maybe the phase differential may no longer be accurate (accurate according to bass-reflex metrics, of course; no bass-reflex phasing is sonically accurate).

Actually, in my tests the bass reflex port phasing is not an issue as long as the port is at least a woofer diameter away. Mine are about two diameters away.

That's why I asked you about the placement on your passive radiator.

By the way I don't necessarily disagree with you about AS vs BR woofers. BR has to be done correctly, which means a tight woofer and a huge box (3cf for 12", 2 for 10", 4 for 15") which is why all the cheap bookshelf speakers use AS.

It's been my experience that most cheap bookshelf speakers use poorly designed BR. Cheap speakers are designed to be sold to people with cheap amps; they need BR.

The only reason I'm aware of for the use of bass reflex is that you can get more noise with less power. If there's some other reason, I'm sincerely anxious to hear it.
You can get great sound out of bass reflex speakers, but they have to be very well designed and constructed.

Here is an old article about my old subwoofer (I notice they have a price on it of $1,250. The bastard went up a lot - about fifty percent - between the time that article was printed and I bought mine a few years later.)
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Intriguing. Does indeed "break all the rules of subwoofer design." The proof is in the pudding -- is it pudding out the bass?


I'm excited about experimenting with sealing the cabinets of my woofers. This project presents a unique engineering challenge though.

Obviously I don't want the full 3CF air dam for AS, so I need to seal off the bottom 2/3 of my cabinets. Somehow -- not sure exactly -- I need to get some lumber in though the 12" speaker holes and create a new bottom. It'll have to be in at least 2 pieces. I'll have to see, I'll have to remove the woofers and experiment with how big a piece of pressboard I can slide in. I can probably use two pieces with a third laid across the top as a connector, screwed in from the top. Fun project for this weekend.

Yes, it does put out the bass. Lots of nice, clean bass.

You might want to consider putting boxes around the mid and tweeter and leave the rest of the enclosure open for the woofer. You don't want too small an enclosure for the woofer. AS enclosures are not 100% sealed. There needs to be a very small vent for the woofer so that the interior air is the same as the ambient air.

Of course, the best thing may be to get the correct surrounds for the woofers and stick with BR.

_Howard wrote:

You might want to consider putting boxes around the mid and tweeter and leave the rest of the enclosure open for the woofer.

Mids and tweets are free-mounted & time-aligned, a la Dahlquist.

_Howard wrote:

You don't want too small an enclosure for the woofer. AS enclosures are not 100% sealed. There needs to be a very small vent for the woofer so that the interior air is the same as the ambient air.

News to me. Never heard this before -- and I've been building speakers for 45 years. IME you need a lot less cabinet for AS, although if it's fully sealed (...as they ALL ARE...) the size isn't that critical. I figure sealing off the bottom of my cabinets ought to be about right.

_Howard wrote:

Of course, the best thing may be to get the correct surrounds for the woofers and stick with BR.

No. It cost something like $250 apiece to have them done, and they're holding up just fine. Just not 'high & tight' like they used to be. The 124a had amazing specs: 4" voice coil, 17Hz resonant frequency, something like 26 lbs in magnet and 250 watts power handling. They still put out the big bass, but now everything sounds... I dunno, flabby.

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NoCoPilot wrote:

_Howard wrote:

You might want to consider putting boxes around the mid and tweeter and leave the rest of the enclosure open for the woofer.

Mids and tweets are free-mounted & time-aligned, a la Dahlquist.

As much as I have read about time-aligned and phase-aligned speakers, I have never become a big fan of them. I know that the alignment is an enhancement and requires high-quality engineering to accomplish excellence, but the improvements are - in my opinion - just too subtle for really old ears to benefit from. If I was younger and had the bucks, I would insist on time-aligned speakers, but phase-coherent is not really of any use to me, as it is position dependent. When you're walking around a nine-foot pool table, the phase coherence is inconsistent.

NoCoPilot wrote:

_Howard wrote:

You don't want too small an enclosure for the woofer. AS enclosures are not 100% sealed. There needs to be a very small vent for the woofer so that the interior air is the same as the ambient air.

News to me.  Never heard this before -- and I've been building speakers for 45 years.  IME you need a lot less cabinet for AS, although if it's fully sealed (...as they ALL ARE...) the size isn't that critical.  I figure sealing off the bottom of my cabinets ought to be about right.

I first heard about the venting of so-called sealed enclosures when an audio engineer mentioned it. I was at his shop for a reason that now eludes me, and he was complaining about some speakers he was working on. These speakers actually had small mechanical vents that closed when the woofers were in rearward travel. According to him, they never worked properly after a few years and the correct way to repair them was to remove them and just leave a small hole. According to him, nearly all AS speakers Not considering cheap shit) had a small vent which allowed the interior air to achieve the same properties as the outside air - humidity, temperature, etc., which improved the equalization of the output waves. Again, it's a very small matter, but real speaker nuts care about things  like that. Or so I hear.

And you may be right about minimizing the size of the woofer enclosure. But that's really difficult to know. Typically, BR and AS speakers use different types of cones and surrounds (the "cones" on my subwoofer are flat), so it's hard to guess what results these modifications will give you. You may even wind up wanting to stuff the enclosures to reduce the free space even further.

NoCoPilot wrote:

_Howard wrote:

Of course, the best thing may be to get the correct surrounds for the woofers and stick with BR.

No.  It cost something like $250 apiece to have them done, and they're holding up just fine.  Just not 'high & tight' like they used to be.  The 124a had amazing specs: 4" voice coil, 17Hz resonant frequency, something like 26 lbs in magnet and 250 watts power handling.  They still put out the big bass, but now everything sounds... I dunno, flabby.

Damn. Those are some serious drivers. Even if there's a problem with the surrounds, it seems that those things should deliver some excellent bass.
Are you using a tube amplifier? Degradation of the vacuum tubes can cause flabby bass.

In any case, I wish you the best of luck with your speaker mods. Do keep us informed, please.

1. Phase coherency is lost if the time-aligned drivers are mounted side by side as they are in Dahlquists. That's why I mounted mine in a line, top to bottom. You are much less likely to move around in the vertical axis while listening. And less.

And the phase coherence is not a minor effect.

2. Your speaker builder friend was blowing smoke up your ass. The whole point of "acoustic suspension" is that the back pressure of the sealed box behind the woofer become part of the suspension. Air pressure itself prevents the cone from traveling freely, allowing looser suspensions and thus smaller magnets with less control. It you vent an AS cabinet it becomes by default "bass reflex" -- and a poorly designed one at that.

3. I have always stuffed my cabinets with acoustic foam, the stuff used for making camp beds and massage tables. It's acoustically dead, meaning it absorbs SPL energy and prevents (or minimizes) standing waves inside the cabinets, without altering the back pressure. (Another consideration is lack of right angles in the cabinet design, as well as cabinet depth less than one wave cycle.) Careful tuning of how much foam, and where it is placed, can radically change the character of a bass reflex cabinet. Standard design feature for me.

4. On disassembling one speaker this evening I was reminded that I bisected the rear planes, trisecting the woofer mount, to minimize cabinet resonance. I also built everything with 1" pressboard. This complicates things.

Incidentally here's what they look like assembled (grill cloth removed):

_Howard wrote:

As much as I have read about time-aligned and phase-aligned speakers, I have never become a big fan of them. I know that the alignment is an enhancement and requires high-quality engineering to accomplish excellence, but the improvements are - in my opinion - just too subtle for really old ears to benefit from.

Just a quick response to this.  

I did not perform any "high-quality engineering" to time-align my drivers.  I knew approximately where they needed to mount -- the front edges of the voice coils should be in the same plane, so they move as a coherent pair, as if they're a single piston.  As you can tell from the photo I mounted the mid ranges on a stationary screw arrangement so I could fine tune them forward and back, once the woofers were installed.  I found the exact center of alignment is audible to within about 1/4 turn of the screw.  When aligned the mid and woofer seem to merge into a single sound source midway between them, if you're listening right in front of the cabinet.

Having aligned the mids, I contemplated a smaller screw arrangement for the tweeter, but discovered that a simple L-bracket, in the proper plane, could be tweaked by bending the bracket.  Simple, effective, cheap. The Yamaha beryllium dome drivers are comparatively easy to align, as opposed to the woofer, because their voice coils come right up to the aluminum cast housing. Aligning the face plates aligns the voice coils.

True, physical time alignment is not difficult (unless there are horns involved), but there is still the matter of delay and shift introduced by the crossovers. Except in the  case of sub-woofers, however, I doubt that they are significant.

Nice looking enclosures. Did you build them yourself?

In case you're interested, here is a lengthy articles about this and related matters.

Not a fan of horns for that reason.

Yes, this is the 13th (and last) speaker system I designed and built. In the 1980s my "speaker building odyssey" was written up in a tiny little specialist magazine, long gone, called "Speaker Builder."

The golden age of "build your own" passed about the same time as the mag.

At the time I was friends with Roger Eakin, Bob Carver and the founders of Speakerlab. There were quite a few of us sharing ideas.