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The Bass System Setup, EQ/Correction Thread


maxmercy

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Continued from another thread.

 

Post details about how you setup your system for best performance at the MLP or a larger listening area and if you would like to get into it, why.  Do you use the room correction in your AVR?  Or do it manually?  Minimum phase or Mixed phase correction?  Experience with both?  Room Treatment?  Sub placement?

 

 

JSS

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Great topic!

 

Generally I would set up manually. Generally I would treat room. Generally I would use multiple sub locations preferably both front and back.

 

But I think specifics really come down to the personal situation. For example if I had a speaker with a great off axis performance I'd treat the first reflection with abfusion, but a speaker with poor off axis I would probably choose to do obsorption.

 

Auto EQ blows, but the learning curve to do better isn't given away for free. It's a lot of info on each of these individual topics to get into the nitty gritty of why and how.

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Hope to see many good contributions here, and also bad ones, and perhaps even some right out ugly ones.

 

"Having a proper low frequency reproduction is something that really makes a huge difference in a sound system.
If the bass is not done right, it really does not matter how good the rest is, the total experience will be lacking.
Good bass is addictive - it has punch, impact, a sense of power and huge scale, and it does not sound boomy or resonating."

 

But we all knew that.

 

 

Bass set-up can be classified in 3 different levels:

 

1. Frequency response - relatively easy, I suspect most systems on here more or less has this already checked out.

2. Impulse response - timing, resonances, capacity for reproduction of peaks - this is where it gets interesting.

3. Sound field - getting the right relationship between pressure (spl), intensity and particle velocity for proper tactile feel - mostly due to random coincidence, some have it and some simply don't, a few systems are designed from the beginning with these properties in mind.

 

Then of course there is a level 0, this is where the typical high-end system is, with no measurements done, no dsp/eq capability and if there is a subwoofer, it is one unit where price seems to be the most important spec.

 

 

I will start with some examples of systems that at least to some degree enters the "level 3".

 

You don't need to look very far, there is a thread here on data-bass about the Bossobass Raptor system - this is not simply another sealed subwoofer, the configuration and placement of several units also help to shape the soundfield.

 

On avsforum I think there are at least 2 presentations of DBA - double bass array - systems.

These installations give complete control of all parameters of the soundfield.

Here is another build presented on the local norwegian nisseforum - in norwegian, look at the pictures, and there are links to docs on DBA.

 

Dipoles and variations have been used for bass systems in 2-ch systems, and they do it because they sound better, not because you get more bass - the principle has obvious flaws if lowest possible extension and maximum output is the goal.

 

Some of these systems are not really well suited for HT, especially the dipoles, I just wanted to show that being aware of sound field properties - velocity, tactile feel - is not entirely new, and some people have taken this quite far.

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Found this article researching minimum phase...

 

http://www.rane.com/note115.html

 

Specifically the section on phase behavior. They essentially say that all equalizers are minimum phase, and that term was derived from a marketing aspect, not engineering.

 

Thoughts? Is this a different context then @sme and @3ll3d00d are discussing?

The article essentially says all minimum phase equalisers are minimum phase :)

 

Try http://www.regonaudio.com/Digital%20Filters%20Part%20I.html (and part 2) for one of the clearest explanations of what it is/means. Actually if you browse his site then you encounter http://www.regonaudio.com/Arion%20Essex.html which is a review of non minimum phase "equaliser" from 1996!

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Try http://www.regonaudio.com/Digital%20Filters%20Part%20I.html (and part 2) for one of the clearest explanations of what it is/means.

After quick skim through those, my impression is that they are well written and explain minimum phase very well.  The article does seem to neglect an important point where it discusses responses that cannot be inverted.  That is, any zeros in the response that are "close" to the unit circle require excessive headroom to invert.  The closer to the unit circle, the bigger the boost required.  Additionally, a filter designed to invert a zero near the unit circle will give unstable results in practice because it will be overly-sensitive to small changes like how many people are in the room and what the temperature and humidity are.

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Bass set-up can be classified in 3 different levels:

 

1. Frequency response - relatively easy, I suspect most systems on here more or less has this already checked out.

2. Impulse response - timing, resonances, capacity for reproduction of peaks - this is where it gets interesting.

3. Sound field - getting the right relationship between pressure (spl), intensity and particle velocity for proper tactile feel - mostly due to random coincidence, some have it and some simply don't, a few systems are designed from the beginning with these properties in mind.

 

Numbers 1 and 2 are the same if you include phase as part of the frequency response.  The frequency response and impulse response are mathematically related by the Fourier transform (and its variants, depending on the type of system you are analyzing).  As for number 3, tactile sensation is indeed a fairly complex subject, but I am 99.9% certain that particle velocity and/or sound intensity are irrelevant to it, unless you're concerned with the cool breeze you feel by putting your hand in front of a subwoofer port.  I'd be happy to discuss my views on this subject further, but perhaps we should start yet another thread for that purpose.

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(3ll3d00d said):

 

 

That is what I mean yes.

 

I can see your argument (with regards to well treated rooms exhibiting fewer excess phase issues) but find it hard to reconcile that with the user community for a product like Acourate (which overwhelmingly seems to consist of boutique systems in custom built rooms). It might be that the improvement is absolutely small but relative differences are easier to detect and hence it is still a critical refinement. Impossible for me to verify that though.

There are a lot of variables.  If room reflections are reduced enough then the room response will become completely minimum phase, but it's also possible for the speaker to make a non-minimum phase contribution to the response, usually in the crossover region(s) of the speaker.  For less than ideal rooms, the speaker dispersion may play a big role.  My front stage uses speakers in the WTW configuration with horns that have pretty limited vertical dispersion down to 500 Hz or so and my side walls are pretty far away.  Even though I only have a rug with rubber underlay on the floor (absorbs at 1 kHz or so and up) and no ceiling treatment, my entire treble response and much of most of the mid range response is minimum phase at the MLP as is my subwoofer response all the way across its range and across my listening area.

 

My OpenDRC units support multiple pre-sets, so I may make frequent comparisons between my own minimum phase and mixed phase filters, as I develop the latter.

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Numbers 1 and 2 are the same if you include phase as part of the frequency response.  The frequency response and impulse response are mathematically related by the Fourier transform (and its variants, depending on the type of system you are analyzing).  As for number 3, tactile sensation is indeed a fairly complex subject, but I am 99.9% certain that particle velocity and/or sound intensity are irrelevant to it, unless you're concerned with the cool breeze you feel by putting your hand in front of a subwoofer port.  I'd be happy to discuss my views on this subject further, but perhaps we should start yet another thread for that purpose.

 

This is the thread that was started for this purpose.

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Placement (subs and seat), polarity, relative phase (digital delay) and level. I don't use post smoothing EQ. An outboard EQ adds gain stage complexity, additional roll off and grossly distorts the input signal.

 

After 20 years in the same room (albeit with different sub configs, treatments and furniture), I know where the best placement for subs and seats are and what the FR will be. Once I A/B'd with and without digital post smoothing EQ:

 

4a0e43788d4daaa3cae8d899aee886a3.png

 

Since no one knows what an explosion effect in a movie is supposed to sound like (no reference), music was used and the consensus of listeners was 1) no difference, and 2) no EQ. Zero votes for the FR with EQ.

 

Without smoothing EQ the response is (+/-) 4.5dB. With EQ, (+/-) 2dB, 20-200 Hz with crossover from subs to mains at 100 Hz.

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Numbers 1 and 2 are the same if you include phase as part of the frequency response.  The frequency response and impulse response are mathematically related by the Fourier transform (and its variants, depending on the type of system you are analyzing).  As for number 3, tactile sensation is indeed a fairly complex subject, but I am 99.9% certain that particle velocity and/or sound intensity are irrelevant to it, unless you're concerned with the cool breeze you feel by putting your hand in front of a subwoofer port.  I'd be happy to discuss my views on this subject further, but perhaps we should start yet another thread for that purpose.

 

Since practical installations often have differences in sound that naturally fall into those 3 categories, this is how I choose to see bass. 

A system can have a flat freq resp, and still lack a precise, tactile hit on transient attacks.

And a system can have tremendous output capacity all the way down to well below 20hz, and still fell like there is no power in the sound, you only get dots in the ears. 

 

This is because the systems are not minimum phase, and because there are differences in the produced acoustic sound field. 

 

In many systems a critical point for getting the impulse response right is setting the delay on front speakers so that they sum up correctly with the subwoofer system.

When this is fixed, the system may often look close to a minimum phase system, so that when the frequency response is reasonable smooth, the impulse response is also correct.

 

Tactile feel is determined by sound field properties.

Without intensity a sound wave can not have a physical effect on anything, and for intensity you need pressure and velocity.

What is less clear is exactly what properties are important in which frequency range, and how the relationship between sound field properties affect how we experience sound.

 

Unless others beat me to it, I will post examples later, on both subjects.

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As for number 3, tactile sensation is indeed a fairly complex subject, but I am 99.9% certain that particle velocity and/or sound intensity are irrelevant to it, unless you're concerned with the cool breeze you feel by putting your hand in front of a subwoofer port.  I'd be happy to discuss my views on this subject further, but perhaps we should start yet another thread for that purpose.

Interested in your thoughts here on the SIL/PVL discussion. This is not what I've observed, but would certainly like to hear a different perspective.

 

Tactile feel is determined by sound field properties.

Without intensity a sound wave can not have a physical effect on anything, and for intensity you need pressure and velocity.

What is less clear is exactly what properties are important in which frequency range, and how the relationship between sound field properties affect how we experience sound.

 

Unless others beat me to it, I will post examples later, on both subjects.

This is how I understand tactile feel as well.

 

Here is a test I performed using an accelerometer to measure tactile feel close to the subs, and further away from the subs using a 15hz sine wave. This test demonstrates that SPL alone is not the sole contributor to tactile feel. Give this equation: Sound Intensity = pressure * particle velocity, the test demonstrates in this example that PVL must be the differentiating factor to the variation in tactile feel.

 

http://www.avsforum.com/forum/113-subwoofers-bass-transducers/1488059-your-home-theater-ulf-score.html#post23676269

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Placement (subs and seat), polarity, relative phase (digital delay) and level. I don't use post smoothing EQ. An outboard EQ adds gain stage complexity, additional roll off and grossly distorts the input signal.

 

After 20 years in the same room (albeit with different sub configs, treatments and furniture), I know where the best placement for subs and seats are and what the FR will be. Once I A/B'd with and without digital post smoothing EQ:

 

4a0e43788d4daaa3cae8d899aee886a3.png

 

Since no one knows what an explosion effect in a movie is supposed to sound like (no reference), music was used and the consensus of listeners was 1) no difference, and 2) no EQ. Zero votes for the FR with EQ.

 

Without smoothing EQ the response is (+/-) 4.5dB. With EQ, (+/-) 2dB, 20-200 Hz with crossover from subs to mains at 100 Hz.

Can you explain how the input signal is "grossly distorted" beyond what the EQ is supposed to do?  This is a bit of a loaded question I guess because there are many different EQ solutions, some implemented more competently than others.  My MiniDSP 2x4 behaves a bit erratically if I try to EQ too low in frequency (< 25 Hz or so) because of it's limited numerical precision.  OTOH, I expect to be able to implement said filters without problems in my OpenDRC-AN (ok, I haven't actually tried it yet to confirm), which uses 32-bit float computation, the same as my simulation software uses.

 

Does your response plot use any smoothing?  When I measure my sub response, I use high resolution measurements and no smoothing at all.  Assuming these figures don't include any data smoothing, I would say +/- 4.5 dB is already pretty good.  On the other hand, if the plot uses smoothing, then +/- 4.5 dB could mean anything, depending on what smoothing was used.  It could easily be covering big peaks and/or deep nulls that are indicative of time-domain problems like ringing.

 

I would expect the subjective improvement going from +/- 4.5 dB to +/- 2 dB (in the unsmoothed response) to be fairly minor and difficult to distinguish with a lot of material, but I'm sure you could find something where it makes a difference.  For example, it's common in some music genres to use bass with strong 2nd order harmonics so that if it gets played on systems with poor bass response, one can still hear the bass line, albeit an octave higher.  With such music, notes in the 50-60 Hz range where you have a dip in your non-EQed response may be masked by the stronger 2nd order harmonics, even though the dip there may have a negligible impact with most other content.

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Since practical installations often have differences in sound that naturally fall into those 3 categories, this is how I choose to see bass. 

A system can have a flat freq resp, and still lack a precise, tactile hit on transient attacks.

No, it can't if the phase is zero or linear.  A system with a flat frequency magnitude response and zero phase will have a perfect transient response.  If the phase linear is linear instead, then it will have a perfect transient response except for a constant delay that depends on the slope of the phase.  This is a mathematical requirement.  If you see any contradiction to this behavior, then either the calculations are in error or the system you are measuring is exhibiting significant non-linear distortion.  (Note that whether a system is linear or non-linear has nothing to do with whether it is linear phase or visa versa; the two uses of the word linear mean completely different things.)  Of course, no real life system has either a perfect frequency response (from 0 to infinity Hz) or a perfect transient response, but the closer you get to flat (with linear phase) over a particular frequency range, the more correct the transient response for sounds at those frequencies will be.

 

This is because the systems are not minimum phase, and because there are differences in the produced acoustic sound field.

 

See above.  If the frequency response is flat including zero phase, then the transient response is perfect and is, of course, minimum phase.  Sub + room systems are not minimum phase in general, but in many particular cases, the subwoofer + room response is minimum phase if you neglect the effects of crossovers, ports, and rolloffs.  Moreover, the non-minimum phase contributions to temporal response from crossovers, rolloffs, and some (not all) port designs are often much less than the temporal response distortions contributed by rooms, whether minimum phase or not, so minimum phase EQ can still fix (e.g.) 90-95% of the temporal response issues in many situations.  Of course, one should always measure to see just how close to minimum phase his/her response.  Programs like REW can be used to provide this information.

 

Tactile feel is determined by sound field properties.

Without intensity a sound wave can not have a physical effect on anything, and for intensity you need pressure and velocity.

What is less clear is exactly what properties are important in which frequency range, and how the relationship between sound field properties affect how we experience sound.

 

Can you please point me to a study that shows that tactile perception of bass correlates with sound intensity?  Not only do I doubt that such a study exists, but I also believe such a study would be hard to conduct because tactile preception is affected by so many other things that would be hard to control for.  Show me a study that correlates tactile preception of bass with sound intensity in a vibrationally inert environment, and I will consider changing my view on this.

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@SME, if you look at measurements of different system, I think you will find that even for those with a reasonably flat and smooth frequency response, there will still be deviations in the phase response.

 

If both phase and freq resp is flat, that would indicate that the impulse response is also covered.

Practical systems consist of parts that are not necessarily put together in such a way that the total response is a minimum phase system.

 

For information on what we already know about tactile feedback, there are several thread on avsforum covering the subject.

I believe there were some good point made in the bossobass-thread as well.

 

Tactile feedback is not simply about feeling the bass in your chest, or shaking the coach.

At the utter extremes, imagine headphones compared to a good, full-frequency range, full-capacity loudspeaker system.

 

On headphones, you can hear the bass, but there is no point in turning up the volume, especially at the lowest frequencies all you achieve is this pressure at the ears.

 

On the speaker system, however, that is a different world.

The very lowest frequencies can be felt as movement - from the floor, sofa, and perhaps pressure sensations in the ears.

As you increase the frequency, you feel the bass like a slightly subtle push on your whole body.

(It is this push that turns the music into a feeling of sound-waves that kind of moves through your body when you turn it up - the louder, the more insane it gets.)

Then around 30-50hz you get a solid punch from the biggest drums, and this sensation from transient attacks continues further up in frequency, 80-120hz is often mentioned as the chest-impact-range.

 

Perhaps others could fill in on my attempt to describe this tactile feel, it is not always easy to describe sound sensations, and others may have different experiences.

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I think nearly all 'tactile feel' is mainly resonant in nature, unless SPL is high enough to literally be hit by a 'blast wave' of bass.  Chest and abdominal cavities, furniture, floors, walls, all contribute to the sensation to a certain extent.

 

Dom,

 

How did you isolate the accelerometer to do your measurements?  Concrete pier or cinder blocks?  If the accelerometer was on a piece of furniture or anything else that could resonate at the freqs tested, you may have just been measuring sympathetic resonances. 

 

SME,

 

Do you use REW with HDMI out to DUT?  If so, what do you use for your loopback reference for phase/timing measurements?

 

 

JSS

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I borrow this post from the bossobass-thread:

 

 

I would like to hear what you guys think of tactile feeling from 50-500Hz, where small room effects rob us of so much SPL/SIL/Impact.  The most tactile movie presentation I ever heard was TF2 at this theatre:

 

CanaTheater9_070909.jpg

 

An old small town Texas theater that was completely renovated and THX certified.  'Punch in the chest' was a very mild way to put it.  More like 'Mule Kick to the chest'.  I have never felt more impact save for at hearing-damage-loud large outdoor concerts.  I know the response did not dig below 25Hz, but it was impressive nonetheless.  So many HTs simply cannot do this, and it may be due to room size and the inherent cancellations therein.

 

Thoughts?

 

JSS

 

Now, this is what we want.

 

I think 2 things are crucial - 1. capacity, 2. impulse response.

To get the first, it is important to notice that this requires capacity from mid bass and up, having long-throw drivers with heavy moving mass simply will not cut it.

Calculate and find what you think is necessary, and double up..

 

My tip for impulse response is to try to adjust front delay so that fronts and subwoofer system creates the best step response, with the highest possible amplitude on the first peak, you can also get indications from the GD and phase - flat is good.

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I generally begin bass setup by placement of subs for best FR at the LP, trying to cancel the most modes possible.  While there are several theoretical approaches, nothing beats experimentation in your own space.  REW has a tremendous room simulation tool, where you are able to change delays and relative levels of multiple subs in a rectangular room.  Then try some of the best solutions and see which ones work best.

 

Once a good placement has been figured out (while experimenting with acoustic treatment for upper bass problems that cannot be solved by placement), integration with mains/surrounds takes place.  Measure, measure, measure, adjust positions, delays, move or place more/less room treatment.  EQ is the LAST thing you do.  LAST.  I cannot stress this enough.  If you have not exhausted ways to solve your FR problems with placement and treatment, EQ is not the panacea it is advertised as.  While mixed-phase correction can correct some problems that room treatment can also address, it can usually only do so at one listening position.  Correcting a specular reflection with EQ is not the way to go about it, treat the room instead.  I see people recommending 'fixing' crossover problems in speakers with FIR EQ filters.  Nonsense.  FIX THE CROSSOVER IN THE SPEAKER, DON'T ADD MORE COMPLEXITY AND ADD MORE TO THE SIGNAL CHAIN AND NOISE FLOOR TO FIX IT.  Fix the root problem.

 

But I understand why people would opt not to fix the root problem, when adding EQ will provide a band-aid.  It's easier than taking a speaker apart or going active and needing more amplification.

 

So many folks are constrained by WAF and such that they expect that they can place subs/speakers anywhere with stained concrete floors and back wall 6" behind the MLP and 'Audyssey/YPAO/Dirac/whatever will fix it'.  Not the proper way to approach things if a good sonic presentation is needed, IMO.

 

What is so strange is that even with so many ways to 'disguise' room treatments, most people still scoff at the idea, almost as if it is taboo to mention it.  

 

 

JSS

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Then around 30-50hz you get a solid punch from the biggest drums, and this sensation from transient attacks continues further up in frequency, 80-120hz is often mentioned as the chest-impact-range.

 

Perhaps others could fill in on my attempt to describe this tactile feel, it is not always easy to describe sound sensations, and others may have different experiences.

I always like this image for this purpose :)

 

post-1440-0-36792100-1428359357_thumb.jpg

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@maxmercy, many good points, and I agree, this is basically what I do also.

 

Start with subs up front, then place more to fill nulls if needed, and certainly the REW sim is very useful for this.

I think the most important is to fill the nulls, if you have holes in the response, no eq can fix that, and if you try, the result will be disaster.

 

I use eq on the subwoofer system, but only to take down peaks, and shape the overall response.

Main speakers have no eq, no room correction.

It is not needed when room acoustics are fixed to a reasonably decent level, and the speakers are born with flat on-axis response and decent directivity.

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Can you explain how the input signal is "grossly distorted" beyond what the EQ is supposed to do?  This is a bit of a loaded question I guess because there are many different EQ solutions, some implemented more competently than others.  My MiniDSP 2x4 behaves a bit erratically if I try to EQ too low in frequency (< 25 Hz or so) because of it's limited numerical precision.  OTOH, I expect to be able to implement said filters without problems in my OpenDRC-AN (ok, I haven't actually tried it yet to confirm), which uses 32-bit float computation, the same as my simulation software uses.

 

Does your response plot use any smoothing?  When I measure my sub response, I use high resolution measurements and no smoothing at all.  Assuming these figures don't include any data smoothing, I would say +/- 4.5 dB is already pretty good.  On the other hand, if the plot uses smoothing, then +/- 4.5 dB could mean anything, depending on what smoothing was used.  It could easily be covering big peaks and/or deep nulls that are indicative of time-domain problems like ringing.

 

I would expect the subjective improvement going from +/- 4.5 dB to +/- 2 dB (in the unsmoothed response) to be fairly minor and difficult to distinguish with a lot of material, but I'm sure you could find something where it makes a difference.  For example, it's common in some music genres to use bass with strong 2nd order harmonics so that if it gets played on systems with poor bass response, one can still hear the bass line, albeit an octave higher.  With such music, notes in the 50-60 Hz range where you have a dip in your non-EQed response may be masked by the stronger 2nd order harmonics, even though the dip there may have a negligible impact with most other content.

 

 

This is just pulling down a peak at the seats using digital PEQ:

 

cd3989bdba6a360acd536db1a3aaed66.png

 

No smoothing on the traces, except for the close mic with no PEQ, which is 1/6 octave smoothed.

 

The EQ was simple enough to apply and if there were no side effects (mainly, in this case, loss of low end response from the unit's roll off) and if everyone would have voted the EQ'd version to be superior, I would prefer the flatter response, no doubt. Alas, no one noticed the difference as being an improvement and the first octave of response fell by over 6dB.

 

Yes, (+/-) 4 or 5dB is pretty good, accomplished without input signal manipulation. That was my point. Most people do not labor over the first steps and simply hit the auto EQ button without knowing how grossly distorted the input signal is. I say grossly because most people have a plunk-and=play response that requires far greater signal manipulation than my example shows.

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Interested in your thoughts here on the SIL/PVL discussion. This is not what I've observed, but would certainly like to hear a different perspective.

Not only do I believe that SIL/PVL are not relevant, but I believe there are far better explainations for why tactile preception differs between systems and why near-field placements often provide tactile perception for the same SPL.

 

Tactile preception of vibration and sound is a very complex subject, one that is probably even more complex than hearing.  Here are some lecture notes on the subject of tactile sensation including vibrotactile sensation.  Our body is capable of sensing vibrations in different parts of our skin, in our muscles, and even in some of our internal orgrans.  We can also hear vibrations in our bones that are transmitted directly to our ears.  Each of these sense modalities is integrated along with our hearing to form a perception.  Each sense likely contributes a different character to our perception, but at the same time sensation via one modality can mask sensations from other modalities.  All of these things affect what we hear and feel in very complex ways.

 

One thing we should all be able to agree on is that tactile perception necessarily requires vibration to be transmitted to the skin or through it deeper into the body.  Vibration is most efficiently transmitted into the body via direct contact with a vibrating solid surface (or liquid surface, if you like to get your bass on in the swimming pool), but transmission directly from the air to the body is also relevant.

 

Let's consider vibration transmitted through solid surfaces first.  The two surfaces that are most likely to transmit vibration to you are the floor, if your feet are in contact with it, and your seat.  All else the same, the stronger these vibrations are, the more bass

you will feel.  One transmission path for these vibrations is between the subwoofer, the floor, your seat, and your body.  Most subwoofers produce mechanical vibrations in addition to sound because they contain moving masses.  It is possible to reduce or eliminate this transmission path by using a dual-opposed woofer configuration or by mechanically decoupling the subwoofer from the floor.  It can be difficult, however, to mechanically decouple the lowest frequency vibrations between the sub and the floor.  Even if the subwoofer is perfectly isolated from the floor or avoids vibrating entirely by using a perfectly balanced dual-opposed configuration, vibration will still be transmitted to the floor by the sound itself.  Generally, the higher the SPL of the sound, the more vibration will be transmitted, so the greatest transmission will happen at the edges and corners of the room and especially at any surfaces near to a woofer.  A balanced or decoupled down firing woofer will still transmit plenty of energy to the floor simply because part of the floor is very close to the woofer where the SPL is very high.

 

Now let's talk about why you are more likely to feel vibrations with a sub placed close by than one farther away.  First of all, the level of vibration in the floor and other room surfaces decreases with distance from the source.  This is true of the sound coming from the sub also.  However, whereas the sound is constrained by your room boundaries vibrations in the floor and walls are free to travel to adjacent parts of the structure.  For this reason, it can be expected that the reduction in vibration level with distance for a floor is more substantial than the reduction in SPL with distance through the air.  Another consideration is damping.  Damping is the phenomenon by which mechanical energy is dissipated as heat.  Air damps sound very poorly except at the highest frequencies.  Damping in solids on the other varies by material, and some materials can damp bass energy fairly well.  The reason damping is important is because it reduces how much vibration is transmitted through solid surfaces over a distance.  If your subwoofer is located relatively far away, then the vibration that is transmitted into the floor around your subwoofer is dissipated a lot more before it reaches your feet and your seat than it would be if your subwoofer was located close by.  I believe these are the most likely reasons that close sub placements yield more tactile stimulation.

 

If you listen with your feet off the floor and you mechanically decouple your seat from the floor, you may be able to further reduce the amount of vibration reaching your body.  However, vibration will still transfer directly from the air to the seat itself.  Suppose you instead listen while standing up on a very heavy and firm surface (like the earth), then you will only feel vibrations that are transmitted directly between the air and you and your clothes.  This can and does happen, and its impact is not at all limited by the distance of the transducer to the listener, as long as the SPL is high enough.  Most likely this is the phenomenon at work for JSS when he felt the "mule kicks" at the movie theater in Austin.  I personally have felt some great bass kick from large scale outdoor sound systems at distances of 1/4 mile or more.  I think all of us have felt the bass from fireworks even from miles away.

 

I have a hypothesis that this air-to-body vibratory transmission is most impressive when the response is fairly flat, particularly in the 60-120 Hz region.  (A slight bump there relative to the other frequencies seem to hurt much.)  As I already aruged on the BossoBass thread, my hunch is that the flatter response delivers the energy of a transient more compactly in time leading to a higher peak SPL and a stronger perception.  I may be wrong here, and I will admit that I have felt quite a bit of "punch" from e.g., ported systems with excessive ringing at their port frequencies that otherwise sound like crap.  On those ported systems, just about anything that punches feels about the same and makes a similar drawn out "oooooph" sound as it happens.  On the other hand, while flatter systems may not hit as hard, they seem faster, they differentiate different sounds (i.e., different kick drums), and they are more likely to kick in general.  With a nice flat system, the kick is often felt strongly even though the sound is barely heard.

 

So if my hypothesis about a flat response is correct, then another good reason that close sub placement yields better kick is because it's response in that critical 60-120 Hz region is likely to be more smooth.  The response is likely to be more smooth with the close placement because the direct sound is (relatively speaking) greater than the reflected sound from the boundaries.  Unfortunately, close sub placements often perform worse at lower frequencies because the wavelengths are so large that the room response overwhelms.  Unless you can put your ears within inches of the woofer, you are still hearing mostly the room at the lowest frequencies.  For that reason, I use mid-bass subs placed close to my seats in conjunction with dedicated deep bass subs located at the front of the room.  I actually get *stronger* tactile response in the deep bass with my subs placed at the front of the room than I did when they were placed close by because the sub/room coupling is more efficient overall.  For me, the setup use gives me the best of both worlds.

 

Now, let me go back to the subject of vibrotactile sensation via transmission from the seat or floor and the issue of non-flatness of this response.  At one point in the past, I experimented with installing decoupling feet on my close-placed mid-bass subs.  I chose a design that isolated most vibrations above 50-60 Hz or so, which is close to the end of their range.  After installing the feet, I was able to roughly verify that they worked as designed by doing before and after SPL measurements.  After installing the feet, I lost about 1 dB SPL overall starting around 50-60 Hz, presumably because the floor was no longer radiating this extra energy into the air.  The subjective change that resulted from doing this was quite dramatic.  I definitely noticed a lot more of the "chest kick" type of feeling I associate with large scale sound systems where I am standing on concrete and too far away from the transducers for solid surface vibrations to significantly contribute to the sound impression.  My experience strongly suggests that the vibrations felt through solid surfaces can overwhelm and mask and those felt through the air, and that the poor transient response of the solid surfaces can substantially degrade the quality of the overall perception.  However, I also did not prefer the sound in the end result, nor did my wife.  I thought the bass sounded very quiet, distant, and poorly defined.  Turning up the sub level 6 dB brought the subjective level back up, but it still sounded distant and poorly defined.  What happened here?  It took me a while to work it out, but I believe the biggest reason we preferred the decoupled sound less is because my in-room upper bass response was very poor with severe dips throughout.  What I was hearing with the decoupling was a total loss of upper bass, even though the measured SPL response only dropped by 1 dB.  The vibrotactile sensations from the floor and sofa were contributing to the preception of the audio and filling in the dips of my in-room response.  It was quite remarkable to realize that much of what I thought I was hearing, I was actually feeling, but my brain was melding the perceptions together.  Now that I've added bass traps and have ditched Audyssey for my own EQ solution, my upper bass response is worlds better.  I still have a ways to go, but once I'm satisfied with how flat it is, I'll probably give decoupling the mid-bass subs another shot because it really did help with the chest kick sensation.

 

From this exercise, I learned that the importance of vibrotactile sensation to bass perception cannot be ignored.  In the vast majority of listening rooms these sensations likely make a considerable contribution both to what we hear and what we feel.  As such, I believe that controlling and equalizing vibrotactile response is an important frontier in the endless quest for better audio.

 

Here is a test I performed using an accelerometer to measure tactile feel close to the subs, and further away from the subs using a 15hz sine wave. This test demonstrates that SPL alone is not the sole contributor to tactile feel. Give this equation: Sound Intensity = pressure * particle velocity, the test demonstrates in this example that PVL must be the differentiating factor to the variation in tactile feel.

 

http://www.avsforum.com/forum/113-subwoofers-bass-transducers/1488059-your-home-theater-ulf-score.html#post23676269

 

I'm sorry, but I don't see any evidence in that test that sound intensity has anything to do with tactile sensation.  You have most certainly demonstrated (even using the rather crude accelerometers in your phone) that solid surface vibrations are increased with your sub placed close by.  I give a very reasonable alternative explaination above for why this occurs.  There is no need to consider the sound intensity or particle velocity to understand this.

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@SME, if you look at measurements of different system, I think you will find that even for those with a reasonably flat and smooth frequency response, there will still be deviations in the phase response.

 

If both phase and freq resp is flat, that would indicate that the impulse response is also covered.

Practical systems consist of parts that are not necessarily put together in such a way that the total response is a minimum phase system.

 

I'm not trying to argue that system/room responses are 100% minimum phase.  However, my experience is that it is often possible using strategic placements and delays to achieve nearly 100% minimum phase behaviour over the sub bass range, and in these instances, a nearly flat frequency response will also have a nearly ideal temporal response.  Certainly it's enough to make an 80-90% improvement in practice in many cases.

 

Rather than argue about whether this actually happens or not, I suggest you measure your own in-room subwoofer response and use a tool like REW to compare your response with the ideal minimum phase response having the same magnitude.  If your subs are well placed and have delays configured so that they play in phase with one another, there's a good chance your room response will be nearly minimum phase.  If not, then you may want to consider adjusting the placements and/or delays to try to achieve this, because if you can and if you have enough headroom, you can EQ with minimum phase filters to a very nearly flat response in time and frequency.

 

Why leave all this potential performance on the table?  The only reason I see is ignorance and confusion over how this stuff works.  Unfortunately, the vast majority of what I read about audio systems on forums and the internet as a whole is just plain wrong.  This is partly due to the fact that this subject is very hard to understand.  Even engineers who specialize in the field struggle.  The other reason is the simple fact that technical correctness rarely helps sell anything, and people are very easily influenced by placebos.

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