Bass null with two speakers but not on their own?

[C0mbat]

Hi all,

I've been using REW and the WiiM peq slots to do some room correction and when I measure the speakers individually (post eq) they measure pretty well. However, when I measure them together it shows a big bass null between 102 and 115hz. This null somewhat existed pre-eq with the right speaker, when measured individually, but post-eq the right speaker measures well. When I move the mic to other positions (within 3-4 feet) the null moves to different frequencies or in some instances disappears. I what's the science behind this null only existing when both speakers are measured together. See screenshot.

 

[ppataki]

This is 'normal' - what's happening is that the phase interaction between the drivers cause cancellation that you see in the graph
One of my friends had the same but very severly (like a 15dB dip between 40 and 100Hz and only when both the front speakers were playing)
We solved that by adding two subs to his system, see the project here
I had a similar issue in my living room and also fixed it with one then two subs
Maybe others here can recommend other solutions too but from my perspective, adding one or more subs is the best way to solve these kinds of issues

 

[C0mbat]

This is 'normal' - what's happening is that the phase interaction between the drivers cause cancellation that you see in the graph
One of my friends had the same but very severly (like a 15dB dip between 40 and 100Hz and only when both the front speakers were playing)
We solved that by adding two subs to his system, see the project here
I had a similar issue in my living room and also fixed it with one then two subs
Maybe others here can recommend other solutions too but from my perspective, adding one or more subs is the best way to solve these kinds of issues

Thanks for this explanation. I actually have a Kef KC62 but was trying to see if I could do without it. I just got a pair of Robson Acoustics Burlington MKll and they have Purifi woofers that extend pretty low. I was using the KEF sub with a crossover at 85hz and was still getting the same dip with that (see screenshot). Do I need to cross over much higher at around 120hz to get rid of it? If so then aren't I wasting a key value proposition of the purifi woofers by using the sub up to 120hz?

FYI the measurements below with the sub are both pre and post eq.

 

[ppataki]

Thanks for this explanation. I actually have a Kef KC62 but was trying to see if I could do without it. I just got a pair of Robson Acoustics Burlington MKll and they have Purifi woofers that extend pretty low. I was using the KEF sub with a crossover at 85hz and was still getting the same dip with that (see screenshot). Do I need to cross over much higher at around 120hz to get rid of it? If so then aren't I wasting a key value proposition of the purifi woofers by using the sub up to 120hz?

FYI the measurements below with the sub are both pre and post eq.

Crossing the sub higher has the risk of the sub becoming more localizable in the room
If you have one sub, try placing it in-between your front speakers to minimize that effect

Measure the sub only in different locations in your room - that will tell you where you shall ultimately place your sub (and then integrate it with your mains to see if you have localization issues or not at a higher crossover point)

aren't I wasting a key value proposition of the purifi woofers by using the sub up to 120hz?

Taking the bass burden off of your mains is always a good idea, even if you have Purifi drivers - doing so will give you more headroom and potentially less distortion in the bass range

If you cannot fix the dip(s) with changing the crossover frequency and playing around with different subwoofer positioning, then you can ultimately try two subs in strategic locations.

 

[C0mbat]

Crossing the sub higher has the risk of the sub becoming more localizable in the room
If you have one sub, try placing it in-between your front speakers to minimize that effect

Measure the sub only in different locations in your room - that will tell you where you shall ultimately place your sub (and then integrate it with your mains to see if you have localization issues or not at a higher crossover point)



Taking the bass burden off of your mains is always a good idea, even if you have Purifi drivers - doing so will give you more headroom and potentially less distortion in the bass range

If you cannot fix the dip(s) with changing the crossover frequency and playing around with different subwoofer positioning, then you can ultimately try two subs in strategic locations.

Thanks so much for your replies. I'm limited to where I can place my sub. I have tried moving it around before just to try and luckily the place where it is came out near the top.

I will try crossing the sub at 120hz and see. However, are there any alternative things I could do to either reduce that dip at 110hz or even move it further down the frequency range so that I could deal with it with the sub at a lower frequency? I'm also limited in being able to change my seating position and speaker position. I can adjust my seating slightly forwards of backwards.

Things like treatment an alternative approach to eq correction?

 

[ppataki]

I'm also limited in being able to change my seating position and speaker position. I can adjust my seating slightly forwards of backwards.

Yes, changing front speaker positions and/or changing listening position could help too - just measure the different options and hopefully one will be better than the other
Even 5cm can make the difference that you need

Physical treatment can be tricky in the bass range - to be honest I am not that knowledgeable in that area, I will leave that for other folks here to comment

 

[DonH56]

As already stated, two speakers act as two sound sources that can combine constructively (add) or destructively (subtract). That is also one reason for adding a sub (or several subs), since the interactions among speakers and room tend to be more noticeable at low frequencies. With a sub you can place the main speakers for the best image and so forth, then place subs for best bass response since very low frequencies in a room are nondirectional.

These may help:

Comb Filter Effects

Comb filter effects are a popular topic of discussion among audiophiles (and many others, of course). However, some audiophiles may not have a good concept of what they look like, why they may (or may not) be bad, and what they can do about them. Or even if they have them and need to do...

www.audiosciencereview.com

Room Modes

Room interactions are often discussed as one of the primary reasons similar (or even dissimilar) systems sound different. If you read the “Reflections and DACs” thread you saw how reflections and standing waves can interact along a transmission line. As it happens, sound waves in a room also...

www.audiosciencereview.com

 

[jeffaegrim]

Hi all,

I've been using REW and the WiiM peq slots to do some room correction and when I measure the speakers individually (post eq) they measure pretty well. However, when I measure them together it shows a big bass null between 102 and 115hz. This null somewhat existed pre-eq with the right speaker, when measured individually, but post-eq the right speaker measures well. When I move the mic to other positions (within 3-4 feet) the null moves to different frequencies or in some instances disappears. I what's the science behind this null only existing when both speakers are measured together. See screenshot.

Same exact thing is happening in my room at 100hz and has been since forever. I can only echo what others have stated already: try moving the speakers around slightly. That’s what I did; the cancellation dip is still there, but appears to be much narrower now. Alternatively, let the sub play a bit higher than the usually recommended 80hz which should help smooth out the dip with the added danger of the sub becoming localizable. Tried that too, works well enough

 

[C0mbat]

As already stated, two speakers act as two sound sources that can combine constructively (add) or destructively (subtract). That is also one reason for adding a sub (or several subs), since the interactions among speakers and room tend to be more noticeable at low frequencies. With a sub you can place the main speakers for the best image and so forth, then place subs for best bass response since very low frequencies in a room are nondirectional.

These may help:

Comb Filter Effects

Comb filter effects are a popular topic of discussion among audiophiles (and many others, of course). However, some audiophiles may not have a good concept of what they look like, why they may (or may not) be bad, and what they can do about them. Or even if they have them and need to do...

www.audiosciencereview.com

Room Modes

Room interactions are often discussed as one of the primary reasons similar (or even dissimilar) systems sound different. If you read the “Reflections and DACs” thread you saw how reflections and standing waves can interact along a transmission line. As it happens, sound waves in a room also...

www.audiosciencereview.com

@DonH56 and @jeffaegrim thanks for your responses. The WiiM doesn't allow you to fine tune the cross over slope but it does allow you to play the low frequencies below the cut off on both the sub and speakers. I wonder if that would be a good way of reducing the likelihood of the sub being obvious in the 80-120hz range? I'll try it out.

 

[DonH56]

@DonH56 and @jeffaegrim thanks for your responses. The WiiM doesn't allow you to fine tweak the cross over slope but it does allow you to play the low frequencies below the cut off on both the sub and speakers. I wonder if that would be a good way of reducing the likelihood of the sub being obvious in the 80-120hz range? I'll try it out.

I am not sure what "play the low frequencies" means; it sounds like running the main speakers full-range? I do not like that idea since it obviates many of the benefits of using subs to offload LF signals from the mains and means more interaction among mains and sub(s).

You could try adjusting the sub's position and/or delay (phase) to see if you can reduce the null.

Crossovers are not "brick walls" instantly reducing the output to 0 on either side, so an 80 Hz crossover may still have significant signal an octave above and below the crossover. Higher-order crossovers roll off faster but have greater phase shift as well, a mixed blessing.

 

[ebslo]

Do you have manual control over the room correction filters on the WiiM? If so, you may be able to fix this with all-pass filters. I do this in my system with a MiniDSP SHD, but I didn't bother with proper mathematical analysis to get the filters right, I just mucked with them until I found something that worked. It was fiddly and time consuming, but it worked. Fortunately, REW allows you to simulate by applying filters to each single-speaker measurement, then use vector-add (or the alignment tool) to see how they combine. Once I had filters that worked in simulation, I ran a final measurement and it matched REW's prediction very well.

 

[Farenheit]

I had a huge null around 60Hz and I fixed it by equalizing both speakers as shown in the attachments.

In short, it is about preventing both woffers from "fighting" where they are out of phase (generally in asymmetrical rooms like mine) by applying deep PEQ and with Q 16 on one speaker and compensating it on the other.

I don't have subwoofers.

 

[C0mbat]

I had a huge null around 60Hz and I fixed it by equalizing both speakers as shown in the attachments.

In short, it is about preventing both woffers from "fighting" where they are out of phase (generally in asymmetrical rooms like mine) by applying deep PEQ and with Q 16 on one speaker and compensating it on the other.

I don't have subwoofers.

Thanks for your response @Farenheit

Can you elaborate a bit more on that please as I'm really interested to try it? You're saying that you reduced the db on one speaker and boosted it on the other at the same frequency point and also did it with a very wide q factor on both? Didn't that result in putting too much strain on one of the speakers by heavily boosting it?

 

[C0mbat]

Do you have manual control over the room correction filters on the WiiM? If so, you may be able to fix this with all-pass filters. I do this in my system with a MiniDSP SHD, but I didn't bother with proper mathematical analysis to get the filters right, I just mucked with them until I found something that worked. It was fiddly and time consuming, but it worked. Fortunately, REW allows you to simulate by applying filters to each single-speaker measurement, then use vector-add (or the alignment tool) to see how they combine. Once I had filters that worked in simulation, I ran a final measurement and it matched REW's prediction very well.

Thanks for this @ebslo

Is there anywhere I can read up on this a bit more. But like an "all filters" for dummies in REW because I'm a bit of a noob lol. I'm not even sure if the WiiM allows me to do it. It has 10 peq slots per channel but not much else.

 

[C0mbat]

I am not sure what "play the low frequencies" means; it sounds like running the main speakers full-range? I do not like that idea since it obviates many of the benefits of using subs to offload LF signals from the mains and means more interaction among mains and sub(s).

You could try adjusting the sub's position and/or delay (phase) to see if you can reduce the null.

Crossovers are not "brick walls" instantly reducing the output to 0 on either side, so an 80 Hz crossover may still have significant signal an octave above and below the crossover. Higher-order crossovers roll off faster but have greater phase shift as well, a mixed blessing.

Yep, understood that it's probably not a great idea to still play the full frequency range on the speakers.

Interesting I already changed the subs phase as an experiment and it significantly reduced another null at about 80hz. It may also help with the 102-115hz null but I'll need to run some measurements at a high cross over to check. Thanks for the food for though.

 

[Farenheit]

Thanks for your response @Farenheit

Can you elaborate a bit more on that please as I'm really interested to try it? You're saying that you reduced the db on one speaker and boosted it on the other at the same frequency point and also did it with a very wide q factor on both? Didn't that result in putting too much strain on one of the speakers by heavily boosting it?

First several considerations:

- My method is self-taught and unorthodox, but it measures me and sounds good.

- My room is asymmetrical and I don't know how it would work in symmetrical rooms.

- Regarding your question "Didn't that result in putting too much strain on one of the speakers by heavily boosting it?: my speakers have 15" woofers and high sensitivity (JBL S3100), but I amplify them with 20w per channel in a 30 m2 room and I have no problems. In fact, I have another YAMAHA P3500 350w professional amplifier and I don't feel the need to use it.

Therefore, I present to you...


"FARENHEIT METHOD FOR DESPERATE NULLS":


The graph with a red line corresponds to the measurement at the L+R listening point without any equalization where the null is observed with a vertex at 62 Hz.

At that frequency I tried applying a PEQ with Q 16 and -10 db on only one of the speakers (the cause could be the left or the right), observing that it largely disappeared, so I applied the inverse equalization on the other speaker and filled the null, as can be seen in the green trace graph.

However, you have to play with different +/- EQs at various points to get a flat response.

Finally, I applied equalization to L+R to flatten the response down to 200 Hz (blue trace graph).

Easy, fast and for the whole family.

 

[C0mbat]

First several considerations:

- My method is self-taught and unorthodox, but it measures me and sounds good.

- My room is asymmetrical and I don't know how it would work in symmetrical rooms.

- Regarding your question "Didn't that result in putting too much strain on one of the speakers by heavily boosting it?: my speakers have 15" woofers and high sensitivity (JBL S3100), but I amplify them with 20w per channel in a 30 m2 room and I have no problems. In fact, I have another YAMAHA P3500 350w professional amplifier and I don't feel the need to use it.

Therefore, I present to you...


"FARENHEIT METHOD FOR DESPERATE NULLS":


The graph with a red line corresponds to the measurement at the L+R listening point without any equalization where the null is observed with a vertex at 62 Hz.

At that frequency I tried applying a PEQ with Q 16 and -10 db on only one of the speakers (the cause could be the left or the right), observing that it largely disappeared, so I applied the inverse equalization on the other speaker and filled the null, as can be seen in the green trace graph.

However, you have to play with different +/- EQs at various points to get a flat response.

Finally, I applied equalization to L+R to flatten the response down to 200 Hz (blue trace graph).

Easy, fast and for the whole family.

Brilliant, thanks @Farenheit. I see that your null is both extremely wide and deep. Mine is shallower and narrower so I wouldn't need to boost as much. I'll have a play over the next week or so and report back.

 

[C0mbat]

Brilliant, thanks @Farenheit. I see that your null is both extremely wide and deep. Mine is shallower and narrower so I wouldn't need to boost as much. I'll have a play over the next week or so and report back.

@Farenheit so I just tried doing this and it works. It's mindboggling because my original measurements, when I use a sub, were measuring that null with both the left and right channels. I'm guessing that the sub it contributing to it even at a 95hz crossover. So the EQs are boosting the left by 10db and the right by 7.9db. But when I boost both channels the null doesn't budge a single db. So instead I inversed the left from a 10db boost to a -11db reduction and lifted the right to a 10db boost. It's cut the null from a -10db dip to a -5db dip. So it's not completely fixed it but it's made a huge difference. I did all of this inversing and changing in increments and each time the delta between -10db and -5db got smaller. I'm sure I could take it further but I'm concerned about boosting it too much as +10db is already big.

All of this happened without having any effect on the rest of the frequency range so I'm looking pretty good now.

While doing this I also experimented with having my door open or closed. Previously I had kept it closed but I'm realised that the bass roll off changes from 20hz to 10hz with the door open. A boost for me as I listen to electronic music

 

[ErLan]

My 2c...

I am also facing a similar Nulls caused by phase misalignment between Rt & Lt speakers that - when combined together - cancel each other.
Repositioning the Lt & Rt improved things a bit, yet not sufficient.
I have also pulled in the 'Subwoofer' as an optional remedy, which nicely allowed me to retire one of the nulls (~50Hz) however the 2nd null is too high (~110Hz) to crossover subwoofer with as it led to its 'detectability'.

I ended up with the following schema:

• Creating a virtual Lt_phase-Rt_phase signal (using REW export of the Lt & Rt phase saparately >> substruction of Lt-Rt phase via excel >> and embedding the result back to a REW export template so it can be uploaded to REPHASE)
• Uploading the file to REPHASE and gently minimize the difference to be close to 0 (using: low Qs, focusing only on the range of 50Hz-150Hz); when done, create a *.wav file.
• Embedding the *.wav file it in a convolver (my case: EqAPO)

Using this allowed me to get phases of Lt & Rt phases very close to each other and clear out the nulls. Impact was notable both qualitatively (Bass can be heard! as well as quantitatively (IACC).

Few points to note:

• Through this path, REW was VERY supporting as a simulation tool.
• I do realize that this can not be deployed in WiiM (unless it hold a convolver? I am not that familiar with this device).
• This approach does require some manual work (which I find fun, and others may not).

But I thought to bring it anyway as so far this is the most impactful result that I was able to get.

 

[C0mbat]

My 2c...

I am also facing a similar Nulls caused by phase misalignment between Rt & Lt speakers that - when combined together - cancel each other.
Repositioning the Lt & Rt improved things a bit, yet not sufficient.
I have also pulled in the 'Subwoofer' as an optional remedy, which nicely allowed me to retire one of the nulls (~50Hz) however the 2nd null is too high (~110Hz) to crossover subwoofer with as it led to its 'detectability'.

I ended up with the following schema:

• Creating a virtual Lt_phase-Rt_phase signal (using REW export of the Lt & Rt phase saparately >> substruction of Lt-Rt phase via excel >> and embedding the result back to a REW export template so it can be uploaded to REPHASE)
• Uploading the file to REPHASE and gently minimize the difference to be close to 0 (using: low Qs, focusing only on the range of 50Hz-150Hz); when done, create a *.wav file.
• Embedding the *.wav file it in a convolver (my case: EqAPO)

Using this allowed me to get phases of Lt & Rt phases very close to each other and clear out the nulls. Impact was notable both qualitatively (Bass can be heard! as well as quantitatively (IACC).

Few points to note:

• Through this path, REW was VERY supporting as a simulation tool.
• I do realize that this can not be deployed in WiiM (unless it hold a convolver? I am not that familiar with this device).
• This approach does require some manual work (which I find fun, and others may not).

But I thought to bring it anyway as so far this is the most impactful result that I was able to get.

@ErLan This sounds like a great approach. I'm limited with what my WiiM can do unfortunately. They do have a really good feature request process so I could ask. What would I be asking for? The ability to rephase individual speakers? Or is it more specific than that? In the mean time I did post just before you and have partially solved the issue with the help of @Farenheit so am pretty happy.