2235H Project Help

[Loren42]

Greetings,

I am trying to understand what might be the problem with my speaker system. The issue is lack of bass.

The 2235H is in a 6 cubic foot ported enclosure with two 4" diameter ports 6" long tuned to 32 Hz. This is the frequency plot at 1 meter:

http://www.mdbq.net/pyramid/2235HPyramid.jpg

This i the near-field of one of the ports:

http://www.mdbq.net/pyramid/2235HPyramidPort.jpg

The box appears to be tuned correctly, but for whatever reason at about 55 Hz the bottom falls out of the bass and I don't know why.

Is that all I can get out of a 2235H?

Any thoughts?

[Robh3606]

Can you measure the FS of the drivers?? Can you run an impedence plot with the woofers in the boxes?? When I got my pair with new recones they were actually a bit under 20Hz. I have a pair in 5 cubic ft and a third in the 4.5 cu ft B380 box. Either set-up works just fine. What happens with a sine sweep?? No phase issues for a stereo pair?

Rob:)

[Loren42]

Can you measure the FS of the drivers?? Can you run an impedence plot with the woofers in the boxes?? When I got my pair with new recones they were actually a bit under 20Hz. I have a pair in 5 cubic ft and a third in the 4.5 cu ft B380 box. Either set-up works just fine. What happens with a sine sweep?? No phase issues for a stereo pair?

Rob:)

Can be done, but 55 Hz seems like a pretty big error. I could do a test in the box and I could also pull one and do a free air impedance test in a day or so.

I am just testing one cabinet set in the middle of the room facing a sofa with a mic on a micstand to do the tests. The program to run the sweep does a pretty good job of canceling room artifacts and the plots are a log sine sweep. I ran multiple plots at near and far field and they come up essentially the same. The plots confirm what my ears tell me; no bass.

So, how far down does your bass extend for you?

[Robh3606]

Do the impedance in the box. That will verify the box tuning. The FS should be 20Hz on the raw drivers do that last after you make sure everything else is OK. The best thing to do is go through your whole set-up and look for a high pass filter in line and verify speaker phase if you are running a pair. If it's phase simply disconnecting one will tell you.

Rob:)

[Loren42]

Do the impedance in the box. That will verify the box tuning. The FS should be 20Hz on the raw drivers do that last after you make sure everything else is OK. The best thing to do is go through your whole set-up and look for a high pass filter in line and verify speaker phase if you are running a pair. If it's phase simply disconnecting one will tell you.

Rob:)

I can't imagine phase being an issue. there is only one 2235H in the cabinet and I am only testing one speaker, so the second channel is disconnected.

[Robh3606]

I am just testing one cabinet set in the middle of the room

Worst place, try moving it to where you plan on setting them up. Could just be room placement issues although to loose the whole bottom is a bit odd.


So, how far down does your bass extend for you?

Solid to below 30hz about 25hz or so for the pair. For the sub a little lower with the BX-63.

Rob:)

[Ruediger]

If You measure at a distance of one meter You measure speaker and room influence.

You could do a nearfield measurement where the direct signal is magnitudes larger than the reflected one.

There is an article from D.B. Keele about this, available at http://www.xlrtechs.com/dbkeele.com/papers.htm,
"Low-Frequency Loudspeaker Assessment by Nearfield Sound-Pressure Measurement".

Ruediger

[grumpy]

You appear to be using Fuzzmeasure (Mac software)... there are instructions
for assembling a simple impedance measurement cable/setup with it.

Both Rob's and Ruediger's help should...er help. :)

A 3 year old nearfield plot of two 4430 systems (albeit with the low pass filter in place)
are here with comparable (with yours) graph ranges and smoothing:

[Loren42]

You appear to be using Fuzzmeasure (Mac software)... there are instructions
for assembling a simple impedance measurement cable/setup with it.

Both Rob's and Ruediger's help should...er help. :)

A nearfield plot of two 4430 systems (albeit with the low pass filter in place)
are here with comparable (with yours) graph ranges and smoothing:

So, you think I am in the ball park?

I did a slew of different measurements with both speakers in near-feld, far-field, and 1 meter with the cabinets placed in various positions in the room and pointing in various directions.

After doing all those tests I found that the rolloff begins at about 40 - 45 Hz and drops at least 15 Hz per octave.

A simple sine wave generator on my Mac and a voltmeter tells me I have a box resonance tuned at about 25 Hz. I need to confirm that because that seems too low for the cabinet volume and two port dimensions (3.95" diameter 6" long).

Here is the crossover:

http://www.mdbq.net/pyramid/LEAP.pdf

Here is a picture of the cabinet:

http://www.mdbq.net/pyramid/pyramid.jpg

Stuffing is 1" polyfill on about 85% of the wall surface. I also have one sheet stretched across the internal bracing about 6" behind the woofer that subdivides the internal cabinet space.

While it looks like the ports are clogged in the picture, that sheet is about 6" behind the ports. I think I may pull that sheet out and recheck the port tuning and frequency sweep when I get some more time to play with it.

[Loren42]

If You measure at a distance of one meter You measure speaker and room influence.

You could do a nearfield measurement where the direct signal is magnitudes larger than the reflected one.

There is an article from D.B. Keele about this, available at http://http://www.xlrtechs.com/dbkeele.com/papers.htm, (http://http//www.xlrtechs.com/dbkeele.com/papers.htm)
"Low-Frequency Loudspeaker Assessment by Nearfield Sound-Pressure Measurement".

Ruediger

That link does not seem to work, even when copied and pasted into the address bar.

[grumpy]

chop off the first http:// ... and take a look at the nearfield paper (#6)

[4313B]

A simple sine wave generator on my Mac and a voltmeter tells me I have a box resonance tuned at about 25 Hz. I need to confirm that because that seems too low for the cabinet volume and two port dimensions (3.95" diameter 6" long).Check the tuning frequency without any internal stuffing. If your tuning frequency is 25 Hz instead of your targeted 32 Hz and your ports are correct then you have a whole lot more volume than the six cubic feet you built for.
After doing all those tests I found that the rolloff begins at about 40 - 45 Hz and drops at least 15 Hz per octave.Measure the driver nearfield and then the port nearfield and sum the response. Response should be solid to ~ 30 Hz and roll off from there. If they sound thin then it's your room.

I suppose you could also have 2234H's instead of 2235H's...


The 2235H is in a 6 cubic foot ported enclosure with two 4" diameter ports 6" long tuned to 32 Hz. The 2235H really shines in 4.5 to 5.0 cubic feet (tuned in the 26 Hz to 30 Hz range). I know people build six cubic foot boxes for them (and I did a few too) but... whatever...

[Loren42]

Measure the driver nearfield and then the port nearfield and sum the response.

I have two ports, so do I add 3 dB to the port measurement and then sum with the nearfield of the woofer?

[Robh3606]

I suppose you could also have 2234H's instead of 2235H's...


I was thinking the same thing. They have been refoamed as well so we don't know what the driver FS is.

Rob:)

[pos]

I have two ports, so do I add 3 dB to the port measurement and then sum with the nearfield of the woofer?
+6dB at these frequencies

[Ruediger]

I checked Your data. Your box should resonate at 35.6 Hz. I ran WinISD, selected the 2235H driver, 170 liter box volume and played a bit with the tuning: 32 Hz box resonate frequency (Your target) is a relative optimum.

The ducts have one end flanged and one end free standing. The end corrections are:

flanged: 0.85 R
free: 0.613 R
together: 1.463 R

So the effective length of each tube should be (6 + 2.93) in = 8.93 in.

Perhaps Your simple sine wave generator has a broken frequency pot?

Your picture shows that You are using a tube amplifier. These things often do have a high output impedance. And Your passive xover will have a series inductor with maybe a high resistance. The following post shows how to calculate the resulting Qt for Your driver. This will have an influence on bass response.

http://audioheritage.org/vbulletin/showpost.php?p=273909&postcount=105

Ruediger

[Loren42]

I checked Your data. Your box should resonate at 35.6 Hz. I ran WinISD, selected the 2235H driver, 170 liter box volume and played a bit with the tuning: 32 Hz box resonate frequency (Your target) is a relative optimum.

The ducts have one end flanged and one end free standing. The end corrections are:

flanged: 0.85 R
free: 0.613 R
together: 1.463 R

So the effective length of each tube should be (6 + 2.93) in = 8.93 in.

Perhaps Your simple sine wave generator has a broken frequency pot?

Your picture shows that You are using a tube amplifier. These things often do have a high output impedance. And Your passive xover will have a series inductor with maybe a high resistance. The following post shows how to calculate the resulting Qt for Your driver. This will have an influence on bass response.

http://audioheritage.org/vbulletin/showpost.php?p=273909&postcount=105

Ruediger

So, you are saying that the effective length of my ports are really 8.93" even though the physical length (with flange) is 6.0"?

I published the link to my crossover design earlier, but here it is again so that you can see what I did:

http://www.mdbq.net/pyramid/LEAP.pdf

The main inductor is a Solen inductor I got here:

http://www.madisound.com/catalog/product_info.php?products_id=903

Yes, that is a tube amplifier that I use for these cabinets.

[Ruediger]

So, you are saying that the effective length of my ports are really 8.93" even though the physical length (with flange) is 6.0"?


1st of all: I may have made a mistake in my calculations, that's always possible :)

Yes, the effective length of a tube is physical length plus an end correction. For the flanged end of a tube it's 0.85 * radius, and for a free standing end it's 0.613 * radius. For cases in between: interpolate.

The 5.5 mH coil does have a DC resistance of 0.63 Ohms. Your amplifier will have an output impedance which cannot be neglected. Ask the manufacturer (easy) or measure it (not so easy). Often a "damping factor" is specified from which You can calculate the output impedance. Your cables will have a resistance (calculate). The sum of these 3 resistances is the "generator resistance" for the Qt-formula. Take this corrected Qt into account when calculating the box response.

Ruediger

[Loren42]

1st of all: I may have made a mistake in my calculations, that's always possible :)

Yes, the effective length of a tube is physical length plus an end correction. For the flanged end of a tube it's 0.85 * radius, and for a free standing end it's 0.613 * radius. For cases in between: interpolate.

The 5.5 mH coil does have a DC resistance of 0.63 Ohms. Your amplifier will have an output impedance which cannot be neglected. Ask the manufacturer (easy) or measure it (not so easy). Often a "damping factor" is specified from which You can calculate the output impedance. Your cables will have a resistance (calculate). The sum of these 3 resistances is the "generator resistance" for the Qt-formula. Take this corrected Qt into account when calculating the box response.

Ruediger

Useful knowledge. Thank you.

[Loren42]

I have what I think are impedance plots for this cabinet:

Here is the raw frequency plot data when plotted with a 829 Ohm resistor in series with the amp and the voltage measured at the speaker terminals:

http://www.mdbq.net/pyramid/pyramidF1.jpg

The program I am using (Fuzzmeasure) is very fuzzy about how to actually generate impedance plots from the frequency sweep and I did my best:

http://www.mdbq.net/pyramid/pyramidz1.jpg

The cabinet is tuned to 28 Hz, which is not too bad.

I did a number of other more detailed sweeps with the cabinet in a number of orientations and I can say that the real rolloff is at 45 Hz. The room complicates things very much.

I have a few other things to try, like reducing the internal fill in the cabinets and take more detailed measurements. I will keep you guys posted. THANKS!

[4313B]

I did a number of other more detailed sweeps with the cabinet in a number of orientations and I can say that the real rolloff is at 45 Hz.Maybe for the woofer but certainly not for the ports unless they are broken. If you are thinking that the woofer itself should be flat down to 28 Hz then you are mistaken. The bottom octave is handled by the port(s).

[Loren42]

Maybe for the woofer but certainly not for the ports unless they are broken. If you are thinking that the woofer itself should be flat down to 28 Hz then you are mistaken. The bottom octave is handled by the port(s).

Good point. However, if the ports are supposed to be contributing to the total picture, "Where's the bass?" If I may coin a phrase.

This is a carefully performed on-axis sweep with the microphone 1 meter from the front of the cabinet. The mic is centered on the cabinet.

http://www.mdbq.net/pyramid/Plot1m.jpg

Should this not be the aggregate of the woofer and ports?

When I put the mic at the listening position at the center of the sofa (magenta trace) and compare that with the 1 meter on-axis plot (green) I get this:

http://www.mdbq.net/pyramid/Plot1m_Sofa.jpg

I have a horrible 50 Hz antinode at that point in the room and both speakers have the sweep tone sent to them equally.

An alternate arrangement for the speakers in the room (both speakers backed tight into the corners) smooths out that 50 Hz hump, but places a strange dip in the high end (blue trace).

http://www.mdbq.net/pyramid/NewOldLayout.jpg

Hard to tell what is room artifact and what is really output from the speaker.

Maybe there is more bass from the speaker than what the 1 meter plots reveal?

[Ruediger]

Your frequency response can result from underdamping by a generator impedance which is too high. If I imagine that influence subtracted from the curve on the plot I see a -3dB point of 30+Hz.

Test 1: short circuit the series inductor in Your xover. That will remove 0.63 Ohms, which is a lot. Then run the test again. Ignore what happens above 500 Hz.

Test 2: get a proper transistor amp (that means: an amp without output transformers) with a high damping factor (200 or better). Keep the series inductor short circuited. Run the test again.

Ruediger

[Loren42]

Your frequency response can result from underdamping by a generator impedance which is too high. If I imagine that influence subtracted from the curve on the plot I see a -3dB point of 30+Hz.

Test 1: short circuit the series inductor in Your xover. That will remove 0.63 Ohms, which is a lot. Then run the test again. Ignore what happens above 500 Hz.

Test 2: get a proper transistor amp (that means: an amp without output transformers) with a high damping factor (200 or better). Keep the series inductor short circuited. Run the test again.

Ruediger

I have a solid state amp, but I doubt the damping factor is that high. Needless to say, I can give it a whirl. Thanks.

Also, if I can man handle this thing outside I could run a sweep in the open yard or maybe face the cabinet up toward the sky, but I wonder about ground effects below the baffle step (210 Hz in this case)?

[Ruediger]

Also, if I can man handle this thing outside I could run a sweep in the open yard or maybe face the cabinet up toward the sky, but I wonder about ground effects below the baffle step (210 Hz in this case)?

I think there is a common missunderstanding. Below a certain frequency the loudspeaker is radiating into full space (4 pi), above that frequency it is radiating into half space (2 pi).

If the speaker is in a room and not in the free field then the sound which is radiated to the rear will be reflected by the wall behind the speaker.

At 210 Hz the wavelength is 1.6 meter. Reflected sound will add to direct sound with quite a phase shift. At 100 Hz the wavelength is 3.4 meter, and the phase shift will be much smaller.

At 50 Hz the wavelength is even 6.8 meter. Direct and reflected sound are almost in phase.

This means destructive interference will happen around 210 Hz, and at low frequencies the interference will be constructive, "bass will not get lost".

To my opinion it is wrong to boost the bass range to correct the baffle step diffraction. That would only make sense in free field. But then a reflecting panel behind the speakers is the more economic alternative.

Ruediger