Subwoofer phase adjustment

[slartibartfast]

My sub has a 0-180 phase adjustment. Now this might be a daft question but does this have any effect when connecting via the sub out of the WiiM Amp. In this case bass management is handled by the amp and the low pass filter on the sub is bypassed. I suspect the phase adjustment is also bypassed as I read that the phase adjustment is only effective at frequencies close to the LPF setting.

 

[harkpabst]

The phase adjustment 0⁰ or 180⁰ should breather be called "polarity", but unfortunately the term "phase" is all over the place. It's the newly added delay setting that allows for continuous phase adjustment.

Generally the polarity switch always affects the entire output of the subwoofer. This is true formthe sub as well as for the WiiM Home app. However, if you are using WiiM's bass management then the mains are high pass filtered, so for frequencies noticeably lower than the crossover frequency there is no meaningful output from the mains anymore. Nothing that could interfer with the sub's output.

The sub knows nothing about big your amp/pre-amp/receiver provides bass management or not. Consequently, the function of the 180⁰ phase or polarity switch cannot change based on that.

If your sub has a dedicated LFE input or a dedicated LFE configuration setting, the with this input being used or this config being enabled the sub might not be affected by the switch ... or it just might.

You really have to look it up in your sub's manual.

 

[rccarguy]

"However, if you are using WiiM's bass management then the mains are high pass filtered, so for frequencies noticeably lower than the crossover frequency there is no meaningful output from the mains anymore. Nothing that could interfer with the sub's outpu"

Not quite. The crossover isn't a brick wall, either 12 or 24db octave slope for sub and/or speakers (if you have that option) there will be interactions between sub and speakers, cancellation, boosting at various frequencies etc. All depending on speaker and sub location, the delay in your subwoofer, crossover slopes, phase crossover setting, gain etc.

 

[harkpabst]

"However, if you are using WiiM's bass management then the mains are high pass filtered, so for frequencies noticeably lower than the crossover frequency there is no meaningful output from the mains anymore. Nothing that could interfer with the sub's outpu"

Not quite. The crossover isn't a brick wall, either 12 or 24db octave slope for sub and/or speakers (if you have that option) there will be interactions between sub and speakers, cancellation, boosting at various frequencies etc. All depending on speaker and sub location, the delay in your subwoofer, crossover slopes, phase crossover setting, gain etc.

"... for frequencies noticeably lower than the crossover frequency" this doesn't matter. 4th order LR is pretty steep.

I didn't mean to write (yet another) all-encompassing compendium on subwoofers. The point I made (maybe not clearly enough) is that the polarity switch's effect is not limited to the crossover region.

Is that OK with you?

 

[slartibartfast]

The phase adjustment 0⁰ or 180⁰ should breather be called "polarity", but unfortunately the term "phase" is all over the place. It's the newly added delay setting that allows for continuous phase adjustment.

Generally the polarity switch always affects the entire output of the subwoofer. This is true formthe sub as well as for the WiiM Home app. However, if you are using WiiM's bass management then the mains are high pass filtered, so for frequencies noticeably lower than the crossover frequency there is no meaningful output from the mains anymore. Nothing that could interfer with the sub's output.

The sub knows nothing about big your amp/pre-amp/receiver provides bass management or not. Consequently, the function of the 180⁰ phase or polarity switch cannot change based on that.

If your sub has a dedicated LFE input or a dedicated LFE configuration setting, the with this input being used or this config being enabled the sub might not be affected by the switch ... or it just might.

You really have to look it up in your sub's manual.

I am pretty sure now that the continuously variable phase setting between 0 and 180 on the sub is only useful when the sub crossover frequency is determined by the sub itself. When you use bass management you need to use the delay setting as you said.

 

[harkpabst]

... continuously variable phase setting between 0 and 180 on the sub ...

Sorry, I got that wrong. A continuous phase adjustment does deserve the name, but is best left alone when the amp provides its own bass management, indeed.

Since it is not needed, stay on the safe side, leaving it at 0⁰ and adjust everything from within the WiiM Home app.

 

[rccarguy]

Just because you have bass management doesn't mean you should set phase to 0.

AVR's do have distance adjust on the sub woofer, but stereo amps don't. Bass management isn't the same as phase/distance alignment.

If the wiim subwoofer out isn't delayable, or if during the calibration process it pings speakers and sub and then calculates ideal distance for each, then you would need to adjust the subs phase response, ideally with variable phase rather than 0/180 direct switch.

Need to plot responses to see how the three speakers interact, at what frequencies.


I did this on my hifi, a combined full range sweep, with both speakers and sub on, then adjust the subs phase every 15 degrees. You can see the interations.

 

[harkpabst]

Just because you have bass management doesn't mean you should set phase to 0.

AVR's do have distance adjust on the sub woofer, but stereo amps don't. Bass management isn't the same as phase/distance alignment.

If the wiim subwoofer out isn't delayable, or if during the calibration process it pings speakers and sub and then calculates ideal distance for each, then you would need to adjust the subs phase response, ideally with variable phase rather than 0/180 direct switch.

Need to plot responses to see how the three speakers interact, at what frequencies.


I did this on my hifi, a combined full range sweep, with both speakers and sub on, then adjust the subs phase every 15 degrees. You can see the interations.

Sorry, but why are you're not listening?

Nobody said you should not or do not need to compensate for delay.

In WiiM Home this can be done entering a delay in ms for either the sub or the mains. Much more precise than any old phase dial. So yes, current stereo amps do offer the same functionality as AVRs.

By all means, leave the "phase" switch or the phase dial on your sub at 0⁰ when using the WiiM Amp or the upcoming WiiM Ultra.

 

[John61CT]

So, is it the case that infinitely variable phase adjustment is in all aspects EXACTLY THE SAME THING as setting time delay in ms ?

Is it always going to be the higher frequency signal that requires the delay?

Can anyone point to resources that explain (ELI5) how to use REW to inform this?

Should the automatic process be run first, then manually just tweaked a bit?

Is the process complicated by the fact that the stereo output signal gets further split downstream e.g. between my main bookshelf pair and midbass couplers use for bass reinforcement down to say 60-70Hz?

 

[harkpabst]

So, is it the case that infinitely variable phase adjustment is in all aspects EXACTLY THE SAME THING as setting time delay in ms ?

That's quite a good and important question, and the answer is no. The truth is that variable phase adjustment as usually found on subwoofer plate amps is technically inferior to the time delay setting. The reason has nothing to do with "infinitely variable" vs "1 ms steps".

Subwoofers with a variable phase control knob implement an analogue all-pass filter. It leaves the magnitude of the signal unchanged and just modifies the phase between 0⁰ and 180⁰. However, there is only one fixed corner frequency, defined as the frequency where the phase angle is 90⁰. The most important consequence is that you can set the 100% correct time shift (equalling the required delay) for one frequency, only (usually 80 Hz). Everything far enough above or below the corner frequency is not time-aligned with the other source, e.g. the main speakers.

This is just an approximation to the solution of the problem we want to solve, different time of arrival of sound waves due to either different distances to our ears or due to delay caused by processing (or both). Setting a latency in ms steps does exactly this, it compensates for a delay for all frequencies. WiiM's chosen resolution of 1 ms is a tad rough, as each ms is equivalent to a distance of ~34 cm or ~12". It's still sufficiently precise compared to the wavelength at typical crossover frequencies (e.g. 4.3 m at 80 Hz). Personally, I'd be more happy with 0.5 ms steps, but I know that going all the way to 0.1 ms steps was probably necessary to silence all the critics.

The all-pass filter still works ok-ish (but not as well as the latency setting) in practice. The reason is the same as why 1 ms steps are usually fine enough: In the low bass and within a room we don't have to care about precision in the centimeter range.

Is it always going to be the higher frequency signal that requires the delay?

No, not always, but most of the time. In most domestic setups the subwoofer sits further from the listening position than the main speakers (in particular with corner placement, which makes a lot of sense if room correction is available). Also, modern subwoofers often use digital signal processing, adding further delay. Since there's no way to somehow speed up the signal from the sub, the mains must be delayed.

Note how this is not directly related to higher vs lower frequencies. You could end up with a setup where a purely analogue active subwoofer has no inherent delay and the sub actually sits closer to the listening position than the speakers (even though the acoustical center of the sub is almost never the center of the cone). In such a scenario it's possible that the subwoofer's signal needs to be delayed, but it's less common.

Can anyone point to resources that explain (ELI5) how to use REW to inform this?

Not from the top of my head, not sure if @dominikz has already pointed to such a resource. Measuring the difference in timing directly is not that easy.

Personally I've always just followed the layman's approach of picking the setting that results in the highest SPL right at the crossover frequency.

Should the automatic process be run first, then manually just tweaked a bit?

Ideally, no manual tweaking should be necessary after using the automatic process, now that the correct microphone is used for the measurement. If the summation of mains and sub doesn't look smooth (e.g. when running RoomFit) it's certainly a valid approach to change the automatically obtained delay setting by +/- one or two milliseconds.

It's probably an even better idea to try out both “phase“ settings (0⁰ a d 180⁰) when measuring the delay. I'd go with whatever setting results in the smaller delay value.

The "phase" switch is actually a misnomer. It switches the polarity of the input signal, no all-pass filter in this case.

Is the process complicated by the fact that the stereo output signal gets further split downstream e.g. between my main bookshelf pair and midbass couplers use for bass reinforcement down to say 60-70Hz?

What are midbass couplers? Sorry, looks like I didn't follow the changes to your setup on ASR as closely as I could have.

 

[John61CT]

Thanks, very helpful.

MBM is another term, boxen designed to reinforce the lower end of the bookshelf / satellite main L/R pair.

With LS50 I've been told the highpass should be 170Hz or even higher.

I plan to go deepest SPL as possible with a trueSub but have limited space per box <1.5cuft

Therefore limited frequency bandwidth.

So far thinking to try getting the MBM down to 60-70Hz.

It may take two "trueSub levels" to get good SPL from 20Hz to there, we'll see via testing, a little bass boost may help, maybe from Wiim Ultra

Thinking bandpass enclosures with PRs a la Wolfs' "Overdrive 10" project aka Kilauea

 

[John61CT]

>> Is it always going to be the higher frequency signal that requires the delay?
...
No, not always, but most of the time. In most domestic setups the subwoofer sits further from the listening position than the main speakers (

In my case I have complete freedom of placement within an 18x24' space, but shared with storage shelving, currently rolling baker's racks, to be replaced over time with IKEA Kallax made to appear "built in" along the wall(s).

> modern subwoofers often use digital signal processing, adding further delay

Mine have no inherent electronics at all, power amps and active crossovers at line level will be external and using DSP sparingly for bass management.

My plan is to try to use placement-only to align arrival times AMAP before even starting to tune the xover frequencies. I plan for Wiim Ultra DSP to govern the "first split" between the deepest trueSub - maybe as high as 60Hz but likely lower. Also a rumble HP filter maybe 10-15Hz

and "the rest" - basically full-range stereo output.

For the remaining split(s) I will first try to use analog crossovers.

The LS50s to get HPF only, will experiment with passive, likely 12dB/octave.

The MBM pair is bandpassed, the LPF carefully blended with above - do you think steeper 24dB/octave Linkwitz-Riley is best there?

(Didja notice that that "above" there carries three meanings simultaneously? the LF boxen act as "basstands")

...

Remember the lowest crossover (and rumble HPF) is already handled by the Wiim Ultra DSP, so that deepest sub must be mono.

if a second in-between pair of trueSubs is required to get good SPL from the deepest mono one(s),

I will probably test my ICBM-1 crossover for the boundary between the top trueSubs and the MBM, can choose 12dB/octave or 36dB slope.

I will also test whether stereo co-located is better than mono freely placed.

...

So my hope is, after iterative measurement / tuning cycles of the FR magnitude side, and testing different slopes

further tweaking of MLP and speaker placement might get the time alignment aspect of phase tuning "good enough" without adding more DSP to the mix in my bass management.

However, I do have ONE miniDSP 2x4 HD I picked up cheap to use for comparison, or "production use" on one "pair of pairs" to further clean up phase / timing if needed, in which case might as well use it for the FR magnitude side.

None of this has anything to do with DRC, I really want to keep that separate, since (parts of) my system will be portable for use in different spaces and also outdoors.

If the miniDSP is not required to help with bass management, it might be helpful with DRC operating on the upstream full-range signal.

I also like OCA's Gsonic approach in that regard, plan to compare it with Roomfit and other tools once I get more powerful convolvers sorted.

But I would REALLY like to avoid my audio systems requiring having a PC always on.

 

[dominikz]

My plan is to try to use placement-only to align arrival times AMAP before even starting to tune the xover frequencies.

Just a note that placement should ideally be chosen such that SBIR nulls are avoided (a bit more on that in this post), since nulls are not adviseable to try and fix with DSP.
Unfortunately, positions where SBIR nulls can be avoided in speakers' and the sub's passbands often won't have matching distances to the listening position.
Luckily, time-of-arrival differences between speakers/subs are easy to fix with a bit of delay in DSP.

 

[John61CT]

Thanks, ever more to study, I take it "room modes" and this SBIR are separate issues?

All boxen in the front channels will be co-located as closely as possible

Mono trueSub(s) freely placed, will try to get everything as well blended as feasible before starting with DSP, at least for a single MLP

 

[dominikz]

Thanks, ever more to study, I take it "room modes" and this SBIR are separate issues?

Indeed, they are completely different acoustical phenomena. Have a look at this post for a bit more information.