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Pieface

So what does this all mean? REW graphs...

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Ok so my reading has lead me to believe there is no current loopback solution for the USB mic/REW setup for working out the time delays and phase of individual drivers in a system and that type of thing (A bit hazy on the exact ramifications of not being able to do this in relation to taking the impulse response measurements you talked about Mait) Posts from April this year from the REW author seem pretty categorical about this.

 

I am guessing you are trying to get me to see if there is cancellation in that 100-200Hz hole between the subs and the mains and/or if the mains just have a giant hole in their frequency response?

 

I have got the impression that I can still use the REW generated impulse response graphs to determine useful information about room/speaker interactions. For example:

 

post-133926-0-67070900-1411089695_thumb.

 

post-133926-0-33077000-1411089699_thumb.

 

As I read a bit more I was able to look at these graphs and see there is a big reflection right around 1 millisecond. A bit of math (or punching the numbers into an online calculator) had me looking for an object approximately 34cm away from the mic...that would be the back of the chair I balanced the mic on then :lol:

 

Well I think that is what it is if I am understanding how to read the graphs. Further spikes on the graph seem to correspond closely enough with floor/ceiling/sidewall distances from a calculator I found. Learning is taking place I think...will procure a mic stand today.

 

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 spikes on the graph seem to correspond closely enough with floor/ceiling/sidewall distances from a calculator I found

 

Yes.  This is the biggest thing to understand.

 

You are looking at a chart plotting the speaker, with the reflections from the environment super imposed on top....  in the bass, you are looking that the dimensions of the room, and the speaker and mic locations relative to each other and the boundaries....  and the higher frequencies you see reflections from thing like the chair the mic is sitting on.

 

 

In order to see the speaker only....  you must limit the amount of time the microphone listens for.    This is often called "gating" or "windowing" the measurement.....   but by reducing the amount of time the mic listens for, you make the response below a certain frequency invalid.   This is because the amount of time the mic listens for, is less than the duration of one cycle of the sound wave.

 

For example.... if you listen for less than 5ms.... then you cannot measure below 200hz, because 1 cycle of a 200 wave lasts for 5ms.

 

 

From memory, REW sets it's default "window" at 250ms  (meaning you can measure low bass) ... but this means the response includes "the room".

 

 

 

 am guessing you are trying to get me to see if there is cancellation in that 100-200Hz hole

 

It's quite common to have a cancellation in this range caused by reflections from the floor or front wall .... which arrive out of phase with the direct sound from the speaker   (a cancellation at 150hz corresponds to a path length difference of 115cm)

Edited by davewantsmoore

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que?

 

You are on the right track.    The graphs you posted in #17 are showing you the room more than anything else.

 

 

If you want to interpret above 200hz .... your next step is to learn how to get a graph of the speaker only  (how to exclude the room)

 

If want to interpret below 200hz .... then as Ken said, you will need to measure in multiple locations   (because moving the mic or speaker will change the response)

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Sorry Dave...the rest of your post wasn't showing when I posted. Thanks for your input.

 

Gating the impulse response to remove the room is sounding familiar, I have definitely read something about this process in the past day or two. I will revisit this in due course.

 

I just picked up a mic stand so all things being equal I think I will be able to take some much improved baseline measurements in the next few days.  Mic set up properly, speaker levels set better etc.

 

Is the typical 2" thick type broadband treatment at first reflection point effective in that 100-250Hz range?

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Is the typical 2" thick type broadband treatment at first reflection point effective in that 100-250Hz range?

 

No... but the beauty of being able to measure is that you can work that out for yourself  (use a blanket or something)      ;)

 

Hint:    You'll discover that the broadband absorption does little in this region .... but that moving the mic or speaker changes it greatly.....  so you'll realise that if you're doing the "does blanket affect 100-200hz" test .... that you will need to keep the speaker and mic in the same location to be able to see the effect of the blanket only.

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Ok so my reading has lead me to believe there is no current loopback solution for the USB mic/REW setup for working out the time delays and phase of individual drivers in a system and that type of thing (A bit hazy on the exact ramifications of not being able to do this in relation to taking the impulse response measurements you talked about Mait) Posts from April this year from the REW author seem pretty categorical about this. I guess it depends on what mic and usb thingy you use.

 

I am guessing you are trying to get me to see if there is cancellation in that 100-200Hz hole between the subs and the mains and/or if the mains just have a giant hole in their frequency response? That was my initial hope yes. From the previous graphs your sub looks like crossing at 80 (tho only half an octave above that is 120, in your dip, it might be stretching it to think it affects it all the way to 200 tho) and sadly, it looks like that hole appears in the mains anyway. I'm wondering if that is room interactions or 'bad' mains.

 

These are the type of 'logical steps' you need to nut out.

 

I have got the impression that I can still use the REW generated impulse response graphs to determine useful information about room/speaker interactions. For example:

 

attachicon.gifimpulse.l+subs 16.9.14.jpg

 

attachicon.gifimpulse.r+subs 16.9.14.jpg

 

Should be easy to find out if that graph does the trick. Take a measurement, then delay the sub (only do one) a known amount, see if the other stays the same and the one you changed moves as expected. IF those graphs are telling us what we want to know (even tho you don't have the loopback going) then as you can see there is a distinct time difference. I cannot see them now as i type, did not think to eyeball that actual time delay. It may not be anything to worry about.

 

(ah, scroll down the page and I can see them. 1ms, if that is the sub (?) then that most certainly will not be the cause of that dip. I doubt it is the sub as it is very consistent between measurements, more likely the back of the chair as you posit). IF that is the case, then without the loopback you might not be able to measure arrival times. Try it. Measure one sub ONLY, delay it in your processor/whatever, measure again and see if it moved.

 

 

As I read a bit more I was able to look at these graphs and see there is a big reflection right around 1 millisecond. A bit of math (or punching the numbers into an online calculator) had me looking for an object approximately 34cm away from the mic...that would be the back of the chair I balanced the mic on then :lol:

 

Well I think that is what it is if I am understanding how to read the graphs. Further spikes on the graph seem to correspond closely enough with floor/ceiling/sidewall distances from a calculator I found. Learning is taking place I think...will procure a mic stand today.

 

I was presuming you are able to delay subs etc. I don't recall you mentioning what capabilities you actually have. WHAT is it you are trying to achieve? Is it 'here is what I have, what can you tell me, I can't do anything about it so it is only curiosity' OR is it, 'I can add room treatment, or move subs, and have eq available, so with this starting point where can I go?'

 

As you can imagine you will get very different responses depending on where you are coming from.

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Hi Mait...

 

I am initially looking at it from a "here is what I have" scenario as I learn how to use the tools properly and understand what they are telling me. Once I am confident I am generating worthwhile data...then I will be asking "where can I go?"

 

The room is a living room so I don't have total freedom to put things where they may measure best. I do however have a few permutations of practical options that allow the room to function. I hope to ascertain which of these is the "least worst" for basic response and integrate the subs with the mains as well as I can. Sub DSP/EQ is a likely starter via a miniDSP solution as it is a ) unobtrusive b ) doesn't cost the earth c ) appears to have easy/good integration with REW.

 

Room treatments will be a sticking point for WAF I should suspect but you never know your luck over the years :lol:

 

The end goal? Listen to some tunes and watch some movies  :cool: 

 

If I can have a bit of fun, learn a bit and get a bit closer to hearing things as the artist intended along the way it seems like pretty good hobby time to me.

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ok, so correct me where wrong, atm you have no means of eq or any other 'sub fiddling options' and limited ability to change things all that much. (At least you have subs!! :) ) You can at least adjust the sub levels?

 

And as such for now it is trying to understand how it all works together. Are you (or were you) happy with your sound prior to doing all this? If so, then yes it is very much an academic exercise. Or were there things you were not quite so happy with which led to this. Eg, the sub sounded over bearing or anything like that.

 

I hope at least you can 'see' the big problem that has been pointed out, hot sub (with peak, often called 'one note bass') and that big hole. Often, without yardsticks, we might not hear that as a problem, after all 'it is what I listen to'. Sometimes it is only after making a change do you go 'man, I had no idea blah blah blah'. OR, you might prefer it the way it was. (but at least you know that after comparing). I am pretty sure I would not like it, but that means jack because at the end of the day it is for your tastes (and so don't let others railroad you)

 

If you can see that the hole could be problematic, then great, that should be your goal, to find out the cause. Poor sub integration? (looking less likely sadly) Poor native mains response? Good native mains response but it is being savaged by the room?

 

So, what are the mains?

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They are Tannoy Little Red Monitor with 3149 Dual Concentric 12" driver. I have 2 x SVS SB12-NSD subs and an Integra DTR6.4 AVR doing the bass management.

 

I am pretty chilled about the whole process. I'll correct my setup errors, remeasure and see where I'm at over the weekend. I have another set of main speakers I can easily drop in to compare against to check the room vs speaker issue through the 100-250Hz region.

 

Sub fiddling options are only a week away these days if/when I decide to go that way. I think I can play around with filters and predicted responses in REW. I will look at this a bit later.

 

I'm not feeling any particular time pressure to reach a "result" if that is what you mean :)

 

 

 

 

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Following this thread closely as I'm attempting to improve my setup with REW atm.

I've learnt a fair bit already by reading this thread alone.

Keep posting your improvements Pie ;-)

Matt

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Cheers Matt.

I will get back to the thread . Had a bub decide to arrive a bit early and priorities changed!

I got some more measurements done. Learned some more but haven't had a chance to organise the graphs etc. maybe I will find some time over the weekend.

This measurement caper is interesting but gee I wish I was a more methodical person. Already full of things to try/revisit next go around that I probably would have addressed if I had considered it or was more familiar with reading the measurements!

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Just a couple of graphs to document what I've been doing in light of the help you guys have given me...

 

First I set about repegging the levels of the mains and the subs with the AVR test tones - Blue trace original, purple trace levels reset with UMIK1:

 

post-133926-0-21377000-1411848256_thumb.

 

I'm guessing the response shape has also changed due to the new mic position using the stand. The nasty dip 100Hz-200Hz remains unfortunately. The sub levels didn't actually come down much at all but the speaker levels went up a bunch. Not the result I presumed going in with my understanding of how the iPhone mic works!

 

Now my measurement session moved to dropping my mains into my preferred location (currently perched on top of a 4 foot entertainment unit)...will post tomorrow what happened :)

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OK so I moved the speakers closer to the sidewalls, further from the front wall and lowered them onto "stands" about 16" tall...

 

I don't seem t have just the left main without the subs in the original position so this is where my lack of method I mentioned earlier shoots myself in the foot a bit but I think you can still see what is happening. Original position in purple, new possie in aqua.

 

post-133926-0-97361200-1411858237_thumb.

 

It seems the new speaker position has pushed the large null slightly higher into the Hz range. A bit unclear if it has reduced the depth of the null or if there is a bit of destructive stuff going on with the subs in the purple trace?

 

The other thing that seems evident to me is that there is a lot more treble energy making it to the listening position and the response looks more consistent. There is a bit of a hole in the response centred around 1.5kHz...I'll have to check the speaker specs as this could be the crossover point between the woofer and the tweeter in this model IIRC. Could this be a cause of the dip?

 

To check that the 100-200Hz null wasn;t the fault of the speaker I subbed in my Boston Acoustic CR8 bookshelf speakers to the same position:

 

post-133926-0-06159900-1411859357_thumb.

 

Allowing for the physical size difference of the speakers I'm pretty happy that it is a room effect rather than a faulty speaker causing the null - should I be? Obviously I need to fool around more and see if I can even out the response more by placement...next time I get the mic out. Could be a while now with the new resident :)

 

I mucked around with sub placement next...I'll post some of that next time.

 

Cheers

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Oops!

EDIT for phone in pocket

Edited by Pieface

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Here is some of the sub stuff I did

 

post-133926-0-97798100-1412230565_thumb.

 

Subs located midwall and playing together. Not as smooth as I had hoped.

 

post-133926-0-05014700-1412230584_thumb.

 

My poor method let me somehow take seperate measurements for corner loading the subs but I didn't get one playing together. I think this one looks quite promising to try next time around?

 

Here it is with the left main super imposed:

 

post-133926-0-90428700-1412230741_thumb.

 

Now I think I understand that if I get them looking good seperately and the combined response is poorer (not summing) that is when I can play with the phase on the subs and /or the delay in the AVR if the crossover to the main is poor?

 

I have also been reading about Speaker boundary interference response and some other small room acoustic stuff.

 

My basic understanding currently is that I have three main issues to deal with for seting up the room as best as possible...

 

Room modes: These are determined by the physical dimensions of the room and in general the parallel boundaries will create the most severe effects on my freqency response and time domain. The tools I have to deal with this are: listening position placement, speaker placement (mains vs subs) and DSP. Even large corner traps will have little effect under 125Hz(?)

 

SBIR - I need to do more work here and continue to refine main speaker placement. I found an SBIR calculator at Ken Tripps site. I think taking more measurments in conjunction with this will help my understand how to minimse the negative impacts. Why do the sidewalls not need a reference to listening distance with this phenomenon but floor/ceiling bounce does? I am also battling a little with how the baffle width impacts this phenomenon. I assume the speaker designer provides output compensation as the speaker loses output to omnidirectionality so I'm not sure how this fits in?? What happens if the relevant sound/impulse played is shorter than the length of time it take the sound to travel to the boundary and return to the speaker - no affect of the frequency output level?

 

These two problems are largely caused by the omni-directional nature of bass frequencies?

 

Thirdly - Strong Early Reflections. This is the direct sound vs reflected sound phenomenon. If the reflection is late enough and loud enough you get a pronounced echo? If it is early enough and loud enough it can influence the perception of the direct sound? Get it at the right time/loudness and you can get an anhancement of the stereo illusion? It seems more relevant to higher frequencies than the first two issues? This is why they can be treated well with thinner panels/regular furnishings?

 

Am I on the right track here or on crack? I am grabbing out my tape measure at any rate and having a play,. I clearly have a way to go getting things positioned as best as possible and I'm learning all the way! :)

Edited by Pieface

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