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Kvalsvoll

Bulding the Room2 listening room

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What are relevant and useful specifications for a subwoofer.

A complete set of measurements, showing frequency response, capacity and distortion, is sufficient to tell how a subwoofer will perform. Then you can see how loud it can play at different frequencies, which is what you need to know for system design, you can see how low it can play and how loud. The graph also gives an indication of usable frequency range upwards.

However, most customers don't really want to see lots of measurements, they do not understand what those graphs mean, and they acknowledge that fact.

For the new Compact Horn subwoofers I did this:

5a91b333b7e84_V110speccs2.png.c565c8ff56b5b519a82a3ea07c14b155.png

The output capacity number and the frequency range gives the necessary information. You want to know the output capacity to be able to dimension your bass-system, and you want to know the usable frequency range to see if it reaches low enough and covers all the range up to the desired crossover.

The less tech-oriented customer still does not make much sense of the numbers, and is more likely to go by what I recommend. That's fine.

The tech-experts needs to be educated on the meaning of those numbers, because they make no sense to them since they are different from what other manufacturers typically publishes.  They don't recognize the meaning of Output capacity, and the frequency range is not the same as frequency  response with specified tolerance limits. This is labor-intensive - requires lots of time and effort to educate and show. Perhaps these customers should be ignored - it's really a question of effort vs. value. One solution could be to make additional specifications and measurements available, so they can see exactly what the performance of the subwoofer is. The graphs still require some explanation.

(The real experts usually get it, so they don't need any more education. They may ask for measurements, if they want more exact information.)

Typical subwoofer specifications are useless. They say nothing about capacity, frequency range specifications are at best unreliable. One English manufacturer speccs a small egg-shaped subwoofer with two 8" drivers as "7.5Hz" - clearly very, very far off from reality. Another manufacturer makes a hairdryer with two 6" or close to that drivers, claiming "14Hz" - I have heard it, and there is no way to get anything useful out of it at that frequency, from what i heard, it struggled hard to do normal bass frequencies. 

Capacity is important to know because this tells how loud the subwoofer can play in the room. This is the number you use to determine how many units you need to achieve your desired spl at the listening position.

Frequency range is the usable range - how low it can play at still somewhat useful output level, and how high up you can set the crossover. For a subwoofer, the frequency response is largely irrelevant, you only want to know the range, and as long as the subwoofer is designed for high sound quality the response will be smooth between lower and upper limit. If the curve is flat or tilted or in some other shape does not matter, because the in-room frequency response will be dominated by the room, and will need adjustments in dsp for optimum performance.

----------------------------

Since the real experts are on data-bass, this is the place to ask for opinions on this - how to specify subwoofer performance.

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I like your format. Even though the room will dominate in the actual measured response, outdoor GP is the best measure, since all devices are on an equal playing field.

A 'FAQ' or frequently asked questions section on your website can answer a lot of the same repetitive questions that can tie up your time replying to inquiries. Well informed customers can then email you for additional information on your products. 

 

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10 hours ago, Ukko Kari said:

I like your format. Even though the room will dominate in the actual measured response, outdoor GP is the best measure, since all devices are on an equal playing field.

A 'FAQ' or frequently asked questions section on your website can answer a lot of the same repetitive questions that can tie up your time replying to inquiries. Well informed customers can then email you for additional information on your products. 

 

FAQ is a good idea. Easy to add subjects.

Those interested enough to consider buying use email. For questions and comments people can use the facebook page, it has both personal conversations and public comments. The blog posts has a comment option.

I am now making a blog post on those specifications, basically the same text as above here, and I will add some examples and measurements to show how this works in practical situations.

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I agree with much of what you say. I've thought about this for a while.

Frequency response of subwoofers should have an output rating attached with it to keep things honest. Otherwise you get things like tiny 10" powered subs claiming 14Hz extension and other nonsense like you mentioned. For subwoofers I would like to specify a "useful" range and not worry too much about +/-3dB or whatever. What I do worry about is how much output a subwoofer is capable of. In my opinion this must be attached to the FR rating. These days almost anything can be equalized flat to very low frequencies. Who cares if it gets flat down to 14Hz if it can only produce 75dB?  The way this would be kept honest is by attaching a detailed output rating to the lowest useful extension claimed.

 Too much detail is overload, but not enough allows specsmanship, or gaming the system. Balancing the 2 is tricky.

The output rating itself has to have all the details necessary to make it comparable and honest. It must note whether the output is calculated or measured, microphone distance, whether it is a peak reading and whether the environment is full, half, quarter-space, etc. Distortion such as via Don Keele's tone bursts would be nice, but not required. Also absolutely necessary is the frequency used for the output rating.

This is what I would like to see used. It keeps things as simple as possible, but provides the necessary detail for some sort of quick comparison.  The attached measurements are from the dual opposed 24" sealed cube with 21Ipal's and a SP1-6000 amp as an example. There is a raw FR and the distortion limited and maximum output bursts. I'm using this one as an extreme example since it is sealed and has ridiculous upper bass output which skews the raw FR by a huge margin. Clearly no one would run this system sans EQ. Almost any bass system these days will employ EQ for response shaping. Certainly any active / turn key sub system for sale. It would be quite easy to make this response shape flattish to an arbitrary low frequency. Say 20-150Hz +/-3dB for example. Let's call that the FR rating and assume that was done. There will be a limit to how loud the sub can be pushed before it starts to limit the low frequency output and the response shape is no longer maintained. In this case the all out max burst of the sub was measured at 113.7dB at 20hz. I'd like to see the output rounded to the nearest 0.5dB so as not to get into splitting data points of less than 0.5dB between units, so let's call it 113.5dB. I'd call this "minimum output" or something like that. Many subs will also define a useful frequency range which is a bit looser (sometimes +-10dB) and beyond the tighter FR spec. This being a rather high output sealed sub we could reasonably call the useful lower FR limit as 10Hz. Let's call the "useful FR" 10-150Hz. That being the case we should also specify the max output at 10Hz since it is claimed to be useful. That would be 102.2dB all out with no THD limit. Rounded down to 102dB. A maximum in band output would also be useful. as we all know it's much easier to make something really loud at 150Hz than at 15Hz. Personally I think all subwoofers should be rated at 63Hz or lower for maximum output. 63Hz is a legitimate bass frequency which will avoid cheating and using peaks in output up above 80Hz (or even an octave higher) for a "sub". If the sub has peak output at a lower frequency great, however the DB the measurements show that to be a very rare sub indeed. 63Hz is also high enough in frequency that any cab calling itself a "sub" should be able to hit it in bandwidth with power. In the case of this sub we would use the 63Hz burst which measured an even 130dB. 

The ratings for this sub would look like this.

Frequency Response: 20-150Hz +/-3dB / Half-space / 2m (assuming after DSP of course) 

Minimum output with rated Frequency Response: 113.5dB SPL@20Hz / Half-space / 2m / Measured

Useful Frequency Range: 10-150Hz

Useful Low Frequency Output: 102dB SPL@10Hz / Half-space / 2m / Measured

Maximum Output: 130dB SPL@63Hz / Half-space / 2m / Measured

 

 

Example FR.jpg

Example Max output.jpg

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@Ricci, I should comment on your excellent post, just too busy with the new subwoofers. If all subwoofers were specified the way you suggest, it would be possible to see and compare models, and decide whether capacity and extension meets requirements.

Set up a system with 4x V110 in Room2, to test it, pictures on facebook (kvalsvolldesign).

Easy to compare going from 1 to 2 to 4 units, with dsp presets ensuring similar frequency response and level - only capacity and sound field intensity/velocity are different. Interesting.

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Going from 1 to 4 units, where are they located in Room2, and how does the placement affect SFI/Velocity?

JSS

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@maxmercy, 4 units stacked (2 units stacked) near corners on front wall, so you get these systems:

1x: FL

2x: FL FR

4x: 2xFL 2xFR

Let's see if it is possible to get a picture:

28378461_934250440067469_376461996637268

28660769_934250510067462_643125018232461

I managed to take a full set of measurements before taking down the system. And they show that the systems have different sound field properties - but which one is better..

The 1x is all over the place, the 2x removes sideways velocity, the 4x removes both sideways and up-down.

Yes, the 4x is better, but at any sane volume, it is hard to justify the double cost compared to 2 units. In a different room, or larger room, the situation may be very different. The 1x holds up surprisingly well, but it does not have enough capacity for that wall-of-sound feel that the 2x and 4x certainly has.

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Interesting findings, form factor for those subs is good.  Do you think there is a correlation to room mode cancellation and velocity measurements?

The front wall looks like it has absorption with a 1D slat diffuser in front, is that new?

JSS

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Your room looks good. What are the dimensions? I can't tell if your ceilings are high or the width of the room is small. 

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It's a very narrow room.  Those are 12" woofers, so you can kind of eyeball the width as maybe 8-10 feet.  I don't recall, but I think the room stats were discussed earlier in this thread.

On 3/5/2018 at 7:45 PM, Kvalsvoll said:

The 1x is all over the place, the 2x removes sideways velocity, the 4x removes both sideways and up-down.

Yes, the 4x is better, but at any sane volume, it is hard to justify the double cost compared to 2 units. In a different room, or larger room, the situation may be very different. The 1x holds up surprisingly well, but it does not have enough capacity for that wall-of-sound feel that the 2x and 4x certainly has.

So with 4 subs, almost all the velocity is in the front-rear dimension?  What do the absolute magnitudes look like in each case?  I.e., the square root of the sum of the squares of velocity in each dimension?

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5 hours ago, Ricci said:

Your room looks good. What are the dimensions? I can't tell if your ceilings are high or the width of the room is small. 

3.35m wide, around 4.6m length - before adding the 20cm front wall absorption.

So this is a small and quite narrow room. It doesn't feel cramped, there are openings on back wall and right wall, and having all walls white i suppose helps.

But sound doesn't care about wall color, especially the width is challenging - little room for treatment to fix it, and since it is narrow, the side wall reflections hardly do anything good.

Turned out quite nice, sounds reasonably good, but this small space requires treatment to work for high quality sound reproduction. Mind that quite many will find themselves in a quite small room, or constricted to a smaller part of a larger living room, for the music and sound.

It is often better to use the longer wall as front wall - turn it 90 degrees - in a small room of such proportions. Then you would sit close to the back wall, which causes its problems, but also the advantage of not having to deal with back wall reflection in the bass range.

There are some pictures earlier in this thread, showing the acoustic treatment with absorption and hard back with poly diffusors.

 

 

 

 

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10 hours ago, maxmercy said:

Interesting findings, form factor for those subs is good.  Do you think there is a correlation to room mode cancellation and velocity measurements?

The front wall looks like it has absorption with a 1D slat diffuser in front, is that new?

JSS

Front wall has been like this since the room was built, it is 20cm Rockwool A with some slats, designed to work from around 100hz and up, which it does.

There is a correlation, yes, but in this room, for this set-up, the frequency response is very similar for all those configurations. This is because the huge cancellation problem is a LENGTH reflection, it must come from the corners on the back wall. I did not believe that was the case, as there is a very large opening on the back wall, but those measurements show that a more plane wavefront from the front actually causes the cancellation dip at 60hz to be larger.

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1 hour ago, SME said:

It's a very narrow room.  Those are 12" woofers, so you can kind of eyeball the width as maybe 8-10 feet.  I don't recall, but I think the room stats were discussed earlier in this thread.

So with 4 subs, almost all the velocity is in the front-rear dimension?  What do the absolute magnitudes look like in each case?  I.e., the square root of the sum of the squares of velocity in each dimension?

Yes, with 4x the measurements show that most of the velocity is head-on, as in a plane wave. It is also interesting to observe that the magnitude in the 0-direction (head-on, normal to the front wall plane) is the SAME for all configurations - it does not increase when sideways and up-down velocity decreases.

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How does the relative impact at a certain SPL change when velocity components are also coming in from the sides and from above/below in the smaller configs?

JSS

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@maxmercy There is not much correlation between velocity and spl.

For the 1x config the spl looks like it follows the sideways velocity to some degree in the range where the 1x config partly fills in the cancellation dip.

Velocity in 0 direction (normal to front wall) has a small dip where the cancellation occurs, but this dip in velocity does not follow the spl exactly. Especially in the 4x config the velocity is present where the spl drops off to 0. This shows that velocity and spl must be out of phase here.

Just measured 3 V110 units. They are reasonably consistent, and matches the design sim quite well. Nearfield measurements at different locations in the horn mouth, and inside the horn path. I don't have any useful outdoor measurements, tried to measure the first one while moving it from the workshop, but with temp below freezing and windy conditions this was hopeless. It is also difficult to measure properly, due to the size of the radiating horn mouth - it is too long for 1m measurements, and the outdoor space is not large enough for good measurements at longer distances, may have to go up to around 4m to get correct results on the V110. Nice to see the smooth response across this wide frequency range, the rather complex internal construction with 4 damping chambers actually works.

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So if the listening position is approximately half-way between the two side-walls and approximately half-way between the floor and ceiling, it makes sense that you see a lot of velocity along those dimensions with a single sub.  The 1st order room mode contributes pressure peaks at each boundary and a pressure null in the middle.  For velocity, the reverse is true with a velocity peak where each pressure null is.  When you place sub(s) at opposing boundaries across a particular room dimension, they cancel the mode, leading to a much more even distribution of pressure and near elimination of the velocity vector in that dimension for those frequencies.

More generally, where standing waves dominate response, the particle velocity at a point in space is usually proportional to the spatial-gradient of pressure, i.e., the rate of change of pressure with respect to spatial location.  (This is more or less the same as saying that they are separated in phase by 90 degrees because [math alert] the derivative of a sine is a cosine and the derivative of a cosine is a negative sine, and so on.)  The pressure gradient is actually highest in the nulls (it quickly rises in either direction), and velocity peaks here.  In contrast, when approaching a rigid boundary, the pressure gradient in the direction normal (i.e. perpendicular) to the boundary tends toward zero as does the particle velocity.  Here's a visualization, from here:

StehendeWellenVeit.gif

Note that if you are concerned about sound intensity, it is essentially zero in both pressure peaks and nulls.  In fact, a key feature of standing waves is that, on average, no net energy is transmitted through space at all, so sound intensity (in the RMS average sense) is essentially zero everywhere.  Instead, the energy of the standing wave oscillates between potential (pressure) and kinetic (velocity) forms and sloshes back-and-forth between the pressure/velocity peaks and nulls in the process.

Of course, the situation changes in the presence of a listener, which acts kind of like a membrane bass absorber.  Sound intensity across the skin depends mostly on frequency (i.e. chest cavity resonance) and pressure because the impedance of the solid/liquid flesh is much higher than that of the air.

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@SME, inside a room the situation is complex, because boundaries are not rigid, there are windows, and sometimes openings, such as in Room2.

These velocity measurements are only another tool to try to get more information, to get a better understanding of what is going on.

Sound intensity and sound field properties affect perception of sound, we know that now. But velocity alone is only one part of this, and especially for higher bass frequencies - where chest slam occurs - it looks like intensity pressure is more important, due to the acoustic impedance properties of our body. To feel the sound moving your clothes, you need velocity.

Also, frequency response and phase behavior have huge significance for perception, both what we hear and tactile.

Just finished testing the V110 in the Moderate Cinema, to find out if it is suitable for movies. It is. But compared to the original T138 horn, it drops off a little below 20hz, above 15hz the V110 has more output, and should sound cleaner and more defined because there are no resonances left around the crossover, where the T138 no longer performs well in the time domain.

I didn't bother doing a proper calibration, did not even use the default dsp settings, but the freq response looked reasonably similar to the T138, so I just left it like that.

And does it sound better? Is it better to have this vertically large sound source? Does the effort put into the advanced design pay off in  better sound quality? 

It does sound different, but not necessarily better. I suspect my lazy calibration approach comes to play here.

The sub bass is experienced as similar, the level drop in the 10-15hz range does not seem to have much significance. The airplane-flyover-scene from Hanna sounds similar, Oblivion works fine, the storm in Kon-Tiki moves the whole house.

Mid-bass is stronger and more powerful. Nice punch, and there is capacity available to turn it up, bass-heavy music at +6dB works fine, and then you add another +6dB on the bass system for that visceral feeling.

Transients with large bandwidth lacks some of the precision and sudden impact. I blame this on my lazy calibration. Example - the cannons in Hunger Games.

All in all, my conclusion is that the V110 works for movies. Same powerful bass transients with impact, like you are hit by a small shock-wave.

Now I will look at the measurements to see if it is possible to see something there that corresponds with what I think I hear.

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Velocity measurements now available:

Sideways velocity (90 degrees):

4x-2x-1x-V110-v-90-768x328.png

Green is 1x configuration.

Vertical (h):

4x-2x-1x-V110-v-h-768x328.png

Red is 4X.

Normal to front wall (0 degrees):

4x-2x-1x-V110-v-0-768x328.png

 

Frequency response has a huge, narrow dip around 60-70hz. This must be caused by reflection from the back wall corners. This dip becomes progressively more deep and narrow as the sound field from the source gets closer to plane wave.

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On 3/9/2018 at 2:16 PM, Kvalsvoll said:

@SME, inside a room the situation is complex, because boundaries are not rigid, there are windows, and sometimes openings, such as in Room2.

These velocity measurements are only another tool to try to get more information, to get a better understanding of what is going on.

Sound intensity and sound field properties affect perception of sound, we know that now. But velocity alone is only one part of this, and especially for higher bass frequencies - where chest slam occurs - it looks like intensity pressure is more important, due to the acoustic impedance properties of our body. To feel the sound moving your clothes, you need velocity.

Also, frequency response and phase behavior have huge significance for perception, both what we hear and tactile.

Of course the boundaries of a real room are not perfectly rigid.  They are lossy, especially at certain mechanical resonance frequencies.  Your room is also open at the rear.  However the side-walls, floor, and ceiling are rigid enough that the sound field between those dimensions is qualitatively similar to what I described.  Pressure still peaks at the boundaries where velocity drops to almost zero, and there are likely dips at certain frequencies .  My point is that the effect of multiple subs on your velocity measurements is entirely consistent with this perspective.

I don't know what you mean by "intensity pressure".  If you mean merely pressure, then we are in agreement that pressure is the most important characteristic for transmission of vibration from the air into the body of the listener.

The proposition that the motion of clothing depends on velocity is interesting, but I suspect the situation is more complicated than that.  It may have more to do with pressure gradients, which may coincide with areas of high velocity as it does for standing wave sound-fields, but high velocity and high pressure gradients don't always present together.  It would be interesting to do some experiments with subs outdoors to see if clothing movement perception is affected by source distance, while pressure is kept constant.  In the far-field of a monopole radiator, pressure and velocity both drop with 1/R, but the pressure gradient drops with 1/R^2.  Thus, if what I suggest above is true, we'd expect less clothing motion at greater distances, even after compensating for SPL.

To your point that frequency response and phase are very important, I totally agree, but I would say it's a lot more complicated than most people think.  And of course, the sub range is only one part of the picture.  The rest of the speaker response also impacts perception a lot, and 100-500 Hz is particularly important for tactile sensation.  Unfortunately, this range is often harmed by speaker placement problems, but there may be ways to fix this with EQ.  This is work in progress for me, but I can say with confidence that a perfectly flat or smooth in-room response is not optimal unless the room is completely dead.  And if the room is completely dead, then you have another problem.  :)

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8 minutes ago, SME said:

but I can say with confidence that a perfectly flat or smooth in-room response is not optimal unless the room is completely dead.

This is the tricky part. If the solution is to just eq to flat, it would be easy. But it isn't.

Especially in the bass range, velocity and intensity matters. And you have to measure at different locations, at least cover different locations in height where the listener is located.

Decay is important. Resonances will affect perceived tonal balance.

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Kvalsvoll,

With all  4 configurations, (1,2,4), for equal SPL with each, which configuration had the greatest perceived impact?

JSS

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@maxmercy, yes, now, that is the question to ask.

Fact is, I did not notice any huge difference. Even the 1x is as good as should be expected, and the 2x and 4x does not really have that much more, though they sound a little bit more tight and immediate, 2x a little better, and 4x even a little more. But I can not say there was a difference large enough in tactile experience to really differentiate the systems.

The 1x hold up with bass-heavy music up to +3dB, enough for some tactile feel. But the larger ones can do much louder, and THAT makes a huge difference.

ULF below 20-25hz is similar, as long as kept within limits.

I still have 2x V110, so I can test some more, perhaps find a way to measure and get objective information.

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@maxmercy, I have never heard one with drivers mounted across the whole wall.

There are practical problems with SBA/DBA solutions - obviously. And - I may be scolded for saying this - I am not convinced they add so much more than a more conventional system can do, when properly set up and calibrated, and you can manage to somehow get rid of the cancellation reflections.

From the experiments when I first found that velocity has significant impact for low bass perception - especially in the 20 - 50hz range - I also had a set-up with very much of that powerful wall-of-sound feel, and part of this has to do with getting more velocity than you get in the steady-state free-field condition. Similar to what you can achieve with near-field subs.

A DBA done right will get predictable and very good results. Other solutions often end up with one part of the freq range having some special and very good performance, but then there are faults in other places. The trick is to achieve that special part, but at the same time be able to fix the bad parts.

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5 hours ago, Kvalsvoll said:

@maxmercy There are practical problems with SBA/DBA solutions - obviously. And - I may be scolded for saying this - I am not convinced they add so much more than a more conventional system can do, when properly set up and calibrated, and you can manage to somehow get rid of the cancellation reflections.

I agree that there should be no advantage to SBA/DBA solutions, provided that "you can manage to somehow get rid of the cancellation reflections", or otherwise remove the effects of the room using another method.  In both cases, one achieves very smooth (near-anechoic) bass frequency response (when viewed without using smoothing) across a wide region of space.

I don't have an SBA/DBA, but I do have DSP-optimized filters to achieve the above near-anechoic ideal.  This condition is maintained up to about 65-70 Hz, above which I have multiple issues preventing me from maintaining an completely smooth response.  This is probably an issue in most other types of near-anechoic sub systems, including SBA/DBA also.

To be honest, I wasn't expecting to have to go back and shape the broad response by ear the first time, and I was even more surprised when I had to shape the broad response *in a different way* after I re-did it with new crossovers.  More recently, I did an EQ overhaul of my mains speakers above 160-200 Hz only, and I had to re-do the broad shape of the subs *again*.  These weren't minor changes either.  Whereas before, boosting below 40 Hz even 0.25 dB caused the bass to go to mud, with the latest config, I was able to boost the low end by several dB without loss of intelligibility.

So at this point, I'm not convinced that any particular way is better than another.  If anything, my work with mid and high frequencies has convinced me that some early reflections are better than none, provided that the speaker is well-behaved.  The main reason so many pros are convinced that early reflections are bad is because they are experienced with using monitors with poor off-axis response, and the elimination of early reflections prevents that nasty off-axis sound from corrupting the better on-axis sound.  With good speaker design, that's no longer an issue.

It does lead me to wonder if maybe early reflections are good for bass too, to a point.  It appears to be well established that modal resonances have negative perceptual consequences, but do early reflections have as much impact as we think they do?  We practically hear through early reflections for mids and highs.  We may also do so for bass, but it probably depends on how much reflected sound energy there is in total and how well it is distributed in time.  At lower frequencies, there is definitely a propensity for the sound field to become quite structured within the room, even if discrete modes are not obvious.  This is a substantial open problem that I plan to devote more time to in the future.

I would strongly caution anyone not to read too deeply into frequency response data.  Because we hear pitch (frequency) and level, it is easy to assume that a frequency response (FR) plot tells us how different pitches will be emphasized, relative to one another.  However, this is far from the truth.  An FR plot with smoothing is largely meaningless because the smoothing discards most of the information that's relevant to perception in the first place.  An FR plot without smoothing and with phase data does contain that information, but it is a terrible visualization of that information.

Time-frequency plots like waterfalls and spectrograms are kind of a step in the right direction, but it is hard to glean quantitative information from them and the information within them is still not weighted very close to how perception weighs the information.  All of this makes sense if you imagine what it would take to analyze an IR to determine the true spectrum of a source within the room.  One must deal with a variety of acoustic interference effects and possible obstructions in the path of the direct sound.  It takes some very clever processing in order to accomplish this with the accuracy that our ears and brain do.

As can be seen, I'm rather short on good advice here.  I am less confident in what I know about bass reproduction than I ever have been, having tried a variety of strategies and having failed, in the sense of not achieving any consistency.  And that's just in one room.  And this is yet another reason why I am very skeptical of the relevance of particle velocity, independently from pressure.  Why?  Because there's so much we don't understand about how pressure response affects perception.  Unless or until one constructs an experimental apparatus in which velocity response can be varied while keeping pressure response *exactly the same*, no one has proven anything with regard to the relevance of velocity response.  In practice, this kind of test is extremely difficult to do.  Almost anything that changes velocity response will change pressure response in some way.  "Close" does not cut it here.

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