Sealed versus Ported Bass, can the difference be measured?

[Keith_W]

With your setup I think you will need to create the sweep file with REW "Generator". On the Top set of tabs on the "Generator" chose "Sweeps" and on the second row of tabs chose "Measurement" then fill in information the same as what you used before. On the third set up tabs chose "Save to file" and set up the channels and timing reference as before and click the "WAV" button. This will create a file you will use to measure. On the main REW Measurements Icon for Playback chose "from file" and navigate to and select the file you just created. Then load up the file into whatever "player" you are using then go back to REW click "Start" and then back to you player and start playing the file. REW will "listen" for the timing reference and when it plays will start the measurement process. You will probably have to create two files, one for the right with the left reference and one for the left with the right reference. Before you do the measurement make sure you have the volume on your player where you want it or you might be surprised. I usually use -20 dB when I Generate the sweep files to help prevent too loud playback accidents.

Why does he need to save a sweep to file instead of playing the sweep directly from his laptop?

 

[Sokel]

Why does he need to save a sweep to file instead of playing the sweep directly from his laptop?

It can be useful to test external players loaded with VST's, or apps using their own filters, etc.
Just testing them as black boxes.

 

[Paul McNeil]

I had a look at your measurements.

Just so you know, this is how you determine what is responsible for what peak in the step response.

Step 1: Import all measurements of individual speakers into the same MDAT.
Step 2: SPL and time align all of them.
Step 3: Open "Overlays" and go to "Step Response".
Step 4: Display the reference curve (this will be the curve with ALL speakers and subs playing together) and fix your eyes on that huge late arriving peak at 100ms.
Step 5: Click on/off each measurement and see which speaker is responsible for that peak.

View attachment 463020

In this case, red is the reference. It's from that SEALED MDAT that you posted a few days ago. Purely for illustration, the purple curve seems to match the shape of the previous measurement. I said "purely for illustration" because the measurements you took are incorrect.

View attachment 463021

Firstly, if these are measurements of the subwoofers alone (from MDAT labelled "SVS 13 only" and "SVS 17 only") why does the measurement extend up to 20kHz? This is clearly a measurement of the subwoofer with the speaker.

View attachment 463022

You see that double peak at the start of the Energy-Time Curve? That means that you measured both main speakers together. This is supposed to be a measurement of the "SVS 13" by itself. I can even tell how off-centre your microphone is. The time discrepancy between the peaks is 220 us, meaning your mic is 75mm off-centre.

View attachment 463023

How can I be so sure that you measured both speakers together? Well, in pink we have "L speaker only" and in blue we have "SVS 17 only". The "L speaker only" measurement does not have a double peak. Sometimes, double peaks can be caused by very very bad mic placement (e.g. a mic which is 37.5mm away from a wall can create a double impulse with 220us delay). But anyway I digress.

TO BE CLEAR, these are the measurements that need to be taken:
- L subwoofer ALONE, with timing reference
- R subwoofer ALONE, with timing reference
- L speaker ALONE, with timing reference
- R speaker ALONE, with timing reference
- Both speakers with both subs, with timing reference

Keith, Here are the files. It looks to me as if the same Step Response problem is present with the subs only (both them). These are time and SPL (80htz) aligned.

1) left sub only
2) left speaker only
3) right sub only
4) right speaker only
5) full system

 

[Paul McNeil]

Here they are individually...

 

[Keith_W]

Thanks for taking those measurements. Anyway let's take a look.

Remember when I told you that I have little faith in REW's automatic time determination? Look at this, I have a hard time believing that your left sub is 1.5m further away than your right sub. It also says that your subs are delayed 12.3ms and 8ms with respect to your main speakers, effectively a distance of 4.3m and 2.7m distance. Now it is possible that both subs have inherent latencies that produce this delay, especially if the subs are DSP enabled. If the DSP is enabled on one sub but not the other, this may be the result. A quick google says that SVS PB-13 subs are DSP enabled. Would you be able to check this? Otherwise, it is likely that REW didn't determine the delay properly.

If you were going to use these delays for DSP, then I would insist on better accuracy and I would suggest that you use a special procedure to determine the delays. But since the purpose of this exercise is for diagnosis, we don't need to do that. We already have a diagnosis.

In green we have the step response of your full system, 2 speakers and 2 subs, from 20Hz - 200Hz. In purple we have your left subwoofer alone. Notice how REW aligns the two curves at the start of the impulse response (i.e. t=0), which means that the big reflection at about 100ms is misaligned.

Now look what happens if I shift the subwoofer to the right by 15ms. Although I have only used the left sub for illustration, your right sub has the same problem.

(I have to note that I am not an REW expert, I used the "Time Align" command. According to the REW manual the "Time Align" command aligns the impulses using the acoustic timing reference, which SHOULD produce the correct alignment, yet I had to manually shift the L Sub measurement by 15ms to reproduce your initial measurement. I did NOT use the "Align IR start" command. This is yet another reason I have little faith in REW's automatic delay measurements!)

Anyway, from this graph alone, you have two problems:

1. Your subs are very poorly time aligned. Since your L sub and L speaker are on the same DAC channel, you have no way of fixing this. The ONLY solution is to put them on separate DAC channels and delaying the L speaker by about 15ms to match the sub. I say "about 15ms" because we don't know what the actual delay is, we need special procedures to determine that.

2. You have a late arriving reflection which is significantly louder than your main impulse. We haven't worked out why yet. The solution to this would be to reposition the subs in your room. Some people would say acoustic treatment, but personally, I never recommend acoustic treatment to fix bass problems because it is too intrusive and it is easier to reposition subs and use DSP.

Here is something else to look at.

This is the Energy-Time Curve (ETC) from your previous measurement "Jul 13 stereo left speaker only.MDAT". At t=0 we have your main impulse. All impulses that follow should be less than 15dB below the main impulse (some people say 20dB), otherwise it has the effect of making music sound smeared. We can see a loud early peak at 2ms which I marked. This may be due to a measurement artefact, e.g. if your microphone is not on a proper boom tripod and it's too close to furniture. But it can also be caused by furniture and walls. 2ms equals a time lag of about 0.7m. There is another smaller peak with a 6ms delay, this equals a time lag of 2.06m. I would give that one a "pass" but that depends on how stringent and OCD you are.

 

[voodooless]

@Keith_W , marvelous!

Since your L sub and L speaker are on the same DAC channel, you have no way of fixing this. The ONLY solution is to put them on separate DAC channels and delaying the L speaker by about 15ms to match the sub. I say "about 15ms" because we don't know what the actual delay is, we need special procedures to determine that.

I think this is only so for the measurement? In any case, both SVS subs have their own DSP, so you can add delay there (plus it will have some sort of propagation delay to account for).

 

[Keith_W]

I think this is only so for the measurement? In any case, both SVS subs have their own DSP, so you can add delay there (plus it will have some sort of propagation delay to account for).

When I saw that you had quoted me I felt a bit of anxiety, what mistake have I made now??

Anyway, using DSP on the subs won't work. Subwoofer DSP only adds delay to the sub. In this case, it's the speaker that needs to be delayed.

This reminds me of the group delay audibility discussion that was happening earlier in this thread. I stated somewhere that the threshold of audibility is between half a period to a period of the frequency in question. So for e.g. a 50Hz wave has a period of 20ms, so the audibility threshold should be about 10ms - 20ms. His subs are delayed 15ms so it should be inaudible, right? Not really ... he also has a huge delayed reflection that arrives at 100ms. That is easily audible.

 

[Paul McNeil]

Thanks for taking those measurements.

Hey, thanks for helping me with this Keith_W!

 

[voodooless]

Anyway, using DSP on the subs won't work. Subwoofer DSP only adds delay to the sub. In this case, it's the speaker that needs to be delayed.

Ah! I bet that DSP cannot be bypassed, and the propagation delay of ADC/DSP/DAC is probably more than those 15ms.

Anyway, he's using an AV10 and Dirac, so it seems to me that adding the correct delays to the individual channels should be trivial.

 

[Paul McNeil]

Remember when I told you that I have little faith in REW's automatic time determination? Look at this, I have a hard time believing that your left sub is 1.5m further away than your right sub. It also says that your subs are delayed 12.3ms and 8ms with respect to your main speakers, effectively a distance of 4.3m and 2.7m distance. Now it is possible that both subs have inherent latencies that produce this delay, especially if the subs are DSP enabled. If the DSP is enabled on one sub but not the other, this may be the result. A quick google says that SVS PB-13 subs are DSP enabled. Would you be able to check this? Otherwise, it is likely that REW didn't determine the delay properly.

Dirac sets the L/R to 10.1 and 10 ms, the left sub no more distant then 0.5M to 4.6 ms, the right also no more distant then 0.5 M to 0.00 ms. I don't believe that I have DSP engaged, using the app. No equalization is applied, nor anything else on the menu, but I'd think these subs apply DSP to the incoming analogue signal as a matter of course. Maybe SVS publishes the resultant delay time. However, the subs must be in phase, no, because playing together they add ~5dB.

 

[Paul McNeil]

Your subs are very poorly time aligned. Since your L sub and L speaker are on the same DAC channel, you have no way of fixing this. The ONLY solution is to put them on separate DAC channels and delaying the L speaker by about 15ms to match the sub. I say "about 15ms" because we don't know what the actual delay is, we need special procedures to determine that.

I can work on this, but the question is how to do it efficiently, rather than 'monkey and typewriter' style?

 

[Paul McNeil]

You have a late arriving reflection which is significantly louder than your main impulse. We haven't worked out why yet. The solution to this would be to reposition the subs in your room. Some people would say acoustic treatment, but personally, I never recommend acoustic treatment to fix bass problems because it is too intrusive and it is easier to reposition subs and use DSP.

Yeah, this is the most worrying to me. I can hear this, no? I do not measure with a proper boom microphone but rather with one mounted on a tripod, 4 inches from the back of a couch, the main listening position. I would rather not reposition the subs, from the near corner locations, which the REW room tool seems to indicate is the optimal position. But I could move one sub (right) to near the opposite back corner of the room. There are other potentially problematic items in the room, a pool table and an 83" TV mounted on a wooden stand, between the L/R pair and subs, in addition to the aforementioned in room fireplace.

 

[Paul McNeil]

Anyway, he's using an AV10 and Dirac, so it seems to me that adding the correct delays to the individual channels should be trivial.

Finally some good news!

 

[Paul McNeil]

When I saw that you had quoted me I felt a bit of anxiety, what mistake have I made now??

Anyway, using DSP on the subs won't work. Subwoofer DSP only adds delay to the sub. In this case, it's the speaker that needs to be delayed.

This reminds me of the group delay audibility discussion that was happening earlier in this thread. I stated somewhere that the threshold of audibility is between half a period to a period of the frequency in question. So for e.g. a 50Hz wave has a period of 20ms, so the audibility threshold should be about 10ms - 20ms. His subs are delayed 15ms so it should be inaudible, right? Not really ... he also has a huge delayed reflection that arrives at 100ms. That is easily audible.

Yeah, this is what bothers me the most. Is it the room? Or Dirac? Or my (amateur) measurement technique?

 

[Paul McNeil]

@Keith_W , marvelous!

View attachment 463362


I think this is only so for the measurement? In any case, both SVS subs have their own DSP, so you can add delay there (plus it will have some sort of propagation delay to account for).

The subs (two on right connected by Y cabling), and the single on the right, on separate channels. All three could be on separate channels, if this would help, but my feeling was that the two SVS 13 cylinders were about the equal of the single SVS 17, and that Dirac should so treat them as equals.

 

[Paul McNeil]

One more thing, I can add a fourth sub, in the back right corner. The sub and wiring are in place.

 

[Paul McNeil]

Maybe I need to hand over more bucks to Dirac, for the ART addition, soon to be available for the Marantz AV10.

 

[voodooless]

Maybe I need to hand over more bucks to Dirac, for the ART addition, soon to be available for the Marantz AV10.

View attachment 463379

Which one do you have now? DLBC?

 

[Paul McNeil]

Which one do you have now? DLBC?

Yeah, Dirac Live with Bass Control.

 

[Keith_W]

The subs (two on right connected by Y cabling), and the single on the right, on separate channels. All three could be on separate channels, if this would help, but my feeling was that the two SVS 13 cylinders were about the equal of the single SVS 17, and that Dirac should so treat them as equals.

I don't know what you are talking about. You have two subs on the RIGHT connected with a Y-cable, and ANOTHER sub on the right on a separate channel?? According to your diagram:

You have two subs on the left, and one on the right??

Anyway, if you have four subs, according to Welti 2004 the subs should be positioned like this:

The second configuration is probably not possible given where the fireplace in your room is.

I had a quick look at the Marantz AV10 manual and I see it has four subwoofer outputs at the back. Whether these are on separate DAC channels, I have no idea. So I don't know why you are using a Y-cable when you have enough subwoofer outputs.

Unfortunately from here on, I can't help you any more. To me those AVR's are DSP black boxes where measurements go in, it does its mysterious thing, and results come out. I really, really, hate them. I don't like software making decisions for me, particularly when I can see with my own eyes that the software is screwing it up. I want to bypass the software decisions and make it do what I want, but those AVR's typically won't let me. And it boggles my mind that a $10k high end AVR expects you to use this piece of junk to take measurements:

Anyway my ranting isn't going to help you, so i'll stop here. Let's just say that AVR's and Dirac don't suit my personality

If you want to improve the DSP of your system, you'll have to ask some people who know how to use AVR's and Dirac. It is very likely that the reason you have bad results is because you have made an error in your measurements or your DSP settings. Maybe someone else on ASR knows, or someone in a HT forum may have more of a clue.