Taking and Interpreting Measurements with REW (FREE eBook)

[xanalog]

...Then goes onto to say many only measure one then calibrate it then move onto the next.
This is what I have done in stereo measured left sub then using REW implement filters, then repeat this for the right.
Based on Floyds advice should I be measuring both subs together then implementing filters to suit? Both subs would have the same filter settings I assume.
....

If you perform the F.Toole's recommended method, you can then compare these results with your old (L then R) method results.
And let us know if you see any differences worth exploring?

 

[Keith_W]

I just watched Floyd Toole's Toronto AES you tube video. Excellent watch. I was intrigued with a point he made at position 1.57.51 regarding subwoofers/woofers.

He states "Always do low frequency measurements with all subwoofers operating". Then goes onto to say many only measure one then calibrate it then move onto the next.

This is what I have done in stereo measured left sub then using REW implement filters, then repeat this for the right.

Based on Floyds advice should I be measuring both subs together then implementing filters to suit? Both subs would have the same filter settings I assume.

In my eBook, I say to always measure each speaker individually with a time reference. I haven't seen that video, so I don't know what he actually said. But I disagree with that recommendation of measuring all the subs together when you are tuning subs. You only do that when you want to see the result. When you are tuning the sub, it is better to measure each sub individually and with a time reference so that you can see what each sub is doing. And even then, it is easy to sum the result of all individual subs (Trace Arithmetic A+B, or Vector Sum in All SPL) to see the simulated result. It will be very close, if not an exact replica of the measured result.

And no, each sub should have a different filter setting, because they are at different locations in the room. When tuning subs, this is the process:

1. Place both speakers and sweep them together. Take note of the pattern of nulls, ignoring the peaks. Take note of where the main speakers roll off. Decide if you want to implement a HPF or run the mains full range. If you want to implement an XO, do it now.

2. Place one sub and sweep. The goal is to find a position with peaks that match the nulls from the speaker. If you are not happy, choose another placement. Repeat until all subs are placed. For the second and other subs, the goal is the same, you want peaks to fill in the nulls.

3. Place the XO on the subs and then do a rough time alignment for all subs to the mains. Then use REW's alignment tool to fine adjust the delay of each sub (aligning the phase) until as many of the nulls as possible are gone.

4. NOW you sweep all the subs together with the mains, and use DSP to chop off the peaks.

5. If there are any remaining nulls ... well, that is where the fun begins

Now, I am no Floyd Toole, I am just a random guy on the internet. But can assure you that this method works.

 

[janbth]

I just watched Floyd Toole's Toronto AES you tube video. Excellent watch. I was intrigued with a point he made at position 1.57.51 regarding subwoofers/woofers.

He states "Always do low frequency measurements with all subwoofers operating". Then goes onto to say many only measure one then calibrate it then move onto the next.

This is what I have done in stereo measured left sub then using REW implement filters, then repeat this for the right.

Based on Floyds advice should I be measuring both subs together then implementing filters to suit? Both subs would have the same filter settings I assume.

Robert

I haven’t watched the full video yet, but I assume Floyd Toole is referring to the multisub technique to even out the frequency response in the room and combat room modes. If you want to do it properly, then use bass management and set the subs to play the exact same signal, i.e. mono. The subs in combination should be aligned to the L+R signal (L and R played together). And yes, any filters you compute for the subs should be the same for both, at least as a starting point for further tweaks.

See for example here:

Multi-Sub Tuning Basics

Tuning multiple subwoofers with miniDSP processor + REW + MSO.

www.minidsp.com

Multisub is Method A.

 

[Flaesh]

the REW's RT calc, here it is

It's not here

V5.31.3

Installed 6.40 Beta 104, here it is, thanks again!

 

[antcollinet]

MODS: I wanted to create this thread in "Audio Reference Library" but I have no permission to do so. Would you be able to move this thread there if you think it is appropriate and delete this message. @amirm @RickS

A few years ago, Amir wrote Room Measurement Tutorial for Dummies Part 1 and Part 2. The third part has never materialized despite many requests. In the meantime, there is a clear need for clear guide on how to take room measurements because we are seeing more and more people take and post their measurements on ASR and making a lot of mistakes in the process.

LINK TO DOWNLOAD EBOOK

I have written two documents on taking and interpreting room measurements and posted it in my Google Drive. You can find both documents in the link above. You may find other free eBooks or resources in that link that I have written and decided to share.

The first document, which I will reproduce below, is an extremely basic guide to how to place your microphone and interpret the frequency response. I have tried to keep it simple and approachable. Note that as time goes on, the eBook will be updated, whereas I can not edit old posts on ASR.

The second document, which I am still writing, is a far more extensive guide that covers some speaker measurements and how to interpret other measurements in REW. At the moment you will be able to see it in Google Docs format, but when it is finished it will be replaced with a PDF.

Stunning piece of work (or at least it looks so without having yet read it)

Thanks for putting it together.

When I am not preparing for my presentation on Monday, I might find time to go through it.

 

[mmares056]

Thank you @Keith_W for your effort in putting this eBook together. I'm finding it very helpful. More reference info to continue learning how to effectively use REW.

 

[Keith_W]

Hello all, REW book 2 ("Advanced") has been updated and uploaded. New in Book 2:

- an extensive discussion on phase
- some errata corrected
- a discussion on different smoothing methods
- some illustrations redone

 

[goryu]

Hello all, REW book 2 ("Advanced") has been updated and uploaded. New in Book 2:

- an extensive discussion on phase
- some errata corrected
- a discussion on different smoothing methods
- some illustrations redone

Link?

 

[Keith_W]

Link?

It's on the first page.

 

[goryu]

It's on the first page.

Thank you!

 

[AlmaAtaKZ]

Massive thank you!

 

[Daverz]

Hmm, when I wrote this eBook my intention was to strictly write about taking and interpreting measurements. And not preparing measurements for DSP. You don't need to gate your measurement unless you are planning to use it for some kind of correction.

I did think about writing about the difference between psychoacoustic smoothing and plain old Gaussian smoothing, but I thought that even that was too much. Just tell people to use 1/6 or 1/12 and that's it. I might remove stuff i've already written from the longer document just to make it shorter and more to the point.

And BTW, if you have a reflective surface like a leather sofa, you don't want to deaden reflections from it. Earl Geddes said that you want it to "stand out like a sore thumb" so that you know where to gate your measurement.

Is how to gate the measurements described somewhere?

I'm thinking that I should also include the 135 pounds of flesh that sits in my Stressless leather chair when taking measurements (with earplugs for hearing protection, of course.) However, the chair back blocks a 0 degree mic orientation and my shoulders block a 90 degree mic orientation, so only negative 90 degrees (pointed down) would work.

 

[Keith_W]

Is how to gate the measurements described somewhere?

Not explicitly described, it's sort of glossed over in the new "smoothing" discussion in the FR section 4.1.

This is a book on interpreting measurements, not on how to use REW for DSP, so while there are small snippets in there about DSP, I do not discuss it in great detail. Writing about DSP is going to be a different kettle of fish.

 

[Daverz]

Not explicitly described, it's sort of glossed over in the new "smoothing" discussion in the FR section 4.1.

View attachment 492501

This is a book on interpreting measurements, not on how to use REW for DSP, so while there are small snippets in there about DSP, I do not discuss it in great detail. Writing about DSP is going to be a different kettle of fish.

OK, so this is for doing quasi-anechoic driver measurements? Or is it still relevant for room correction?

 

[Keith_W]

OK, so this is for doing quasi-anechoic driver measurements? Or is it still relevant for room correction?

The difference between "room correction" and "loudspeaker correction" is: with the former you want to INCLUDE the effect of reflections. So no gating. With the latter, you want to EXCLUDE reflections - i.e. a quasi-anechoic measurement - so you need to take speakers outside, elevate, and gate to remove reflections.

I should rename the book "Room measurements with REW" and remove that section on loudspeaker measurements. There is a LOT to say about taking quasi-anechoic loudspeaker measurements, particularly if you are a hobbyist and do not have a Klippel or anechoic chamber. The workarounds can fill up a book on its own. I think that REW book is already too long. It's >80 pages and it's getting to the length where people won't read it and it will lose it's impact.

 

[Daverz]

Having furniture in the way will contaminate your measurement with early reflections. My position is that you want to see these reflections in your measurement, along with the influence these have on your frequency response. Large pieces of furniture will affect measured bass response, and you want to correct for it.

I'm still going down this rabbit hole of what the listening chair does to the sound and how to correct for it (or if that's even desirable).

I did some experiments to see what the effect of my chair (a Stressless leather recliner) would be on the measurements. 1/3 octave smoothing used so the effect is easy to see:

I then divided out the chair-free measurement using trace arithmetic:

To hear what that first low mid bump was doing, I had REW create a single PEQ to flatten it. Then I can switch the PEQ in and out while listening to reference tracks (e.g. Fast Car)

The PEQ is -5.9 dB centered at 304 Hz with a Q of 1.46. With the PEQ switched out, the midrange is noticeably thickened too much.

 

[Mort]

I am halfway through the advanced book. Thank you @Keith_W

Likely Dumb question I can not figure out regarding timing references. I notice people , mess this up a lot. Maybe this needs to be added?

The question is 'After I do the first reference speaker, where and how do I link the timing of second speaker to the first?'

I just get the same options from REW. Shouldn't I see the referring speaker as a timing option?

 

[Keith_W]

You're welcome.

Re: timing references. I have to limit the scope of this book, otherwise it will get too long. To me, timing references are a DSP thing. I COULD include a whole bunch of DSP stuff in there, but the book would be 3x as long. DSP is an endless rabbit hole and there is already plenty of content on Youtube showing you how to use REW for DSP, including from our friend @OCA.

 

[voodooless]

Thanks for this! This is very comprehensive

Asking for a dyslexic friend: will there be an audiobook version ?

 

[Keith_W]

You are most welcome, the fact that people find this helpful is enough satisfaction for me I have no plans for an Audiobook version. But a quick Google found some PDF to audiobook converters - Speechify and Eleven Labs. I can't vouch for either of those websites.