Advice for the equalization of low frequencies

[Stefano_Milano]

I have read hundreds of posts related to eq. But I still have a couple of doubts.
I have read that the best result of handling bass resonances in the room is to lower the peaks using as few filters as possible.
I have also read that it is useless to try to pull up the cancellation peaks.
What do you recommend? Equalizing peaks without boost, leaving negative peaks? At what height should the house curve be placed? towards the end of the negative peaks or in the middle? Should I set to VAR smoothing or 1/12?
I have Elac Reference 6.2.
Thanks so much!
Stefano

 

[FeddyLost]

I have also read that it is useless to try to pull up the cancellation peaks.

Not completely useless, but it makes a lot of strain for speakers and amp, while giving mediocre results.
For example, if you have pair of 15" subwoofers and small room, you can try to pull up cancellations while measuring distortion at this frequency.
If you have small bookshelf speakers, +10-15 Db in pair of bass peaks is a good way to damage something. Especially if you pulling up frequency below bass reflex tuning.

 

[Wayne A. Pflughaupt]

Not sure why you believe anything posted here will be better than hundreds of other posts you’ve read, but I’ll play along.

For starters, any subwoofer equalizing is going to tax headroom, whether you use positive- or negative-gain filters.

The common situation is that there are one or two peaks in the room. Those hot frequencies are what determined the volume setting you set the sub for before equalization. Eliminate (cut) those peaks and you’ll now perceive that your sub is too quiet. So, you have to turn it up to compensate. Well guess what? Now you’re driving your sub harder than you were before. Say goodbye to any headroom you thought you saved by not using any boosted filters.

Also, consider what happens when you make a series of cuts. It shouldn’t take much to figure out that you now have peaks in between them!

For instance, consider the graph below, which depicts the equalizer’s electronic response after equalizing. In other words, the equalization changes the electrical signal that was originally flat (i.e. the flat blue line, which also includes the slope of the crossover). So, did we make three cuts here...

...or did we make three boosts?

See what I mean?

Bottom line, there’s no free lunch. In most cases, equalization requires that you have ample headroom going in, because you’re going to end up with less than what you started with. If you don't have ample headroom going in, you shouldn’t equalize, or equalize minimally.

Now that we know that using only negative-gain filters gets no benefit, what about trying to correct depressions? Common wisdom holds that you shouldn’t boost troughs or nulls in response because it doesn’t accomplish anything. We’re told that nulls are caused by phase cancellations, where the sound reaching the measurement point is a combination of the original direct soundwave combined with a reflected soundwave that is 180˚ out of phase (one half wavelength). When you add a gain filter at the depressed frequency, not only does the direct sound increase by the number of dB of the filter, but unfortunately the 180˚ out-of-phase signal also applies an equal and opposite signal to counteract. The result is that your dip is still there and you have wasted the gain you've thrown at it.

This theory is technically correct, but it’s based on the silly expectation that bass emanates from a subwoofer like a laser, with the primary “sound beam” reflecting off a perfectly-perpendicular wall and meeting the original “sound beam” half-way in between, 180˚ out of phase, causing cancellation.

Of course, the theory falls apart with the realization that bass frequencies are omnidirectional, not directional. They discharge from the subwoofer in all directions and in like manner bounce around the room in all directions. A quote from this article explains things nicely:

These nulls are related not only to the distance from the rear wall, but also from the other walls, the floor, and the ceiling. So in order to create a deep null a precise balance is needed, and that balance is easily disrupted by the many other reflections bouncing around the room.

In other words, since bass is omnidirectional, sound is reflecting at every angle from every boundary in the room. So while there may well be a point and / or frequency that exhibits wholesale cancellation due to a reflection being 180˚ out of phase with the original signal, there is no shortage of other, non-canceled reflections present for an equalizer to work with. Plus, the equalizer introduces phase changes itself . This is why you’ll seldom see a case where a depression won’t respond to EQ.

The other problem with the “never boost nulls” theory is that it presumes all depressions, dips, troughs etc. in response are nulls. The explanation above should be sufficient to show that true nulls are pretty rare. Whenthey do occur, it’s usually the result of a poor subwoofer location.

It’s pretty easy to identify nulls: Typically they are deep and narrow and won't respond to equalization, as we see with these graphs:

Here’s the second graph, but with EQ. As you can see, the nulls didn’t budge.

By contrast, here’s a graph of a sub with a rather severe depression that boosted just fine with equalization. Obviously it wasn’t a null.

Regarding the number of filters to use, people really tend to over-equalize. The reality is, all you really need to do is address the worst peaks and troughs in response. Chasing every little ripple in response trying to get a picture-perfect graph is pointless, because filters with small gain values (1-2 dB) and narrow bandwidth – they aren’t audible. You can switch them in and out and not hear a difference. So what’s the point? About the only way a 2-dB filter will be audible is if it’s addressing a very broad area, like a half-octave or wider.

Remember that filters introduce phase shift. Thus, their “best-practices” purpose is using phase shift to counter-act a response issue that has been caused by phase issues. Typically that takes only a few filters.

Hope this helps.

Regards,
Wayne A. Pflughaupt

 

[Jimbob54]

Not sure why you believe anything posted here will be better than hundreds of other posts you’ve read, but I’ll play along.

For starters, any subwoofer equalizing is going to tax headroom, whether you use positive- or negative-gain filters.

The common situation is that there are one or two peaks in the room. Those hot frequencies are what determined the volume setting you set the sub for before equalization. Eliminate (cut) those peaks and you’ll now perceive that your sub is too quiet. So, you have to turn it up to compensate. Well guess what? Now you’re driving your sub harder than you were before. Say goodbye to any headroom you thought you saved by using any boosted filters.

Also, consider what happens when you make a series of cuts. It shouldn’t take much to figure out that you now have peaks in between them!

For instance, consider the graph below, which depicts the equalizer’s electronic response after equalizing. In other words, the equalization changes the electrical signal that was originally flat (i.e. the flat blue line, which also includes the slope of the crossover). So, did we make three cuts here...

...or did we make three boosts?

See what I mean?

Bottom line, there’s no free lunch. In most cases, equalization requires that you have ample headroom going in, because you’re going to end up with less than what you started with. If you don't have ample headroom going in, you shouldn’t equalize, or equalize minimally.

Now that we know that using only negative-gain filters gets no benefit, what about trying to correct depressions? Common wisdom holds that you shouldn’t boost troughs or nulls in response because it doesn’t accomplish anything. We’re told that nulls are caused by phase cancellations, where the sound reaching the measurement point is a combination of the original direct soundwave combined with a reflected soundwave that is 180˚ out of phase (one half wavelength). When you add a gain filter at the depressed frequency, not only does the direct sound increase by the number of dB of the filter, but unfortunately the 180˚ out-of-phase signal also applies an equal and opposite signal to counteract. The result is that your dip is still there and you have wasted the gain you've thrown at it.

This theory is technically correct, but it’s based on the silly expectation that bass emanates from a subwoofer like a laser, with the primary “sound beam” reflecting off a perfectly-perpendicular wall and meeting the original “sound beam” half-way in between, 180˚ out of phase, causing cancellation.

Of course, the theory falls apart with the realization that bass frequencies are omnidirectional, not directional. They discharge from the subwoofer in all directions and in like manner bounce around the room in all directions. A quote from this article explains things nicely:

These nulls are related not only to the distance from the rear wall, but also from the other walls, the floor, and the ceiling. So in order to create a deep null a precise balance is needed, and that balance is easily disrupted by the many other reflections bouncing around the room.

In other words, since bass is omnidirectional, sound is reflecting at every angle from every boundary in the room. So while there may well be a point and / or frequency that exhibits wholesale cancellation due to a reflection being 180˚ out of phase with the original signal, there is no shortage of other, non-canceled reflections present for an equalizer to work with. Plus, the equalizer introduces phase changes itself . This is why you’ll seldom see a case where a depression won’t respond to EQ.

The other problem with the “never boost nulls” theory is that it presumes all depressions, dips, troughs etc. in response are nulls. The explanation above should be sufficient to show that true nulls are pretty rare. Whenthey do occur, it’s usually the result of a poor subwoofer location.

It’s pretty easy to identify nulls: Typically they are deep and narrow and won't respond to equalization, as we see with these graphs:

Here’s the second graph, but with EQ. As you can see, the nulls didn’t budge.

By contrast, here’s a graph of a sub with a rather severe depression that boosted just fine with equalization. Obviously it wasn’t a null.

Regarding the number of filters to use, people really tend to over-equalize. The reality is, all you really need to do is address the worst peaks and troughs in response. Chasing every little ripple in response trying to get a picture-perfect graph is pointless, because filters with small gain values (1-2 dB) and narrow bandwidth – they aren’t audible. You can switch them in and out and not hear a difference. So what’s the point? About the only way a 2-dB filter will be audible is if it’s addressing a very broad area, like a half-octave or wider.

Remember that filters introduce phase shift. Thus, their “best-practices” purpose is using phase shift to counter-act a response issue that has been caused by phase issues. Typically that takes only a few filters.

Hope this helps.

Regards,
Wayne A. Pflughaupt

Brilliant post. Sticky please @BDWoody

 

[RayDunzl]

Opinion from experience here:

I can reduce peaks.

Nulls due to phase cancellation cannot be filled.

I can hear and be annoyed by peaks. There are two in the bass. Or, there is one big one with cancellation occurring where it would have peaked.

I can't hear nulls, and didn't know it (they) were there before measuring. I have a deep hole around 48Hz,

When the bass is mono - the 48Hz null is measurably pronounced. When the bass is stereo, the null can be lessened or even disappear because the phase is not the same.


Examples:

Left, Right, and Both speakers, 1/3rd smoothing, no EQ:

With both speakers playing, the SPL measured increases by 6dB - coherent summing, except at 48Hz where the phase becomes incoherent.

Left, Right and Both speakers, 1/3 smoothing, "flat" EQ target,

Neither left nor right speaker has a severe dip at 48Hz.

Dip at 48Hz remains when both speakers are playing in mono - phase cancellation at the listening position due to asymmetrical (not a closed rectangle) room.

Result: the boom experienced at higher listening SPL is gone. Dip isn't noticed. Nobody complains about "no curve".

---

Experiments showed it is possible to measurably kill the mono null at 48hz by boosting one channel around that frequency and deeply cutting (notch) the other, preventing the phase problem by eliminating half of the signal that creates the phase problem..

 

[Stefano_Milano]

Not sure why you believe anything posted here will be better than hundreds of other posts you’ve read, but I’ll play along.

For starters, any subwoofer equalizing is going to tax headroom, whether you use positive- or negative-gain filters.

The common situation is that there are one or two peaks in the room. Those hot frequencies are what determined the volume setting you set the sub for before equalization. Eliminate (cut) those peaks and you’ll now perceive that your sub is too quiet. So, you have to turn it up to compensate. Well guess what? Now you’re driving your sub harder than you were before. Say goodbye to any headroom you thought you saved by not using any boosted filters.

Also, consider what happens when you make a series of cuts. It shouldn’t take much to figure out that you now have peaks in between them!

For instance, consider the graph below, which depicts the equalizer’s electronic response after equalizing. In other words, the equalization changes the electrical signal that was originally flat (i.e. the flat blue line, which also includes the slope of the crossover). So, did we make three cuts here...

...or did we make three boosts?

See what I mean?

Bottom line, there’s no free lunch. In most cases, equalization requires that you have ample headroom going in, because you’re going to end up with less than what you started with. If you don't have ample headroom going in, you shouldn’t equalize, or equalize minimally.

Now that we know that using only negative-gain filters gets no benefit, what about trying to correct depressions? Common wisdom holds that you shouldn’t boost troughs or nulls in response because it doesn’t accomplish anything. We’re told that nulls are caused by phase cancellations, where the sound reaching the measurement point is a combination of the original direct soundwave combined with a reflected soundwave that is 180˚ out of phase (one half wavelength). When you add a gain filter at the depressed frequency, not only does the direct sound increase by the number of dB of the filter, but unfortunately the 180˚ out-of-phase signal also applies an equal and opposite signal to counteract. The result is that your dip is still there and you have wasted the gain you've thrown at it.

This theory is technically correct, but it’s based on the silly expectation that bass emanates from a subwoofer like a laser, with the primary “sound beam” reflecting off a perfectly-perpendicular wall and meeting the original “sound beam” half-way in between, 180˚ out of phase, causing cancellation.

Of course, the theory falls apart with the realization that bass frequencies are omnidirectional, not directional. They discharge from the subwoofer in all directions and in like manner bounce around the room in all directions. A quote from this article explains things nicely:

These nulls are related not only to the distance from the rear wall, but also from the other walls, the floor, and the ceiling. So in order to create a deep null a precise balance is needed, and that balance is easily disrupted by the many other reflections bouncing around the room.

In other words, since bass is omnidirectional, sound is reflecting at every angle from every boundary in the room. So while there may well be a point and / or frequency that exhibits wholesale cancellation due to a reflection being 180˚ out of phase with the original signal, there is no shortage of other, non-canceled reflections present for an equalizer to work with. Plus, the equalizer introduces phase changes itself . This is why you’ll seldom see a case where a depression won’t respond to EQ.

The other problem with the “never boost nulls” theory is that it presumes all depressions, dips, troughs etc. in response are nulls. The explanation above should be sufficient to show that true nulls are pretty rare. Whenthey do occur, it’s usually the result of a poor subwoofer location.

It’s pretty easy to identify nulls: Typically they are deep and narrow and won't respond to equalization, as we see with these graphs:

Here’s the second graph, but with EQ. As you can see, the nulls didn’t budge.

By contrast, here’s a graph of a sub with a rather severe depression that boosted just fine with equalization. Obviously it wasn’t a null.

Regarding the number of filters to use, people really tend to over-equalize. The reality is, all you really need to do is address the worst peaks and troughs in response. Chasing every little ripple in response trying to get a picture-perfect graph is pointless, because filters with small gain values (1-2 dB) and narrow bandwidth – they aren’t audible. You can switch them in and out and not hear a difference. So what’s the point? About the only way a 2-dB filter will be audible is if it’s addressing a very broad area, like a half-octave or wider.

Remember that filters introduce phase shift. Thus, their “best-practices” purpose is using phase shift to counter-act a response issue that has been caused by phase issues. Typically that takes only a few filters.

Hope this helps.

Regards,
Wayne A. Pflughaupt

Thanks for your explanation!
Stefano