Zaph ZRT 2 way build; bass tuning question

[monroe]

I recently completed a Zaph ZRT 2 way build.

The cabinets are 3/4 inch plywood, to the same dimensions as the recommended Madisound enclosure. I ordered the upgrade crossover (with the foil inductor).

I used the default port tuning: 30 Hz, 2 inch port, using only the outer and inner flare.

The speakers sound pretty good to my ears, with one major caveat: lack of bass.


Description of the issue:
-------------------------

My only basis for comparison is my Seas Odin kit, which was hooked up in the same room and to the same amplifiers and sources. The Odin's bass was occasionally a bit boomy (some notes tended to be a bit louder than others), but it was overall quite a bit louder than the ZRTs.

Bass sounds ok (althogh quiet) on the ZRT when only bass instruments are playing, but becomes buried in the mix when other instruments come in.

From doing sweeps with a test program, it sounds like there is major rolloff below 100 Hz and there is very little sound below 70 Hz, at least as compared to the Odins.



Things that I have tried:
-------------------------

I have verified that the woofers are wired in phase.

I have tried putting in a shorter port, using only the front half of half of the port assembly (~1 inch): this seems to increase the amount of bass slightly, but the bass is less precise.

When I remove the port entirely, running the system with the large port-hole in the baffle open, the bass gets much louder. However, it is not controlled and has something of a one-note quality.

I have tried listening through two different amplifiers (a Marantz solid state amp rated at 25 watts in class A and twice that or so in AB, and a Consonance push pull el34 tube amp at ~40 watts) and have found that the speakers exhibit a similar lack of bass response with both amps.

I don't know if it's of any significance, but strangely, with the tube amp, the sound is clearer when the speakers are plugged into the 4 ohm tap than the 8 ohm tap (the ZRTs are 8 ohm speakers I believe). There is mild but audible distortion on the 8 ohm tap which prompted me to try the 4 ohm. I only ever ran the amp with the 4 ohm tap for the Odins.

I had discussed this issue on the diyaudio forum, here, and had tried some of the suggestions:

http://www.diyaudio.com/forums/multi...-braces-2.html

[Flagenbush]

Monroe,

This is what I love about these forums... you get to learn from the efforts of others...

The apparent lack of bass could be due to any number of things, not the least of which is that the ZAPH design itself isn't as "bassy" as you would like. However, I don't think it is the ZAPH design in this case as just about any speaker can be tuned better than the first attempt by the builder (unless you are unusually lucky at the first go).

Looking at your link to the other forum I see a nice photo of the inside of your cabinet. Like you, I wonder about the holes in the bracing. Although I am curious whether the area of the holes is enough, I'm even more curious if the smaller holes are somehow "pretuning" your sound to a higher frequency before it gets to the port. Now, I don't know the answer to this, but it is something I thought I might one day experiment with. Maybe someone out there has experimented with this first hand and can tell you whether it makes a difference.

I'm also wondering about the foam padding you are using? Is it open or closed cell? If it is closed cell it probably isn't helping anything.

How well is your speaker sealed? Are there any potential leaks? the only "leak" should be the port itself.

If the foam you are using is open cell and the cabinet is air tight you might want to experiment with MORE insulation.

Less than a year ago I made a modified version of the ZRT 2.5 and I tried many different things. I had 1/2" carpet felt on the sides (in some places 3/4") and stuffed the bottom with fiberglass insulation. At first listen they had outstanding imaging, but lacked low mid and bass bass (guessing between fs and 200Hz). I added more insulation and found that the impedance peak dropped noticeably and the speakers became more "alive". Of course if the resistance of the impedance peak drops, the transducer becomes more efficient near its fs, so it makes sense. My final effort was to put insulation anywhere I could that it would not block air getting to the passive (think port, in your case)... that seemed to be the ticket. (I too have bracing, but my holes are a single 8" diameter hole in each 3/4" thick plywood brace. My cabinet sides are all 1" laminated plywood.)

This (adding more insulation) by itself may be all you need to do. But you may want to experiment with the bracing hole sizes as well as the foam padding after trying more insulation.

Please let us know what you try and the results. Trying different things is half the fun... so have fun!

Brian

[monroe]

Hi Brian,

Thanks for the tips! I was wondering about the stuffing; I have some stuffing in the lower cavity: as much as would comfortably fit. I'm sure I could squash more in though, and Madisound supplied quite a bit more than I needed to just pad the lower cavity. I will add more foam and observe the difference.

As far as the holes in the braces go, the picture on diyaudio shows the cabinet before I openened the holes up. I joined each of the small holes in the photo, so each brace now has a single very large hole through the centre. There should be fairly good airflow from top to bottom and there are no obstructions between the port and the driver.

I lined the cabinet with the open cell acoustic foam that was included with the Madisound kit. Might carpet padding yield an improvement?

[Flagenbush]

Monroe,

First a few disclaimers... I am not a speaker building guru or engineer. I am a scientist with a dream to one day create my own speaker line (delusions of grandeur no doubt). Because I am not an actual engineer or an extremely seasoned builder of speakers my nomenclature may be a bit weird at times as well as my advice/thoughts. I can only hope that if I start heading you down the wrong path that someone, like John Krutke (ZAPH), will jump in and politely correct me in my ignorance and inexperience. It is, after all, John's design :)

OK, here are my thoughts and take on what you might have going...

I'm wondering now about the placement of your bracing in relation to the drivers and port. For instance, in my designs I typically put bracing between each driver. IE: each driver is separated by a brace... the larger the driver, the larger the spacing between bracing. Thus, my tweeter has the smallest space between bracing or bracing and side. This is simply my intuitive attempt to keep higher frequency cabinet resonances with the higher frequency drivers and lower frequency cabinet resonances with lower frequency drivers. Of course I'd like my speaker panels to not resonate at all, but short of some exotic vacuum panel this is simply not possible. (The vacuum panel idea has already been patented by someone else... oh well... hard to implement anyway...)

As far as bracing composition goes... being the perfectionist that I am, I had concerns about internal reflections and standing waves, so I pad my bracing with 1/8" cork overlain by 1/8" felt. Every cabinet panel now gets 1/8" cork overlain by at least 3/4" wool felt blend (>60% natural wool). Whether wool felt is better than "acoustic foam" is probably a matter of debate... but with wool I don't have to worry about dry rot in 10 years. For fill I use fiberglass insulation, BUT my speakers are sealed without ports, so it is safe to do so. In my opinion ported speakers should use some sort of synthetic or natural wool that is not a health concern instead of fiberglass.

(It is my personal belief that the cabinet/enclosure construction is way understated and overlooked by many diyers as well as main stream manufacturers.)

Looking at your photo I imagine that you are stuffing the bottom cavity with fill, but perhaps your bottom brace is prohibiting the full effectiveness of the fill? IE it might be blocking the fill from effectively damping internal reflections?

Rather than adding more foam to your panels, I would try more fill. Try to work it with your hands and fingers so it is uniform 1-2" relaxed thickness and cut it to fit panels and put it over your foam/felt (that's how I do my insulation).

Cheers,

Brian

[Andy_G]

If you have the components sitting around, try this x-o instead.

http://members.optusnet.com.au/~grad.../Jarrah_XO.bmp

(iirc, I used .13mH rather than .15mH for the small coil)

[monroe]

Would this crossover work with the scan speak drivers? It looks quite different than the ZRT crossover.

[monroe]

Brian,

I have tried your suggestion of stuffing the bottom of the cabinet as heavily as possible and I think that there has been an improvement in the bass. Its not dramatic, but my sense is that heavy stuffing in the bottom two sections gives stronger bass than moderate stuffing in the bottom section alone.

I suspect that part of the problem may be related to a too-high tweeter level and have ordered some additional resistors to test that theory.

[Andy_G]

Doh.. sorry, I saw the SR71 on another thread and I guess it was on my mind.. sorry.

[Flagenbush]

"Brian,

I have tried your suggestion of stuffing the bottom of the cabinet as heavily as possible and I think that there has been an improvement in the bass. Its not dramatic, but my sense is that heavy stuffing in the bottom two sections gives stronger bass than moderate stuffing in the bottom section alone.

I suspect that part of the problem may be related to a too-high tweeter level and have ordered some additional resistors to test that theory."

Monroe,

Could be you'll never get the bass you want from the ZRT 2-way... The Odin has two mid bass drivers and the ZRT 2-way only one. But I still think even more stuffing might be worth a try. I know this is counter-intuitive because it would seem that by adding more stuffing you reduce the internal volume of the cabinet and it would seem that would raise the Fs of the system higher.

However, in the reference book "Testing Loudspeakers" by Joseph D'Appolito it is illustrated that adding stuffing can effectively increase the cabinet volume by some 20%, thus reducing the Fs of the system.

A couple of cautions with more stuffing 1) You must have a reasonably clear path from the mid bass to the port. The path doesn't have to be straight or smooth curve, it just has to be there... after all this is not a transmission line design. 2) More stuffing may also affect the mid and perhaps tweeter response as well. So in correcting one problem, you might create another... only way to know is to try.

Typically Zaph crossover designs are spot on and flat. If your tweeter is "too bright" for the content you are playing you might want to try an equalizer correction if your amp has that capability... the ole X-curve dilemma. Some recordings are simply too bright in the high end.

Guess what I'm trying to say is, "stuff it" 80-90% full. After that you can tell me to "stuff it" as well. I guess that would only be fair... :)

Brian

[Loren]

>Monroe,
>
>First a few disclaimers... I am not a speaker building guru or
>engineer. I am a scientist with a dream to one day create my
>own speaker line (delusions of grandeur no doubt). Because I
>am not an actual engineer or an extremely seasoned builder of
>speakers my nomenclature may be a bit weird at times as well
>as my advice/thoughts. I can only hope that if I start
>heading you down the wrong path that someone, like John Krutke
>(ZAPH), will jump in and politely correct me in my ignorance
>and inexperience. It is, after all, John's design :)
>
>OK, here are my thoughts and take on what you might have
>going...
>
>I'm wondering now about the placement of your bracing in
>relation to the drivers and port. For instance, in my designs
>I typically put bracing between each driver. IE: each driver
>is separated by a brace... the larger the driver, the larger
>the spacing between bracing. Thus, my tweeter has the
>smallest space between bracing or bracing and side. This is
>simply my intuitive attempt to keep higher frequency cabinet
>resonances with the higher frequency drivers and lower
>frequency cabinet resonances with lower frequency drivers. Of
>course I'd like my speaker panels to not resonate at all, but
>short of some exotic vacuum panel this is simply not possible.
> (The vacuum panel idea has already been patented by someone
>else... oh well... hard to implement anyway...)
>
>As far as bracing composition goes... being the perfectionist
>that I am, I had concerns about internal reflections and
>standing waves, so I pad my bracing with 1/8" cork
>overlain by 1/8" felt. Every cabinet panel now gets
>1/8" cork overlain by at least 3/4" wool felt blend
>(>60% natural wool). Whether wool felt is better than
>"acoustic foam" is probably a matter of debate...
>but with wool I don't have to worry about dry rot in 10 years.
> For fill I use fiberglass insulation, BUT my speakers are
>sealed without ports, so it is safe to do so. In my opinion
>ported speakers should use some sort of synthetic or natural
>wool that is not a health concern instead of fiberglass.
>
>(It is my personal belief that the cabinet/enclosure
>construction is way understated and overlooked by many diyers
>as well as main stream manufacturers.)
>
>Looking at your photo I imagine that you are stuffing the
>bottom cavity with fill, but perhaps your bottom brace is
>prohibiting the full effectiveness of the fill? IE it might
>be blocking the fill from effectively damping internal
>reflections?
>
>Rather than adding more foam to your panels, I would try more
>fill. Try to work it with your hands and fingers so it is
>uniform 1-2" relaxed thickness and cut it to fit panels
>and put it over your foam/felt (that's how I do my
>insulation).
>
>Cheers,
>
>Brian

Bracing and dampening a cabinet should follow these basic guidelines.

1. Bracing is designed to divide each panel so as to reduce its tendency to vibrate and move the vibration frequency point upward in the spectrum. A good rule is to divide each panel unequally. In other words, don't run a divider down the middle of panel and keep bracing no more than about 5" apart. That distance really depends on the wall thickness of the cabinet and the panel size.

2. There are surface treatments like Damplifier Pro ( http://www.secondskinaudio.com/vibra...lifier-pro.php ) that are a heavy rubber like, self-stick, sheet that is applied to the interior surface of the cabinet. There are also variants and you can even use the same stuff applied to submarine hulls if your pockets are deep enough.

3. Some people have stapled roofing shingles onto the interior surface. Others have used roofing tar painted on the inside This is cheap, but the binding agents in the tar/oils of the material will outgas and I would worry about smells and possible attacking of the glues used in the driver's cone. That stuff takes a long time to dry, too.

4. Rockwool is an excellent sound absorbent material that can be applied internally to a cabinet walls (in addition to the Damplifier material if you want) and can be purchased from http://www.atsacoustics.com/cat--Fib...ards--106.html in sheets with a density rating of 2.5 lbs/foot should be adequate.

5. Stuffing free fill into a vented enclosure and particularly a sealed enclosure increases the apparent internal volume.

6. A good starting point for treating cabinet walls with rockwool, fiberglass, polyfill, etc. is 50%, then add delete until you get the desired sound. The back of the cabinet should get particular attention.

7. I would be remiss for not mentioning Warfdale's unique treatment on some of their cabinets. They build two layers for the cabinet wall and fill the center cavity between the layers with sand. I have known of others using sheet rock sandwiched with MDF and the ultimate is simply building the whole cabinet out of concrete!