Bulding the Room2 listening room

[Kvalsvoll]

What is going on between 200 and 700Hz?  Lots of suckouts there.

 

JSS

 

Unfortunately those reflection cancellations have to be fixed, and that will not be easy.

Some of this is caused by the chair in the lp, but if you look at the graph of the freq resp, one of the measurements is with and the other without this chair, so that does not explain everything.

The dip between 1k-2k is the floor, and that one is easy to fix.

[SME]

It may be better.

But the acoustic properties of the wall behind will have greater impact.

 

This is a problem mainly for surround and center speakers, because they are meant for placement against a wall.

The problem is worse for wall placement because the distance between the front baffle and the wall is so small that the cancellations and peaks will end up far into the midrange frequency range.

 

For the S1.2 surround speakers the shape of the cabinet gives a smoother transition to the wall boundary, for the C1 center I designed the baffle a little wider.

 

I'm arguing that the slanted extensions also reduce the negative effects of the wall.  Think about what happens as the angle between the extensions and front baffle approaches 0 degrees.  The smaller the angle, the more it looks like it's just a continuous baffle with woofers flush-mounted to it.  Of course, the smaller the angle, the wider the slanted piece as well, so there is a practical limit, depending on how much space you have to work with.

[TTS56A]

Nice fast decay from 80 hz up. Is the response equalized? It looks very flat from 25 to 100 hz.

 

Mmm and what causes that steep roll off at 20 hz? Maybe it is due to the lowest room mode, or just lacking with room gain.

[Kvalsvoll]

Nice fast decay from 80 hz up. Is the response equalized? It looks very flat from 25 to 100 hz.

 

Mmm and what causes that steep roll off at 20 hz? Maybe it is due to the lowest room mode, or just lacking with room gain.

 

Yes, isn't it - from 80hz..

The room is a bit lively in the lower bass range, but as you can see the decay is still reasonably smooth, there are no huge resonances.

This can be improved in different ways, but with 2 subwoofers only the only option is to add more damping, and that will be difficult, so have to live with this.

 

The bass system consist of 2 S6-14 compact horn subwoofers and amplifier with dsp - very small, small 6" driver, limited to 30hz extension, addictive bass quality.

Explains the 30hz roll-off.

 

The response is equalized in the dsp, peaks and resonances are attenuated, by looking at the decay/time-response and the frequency response.

There is a hole around 60hz, which can not be filled, but the dip is reasonable.

The eq improves this system quite a lot, both frequency response and decay improves very much.

It could be fine tuned more accurately to get an even smoother frequency response, but that does not necessarily lead to better sound, it is important to look at both time and frequency domain. 

 

Sound good now, but those small subwoofers can only do so much, physics still applies.

 

Here is the velocity response, actually improved a bit in the bass:

 

Red: spl

Blue: 0 degrees on-axis

Brown: 90 degrees sideways

Green: Horisontal

[Kvalsvoll]

@SME wanted to see spectrograms.

Different analysis for each purpose, and the spectrograms are especially useful for seeing what happens in the very early time range when the signal starts.

 

Here is a good example of what you can see in those:

 

 

This is the Room2 system with 2x S6-14 subwoofers.

Timing between sub system and mains are set by setting subwoofer distance, which the processor interprets as time delay on the mains.

Here this is set using the typical method, the frequency response sums up correctly, and the sound is the usual bad-subwoofer-integration-bass.

 

In this spectrogram we see why - the bass is actually delayed from the mains, causing very poor transient performance.

 

By delaying the mains A LOT more, the timing improves, and it still sums up correctly:

 

 

It is possible that this could be improved even further, but the processor does not have enough time delay available.

 

Here is the spectrogram from the Moderate Cinema, here we see the timing is accurate:

 

 

[SME]

Excellent find using the spec!  You can clearly see in the first picture that before the delay fix, the sub impulse arrives separately, at a much later time, than the impulses for the mains.  In the second picture, you can see that the sub and mains impulse arrive together, even though the sub impulse is somewhat delayed.  That's actually a lot better than in the first picture, and in fact, the delay I see there doesn't look that bad at all.  How does the kick drum sound before vs. after?

 

Don't forget that a textbook LR4 crossover involves a full phase rotation, which contributes delay when it's working properly.

 

Your specs also reveal a break in the impulse in the 400-700 Hz range.  Can you decrease the window size a bit?  Something like 4 ms might do it.  By doing so, you can try to discover the timing of one or more coincident early reflections that's spoiling the response there.  If with the smaller window, you can't see anything and all you see is an apparent lack of response in the direct sound there, then that means that the problem occurs very early and involves some combination of baffle diffraction and reflections from walls very close to the speaker.  Another possibility is poor crossover integration, which could leave a null in the direct sound but still put lots of energy into the room that arrives later.  At 500 Hz, you probably won't be able to distinguish reflections closer than 2 ms or so apart, so if you can't see a discrete reflection in the spec, then the interfering problem likely involves sound travelling less than 2 additional feet (relative to the direct sound) before reaching the listening area.

[Kvalsvoll]

..

How does the kick drum sound before vs. after?

 

Don't forget that a textbook LR4 crossover involves a full phase rotation, which contributes delay when it's working properly.

..

 

..and that is why the "textbook" crossover does not work, but as you can see, this can be fixed.

A "data-bass only exclusive audio secret" - set the delay to get the best possible impulse while retaining correct frequency domain summation.

 

The difference is huge.

The bass suddenly becomes defined, precise with a natural and powerful transient attack.

The first calibration sounds like there is too much bass, and still lacking in power and impact.

 

I find it interesting when you can actually see a correlation between what is heard and what is measured, like in these spectrograms.

It is then possible to use the measurements to set up a system, and get predictable results, without needing "golden ears" to get it right.

The timing is not exact through the bass range, but it may be difficult to improve this much further due to room acoustic properties and placement.

The big system in the Moderate Cinema measures exact timing, and the bass is also perceived as better - more defined, more precise attack.

But that system has much larger bass horns, so there may be other properties that affect the result.

 

The dips in the 200hz-up range must be fixed, and here I can also use spectrograms to try to find out what is going on, but the decay plot will also have useful information.

But there are other issues with this room that must be addressed, and after fixing those, the response in this frequency range may have changed.

[Kvalsvoll]

The Room2 process has different stages, where focus is on fixing one problem at a time:

 

1. Empty room: Analysis and listening.

2. Decay and boundary interference: Front wall absorption.

3. Early reflections: Ceiling and side wall absorption.

4. Sound stage: Adjust reflections and diffuse sound field to achieve desired sound stage.

5. Adjustments: Analyze and listen, then make adjustments as needed.

 

Step 4 is happening now.

The sound stage - placement of instruments and sounds in the 3D perspective, and perceived physical size and room dimension of the recorded event.

 

I know how I want it to be, and it does not sound like that at all after placement of absorption for fixing early reflections and low frequency decay.

 

I want a larger stage, extending far beyond the R/L speakers, with the recorded room extending from the speakers and seamlessly backwards into my room towards the listening position.

Instruments and sound shall have physical presence and size, not only a precise location.

 

This is achieved by adjusting the reflection pattern in the room.

It is difficult in a small room, controlled directivity speakers makes it possible.

 

This more or less like it is now - a small and "dry" rendering of the recorded event, somewhere between the speakers:

 

 

This is how I want it to be:

 

[Ricci]

I hate to say it but you may need a wider room and a greater spacing of the speakers for that effect.

[Kvalsvoll]

I hate to say it but you may need a wider room and a greater spacing of the speakers for that effect.

 

I hate to say it, but you are absolutely right.

 

It is possible to improve it, it is already better, but there is a problem with the close walls, it may be difficult because the reflections are too close.

I would also like to move the system sideways, so that speakers and listening position is offset  from dead center, and how to do that..

 

The problems with a too small space is what many have to live with, but it can be improved.

The Moderate Cinema room is only 4.1m wide, and the sound in that room is quite good, with a large enveloping sound stage.

[Ricci]

Have your tried moving the LP closer to the speakers so that the angle of coverage between yourself and the 2 speakers is larger? This can work sometimes as well but isn't practical in many cases or causes other problems.

[SME]

I hate to say it but you may need a wider room and a greater spacing of the speakers for that effect.

 

I disagree with this, but I could be proven wrong as I do listen in a 20 foot wide room with speakers spread about 10 feet apart.  That said, my side-wall reflections from my left and right speakers are pretty weak and diffuse, never exceeding -10 dB in level and mostly under -20 dB.  These are likely just barely above the threshold of detection and well below the threshold level for image shift.  I hear plenty of width and depth in the sound-stage on my system, even when sitting well outside the sweet spot, however, the effect does vary a lot by content.  I also have a wall about 18" behind my head with 6" thick absorbers optimized to completely kill that reflection, so essentially no sound arrives at my ears from behind.

 

A crucial question is whether you want to reproduce the content accurately or you want to hear an enlarged sound-stage with *all* content, regardless of whether it is accurate.  I personally strive for accuracy because I don't want to do harm to material that actually is recorded/mixed well.  I believe that for accurate sound-stage reproduction you want good phase accuracy for discrete imaging that extends well beyond the sides of the speakers (and better imaging in general).  That can be achieved with good speaker design, good control of early reflections, and ensuring decay isn't too high across the frequency range.  In other words, imaging to the outside of the speakers should work fine in the setup depicted here with content that images this way.

 

Another crucial characteristic is envelopment.  Reverberant sounds captured or synthesized in stereo contributes the sense of envelopment, which is very pleasurable.  It goes a long way toward making the listener feel like he/she is present at the live event.  Enveloping sound seems to emerge from all directions at the same time without any particular direction being emphasized more than another.  The brain seems to be specially well equipped to perceive diffuse fields and to process this sound differently from the direct sound, as long as the reverb isn't too great.  In my sweet spot, I hardly notice the lack of sound coming from behind, even though that's impossible due to the close wall with absorption on it.  Envelopment provides a convincing illusion.  (I do notice if I rotate my head instead of looking directly forward.)  The ratio of direct to enveloping sound is probably also one of the most important depth cues the brain has to work with.

 

I don't know this for certain, but I would guess that making the listening room too dry causes a loss of envelopment, which diminishes the contrast in direct-to-reverberant sound between instruments residing at different depths in the recording, thus flattening the sound-stage.  I believe the solution is to retain more late arriving mid and high frequency sound energy and ensure that that energy is as diffuse as possible.  I know adding diffusers into my room made a world of difference in as far as envelopment is concerned.  I also know that music is typically mixed and mastered in relatively live spaces, so any adjustments that need to be made to the level of reverb in the recording will tend to be tailored for rooms that are more live.

[Kvalsvoll]

Have your tried moving the LP closer to the speakers so that the angle of coverage between yourself and the 2 speakers is larger? This can work sometimes as well but isn't practical in many cases or causes other problems.

 

Moving the lp closer up front is possible, but the distance to the speakers gets smaller and the direct sound to reflected ratio is even greater, and that does not create more space, and the sound stage is still limited to somewhere between the speakers. 

 

The distance is closer to an equidistant triangle than what it looks like form the pictures.

[Kvalsvoll]

...

 

I don't know this for certain, but I would guess that making the listening room too dry causes a loss of envelopment, which diminishes the contrast in direct-to-reverberant sound between instruments residing at different depths in the recording, thus flattening the sound-stage.  I believe the solution is to retain more late arriving mid and high frequency sound energy and ensure that that energy is as diffuse as possible.  I know adding diffusers into my room made a world of difference in as far as envelopment is concerned.  I also know that music is typically mixed and mastered in relatively live spaces, so any adjustments that need to be made to the level of reverb in the recording will tend to be tailored for rooms that are more live.

 

Too dry does not work well.

 

In a larger room, with large horn speakers, it may not be necessary to add any absorption for early reflections, because all early reflected energy is already very low in level.

 

I find it interesting to notice that accuracy and detail up front in the soundstage is not compromised, but rather enhanced by later reflections from the room.

If all early reflections are controlled and low in level, it does not seem like a more live room destroys anything.

 

The back of the Room2 has no absorption, part form the last ceiling absorbers.

But it is still too dry sounding.

The radiation pattern from the speaker focus the sound towards the back of the room, and so it is possible to do something about it by directing some of that sound back towards the listening position.

[maxmercy]

How does a 'too dry' room handle >2 channel sound, in your opinion?

 

JSS

[Kvalsvoll]

How does a 'too dry' room handle >2 channel sound, in your opinion?

 

JSS

 

I should have a too dry room to test this in before giving an answer, but I can live with that. 

In any case, the Moderate Cinema is a very dry room compared to what most people have, it's just that there is a significant difference from dry and controlled to too dry.

 

Assume we are talking about movie sound here.

 

A too lively room will not sound good for surround, but too dry will also compromise the experience.

 

Experience shows that it is not necessary to do much additional acoustic treatment for multichannel, such as trying to absorb early reflections from every single speaker - which would need damping on every surface in the room.

 

It turns out that the radiation pattern from the surround speakers determines how the surround sound is perceived.

With correct radiation the placement of sound objects and envelopment is not compromised if the room has some reflection and liveliness.

And some reflections actually help to make the sound from the different speakers blend in seamlessly.

 

The surround sound in The Moderate Cinema is very good. 

This is the result of the radiation pattern of the surround speakers and the room acoustics, combining to create this immersive and seamless and at the same time accurate presentation. 

 

In a multichannel room the ceiling should have acoustic treatment/absorption spread out across the whole surface, not just in first reflection points like it is often suggested for 2-channel.

Doing this will not compromise 2-channel sound, actually I will recommend using the same approach also for 2-channel only rooms.

[SME]

Interesting points here.  AIUI, rooms used for movie sound can be and maybe should be a bit drier than those used for two channel music, at least in theory.  I think the reasoning is that a movie soundtrack relies more on the surrounds for ambient reinforcement than on the room itself.  This reasoning makes sense where surrounds are primarily used for reproduction of diffuse sounds, but things are a bit less clear for modern soundtracks where surrounds (and overheads) are also used as discrete sources.

 

However, as far as I know, recommendations for room reverb times in movie mixing facilities haven't really changed.  The recommendations are probably what's most important in so far as achieving a result consistent with the studio.  Multichannel music is another matter entirely.  YMMV, big time here.

 

One point worth bearing in mind is that a room can be so dry as to be uncomfortable to the occupants.  That's definitely something to avoid.  For example, people are known to actually get nauseous in anechoic chambers because the acoustics are perceived to be so unnatural.  I personally think a room a bit on the live side is okay if the sound is well diffused and the power response is reasonably smooth and balanced.  Indeed, a big difference between large and small rooms in general is that large rooms with similar treatment will have longer decay times.  In a small room, it doesn't take much absorption at all to suck out a lot of the energy, particularly in the mid and high frequencies, yet the early reflections are very detrimental if not absorbed.  The bass is especially troublesome because it's hard to adequately diffuse sound in the room below the Schroeder frequency.

 

I have to admit, I envy you, Kvalsvoll, for having multiple rooms within which to experiment.  Your comments about not needing to worry as much about absorbing early reflections from surrounds is reasonable to the extent that surrounds are primarily used for ambiance.  However, where those channels are used for discrete sources, my bet is that they will still image better on their own and in conjunction with the front speakers if their early reflections are controlled.  But there are definitely compromises to be made here for reasons I gave above.  Just about all decisions concerning treatment involve some compromise.  Measurements are an essential part of the process.  Subjective evaluation is also important because it's not always clear how best to analyze and interpret the measurements.  Knowledge of psychoacoustics are very helpful here, but even this provides only incomplete guidance.

[Kvalsvoll]

Thank you for sharing your thoughts, @SME.

 

While waiting for the paint to dry on some mounting brackets for another diffuser for Room2, is a good time to post about the progress, and some general thoughts on sound.

 

The Room2 gets closer and closer to the Moderate Cinema room as things are adjusted and improved, but I can't help it, it sounds like a little version of the other room - soundstage is smaller, instruments are smaller, there is too little body on smaller drums and instruments in the lower midrange, and when you turn up the volume the bass lacks some of the realism and impact.

 

Some of this is caused by differences in the loudspeakers, some if it is room acoustics.

 

@SME, reverb times (T60) are not very useful information for small rooms, in those rooms here the T60 values typically lingers between 0.1 - 0.2s across most of the frequency range, there is simply not enough diffuse and late energy to get a meaningful reverb.

 

But that does not mean the space is dead.

People usually think the surround speakers are also playing with 2-channel stereo, in the Moderate Cinema.

This is because the sound fills the whole room, like you are sitting in the room where the recording took place. 

But the precision and placement of instruments are not compromised, everything has a precise location up front, and instruments and sounds take on a physical presence - they get size and body.

It is the radiation pattern of the speakers that makes this possible - precise direct sound with little early reflected energy, and lots of delayed reflections from the back of the room, which gets diffused and spread by objects on the back wall and side walls.

 

In Room2 I have added diffusors on the side walls close to the back wall, to create more reflections. 

I am now adding one more at the back wall - ceiling transition.

[SME]

Thank you for sharing your thoughts, @SME.

 

While waiting for the paint to dry on some mounting brackets for another diffuser for Room2, is a good time to post about the progress, and some general thoughts on sound.

 

The Room2 gets closer and closer to the Moderate Cinema room as things are adjusted and improved, but I can't help it, it sounds like a little version of the other room - soundstage is smaller, instruments are smaller, there is too little body on smaller drums and instruments in the lower midrange, and when you turn up the volume the bass lacks some of the realism and impact.

 

Some of this is caused by differences in the loudspeakers, some if it is room acoustics.

 

@SME, reverb times (T60) are not very useful information for small rooms, in those rooms here the T60 values typically lingers between 0.1 - 0.2s across most of the frequency range, there is simply not enough diffuse and late energy to get a meaningful reverb.

 

I partly disagree, at least for rooms that aren't too small or dry.  While it is very true that the reflected sound field in a small room cannot be accurately characterized as completely diffuse and reverberant, I believe that much of the decaying sound does become quite diffuse in the late part of the tail.  The clear exceptions to this are modal resonances and slap echo.  Hopefully your room treatments have done a lot to reduce these.  In your decay plot posted here, the response becomes a bit more smooth which each time slice, except for the bass where some modal effects linger.

 

About that decay plot.  Depending on how you look at it, there is a broad dip in late energy response, from about 300 to 3kHz, or there is a moderately broad peak in moderate-to-late energy response between 2-6 kHz or so.  I'm not sure if these features are really important in your case because your room is much drier than mine, but my recent experimentation with signal shaping of my speaker responses in my room suggests that having a relatively smooth response in the decaying sound is a very good thing for envelopment, perception of depth, and imaging outside the two speakers.  If your mid-to-late tail is not too weak to be heard at all, most likely the content at 2-6 kHz is audibly masking the content within the rest of the mid-range.

 

I mention this because my decay plots had similar (if less severe) features when I used signal shaping with a target curve (for smoothed frequency response) that was close to flat from 300-10000 Hz.  In my decay plots, I also had a peak at the lower end of my horn's response.  In my case in the region ~1.5-4.5 kHz.  My broad dip was in the region of about 300-1500 Hz.

 

Since then, I have adjusted my signal shaping target curve to achieve a smoother decay profile, keeping the MLP response reasonably smooth but compromising flatness.  I believe this has made for a significant improvement in sound quality.  What changes did I make to my target curve?

1. I added a 1-1.5 dB "BBC dip" in the 1.75-4 kHz area.  I did this in part because it avoids peaks in that range at some off-axis seats.
2. I added a slight (~0.75 dB/octave) tilt toward the bass, starting from 2 kHz.  This tilt was effectively an extension of the tilt I already had for the 20-200 Hz range.
3. I adjusted the delay between drivers in the active crossover so that they don't cross completely in-phase.  I aimed the lobe somewhat above the listeners' heads and then boosted output at the crossover point to make up for the SPL loss.

With these adjustments, my decay profiles are basically flat from 500-6000 Hz and ramp up only very slightly from 100-500 Hz.  My MLP response only deviates from flat in so far as having a very slight house curve / bass rise.  When I added the boost to the lower mid-range to comply with the new curve, I was actually a bit surprised by how much better it sounded.  I'm not surprised that I had to modify my target curve for these new speakers.  Actually, my old speakers had a directivity problem that couldn't really be fixed without leaving a big on-axis hole at 3 kHz, so instead I just lived with the big power response peak there.

 

I suspect that if this approach were to work for you in your small room, you would have to use a more aggressive tilt.  The reason is that the peak and dip in your decay plots are more severe.  Still, if you have any DSP/EQ/signal-shaping capabilities, I'd definitely suggest trying a house curve here.  Perhaps it could help with getting more bass into the room before loudness becomes a problem.  My own target curve fits into a +/- 3dB window (except for the top octave), but I know some people here use target curves that are more like +/-5 dB or even +/- 10 dB.  The latter seems like it might be too much, but I'm not inclined to judge without having heard the system and the room.  I am very curious as to why tilted responses often sound better in different circumstances and what it is about the room and/or speakers that dictates what house curve sounds best.

 

Let us know what you hear if you give it a try.

[Kvalsvoll]

I partly disagree, at least for rooms that aren't too small or dry. 

...

 

Oh.. long post, but.. I like it, will read it later.

 

Just wanted to clarify about the RT60 issue:

 

I must admit I can not resist to look at the RT60 myself, even though it is "wrong".

It tells the story of the room in a very quick glance, you can immediately get an idea of whether the space is kind of dead or more lively, or simply too much room.

And you can see of the decay is smooth and even across the frequency spectrum.

 

The problem is when the decay is very short and not necessarily smooth. 

The RT60 values are calculated from the decay rate within a given reduction in level, and when this reduction is not following a smooth decay line, the number does not represent the actual decay rate.

You can see this when the 20dB, 30dB and Topt does not match up.

[Kvalsvoll]

@SME, I have already tried that. 

 

Various eq settings on the F2 speakers, and also 2 other speaker systems.

 

If the F2 was to be used like it is now, and the room was like when those measurements were taken, then eq would be a good solution to improve the overall sound.

Because what you say is true, the frequency response of the decay affects perception of tonal balance.

 

However, it has improved some, and this is more of a problem with the speaker - which is not completed - than the room.

Decay for other speakers show different decay response, and as a general rule, the decay response tend to be similar in different rooms, this is a result of the speaker radiation pattern.

 

In the other room, the F1 speakers have a smooth sloping frequency response, and what is interesting is that the decay is also smooth and sloping with  progressively more tilt for increasing time.

 

Speakers with horn and controlled radiation pattern is interesting.

Early decay energy drops very fast, but late energy has a relatively high level.

When frequency response is equal, they tend to sound brighter than trad hifi-speakers.

[SME]

Wow, great points!  While I believe the room can also play a considerable role in the frequency-dependent decay response, the overall shape of the decay response has a lot to do with the speaker power response.  I'm glad to find someone else who agrees that speaker power response, or more specifically, the manner in which the speaker radiation pattern interacts with the room has a considerable impact on the sound of the speakers.

[Kvalsvoll]

@SME: Yes indeed it has.

 

Speaker radiation is one of - maybe the most - important single parameters for sound.

 

Comparing those 2 rooms, the room also contributes, and more for lower frequencies.

Decay in low mid - mid is much drier in the new Room2, and that is not only due to differences in speakers, it is because the room eats more energy in that frequency range.

 

And the interesting point is that even though the decay is slower in The Moderate Cinema, the lower midrange is perceived as better - better imaging, more physical, more presence. 

That may have to do with the spectral balance - frequency response is smoother overall, no huge dips in the midrange, and the slower decay means you get this smooth, sloping decay response.

 

Building SBIR/close to speakers absorbers to fix the low mid issues now, will be finished in a couple of hours, one is already done, one more to go.

[SME]

The audience is in suspense ...

[maxmercy]

Look forward to seeing how the SBIR treatments change things.

 

JSS