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post #1 of 15 Old 03-26-2019, 02:55 PM - Thread Starter
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Subwoofer Localization Studies

Hey all, I've always been curious about low frequency localization and I have always tried to push my crossover frequency higher than the norm, sometimes up to 120 but usually around 100Hz. Anyway, I've been trying to find actual studies on localization and I'm going to summarize a few so that maybe it will help others. All of these were found in the AES site under the E publications.

The 1st study was "Loudspeaker Reproduction: Study on. the Subwoofer Concept" and was an older study that was pretty basic, it compared a subwoofer in between the mains, in a corner and a sidewall and use 3rd order butterworth filters. This study concluded that the subwoofer should be crossed over no higher than 100Hz in the corner or sidewall but up to 140Hz when positioned centrally between the mains.

The next interesting study is "Detection of Two Subwoofers: Effect of Broad-Band-Channel Level and Crossover Frequency" and examines the effect of crossover frequency and whether dual subs helps with localization. This study also concluded that as long as the crossover was 100Hz or under that the crossover frequency wasn't critical and unsurprisingly that dual subs were harder to localize than a single subwoofer.

Another interesting study was "Detection of subwoofer depending on crossover frequency and spatial angle between subwoofer and main speaker" and seemed to be the most in-depth, it considered placement as in front of the listener out to the sides to see if the spatial angle made a difference in localization. This study concluded that the highest acceptable crossover frequency was 120Hz before localization occurred.

Finally "Perceptibility of direction and time delay errors in subwoofer reproduction" concludes that there is "Very little directional information below about 200Hz" and "none at all below about 100Hz." The author also mentions that if you set the crossover below 100Hz, 1 of the practical advantages of a subwoofer is lost in that large main loudspeakers are then required. The 2nd part of this study concluded that delay errors in the mains relative to the subwoofer were much more disturbing to listeners, which underscores the importance of time-aligning your subs with your mains.

None of these studies go into the importance of setup but what I have learned about using higher crossovers is that both the mains and subs need to have a smooth rolloff, usually through EQ, and the levels should be closely matched. For the people who like to turn the sub up 5-10db above the level of their mains, I could see a lower crossover being necessary. Anyway, hopefully this helps someone wondering about crossover frequencies.
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post #2 of 15 Old 03-27-2019, 12:34 AM
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I've read some studies and with many of them I've had quibbles with their methodologies. There are many things which matter, can influence results, yet aren't always addressed:

Was the study with headphones or speakers?

Where the speakers in a reverberant room or an anechoic chamber?

Where the presentation levels measured with probe mics at the ear drums or do you assume one mono mic placed rough where they sit tells you everything you need?

How was the sub calibrated and was room correction applied?

What's the slope of the filter for the crossover frequency used?

Full range speakers or subs?

Was the test signal test tones or music?

What kind of music?

What kind of test tone?

Was it in isolation or was there concurrent music/movie content and what kind? [We perceptually fuse deep bass to the nearest higher frequency source we experience due to what's called the "proximity effect" and then even though there's not a shot in hell you'd localize the clean 20Hz tone in isolation you perceptually would say "I definitely hear it from the left." when there's simultaneous higher content to tie it there. Get it?]

What controls were in place to make sure the listeners weren't accidentally getting directional cues in the way of "tells" [giveaways] as to where the sound came from such as higher harmonics distortion? [People who don't think subs have THD are living in a fantasy.]

What controls were in place to make sure the listeners weren't accidentally getting directional cues in the way of "tells" [giveaways] as to where the sound came from such as sympathetic vibration such as rattling dishware, light fixtures, subwoofer connection wires buzzing against the cabinet, port chuffing noise, etc.

---

Many of these studies are conducted by people with good psychoacoustic knowledge but very little room acoustics and subwoofer placement skills. These are different skill sets and you don't even learn about them at the same college. It would be like expecting a top flight race car designer to be a top flight race car driver. Sorry, they have very little to do with each other.

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post #3 of 15 Old 03-27-2019, 07:47 PM - Thread Starter
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Those are all good points but since you read all of the studies you're referring to you know the answers to your questions I presume? Based on the methodology do you think the suggested crossover points should be lower or higher?
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post #4 of 15 Old 03-27-2019, 08:35 PM
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I have not read all the studies on subwoofer loacalization, as I wrote, I've read some.
---

Another issue which needs to be discussed is practicality. Lets say theoretically you discover you yourself are very sensitive to it under nearly all conditions, with careful controls in place, all the way down to 32Hz. So does this mean you need to buy giant main speakers, good all the way down to 32 Hz, and you'll then crossover the lower content to your sub? If you did that you'd miss out on tons of the advantages of using subs and your sub wouldn't even make a peep for a lot of music and movies. Seems an expensive and bulky way to go, if you ask me.

I was trained by THX technicians* when it came out and here's how they described Tomlinson Holman's [he's the letters T and H in THX, if you didn't know] decision to settle on 80Hz, the world's defacto standard used in all THX products and commercial cinemas, using rounded figures (and my memory. This was decades ago after all]:

- if we limit ourselves to placing the subs in the front hemisphere, not rear, crossing over at about 40Hz is the safest bet to absolutely ensure we never get any accidental localization cues in any content whatsoever, including specially designed test signals presented in isolation from any other content, for 95-99% of viewers.

-- if we limit ourselves to placing the subs in the front hemisphere, not rear, crossing over at about 80Hz, it is the safest bet to reasonably ensure we almost never get any accidental localization cues in all typical content, barring specially designed test signals presented in isolation from any other content which can't take advantage of the proximity effect, a signal movie watchers are never exposed to, for about 90% of viewers, plus there are gigantic practical and economic advantages to this such as all the main speakers being a much smaller size, a fraction of the price, easier to hang on the side walls, lower power needs, etc.

* I am not THX certified however, in fact that concept didn't even exist the year I received my informal training.

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post #5 of 15 Old 03-27-2019, 08:43 PM
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In theory, barring any complications, tells from THD, sympathetic vibrations, etc., and cognitive bias, one can test themselves for what frequency works best for an isolated tone with this test:
https://www.audiocheck.net/audiotest...calization.php

Might I ask what prompted your interest in this topic? Setting up some subs for instance?
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post #6 of 15 Old 03-28-2019, 07:51 AM - Thread Starter
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Originally Posted by m. zillch View Post
In theory, barring any complications, tells from THD, sympathetic vibrations, etc., and cognitive bias, one can test themselves for what frequency works best for an isolated tone with this test:
https://www.audiocheck.net/audiotest...calization.php

Might I ask what prompted your interest in this topic? Setting up some subs for instance?
I've used that test and others and that is part of what got me curious because honestly even at 200Hz, bass seems hard to localize. Also, I think many people generally cross their speakers over too low, especially when you consider most people have receivers that are using the THX slopes of 2nd order high pass/4th order low pass with ported mains, which don't exhibit the specified rolloff of sealed speakers. If receivers could do a 4th order high pass, 80Hz wouldn't be so bad and for sure it's a good general crossover point, all things considered.

But the real reason I've always tried pushing my crossover a bit higher than the norm is because I use LS50s as my front 3 and the more bass you can remove from them the better they sound since the woofer acts as the waveguide, I'm basically trying to minimize cone excursion. If I had towers I would most likely cross them over at 80 since a higher cross wouldn't buy me much with more capable mains. My mains also have a null at 90Hz, so letting the sub play higher eliminates that, actually many people seem to have dips in the 80-100Hz range, so I think higher crossovers could help with that.

Setup is definitely more critical with higher crossover points as well and measurements are pretty much required here. I usually shape the response of my mains with the crossover in place so they achieve a smooth rolloff in the bass and then use the subs to flatten it out. I also add a 1st order butterworth low pass to cut off the highs just a bit steeper, which seems to help. In the end, it's all a system and everyone has to decide what's best given their speakers, subs, electronics, room, treatments, etc.
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post #7 of 15 Old 03-28-2019, 12:18 PM
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Originally Posted by aarons915 View Post
I've used that test and others and that is part of what got me curious because honestly even at 200Hz, bass seems hard to localize. ...
Human hearing is most sensitive to midrange frequencies, and it is those that are easiest to localize. (It turns out, one could have one super-tweeter for all channels for very high treble, without people localizing that, too, but that is not done because it is easy and cheap to just put in a proper tweeter in all channels and it does not take up a lot of space, unlike the situation with deep bass.) And different people are more sensitive to localization than others.

Also, as already discussed above, different setups allow for different crossover points (as, for example, the subwoofer located between the front left and right speakers, versus putting the subwoofer behind you, etc.).

So, yes, 200Hz may not be overly easy to localize. Especially if we are not talking about a pure tone, but are listening to music in which there is some 200Hz content as well as many other frequencies at the same time.

As a practical matter, it is good to have a standard that people can use, instead of trying out complicated testing in their rooms. Most people seem to have enough trouble setting up their systems without adding more difficulties. This is where something like the THX standard comes in handy, as it is a reasonable choice for most cases, and allows one to have reasonably small main speakers (though I would not want to go with smaller than 5" woofers with it).

Of course, using a higher crossover has some advantages, as it will allow one to use even smaller speakers, or allow for higher volumes (since the woofer in the main speakers is no longer reproducing those frequencies sent to the subwoofer).

I get the impression that what you are looking for is validation in using a higher crossover frequency than 80Hz. Well, with the KEF LS50 and with careful positioning of the subwoofer(s) and careful setup, setting it higher may work out better, particularly at higher volume levels. In other words, if it works better in your system to set the crossover higher, go ahead and do so. Don't worry if you are using a higher than the norm frequency, because your system is unique and, if carefully set up, a higher frequency is fine (as you should realize from the research you have done). And, presumably, after you set it up, you are not going to be spending a lot of time listening to test tones but are instead going to be listening to music, and so what matters is what you can localize with music, not test tones. Which means that one can generally set the crossover higher than would work with test tones without being able to localize the subwoofer from the sound.

But for people who are not going to carefully measure things, I recommend that they get speakers suitable for an 80Hz crossover and go with that. Notice, this recommendation is not an absolute requirement, but is one to make things work well for people who are unable or unwilling to go to the trouble of taking careful measurements and taking great care in setting things up.

When I first added a subwoofer to my current main 2 channel system, I spent a few hours playing with a meter and test tones and adjusted the phase and the crossover frequency and level for the subwoofer. (Most people are not going to do that or go to such trouble to get things right, so simple rules of thumb are useful for getting good sound, which may not be ideal but are still quite good.) I ended up with something I would not have guessed would be best beforehand, though looking at the measured frequency response of my speakers helps explain why I ended up with an unusual situation. If you are carefully measuring in your room with your speakers, then you may well end up with unique settings for what works best. Don't let that worry you, that it may be quite different from what people typically do.

If you are worried about what other people think about it, you don't have to tell them what crossover setting you have selected. You have done due diligence in looking at the research on the question such that you should know that a higher crossover setting can be fine. If people who are ignorant of these studies think your crossover is too high, don't worry about their opinions. The easiest way, generally, is to just not tell them about your particular settings.

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post #8 of 15 Old 03-28-2019, 12:30 PM
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If you keep the sub in the front of the room, ideally somewhere between the L and R mains, you probably can get by with using a higher crossover without issue. If you've tried it out and indeed don't localize sound to the sub then this means for you, you are doing fine. What counts is how the individual listener hears things and deep bass localization seems to vary per individual.


You mentioned dips in the 80-100Hz range. I just thought to point out this is likely room placement related and not the speakers themselves which seem fine there:

source: https://soundstagenetwork.com/index....nts&Itemid=153

In A/V reproduction accuracy, there IS no concept of "accounting for personal taste/preference". As art consumers we don't "pick" the level of bass, nor the tint/brightness of a scene's sky, any more than we pick the ending of a novel or Mona Lisa's type of smile. "High fidelity" means "high truthfulness", faithful to the original artist's intent: an unmodified, neutral, accurate copy of the original master, ideally being exact and with no discernable alterations, aka "transparency".
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post #9 of 15 Old 03-28-2019, 12:38 PM
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Originally Posted by Jack D Ripper View Post
If you are worried about what other people think about it, you don't have to tell them what crossover setting you have selected. You have done due diligence in looking at the research on the question such that you should know that a higher crossover setting can be fine. If people who are ignorant of these studies think your crossover is too high, don't worry about their opinions. The easiest way, generally, is to just not tell them about your particular settings.
That sounds like a good plan!

In A/V reproduction accuracy, there IS no concept of "accounting for personal taste/preference". As art consumers we don't "pick" the level of bass, nor the tint/brightness of a scene's sky, any more than we pick the ending of a novel or Mona Lisa's type of smile. "High fidelity" means "high truthfulness", faithful to the original artist's intent: an unmodified, neutral, accurate copy of the original master, ideally being exact and with no discernable alterations, aka "transparency".
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post #10 of 15 Old 03-28-2019, 02:13 PM - Thread Starter
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Originally Posted by Jack D Ripper View Post

As a practical matter, it is good to have a standard that people can use, instead of trying out complicated testing in their rooms. Most people seem to have enough trouble setting up their systems without adding more difficulties. This is where something like the THX standard comes in handy, as it is a reasonable choice for most cases, and allows one to have reasonably small main speakers (though I would not want to go with smaller than 5" woofers with it).
I agree with everything you've said and 80Hz is a very good crossover point for most people when you consider all of the variables.

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Of course, using a higher crossover has some advantages, as it will allow one to use even smaller speakers, or allow for higher volumes (since the woofer in the main speakers is no longer reproducing those frequencies sent to the subwoofer).
Agreed again, I don't listen particularly loud but assuming there is no localization below 100Hz, there really aren't any benefits of going lower anyway, even though I could use 80Hz no problem and have in the past. My big problem with 80Hz comes from the typical receiver that uses a 2nd order high pass, it's just not enough of a slope and forces smaller speakers to play way too much bass. So it stresses the woofers more than necessary and isn't going to blend very well with the sub either. Sealing the ports takes care of this issue but I notice most still keep their ports open.

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I get the impression that what you are looking for is validation in using a higher crossover frequency than 80Hz. Well, with the KEF LS50 and with careful positioning of the subwoofer(s) and careful setup, setting it higher may work out better, particularly at higher volume levels. In other words, if it works better in your system to set the crossover higher, go ahead and do so. Don't worry if you are using a higher than the norm frequency, because your system is unique and, if carefully set up, a higher frequency is fine (as you should realize from the research you have done). And, presumably, after you set it up, you are not going to be spending a lot of time listening to test tones but are instead going to be listening to music, and so what matters is what you can localize with music, not test tones. Which means that one can generally set the crossover higher than would work with test tones without being able to localize the subwoofer from the sound.
Definitely not looking for validation, I've been using higher crossovers for years with varying levels of success. I was really just trying to give something back to the forums since I've learned so much here and I wanted to put forth actual science instead of just my opinion on the matter. I see many people in the REW thread and elsewhere that have room problems between 80-100Hz but they're afraid of crossing higher than 80 because they believe they will have localization and maybe that's true but you have to try it out in your setup and maybe even tweak a few things before deciding that.

The main reason for trying to push the crossover frequency higher in my case is because I've read that 2-way coaxials are more susceptible to IMD distortion than other speakers, since the cone acts as the tweeter's waveguide, so originally I was just trying to remove as much bass as possible from my mains while still not localizing the sub.
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post #11 of 15 Old 03-28-2019, 02:23 PM - Thread Starter
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Originally Posted by m. zillch View Post
If you keep the sub in the front of the room, ideally somewhere between the L and R mains, you probably can get by with using a higher crossover without issue. If you've tried it out and indeed don't localize sound to the sub then this means for you, you are doing fine. What counts is how the individual listener hears things and deep bass localization seems to vary per individual.


You mentioned dips in the 80-100Hz range. I just thought to point out this is likely room placement related and not the speakers themselves which seem fine there:
Yes I have dual subs symmetrically placed, which is even better for not being able to localize the bass. Sometimes I feel like I am localizing the bass but then I switch to 80Hz and that feeling doesn't change much, I think it's just because of how bass feels like it's coming from all around the room since the wavelengths are so large.

And yes nearly all bass problems under the room's transition frequency are room related but the reason why 80-100hz is so common with stand mounts is because the woofer is generally right around 3 ft off the ground which causes the infamous floor bounce cancellation right in the crossover region.
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...

Definitely not looking for validation, I've been using higher crossovers for years with varying levels of success. I was really just trying to give something back to the forums since I've learned so much here and I wanted to put forth actual science instead of just my opinion on the matter. I see many people in the REW thread and elsewhere that have room problems between 80-100Hz but they're afraid of crossing higher than 80 because they believe they will have localization and maybe that's true but you have to try it out in your setup and maybe even tweak a few things before deciding that.
...
Okay, sorry, my mistake. You are definitely correct that experimentation with one's equipment can render the best results. And that 80Hz is not always the best choice (even though it is often a good place to start). People who are capable and willing to experiment should do so if they are having any problems. Of course, if they are not having any issues, then leaving well enough alone is fine.

I also agree with you that many people seem to want to push their main speakers to do more frequencies than are ideal. In my case, my speakers are rated 50-40KHz +/-3dB, and I use an 80Hz crossover. I don't think it would be good to push the crossover lower, and since I am not having any known issues, I don't think I need to push it higher, either. But if I were having issues with upper bass, then a change may well be in order. Certainly, it would be a good thing to experiment with if I were having issues.
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Reffering to studies for such matter seems a bit funny to me. Maybe as a rough guidline but nothing else. Took me literarly 30 minutes to create a simple worst case scenario where I've put sub 6 feet left of MLP and experiment with crossover frequencies, slopes, sub levels and curves to determine what is and isn't localizable.
Lets just say every case needs individual approach, depending on preference and tolerance of user and of course their room/setup.

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Reffering to studies for such matter seems a bit funny to me. Maybe as a rough guidline but nothing else. Took me literarly 30 minutes to create a simple worst case scenario where I've put sub 6 feet left of MLP and experiment with crossover frequencies, slopes, sub levels and curves to determine what is and isn't localizable.
Lets just say every case needs individual approach, depending on preference and tolerance of user and of course their room/setup.
Referring to studies is a good way to make sure that one is not going far wrong. If one is "hearing" where 20Hz is coming from, then one should be looking for other things, like rattling noises and distortion that isn't 20Hz. Often, people who don't pay attention to studies end up believing all sorts of nonsense. Here is an extreme example:

http://www.machinadynamica.com/machina31.htm

I am willfully using an extreme example to be less controversial than more common delusional beliefs. There is a lot of quackery in audio.

But, yes, doing your own testing is a good idea, to find out how far one can push matters. But one should remember that feeding a pure test tone to a subwoofer does not mean that the subwoofer is only producing a pure test tone. This will be particularly relevant in cases of driving the subwoofer hard or at lower frequencies than it can satisfactorily reproduce. And one should also remember the fact that human bias can enter into things, so, again, one needs to take care in one's testing to be sure that one's test really tests what one is trying to test.
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post #15 of 15 Old 03-29-2019, 04:55 PM
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I did my own testing decades ago, along with a group of friends and customers. I think it was about 25 people and we conducted several experiments over several weeks. It was a PITA to set up, and I do not have the data after all these years. We were trying to verify the AES study at the time, and this was when subs were a new thing and very few were available. Anyway, the majority of the group could no longer locate the sub at around 80 Hz, with a range from roughly 60 to nearly 100 Hz. That is, below 60 Hz nobody could tell, and above 100 Hz everyone could tell, where the sub was positioned. Surprisingly the group was pretty tight around 80 Hz IIRC; I expected more of a gradient.

In addition to finding places to run tests, one of the things we had to watch was that, as we increased power to the subs, harmonic distortion was readily apparent and made it easy to localize the sub. We started with three fairly small subs placed around the listeners and ultimately set up four stacks of 2 subs.

The distortion problem is mainly what led me to design my own servo sub about that time. Actually, it was mainly money, since as a college kid working my way through I couldn't afford a nice sub so had to do it myself or go without.

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