RPG Modex Plates (35Hz) placement question

[stevekale]

Did you try measuring with the existing Modex Plates on the back wall? BTW, how much do they cost in the UK?

I stayed several times in the zone of South Kensington in my youth with colleagues when visiting London - the zone is plenty of small non expensive hotels always full of young people. I remember we changed trains at Notting Hill Gate to go shopping electronic parts and hardware at Tottenham Court Road and LPs at the HMV of Oxford Street. Sometime in the end of the 70's ...

In the end I didn't. Jason continued to recommend the front because of the likely pressure build up in the alcoves and I just can't get one in the corner on the right of the rear wall without impairing the door. They're £490 for the Type 2 in the UK ($762). That's with 20% VAT included (which one would not pay if exporting). I'm glad to hear Nyal's comment re where they're produced. I was beginning to wonder if the European model somehow cut corners! It's unusual to find something cheaper here than in the US.

London has changed a lot since the rather depressed state of the UK in the 70s. It's now truly the capital of the world (until Shanghai takes over).

EDIT: Having said all of that, I am fast coming to the conclusion that I should add another two on the rear wall - even if it slightly impairs the door (and even if they have to be removed when moving certain large objects out the door)

 

[stevekale]

Well after a day of chasing my tail inside instead of enjoying the sunshine on my day off work, here is where I ended up (I've updated the room schematic). Still a lot to do.

Left spectrogram:

Right spectrogram:

 

[Nyal Mellor]

Well after a day of chasing my tail inside instead of enjoying the sunshine on my day off work, here is where I ended up (I've updated the room schematic). Still a lot to do.

Left spectrogram:

Right spectrogram:

That's just plain nasty! What do you hear?

 

[microstrip]

Can you show 1/24 octave averaging frequency response measurements only between 20 and 200 Hz? The 1/3 octave ones are not enough for diagnostics.

 

[stevekale]

 

[stevekale]

That's just plain nasty! What do you hear?

Well I wouldn't say it sounds awful - but then again I don't have a reference for "great". I can hear the improvements introduced by the modest amounts of acoustic treatment I have done. First the heavier rug (the floor is hard tile and the previous rug smaller and thinner). Then the Modex Plates. Then the Skyline panel. Lastly, the adjustments I made to the sub setup and crossovers. It keeps getting better. At the moment I have no first reflection point treatment (I'm thinking about Bad Panels). I also know the issues at c35Hz and 70-100Hz associated with room modes will linger post first reflection treatment. I guess I just need to keep adding until I get to the point where compromises are reached. I am wary of over-investing in a bad canvas. A large Bad Panel on the bare left wall (first reflection) won't cost much. I hear the curved panels are better for diffraction but do they radically compromise absorption versus a 4" straight panel? (I've not seen absorption graphs for the curved panels.) Two more Modex Plates is a relatively bigger plonk.

 

[theguesswho]

Hi Steve, To me your response looks good, not great but good. If you compare it to Bruce Browns response of his new Wilsons (or his old MM7s) your response has less variation than his. He says that his speakers sound great and that he is only looking for the last 2-3 percent, so I think you are close. Plus you say it does not sound bad, and how could they sound bad, you have nice equipment.

Nyal is more interested in graphs than listening, don't listen to him. Or perhaps he can give you some advice instead of criticizing your setup.

Wendell
PHP143

 

[Nyal Mellor]

Hi Steve, To me your response looks good, not great but good. If you compare it to Bruce Browns response of his new Wilsons (or his old MM7s) your response has less variation than his. He says that his speakers sound great and that he is only looking for the last 2-3 percent, so I think you are close. Plus you say it does not sound bad, and how could they sound bad, you have nice equipment.

Nyal is more interested in graphs than listening, don't listen to him. Or perhaps he can give you some advice instead of criticizing your setup.

Wendell
PHP143

I do apologize if I was too off the cuff in my last comment. If you look back through the thread I have tried to provide assistance.

The issues are very clear with the spectrogram. The issue is with the steepness of the sides of the peaks and valleys. This indicates very little modal control. These things are very audible, at least to my ears. The audible signature is one of a lack of articulation and definition around bass notes. Further bass notes when energized do not decay as rapidly as they should limiting perceived dynamics and bass impact.

The below spectrogram is not perfect but meets my targets and sounded very good. Look at the steepness of the contours and the overall smoothness in the response. This is more like what good looks like.


It's clear that Steve needs to add a lot of modal control in the region sub 120Hz. The most foolproof way to do this, without adding issues elsewhere, is to add a bunch of Modex Plate Type 2s. Other solutions would involve multiple sub arrays and EQ.

 

[Roger Dressler]

Steve,

The idea of using the 2 mains plus a sub to combat modal issues only works when all three carry the same signal. Now we often figure that bass is fairly mono in a stereo recording, so that will work as intended with the derived subwoofer signal. However, it looks like your tests are being run as L and R channels separately, so I'd say try driving them as mono and see where you are in the bass. That would also be the condition to use when tweaking subwoofer gain and delay.

The other thing I would try, were it my system, would be to run the mains bass managed, crossed over at 80 Hz. There's a chance the near-field subwoofer on its own will work better than the mains. Worth a few minutes to see anyway.

 

[stevekale]

Nyal, your help is very much appreciated. The points you make are what directed me to the purchase of 2 Type 2 Modex Plates to begin with. I hadn't counted on needing 4. ;-) I will keep tinkering and update this thread as I go along.

Wendell, always good to be reminded that chasing the last few percent shouldn't distract us from enjoying what we have! The question is whether it's 3%, 10% or 20% - I don't have a feel for this.

Roger, very good point re running all speakers at once for low end tests. I will try to run that test on Monday. Tomorrow, I am going fly fishing!

 

[stevekale]

However, it looks like your tests are being run as L and R channels separately, so I'd say try driving them as mono and see where you are in the bass.

Hi Roger, I did these tests tonight. The optimised settings didn't change. Here are some results:

Low Frequency Response (FAIL - Mellor and Hedback critieria)

Mid Range Decay (PASS)

Low Frequency Decay (FAIL)

Consistency of Mid-Range Frequency Response (PASS)

(from earlier measurements)

 

[stevekale]

Understanding phase and delay

I've continued to invest in improving the acoustics of my room. I added a couple of 100mm BAD Panels (hung on batons for a further 20mm gap) at first reflection points (albeit the right panel was constrained in size due to the window). I haven't spent a lot of time analysing the results at this point but the audible impact was immediate. I took a couple of measurements quickly last night and while I still have a battle with frequencies and decay below 150Hz or so, mid-range decay (RT60) has improved and same with early reflections (ETC).

Revisiting measurements made me think about sub phase and delay again. I'm thoroughly confused about these. Back on page 5 of this thread I posted the results of a few scenarios with phase and delay settings for my sub. To summarise, I am running my front speakers full range and also sending a low end signal to my sub, the idea being 3 sources of below-80Hz signal are better than 2 (or 1). I found that reversing the phase on the sub helped a lot. This was improved by delaying the signal to the sub by 8ms. (The reason why my memory was jogged on this is because in moving the sub I had inadvertently flicked the phase switch on the sub and when I did the measurements last night low end response was was off again.)

I am struggling to understand the cumulative effect of reversing phase at the sub and delaying the signal 8ms via the Theta CB and why it makes the impact I observe. The sub isn't materially further away from the seating position than the other two speakers. It is behind the seating position and centred on rear wall (so triangular source of low-end). I'm wondering if setting the sub using RTA in this manner ignores timing and affects decay.

Regards


Steve

 

[dallasjustice]

You want all of the sound to get to the seated position at the same time and in the same phase. Is your sub the same distance to seated position than the main left/right? If that's correct, then maybe delaying the sub helps even out the group delay. You would want to measure the group delay of your sub and then compare that with the group delay for your main left/right. I don't have any experience doing this but I would guess that the group delay would usually be more with the sub than with the main left/right compared with most subs since subs use DSP onboard and passive speakers don't use DSP. Nyal is the master of this stuff, so I would ask him.

I've continued to invest in improving the acoustics of my room. I added a couple of 100mm BAD Panels (hung on batons for a further 20mm gap) at first reflection points (albeit the right panel was constrained in size due to the window). I haven't spent a lot of time analysing the results at this point but the audible impact was immediate. I took a couple of measurements quickly last night and while I still have a battle with frequencies and decay below 150Hz or so, mid-range decay (RT60) has improved and same with early reflections (ETC).

Revisiting measurements made me think about sub phase and delay again. I'm thoroughly confused about these. Back on page 5 of this thread I posted the results of a few scenarios with phase and delay settings for my sub. To summarise, I am running my front speakers full range and also sending a low end signal to my sub, the idea being 3 sources of below-80Hz signal are better than 2 (or 1). I found that reversing the phase on the sub helped a lot. This was improved by delaying the signal to the sub by 8ms. (The reason why my memory was jogged on this is because in moving the sub I had inadvertently flicked the phase switch on the sub and when I did the measurements last night low end response was was off again.)

I am struggling to understand the cumulative effect of reversing phase at the sub and delaying the signal 8ms via the Theta CB and why it makes the impact I observe. The sub isn't materially further away from the seating position than the other two speakers. It is behind the seating position and centred on rear wall (so triangular source of low-end). I'm wondering if setting the sub using RTA in this manner ignores timing and affects decay.

Regards


Steve

 

[stevekale]

I haven't measured yet but if anything it would be a little closer. See the diagramme on pg 1. If it were just a case of a small delay then I'd get it. But a reversal of phase and a delay? (Phase is something I struggle with.) "Group" delay?

EDIT: Am I right in understanding that a polarity/phase switch on something like a subwoofer completely inverts the signal being processed? Presumably the imposed delay is then frequency dependent and quite different from a linear shift imposed at a processor? As you can see I am quite confused and wondering if my approach of simply running an RTA and fiddling with all controls (sub phase switch and processor) until I get the best SPL response curve is the right way to go.

 

[Nyal Mellor]

I haven't measured yet but if anything it would be a little closer. See the diagramme on pg 1. If it were just a case of a small delay then I'd get it. But a reversal of phase and a delay? (Phase is something I struggle with.) "Group" delay?

EDIT: Am I right in understanding that a polarity/phase switch on something like a subwoofer completely inverts the signal being processed? Presumably the imposed delay is then frequency dependent and quite different from a linear shift imposed at a processor? As you can see I am quite confused and wondering if my approach of simply running an RTA and fiddling with all controls (sub phase switch and processor) until I get the best SPL response curve is the right way to go.

See figs 1-5 on this page and surrounding text, should explain phase vs. delay etc.

 

[stevekale]

Thanks Nyal. Reading the first few sections there is exactly what I am worried about. (I will continue reading from "Types of Main Speakers" and onwards.) I made my adjustments for "phase" at the sub and delay at the processor by looking at the frequency domain, and am inherently uncomfortable (rightly or wrongly) with making the phase reversal at the sub. I guess the question is, "What's the right way to do this?" I only have REW and a tape measure as tools!

 

[stevekale]

I measured the distances to sub versus the front speakers. The sub is marginally closer (within 10cm).

It seems odd to me that internals of the sub (crossover network - it is receiving a fully balanced "LFE" input, amplifier) would have a group delay (if my understanding of this term makes sense) so dramatically different from a full range signal going to my Krell FPB-200 amp and on to front L/R speakers.

The link Nyal posted suggests delaying the front L/R channels and the sub even more so and going from there. (My sub doesn't have a fine phase adjustment, just "normal" or "reversed".) My concern with delaying the front L/R is that when I switch to a video 5.1 setup isn't my lip synch going to be miles off? I guess I just need to play around some more (perhaps playing an 80Hz signal via REW).

 

[Nyal Mellor]

I measured the distances to sub versus the front speakers. The sub is marginally closer (within 10cm).

It seems odd to me that internals of the sub (crossover network - it is receiving a fully balanced "LFE" input, amplifier) would have a group delay (if my understanding of this term makes sense) so dramatically different from a full range signal going to my Krell FPB-200 amp and on to front L/R speakers.

The link Nyal posted suggests delaying the front L/R channels and the sub even more so and going from there. (My sub doesn't have a fine phase adjustment, just "normal" or "reversed".) My concern with delaying the front L/R is that when I switch to a video 5.1 setup isn't my lip synch going to be miles off? I guess I just need to play around some more (perhaps playing an 80Hz signal via REW).

Sealed box subs have more group delay than main L/R speakers because they often have a load of EQ to make them flat. The EQ adds group delay. The subs can also have additional delay associated with internal DSP processes if the EQ is done in the digital domain (as a lot are these days). Main speakers do not normally have EQ or DSP associated delays. So that's what accounts for the differences.

The processor should automatically adjust things to keep audio/video in sync. In any event up to around 20ms audio/video sync error is hard to detect.

There are many techniques to properly integrate subs. It depends on how you are running them. In your room you probably want to place and integrate the subwoofers for room mode cancellation. It's kind of an advanced topic but something that I do all the time with clients. With two speakers front and one sub rear you can cancel the main front / back room odd order axial room modes, assuming you pick the right crossover points, sub level, sub delay, etc.

 

[stevekale]

Thanks. Now see, you just need to pop on over to London!

My sub is a relatively old Rel Stentor II. I suspect there isn't a lot of DSP going on. It is rated 16 Hz - 120 Hz. I note it has a port underneath and the driver is downward-firing. I have the Rel crossovers set "wide open" so that in essence the full signal it is sent is delivered.

The front Egglestonworks Andra III are rated to -3dB @18Hz.

The Theta Casablanca is set to send the front L/R full range and copy to the sub a LF signal driven by Butterworth crossover starting at 80 Hz, slope 24dB (both high and low).

I guess I have a decent amount of fiddling ahead of me and I suspect I should avoid flicking the phase reversal switch on the sub. Can REW help at all? (I assume RTA is out.) Otherwise I have to sit there with an 80Hz tone and try to judge the sweet spot?

(thanks re the lip synch question)

PS: I'm now quite happy (for me) with my mid range decay / ETC. The BAD Panels helped decay a little down as low as c120Hz but I've still got battles 60-120Hz and 25-45Hz. A couple more Modex Plates might well be ideal but likely not doable for aesthetic reasons (and door interference). I'm wondering how many Modex Corners are necessary to make a difference. I could only place these in the front L/R corners and likely run out of room for further treatment after adding 2 high in each corner. After that, it's a wait for Theta's implementation of Dirac Live.

 

[Nyal Mellor]

Thanks. Now see, you just need to pop on over to London!

My sub is a relatively old Rel Stentor II. I suspect there isn't a lot of DSP going on. It is rated 16 Hz - 120 Hz. I note it has a port underneath and the driver is downward-firing. I have the Rel crossovers set "wide open" so that in essence the full signal it is sent is delivered.

The front Egglestonworks Andra III are rated to -3dB @18Hz.

The Theta Casablanca is set to send the front L/R full range and copy to the sub a LF signal driven by Butterworth crossover starting at 80 Hz, slope 24dB (both high and low).

I guess I have a decent amount of fiddling ahead of me and I suspect I should avoid flicking the phase reversal switch on the sub. Can REW help at all? (I assume RTA is out.) Otherwise I have to sit there with an 80Hz tone and try to judge the sweet spot?

(thanks re the lip synch question)

PS: I'm now quite happy (for me) with my mid range decay / ETC. The BAD Panels helped decay a little down as low as c120Hz but I've still got battles 60-120Hz and 25-45Hz. A couple more Modex Plates might well be ideal but likely not doable for aesthetic reasons (and door interference). I'm wondering how many Modex Corners are necessary to make a difference. I could only place these in the front L/R corners and likely run out of room for further treatment after adding 2 high in each corner. After that, it's a wait for Theta's implementation of Dirac Live.

Actually I am coming over late Dec for a few days. Mostly to see friends but I might be able to squeeze in a couple of hours. PM me if interested.

I did a room with Theta Casablanca and Aerial 20Ts not long ago. We ended up crossing over the mains at 80Hz and using subs + EQ to flatten out the bass. Worked wonderfully and was a better option for the client than adding a bunch more Modex plates...

Haven't looked into your room in great detail but from what you describe likely a multi-sub room mode cancelling array plus EQ plus some additional treatment for the 60-120Hz range and maybe rolling off the mains early would be the solution to fix your bass issues.