JBL310s subwoofer polarity

[witwald]

The back volume control on the LSR310S appears to be one of those continuously variable ones (see below). Does it actually have steps along the way from –∞ to Max?

At least for me, it would be hard to find the position where the output of the two mains matches that of the LSR310S without performing measurements. The midway points on the main speakers and the subwoofer are likely to be quite different output levels.

I'm glad that regular polarity seems to produce the most bass for you, as that is definitely what I would expect to have happen.

If you happen to have a rectangular room, I would try and use a room mode calculator to work out where the low-frequency nulls are located. If you happen to be sitting close to one of the nulls when monitoring, your bass can sound a bit weak in that frequency range. It's a rather pronounced effect, and will skew your perception of the bass that's present in your mix.

 

[witwald]

To achieve the type of phase inversion you are speaking about (3.5ms) if the subs are very close to the speakers, which mine are, in terms of degrees of shift, what might I begin experimenting with? As I mentioned, currently my subs are both shifted 180 degrees and crossed at 79Hz.

At 79 Hz, the wavelength of sound is about 345/79 = 4.367 metres. In 3.5 milliseconds, sound travels 345*0.0035 = 1.208 metres. In terms of fractions of a wavelength at 79 Hz, that amounts to 1.208/4.367 = 0.2766. It therefore is equivalent to a phase difference of 0.2766*360 = 99.6 degrees. This only about 55% of a phase shift of 180 degrees.

 

[A Surfer]

At 79 Hz, the wavelength of sound is about 345/79 = 4.367 metres. In 3.5 milliseconds, sound travels 345*0.0035 = 1.208 metres. In terms of fractions of a wavelength at 79 Hz, that amounts to 1.208/4.367 = 0.2766. It therefore is equivalent to a phase difference of 0.2766*360 = 99.6 degrees. This only about 55% of a phase shift of 180 degrees.

Thank you so much, this is an area that I have little knowledge in and so the above was very helpful. So I should try a phase shift of 100 degrees then. In theory, assuming I have described things accurately I should find the bass integration better then is that correct?. Much appreciated.

 

[witwald]

@A Surfer Assuming that you are using an AVR to drive your main speakers and the subwoofer, I'd like to know which AVR you are using, as well as the settings of its bass management filters. Of course, as the SB-2000 Pro has onboard filtering, you may be running a stereo amplifier's preamp outputs into the SB-2000 Pro and getting it to do the low-pass and high-pass filtering, before sending back the high-pass filtered signal to go into the power amp inputs on your stereo amplifier. If so, which stereo amplifier are you using. That type of extra information would be handy for me.

From what I gather from your descriptions, you have set the filter on the SB-2000 Pro to 79 Hz. However, if you are using an AVR, I don't know what filter setting you are using on the main speakers. The choice of where the filtering occurs can have a significant affect on the summed response from the subwoofer and the main speakers, due to a combination of phase response and amplitude response shapes.

You mentioned that you are using the subwoofers in a stereo configuration, and that they are each placed flush with their partner main speaker (I like that configuration). As a result, the time delay difference to the listening position between the output from a subwoofer and its partner main speaker would be negligible in terms of a fraction of a wavelength. A simple wiring diagram illustrating the connections that you are presently using would be informative, so if you can prepare one that would be helpful.

 

[A Surfer]

@witwald, I will certainly do my best and get you some pictures this weekend if that is ok. My stereo is setup where I live on the weekend as for work I live in another city during the workweek. You are correct, I am using a NAD M3 from the preout to the subs. At one point I was high passing the mains at 40Hz using the built in crossover on the M3, but I could never get a satisfying level of bass in the space so currently the mains are getting the full signal.

Previously I had the same system in a rather small room by way of comparison to where the system is now, and in that small room it had no problem pressurizing the space. It was a 12 x 13 room with 10 foot ceilings and generally sounded pretty darn good in that space. I have since moved the system to a basement in a house that is about 13' wide and 29' feet long with sadly a much lower drop ceiling that is probably 6.5' high so that is less than ideal. Pictures will help so I will certainly provide them. Because there is an open staircase at the back of the room I assume the larger space is even harder to pressurize.

I will provide a drawing in the interim a little later this week, and thank you very, very much for offering my the benefit of your experience. It is greatly appreciated. Cheers.

 

[witwald]

@A Surfer A value of 40 Hz for a crossover between subwoofer and main speakers seems a bit low. Although it can work, of course, you wouldn't get a lot of benefit from reduced cone excursion in the main speakers. 60 Hz would probably be the lowest I would go, especially since your subwoofers have excellent low-frequency extension.

I have taken a look at the owner's manuals for the NAD M3 and the SB-2000 Pro, and I have a few questions.

You mentioned that you are using the NAD M3 from its PRE OUT to the subwoofers. Can I assume that the subwoofers are connected to PRE OUT 1, which is full range (unfiltered), as per the M3's owner's manual, and PRE OUT 2 is connected to the M3's MAIN IN using RCA patch cables?

The SB-2000 Pro's Owner's Manual mentions that the Low Pass Filter control allows you to control the upper frequency cut-off to blend with main speakers. It also allows you to adjust the slope, which is the rate at which the low pass filter rolls off. Can you tell me what slope you have selected? I note that it allows settings of 6dB, 12dB, 18dB, or 24dB/octave for slope. Depending on which one you choose, this will also affect the appropriate choice of phase/polarity, and will have a big effect on the blending between subwoofer and main speaker that can be attained.

What high-pass filter setting have you chosen on the M3 to be applied to PRE OUT 2? I assume that it would be 80 Hz in order to have a reasonable chance of being complementary to whatever low pass filter setting has been chosen in the SB-2000 Pro.

Since you mentioned a 79 Hz frequency, I assume that it is the setting that you've used on the Low Pass Filter on the SB-2000 Pro.

A simulation using 2nd-order low-pass and high-pass filters produces the summed response as shown below, with the subwoofer phase inverted (180 degrees of phase shift).

Here is a simulation with the 0 degrees of phase shift on the subwoofer. Note the large dip between 60 Hz and 200 Hz, due to phase cancellation between the two outputs. I can now see why you preferred the 180 degrees phase shift setting.

A possible solution to get better blending is to choose a 4th-order Linkwitz-Riley low-pass filter slope on the SB-2000 Pro, set to 60 Hz, with the phase set to 0 degrees. Keep in mind that having different filter cut-off frequencies for the high-pass and low-pass sections is not a problem. That's because we are trying to get a complementary acoustic response through the crossover region, and these filters can attain that goal quite well. The simulation suggests the following results might be achievable:

 

[A Surfer]

@witwald That is fantastic and extremely helpful. I must admit that I have been simply mired in my lack of understanding. I will most certainly try these suggestions this weekend when I have access to my system. So if I understand you correctly I should set the mains to be passed at 80Hz and with the subs pass at 60Hz at 0 degrees. I am not entirely sure what slope I should use though. I have been using a 24dB slope up to now. Is that still appropriate? Again, many thanks for taking the time to work with me. I also very much appreciate the graphs as I am a visual learner. Cheers.

 

[witwald]

It's good that you've been using the 24 dB/octave slope on the subwoofer's low-pass filter. It reduces the region of interaction. Go ahead and try the mains high passed at 80 Hz and the subs at 60 Hz with phase shift set to 0 degrees. Hopefully the results will be reasonable to you, and maybe even better than when using your previous settings that you had decided on.

You may have noticed that the filtered response curves of the subwoofer and the main speaker are not quite quite symmetrical either side of the actual –6 dB crossover frequency, which it turns out is at about 62 Hz, and corresponds to the 84 dB level on the graph. One of the reasons that the crossover frequency is lower than the "80 Hz" setting on your M3 is that the high-pass filter used there is only –3 dB at 80 Hz, from which I've guessed that it's a Butterworth filter type. When that 2nd-order high-pass filter interacts with the nominally 4th-order Butterworth-shaped bass response of the main speakers, the acoustic response shape of the main speakers through the crossover region is similar to a Linkwitz-Riley design, which complements the 4th-order Linkwitz-Riley response used by the subwoofer.

The little undulations in the summed response are a result of a slight mismatch in phase response and amplitude response between the low-pass and the high-pass outputs, and there's really nothing that could be done about those without resorting to some local equalization. Those undulations are very small in magnitude, and well within the range of variation that's inherent in these types of theoretical approximations.

Running such simulations and producing and seeing these types of plots does allow one to better understand the cause and effect relationships a little better. It can illustrate how it's possible to achieve reasonably flat summed responses with many combinations of filter slopes and cut-off frequencies. Aiming for a 4th-order Linkwitz-Riley system does tend to reduce the options, and achieves good results. Hence its popularity in pro audio circles, as well as home AV receivers with bass management capabilities.

 

[A Surfer]

@witwald Great and useful explanation. I have always been quite interested in such things but sadly having returned late in life to university and then onto a new profession I have never found myself with much time. So I am even more grateful that you have been so willing to share the knowledge that you have gained from what I can only guess is professional experience. Many thanks again and I will be sure to let you know this Friday evening how the configuration works out.

May I ask another favour? If possible, is there a way with a visual depiction you could illustrate how the different slope settings interact to influence the region of interaction? No pressure though, you have already been very generous with your time. Cheers.

 

[witwald]

@A Surfer I've gone and done some simulations where I've varied the filter order on the output of the subwoofer. The low-pass cut-off frequency has been varied to achieve a relatively smooth blend, as best I could without spending lots of iterations trying to optimise it. This also necessitated some minor adjustments to the output level on the subwoofer from one design to the next. Here are the results, where you can see that filter orders of 3 and 4 can achieve very good results.

1st-order Butterworth low-pass filter, cut-off frequency 30 Hz, positive polarity

2nd-order Butterworth low-pass filter, cut-off frequency 45 Hz, positive polarity

3rd-order Butterworth low-pass filter, cut-off frequency 48 Hz, positive polarity

4th-order Linkwitz-Riley low-pass filter, cut-off frequency 56 Hz, positive polarity

 

[A Surfer]

@witwald Please forgive my ignorance, in terms of interpreting the above outputs:

1) What does the reference angle imply and what ideally does one wish to see?
2) Where the sub and mains cross over is it best to see a smooth reference angle without dips?

When I look at the outputs you provided the 4th order at 56Hz, positive polarity looks the smoothest if I am using the reference angle to guide that evaluation. The graphs are labelled positive polarity. Does that mean the phase shift on the subs, or something altogether different? Finally, I cannot change the type of filter being used so are they a function of the phase chosen on the sub? Clearly my ignorance is showing (at least I'll admit it!).

 

[witwald]

In the frequency response plots that I've provided, the black line labelled as "Reference angle" is simply referring to the fact that we are looking at a simulation that is "on axis" for the chosen "design". It also represents the total summed response of all the acoustic sources in the model, and in our case there are two of them that are included in the simulation.

You've hit the proverbial nail on the head: where the subwoofer and the main speakers cross over is where we want to see a smooth and relatively dip-free response as obtained for the reference angle curve.

With this particular simulation, which involves approximating the bass response of the SB-2000 Pro, the response that we are looking to achieve would look something like the following. This is –3 dB at 19 Hz, with a roll-off that between 18 dB/octave to 24 dB/octave, as this would match the quasi-anechoic data provided by SVS. Keep in mind my model is only approximate, but it does represent the response of the SB-2000 Pro, as provided by SVS, to a reasonable degree without spending a lot of time tweaking it.

 

[witwald]

@witwald
When I look at the outputs you provided the 4th order at 56Hz, positive polarity looks the smoothest if I am using the reference angle to guide that evaluation.

The summed response, represented by the "Reference angle" curve, is the one that we would like to be as smooth as possible. Ideally, the curve would be perfectly flat, indicating a perfect summation of the low-pass and high-pass filtered acoustic outputs from the subwoofers and the main speakers.

The graphs are labelled positive polarity. Does that mean the phase shift on the subs, or something altogether different?

On the SB-2000 Pro, there is an option in the mobile app for setting Positive (+) or Negative (–) polarity on the subwoofer. I just wanted to make it clear which settings were used in each simulation, as there might have been one that produced a better response if Negative subwoofer polarity was set. There wasn't though.

Also note that there is no phase adjustment in any of my models, which is equivalent to Phase setting on the subwoofer of 0 degrees. While on the topic of phase, I'm not sure how SVS have implemented that adjustment. It could be just a simple time delay, with the phase in degrees referenced back to a time delay calculated on the basis of the chosen cut-off frequency of the low-pass filter. That would be easy enough to implement in the DSP code.

Finally, I cannot change the type of filter being used so are they a function of the phase chosen on the sub?

The SB-2000 Pro allows you to adjust the slope of the low-pass filter that is being used, as well as its cut-off frequency. You should be able to change the slope from the mobile app. The options are: 6 dB, 12 dB, 18 dB and 24 dB. The number refers to the slope in dB/octave. It is common for odd-order filters (6 dB and 18 dB in our case) to be based on a Butterworth filter shape. The 12 dB/octave filter is likely to be a 2nd-order Butterworth filter shape too, but it could actually be a 2nd-order Linkwitz-Riley shape. Unfortunately the documentation doesn't specify these details. The 24 dB filter has been assumed by me to be a 4th-order Linkwitz-Riley shape, as this is often mentioned in things like the THX home theatre standard. It also happens to provide excellent blending even when the acoustic output produced by the main speakers and their high-pass filtering is not entirely complementary to the Linkwitz-Riley shape of the low-passed acoustic output from the subwoofer. As you can see, the entire process is beset by many approximations and assumptions.

However, it is important to note that your SB-2000 Pro provides a 1-hertz frequency steps for setting the low-pass filter cut-off frequency, unlike many (most?) AVRs. Hence you can probably achieve a very good blending of the subwoofer with the main speakers as you have a lot of very fine control over the process. That's even before possibly applying some parametric equalisation (PEQ) that the subwoofer is capable of doing.

 

[witwald]

@A Surfer I've had to do another simulation for the sake of completeness. This one uses a 4th-order Butterworth filter instead of the 4th-order Linkwitz-Riley filter topology (shape). I've adjusted the output from the subwoofer to get as smooth a blend as possible. As you can see, we are left with a ±1.0dB undulation through the crossover region, which is still quite good, but not as good as the ±0.5dB obtained earlier when the 4th-order Linkwitz-Riley topology was used on the low-pass filter.

After I lodged a question with SVS tech support, they informed me that all slopes of the low-pass filter in the SB-2000 Pro use a Butterworth topology. Hence my simulation of that choice, with the results as shown below.

You may not be aware that the THX specification calls for a 4th-order Linkwitz-Riley low-pass filter on the subwoofer, and a 2nd-order Butterworth high-pass filter on the main speakers. THX also has some other requirements, but their choice of system filtering generally works well even for non-THX compliant systems. The 4th-order (24dB/octave roll-off) Linkwitz/Riley alignment has both a steep slope and a rapid transition to that slope, which enhances the benefits of that slope near the crossover point. As you no doubt know, the high-pass side helps to reduce excursion and power requirements on the main speakers, while the low-pass minimises the more localisable content at higher frequencies when the crossover frequency is in the range from 60hZ to 80Hz. The benefit of the THX audio engineering choices has been supported by the results of my simulations.

4th-order Butterworth low-pass filter, cut-off frequency 52 Hz, positive polarity

 

[A Surfer]

@witwald Wow, thank you so much for going beyond your already exceptional effort and confirming things with SVS. It also makes me feel good that for somebody on a budget it does seem that the SB2000 Pro was a good call. I had to liquidate what was left of my beloved headphone collection to fund the dual subs, but it was worth it. Previously I had two REL T-Zero subs, which I found to be very musical and capable in small spaces, but clearly for a larger room they were not going to be able to pressurize the space.

So returning to things at hand, I will be setting the polarity to positive (I have left it at default so whatever that is) and I will set the crossover on the subs at 52Hz and on the M3 which from all I can find out, the preamp uses 2nd order analogue filters, I would be setting the high pass at 60Hz. Does this sound like I am implementing your suggestions correctly?

Again, many, many thanks for this help. I imagine others will read through this thread and benefit from your generosity. Not that it matters a great deal perhaps, but these are the specifications that NAD publishes for the M3:

PRE-AMP SECTION
Line level inputs Input impedance (R+C) 150kΩ/500pF
Balanced Input impedance (R+C) 120kΩ/100pF
Input sensitivity, rated power 446mV Frequency response (5Hz - 70kHz)1 <+/-0.3dB

POWER AMP SECTION
Continuous output power into 4/8Ω2 180W (23dBW)
Rated Distortion (THD 20Hz - 20kHz) 0.004%
Clipping power 220W (23.4dbW)
IHF dynamic headroom at 4Ω +4.2dB
IHF dynamic power at 8Ω >280W (24.5dBW)
IHF dynamic power at 4Ω >480W (26.8dBW)
IHF dynamic power at 2Ω >785W (29.0dBW)
Damping factor (ref. 8Ω, 50Hz) >150
Input impedance 20kΩ / 680pF
Input Sensitivity (for rated power into 8Ω) 1.38V
Voltage gain 29dB
Frequency response; 20Hz-20kHz +/-0.03dB
Frequency response; at > 80kHz -3dB
Signal/noise ratio; ref 1W >107dB (AWTD)
Signal/noise ratio; ref rated power >130dB

Cheers.

 

[witwald]

It also makes me feel good that for somebody on a budget it does seem that the SB2000 Pro was a good call. ... Previously I had two REL T-Zero subs, which I found to be very musical and capable in small spaces, but clearly for a larger room they were not going to be able to pressurize the space.

I needed to verify the full details of the low-pass filtering used on the SB-2000 Pro, as it has to be known in order to have any chance of the simulations being meaningful. Unfortunately the SVS documentation didn't have the all the details that I needed. However, SVS were very quick to respond to my question when I asked, and I was grateful for their excellent product support.

The SB-2000 Pro does seem like an excellent performing subwoofer. It blends excellent bass extension with a relatively small size, and when used as a stereo pair should go well in terms of maximum output. The slower roll-off than a vented subwoofer also tends to give you a little bit more bass output than the PB-2000 Pro by the time you get to 10Hz. In comparison, the 6.5" driver in the REL T-Zero is way behind in the size stakes compared to the 12" driver in the SB-2000 Pro. At 20Hz or thereabouts, the REL T-Zero just wouldn't have much output, as it is claimed to go down to 37Hz or so.

So returning to things at hand, I will be setting the polarity to positive (I have left it at default so whatever that is) and I will set the crossover on the subs at 52Hz and on the M3 which from all I can find out, the preamp uses 2nd order analogue filters, I would be setting the high pass at 60Hz. Does this sound like I am implementing your suggestions correctly?

Almost, but not quite. You will need to set the crossover on the M3 to 80Hz, not 60Hz. It might look like that on the graph, but I did the modelling using a 2nd-order Butterworth filter with its –3dB cut-off frequency set to 80Hz, and with a nominal low-frequency response included for your main speakers. This seemed to best approximate the filter in the M3 at the 80Hz setting. It then has the benefit that the high-pass filtered acoustic response of the main speakers then comes out to –6dB at around 65Hz. This then matches quite well with the low-pass filtered response of the subwoofer when a 4th-order Butterworth filter is used with its –3dB point at 52Hz. This results in a –6dB subwoofer response at about 62Hz., which permits the blending to occur as illustrated in the previous graph.

Note that, to some extent, you should be able to set the subwoofer level by ear to suit your needs. Otherwise, you will need to use some kind of measurement procedure to set the subwoofer at a suitable level to get a good blending.

...these are the specifications that NAD publishes for the M3:

I took a look at the specifications, and they seem quite good. Plenty of power available into both 4 ohm and 8 ohm loads, which is good. I liked the fact that the M3 has a damping factor (ref. 8Ω, 50Hz) >150, which will ensure that your main speakers see a relatively flat frequency response when connected to the frequency varying impedance of your main speakers.

Good luck with your trials on the weekend. Just make sure that you make a note of all the settings that you have previously found to work for you. Just in case my assumptions make my simulations less accurate than I hope they are. I'll be interested to hear your comments about the results in due course.

 

[witwald]

@A Surfer I've thought of a viable way to remove the small undulations in the configuration that I simulated earlier. It involves raising the cut-off frequency of the 4th-order Butterworth low-pass filter to 62Hz. This introduces a small bump into the low-frequency response, but has the effect of removing the small dip that was present in the earlier design.

As the bump is quite broad and relatively symmetrical, it can then be readily equalised using one of the PEQ modules that are available on the SB-2000 Pro. Below is the finished result when I add in the PEQ (Parametric EQ) at a centre frequency of 58 Hz, a Q factor of 2 and a boost of –1.8dB (which is of course a cut).

4th-order Butterworth low-pass filter, cut-off frequency 62 Hz, +ve polarity, PEQ F=58 Hz, Q=2, Boost=–1.8 dB

If we turn off the PEQ, then VituixCAD shows the following result, with a bump of around 1.2dB at 58Hz.

4th-order Butterworth low-pass filter, cut-off frequency 62 Hz, +ve polarity, PEQ off

 

[A Surfer]

@witwald That is really interesting and I can see the merit in that. Tomorrow night can't arrive soon enough! Let the trials begin. I will definitely let you know the results, and many thanks for taking so much interest in my education! I still have a few conceptual connections to make through application, but I most certainly have learned and I have no doubt others will also benefit. And good advice about the settings I like being written down. Cheers.

 

[A Surfer]

@witwald So many findings to report and I am hoping to present them in a decent order and with proper, but readable detail. Or so I hope. So to situate you about where my system came from in terms of configuration, but your assumptions built into the modelling were excellent I believe.

My M3 was not attenuating the signal to the mains at all, so full range on the biamp setting there. Now as per your modelling I moved the biamp high pass to 80Hz.

The subs were already configured to positive polarity and 24dB slope. The previous crossover point was actually 80Hz and the volume was (and remains -14dB).

I configured each sub as per your model so crossover now moved to 58Hz and used PEQ settings of 62dB, Q=2 and Gain = -1.8dB.

So what were the results? Very, very, engaging and extremely clean. The amplitude of the overall low frequency energy dropped by probably -2dB; however, the integration became incredibly tight, just extremely detailed tracking, probably what subjectively people often refer to as "fast". Keep in mind that I have been listening to audio systems since the 1980s and I have also played the drums since the 1980s. I believe I have a pretty well honed hearing brain.

I have the last decade grown accustomed to a warmer sound where likely time alignment is out, but it provides a warmer, albeit very mildly slurred sound in terms of detail and low frequency integration. A warm, but not truly destructive bloom in the low-mid bass.

The sound has changed on literally every domain of interaction, or so I think. The midrange and high frequencies were always present, but the clarity and definition is now, I feel, significantly increased so much so that in some ways the speakers have become liberated in these regions and as you can imagine with such effects, the perceived sound is substantially different. I think it is clearly far more neutral and accurate than my previous settings.

Negative Girl is one of my favourite test tracks and comes from the Steely Dan masterpiece Two Against Nature. This album is where I think some of the finest recording adjustments between the highly dynamic recordings of the 70s meets an artful increase in amplitude and frequency depth through talented mastering and engineering with compression. As I am sure you know, the two principles in Steely Dan were absolute sticklers' for recording fidelity.

Now the tracking of the drivers is uncanny and the separation and imaging is just unbelievable. The "fatness" of the bass is of course reduced, but that just means new domains of integration become apparent. Very interesting. I have become very accustomed to my almost Harman like preference for warmth so before I can say which presentation I truly prefer, I need to spend time with these changes. Thank you so much, what a fantastic experiment this will be. I may try crossing the M3 at 60Hz to see if it splits the difference.

 

[witwald]

@A Surfer I was pleased to read your report on the set up and subsequent listening experience. It can be difficult to do simulations using a good enough set of assumptions/approximations, as these can easily affect the quality of the computed results. I'm glad that you're finding the integration between subwoofer and mains to "tight".

In regard to one of your chosen test tracks, "Negative Girl" by Steely Dan, I've had a listen and I think it offers a good test of the complete system. There are a lot of distinctive elements to the sonic features that have been placed into the recording. The whole album is pretty good, and I'm enjoying revisiting it now as I type.

I've analysed the energy content of "Negative Girl" and obtained the following power spectrum. I can see that there is a very strong bass peak at 32Hz, and another at about 43Hz, with significant energy down to 25Hz or so. I was a little surprised to see the plateau of low-frequency energy between 9Hz and 20Hz, which is only 15dB below the level of the peak at 32Hz.