Finally, Room response measurements

[Kruppy]

Well, after months of himming an hawing about learning how to do it and then actually playing around with the software, I've finally run some frequency sweeps through my system with the Room Eq Wizard that I down loaded from the Home Theather Shack (Thanks Sonnie!).

I'm not quite sure what to make of them yet and I'm hoping for some comments from our members with more experience than me (JonFo where are you!).

Set up: Room Eq Wizard (free) run off of my IBM work laptop, SoundBlaster MP3 external sound card (~$20) and my RadioShack analog SPL meter (~$45). The SPL meter was aimed in-between the speakers, I've been told that the accepted way is to have the meter at a 45 deg angle upwards, so I'll have to run another sweep when I'm able.

Levels were set to target 75 dB.

From what I understand or have been told is that the RadioShack SPL meter is good up to about 5k Hz. Notice on my graphs the peak and then drop off at the higher frequencies. I have know idea how accurate these results are.

I'm looking for as any advice/comments.

One other thing to note: The first graph uses 1/3 octave smoothing (which I've been told tames the reflections and comb filtering, the graph without smoothing is really messy in the upper frequencies). The second graph from 15-200Hz does not utilize the smoothing feature.

 

[JonFo]

...

I'm not quite sure what to make of them yet and I'm hoping for some comments from our members with more experience than me (JonFo where are you!).

...

Kruppy, You called? :yell:

Sure, post a measurement graph and I'm all over it like a cheap suit.

First, wanted to check if you used the RatSack Mic correction files in your measurements?
I think EqWizard has that option.
Also, at what distance was this measurement made (mic to front of panel)?

Anyway, the response looks like I'd expect from an SL3. And you actually do not seem to have super-nasty room modes either, so you are either lucky, or have treated the room a bit already.

As you can see from this measurement of an SL3 panel without crossovers, they drop off pretty sharply after 6Khz or so. So your big hump at 6K tells me the passive crossover is EQing for that, which gives a bump there.
The other factor that will cause a bump (or dip) in the 4 to 7Khz range is rear wall reflections. You can see that in the non-smoothed measurements, or specifically, in Impulse responses.

Please post an impulse response of one of the speakers measured at 2meters from the panel. This will tell us how much energy is coming back through and at what delay. Also post companion Freq. graph.
You might want to experiment with diffusion and absorption behind the panel. Typically a broad band absorption of 250Hz to 4K will do wonders.

Regarding the bass region, you can see that the crossover from panel to woofer is a problem area (and always will be in most hybrid ESL’s with passive X-overs). You clearly see that the SL3’s crossover of 250Hz is where there is big dip in the mid bass. But I also note that you overall mid-bass is depressed by an avg of 2 or 3db. So a bit of EQ there would be helpful (or the ‘bass contour’ is on. Make sure it’s off, as it gives a -3 cut from 40 to 150Hz).

I’m also going to guess that the woofer is sort of weak at this point in its life. (what year were your speakers made in?) So another way of improving bass is to upgrade those.
Might want to do distortion measurements of 60 to 300Hz to confirm.

The big hole at 80hz is probably from your crossover to the sub. Playing around with polarity and placement might address that dip, just make sure the rest of the bass is OK. Sometimes fixing one problem exposes others. It could also just be a room mode at that frequency.
Although the accompanying big dip at 160hz (second harmonic of 80) tells me it could be phase.

Take extra measurements at different locations to see if the dip stays relatively constant, in which case it’s probably phase, and not room modes.
Finally, your measurements must include corrections for the RatShack mic, as that is some pretty smooth and extended bass. Congrats, that’s the hardest to get right.

Hope this helps. Interpreting measurements is both a science and art that is well worth the effort to learn. And don’t believe everything I say about it either, as I’m still learning

 

[Kruppy]

JonFo,
First, thanks for taking the time to go though my graphs. I did notice certain things, but I was starting wonder if it really meant anything at all.

Here's some answers to your questions and comments (sorry, I haven't figured out the multiple quote feature):

Yes, I did use a cal file for the SPL meter. REW loads the RadioShack cal files into it's program (all readily available from the HomeTheaterShack website).

All measurements were done from my seated position about 13 feet from each speaker.

I do not have any room treatments, yet, so call it luck that I have the responce that I have. My purpose for this was to help me identify whether treatments would be benificial for my set up.

I've added some graphs here. The first being the non-smoothed full frequency plot of the same data posted yesterday. As you can see, the higher frequencies get messy.

The second is the impulse plot, which I really have no idea how to decipher (little help, please).

My SL3 bass "contour" is set to flat, so no it's not set at -3 dB, although looking at the data, it sure looks like it is set at -3 dB.

Concerning the drop at 80 Hz, I actully got the same phase comment from a guy at the "Shack". Problem is....I'm not using a sub. SL3s only, so I'm guessing that this is a room node. I do feel like I get better bass responce a couple of feet back from my listening position, which unfortunately is not a location that is workable for me.

Thanks again, I appreciate the comments.....:bowdown:

 

[JonFo]

Ok, the dip at 80 is pretty much a room mode then. Only fix is to get some bass traps that operate at and below 80hz, which means, they will be big, or you'll have to use many in corners, etc.

more to come later...

 

[Kruppy]

Ok, the dip at 80 is pretty much a room mode then. Only fix is to get some bass traps that operate at and below 80hz, which means, they will be big, or you'll have to use many in corners, etc.

more to come later...

I'm looking forward to your comments :bowdown:

 

[Kruppy]

JonFo,
I failed to mention something about the impulse plot above, this graph was generated from the sweep data from my listening postion, not from the 2m distance you requested.

 

[sleepysurf]

Room correction for the car!

Not to hijack this great thread, but here's a very interesting preview of JBL's upcoming MS-8 DSP amplifier with built-in "room correction" for your car! It was inevitable that somebody would carry this technology into the auto market!

http://www.audiojunkies.com/blog/783#continued

 

[JonFo]

Kruppy, regarding the Impulse response, here is my view:

Good to see there is not a significant rear wall bounce included. But then, it makes me question the windowing used to produce that graph. Usually, the window (in time) is a couple of ms. If gated to <500ms (is that what the dashed verticals indicate in the program?), then it’s hard to tell speaker from the room reflections you have.

The ‘ideal’ impulse response would have one slim spike at the onset of the signal, with less than a ms or two of decay. The extended decay shown in your graph is probably the SL3 woofer contributing that. Panels decay very fast.

Also, as you measured at your seating location, you are also recording all the room reverb as well (assuming a 500ms window).

As seen in this plot of my SL3XC, the SL3 panel and associated mid-bass array can decay at less than 0.2ms, whereas the old Logos, was more like your SL3, taking about 3ms to decay the impulse.
A sub can take up to 20ms to go through the impulse (and first ‘ring’).


We’d need a 1m or 2m shot with mic placed at 4’ ft high to better see what the woofer is doing in terms of THD.

 

[Kruppy]

JonFo,
Again, because I’m treading on new ground here, I’m not sure if I’ve done this correctly.

I went back and loaded my freq sweep data back into REW so that I could play around with the impulse curve (along with going though the help files a bit). Anyways I’ve changed the Y axis from dBFS to %FS as suggested in the REW help files and changed the Time (X) axis to zoom in on -2 to 10ms and have obtained a graph much like the one you posted of your center channel comparison. How does this look?

 

[Kruppy]

Starting to goof around with waterfall plots now.....

 

[JonFo]

JonFo,
Again, because I’m treading on new ground here, I’m not sure if I’ve done this correctly.

I went back and loaded my freq sweep data back into REW so that I could play around with the impulse curve (along with going though the help files a bit). Anyways I’ve changed the Y axis from dBFS to %FS as suggested in the REW help files and changed the Time (X) axis to zoom in on -2 to 10ms and have obtained a graph much like the one you posted of your center channel comparison. How does this look?

Kruppy, yes this helps see things much more clearly.

From this impulse response I think I can tell the following:

The panel is roughly 3’ from the front wall (5.5ms, which is equal to 6.1ft of distance, of the secondary impulse is divided by two, as half the time is spent traveling from panel to wall, the other half, wall to panel).

The speaker is roughly 2.5’ to 3’ from the side wall (the 7.5ms or 8.4ft, to that impulse are accounted for as 3 ft to back wall (from previous impulse calc.) leaving 5.5ft of extra travel to bounce off front wall to side wall and then add to the 13’ to listener. The 5.5 is divided roughly by thirds meaning the bounce from front wall to side wall is about 4’, plus side to listener of about 2’.

This means you ‘first reflection’ of the rear wave bounce from the panel is roughly 15’ from the listener along the side wall.

Again, without doing detailed trig, these are my best guesses from the data. It would be interesting to see how well they correlate to actual. Was I close?

Anyway, what this tells me from a room treatment perspective is that you could improve the soundstage by pulling the speakers out a bit more. What this does is bring the rear wave and side bounce impulses more in alignment. But watch the phase, as you do not want them to cancel totally or reinforce too much.
What you want to see is one slight broader secondary impulse whose amplitude is no bigger than the second impulse shown in your graph above.

The best option IMHO is to treat the front wall with some absorption and diffusion. Ideally, one RealTraps diffuser would be placed behind the speaker on the front wall, and another on the side wall at first ‘rear wave’ reflection point.
The <600Hz absorption this diffuser has will also improve the mid-bass clarity, as the out-of-phase rear wave is muted by the absorption, making the front radiated signal stronger (no phase cancelation) and clearer (no time smears).

The high frequency will get spread out in time and slightly muted by the diffuser behind the panel. This deflects much of the energy that is coming back through the panel now (which results in major comb filtering as seen the freq. plots). While you will still see comb filtering with a diffusor, it will be minimized and spread across a larger spectrum (see this plot from the SL3XC with diffusion).
It can also improve spatial perception by giving a ‘broader’ soundstage, as there are more reflection points created by the diffused signals.

I’d also recommend absorption of the lateral bounces (if any) of the front of the panels signal on the side walls. Usually, the ML’s are toed in such that the side wall is not a factor for the signal emanated from the front. But precisely because they are toed, the rear wave is given a clean bounce shot from front wall to side wall as described above.

This is one of the fun aspects of dealing with diploes, in that the rear wave can help or hinder the ‘sound’ of the speaker in so many ways that sensitive to placement, treatments, etc.

 

[JonFo]

Kruppy, the Waterfall plot tells me that you definitely have a room mode at your listening position. The peak at 40hz is clearly the fundamental, with 80 and 160 being nulls at your seating position.
You also have another axial mode at 45hz that is pretty nasty.

What were your room dimensions (width, length and height) again? Is it a closed rectangular room or is it open to other rooms?

In any case, you really need to get in touch with Ethan at RealTraps, post that waterfall plot, and they’ll recommend the appropriate products for bass trapping and smoothing out the plot.

 

[Kruppy]

Jonathan,
Again, thanks for the replies.:bowdown:

I'm in the process of putting together a diagram/dimensions of my room that I'm hoping to post in the next day or two that will help out. Yes, your guestimate on panel location is very close.

Right now you are confirming what I've suspected my situation has been:

1) I've not been happy with bass response from my listening postion as can be seen from my null and lowered bass output from the freq sweep and waterfall plots.

2) In addition I've not been too happy with the upper frequecies which I guessed were from rear/side reflections mostly as a result of speaker toe in. My soundstage/imaging has never been sized correctly or in the right location (cymbals for example can sound like they're 5-10 feet from the drum set.) Instruments just appear out of place to me. From the begining I've had to compromise imaging for bass and bass for imaging.

I see acoustic treatments in my future. My room is in a state of change right now. I'm hoping in the next couple of months to change my rack and add/replace some shelving which will be detailed in my room diagram post.

This has been a great learning experience for me.

More to come......

 

[Kruppy]

Ok, I have a bit of trouble with file size here. This is a close up of the listening area in my basement. I was hoping to post the whole basement floor plan but the file size is too big. I'll work on making it smaller.

I'm hoping to run a few more sweeps today, with the speakers in different locations.

 

[JonFo]

Hi Kruppy,

Looks like I was right about position

Given the room layout, I’d definitely get the Difusors to put behind the SL3’s and some corner mondo traps for the front corners

I’d also treat the side wall with some high frequency absorption, which could be some microtraps or a more WAF friendly hanging fiber art decorations. Something like the artistic carpets sold at home improvement stores or furniture stores.

But test this with and without side-wall absorption before committing, sometimes people prefer the wider soundstage the added side reflections present for music.

Remember, this is different than point source speaker first reflection stuff, this is the rear wave of the dipole, delayed about 7ms, which can add to the ‘ambience’.

It might also be advisable to put a micro trap on its side and tilted against the wall under the screen to help a bit with bass modes and rear wave from center channel.

Have fun playing with this

 

[C.A.P]

Man Kruppy I hope we can get together with this tool . I would love to do my room as with all the stone and reflective surfaces it may be a maze of peaks !