The "Walking Mic" test procedure

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Soundman2020
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The "Walking Mic" test procedure

#1

Postby Soundman2020 » Tue, 2019-Oct-01, 13:24

.
The "Walking Mic" test procedure

Here's a set of instructions I wrote a while ago, on how to do what I call the "walking mic" test in your studio.

This is a very boring, tedious procedure using REW (if you don't know what REW is, then see here: How to calibrate and use REW to test and tune your room acoustics ), and this procedure is designed to help you better understand the acoustic issues in your room, as well as to identify which ones are which, find the best spot for your mix position, and other things too.

Here's the procedure, in PDF, for you to download and follow:

Soundman--REW-walking-mic-test-procedure--V2.2.pdf
(99.62 KiB) Downloaded 185 times
Soundman--REW-walking-mic-test-procedure--V2.2.pdf
(99.62 KiB) Downloaded 185 times


And here's an example of what the results might look like, just for the Frequency Response curve:

REW-FR-sample-walking-mic-2--15-500.png
That's from a real studio while under construction, not just invented stuff, or laboratory measurements. Each line is the frequency response at one specific location in the room.

Have fun! :)

- Stuart -



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Re: The "Walking Mic" test procedure

#2

Postby Soundman2020 » Fri, 2019-Oct-04, 19:11

I'm adding an animated GIF file below, showing the changing waterfall plots for the same room that I posted the set of frequency response curves, above, from doing the "walking mic" test in a studio under construction.

This is all of the waterfall plots for the locations between 18" in front of the mix position, and 36" behind it. In other words, it covers a range of a little more than four feet (about 130 cm). This is what you would hear if you were to walk between those two spots, form the front going to wards the back of the room. In the graph, you can see the location where each measurement was taken at the bottom of the image, with the name "Final Walk" followed by a number. Positive numbers are inches in front of the mix position, negative numbers are inches behind it, and the mix position itself shows up as "0000". This animated graph shows the same frequency range as the FR graph above: from 15 Hz. to 500 Hz. (the entire low end of the audio spectrum, and about one third of the mid range).

Soundman-studio-design-forum--Walking-mic--REW--WF-4-Front-to-Back-SML.gif
Soundman-studio-design-forum--Walking-mic--REW--WF-4-Front-to-Back-SML.gif (2.57 MiB) Viewed 1329 times
Soundman-studio-design-forum--Walking-mic--REW--WF-4-Front-to-Back-SML.gif
Soundman-studio-design-forum--Walking-mic--REW--WF-4-Front-to-Back-SML.gif (2.57 MiB) Viewed 1329 times
(The animation holds the first and last frames for 1 second, so you can see where it starts and ends, and it also pauses at the "00" location (mix position), to make it easier to identify that spot.) (Higher resolution version here)

Now you can see why I call this the "walking mic" test! This is what you would hear as you walked from the front of the room towards the back.

As you can see, there's a fairly large variation in the perceived modal response around the mix position. This is often surprising to many people, because its intuitive to think that the response of the room is fixed, not changing much, and it sounds the same everywhere, but it does not. Bare in mind that the measurements in this set are separated by just two inches (5cm): there's pretty big differences between frames in the animation! You can also see why it is not a good idea to base your choice of mix position ONLY on the frequency response. The location with the smoothest frequency response (-6") is not the same as the one with the best time-domain response (00").

Note that this series of measurements was done early on in the room treatment process, not at the end. Some parts of the room had been treated, but not all of it by any means! The purpose of this test was to check what was still going on in the room, and what treatment was still needed. (Don't get confused by the name "Final Walk": that was just because there was an earlier set before this, where some measurements were invalid, so we had to re-do the entire set to get to this "final" version.... at that stage of the treatment). This test was also to check that the room was working as designed: We did not do this test to find the best position, but rather to confirm that the best position was where it was supposed to be: at the mix position. (You can see that this turned out to be correct).

Also, note that there was a faulty mic cable here, with a grounding issue, and in some of the measurements it picked up 60Hz electrical "buzz" due to that. You can see that in all of the tests from +6" to +18" as a sharp spike at 60 Hz. That's not a room acoustic problem: its just a cable problem! Since we already knew about it, and it didn't affect what I was looking for, there was no need to re-do those tests. So just ignore that spike.

Later I'll post some similar animated graphs of other aspects of the same room, to show more detail.

- Stuart -



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Re: The "Walking Mic" test procedure

#3

Postby Soundman2020 » Fri, 2019-Oct-04, 20:28

... And another one: Same room, same data, but this time here's the "Spetrogram" animated gif. Same as above, it holds the first and last frame for a bit, and pauses on the "00" spot.

Soundman-studio-design-forum--Walikng-mic--REW--SP2-SML.gif
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Soundman-studio-design-forum--Walikng-mic--REW--SP2-SML.gif
Soundman-studio-design-forum--Walikng-mic--REW--SP2-SML.gif (1.72 MiB) Viewed 1327 times
(Higher resolution version here)

Here too you can see the large difference between adjacent frames in the animation, even though they are only 2" apart in physical space.

- Stuart -



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Re: The "Walking Mic" test procedure

#4

Postby Soundman2020 » Fri, 2019-Oct-04, 21:03

So why are these graphs more important? And why is the mix position at that location, rather than a bit further forward or backward? And why not just use the frequency response? Here's the real issue: The animated graph below shows the frequency response overlaid with the PHASE response. The second line on this graph shows what the phase is doing for each frequency band...

Soundman-studio-design-forum--Walking-mic--REW --FR-PH2-SML.gif
Soundman-studio-design-forum--Walking-mic--REW --FR-PH2-SML.gif (1.32 MiB) Viewed 1326 times
Soundman-studio-design-forum--Walking-mic--REW --FR-PH2-SML.gif
Soundman-studio-design-forum--Walking-mic--REW --FR-PH2-SML.gif (1.32 MiB) Viewed 1326 times
(Higher resolution version here)

You can see that for most locations away from the mix position, the phase response is not smooth: there are sudden swings, especially a couple of feet behind the mix position. Obviously you don't want to sit with your head in a spot where the phase is swinging about wildly. As you can see from this one, there location where the phase is smoothest, is exactly at the mix position. Actually, it's about an inch back, so in this case we did move the mix position back one inch.

I'll check with the owner of this room to see if he'll do another walking mic test in the final completed room, in order to compare how it actually turns out, with all the treatment and tweaking complete. So do check back at some point, to see if there's an update here.

If you are seeing these graphs as a visitor to the forum, then please do register and join in! Also, feel free to link to this page from other places: there's not much solid information like this available on the internet about measured acoustic response at different locations in rooms, and especially not from real studios in the real world!


- Stuart -



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Re: The "Walking Mic" test procedure

#5

Postby Soundman2020 » Sat, 2019-Oct-05, 01:30

OK, one more. A not-so-usefull-but-rather-cool one. The animated graph below is the complete walk from the back wall to the front wall of the same studio as all the other graphs on this page. OK, so its not ALL the way from back wall to the front wall, because REW only allows up to 99 measurements in one MDAT file and the room is longer than 198 inches. So this animation runs from 102 inches (2.6m) behind the mix position, to 90 inches (2.3m) in front of it. The rest of the room isn't that interesting anyway, but this gives you nearly 6 meters (16 feet) of "walk". In this case, you are starting at the BACK of the room, and walking FORWARDS, towards the speakers.

Soundman-studio-design-forum--Walking-mic--REW--WF-Long-Walk-102+90!.gif
Soundman-studio-design-forum--Walking-mic--REW--WF-Long-Walk-102+90!.gif (4.72 MiB) Viewed 1341 times
Soundman-studio-design-forum--Walking-mic--REW--WF-Long-Walk-102+90!.gif
Soundman-studio-design-forum--Walking-mic--REW--WF-Long-Walk-102+90!.gif (4.72 MiB) Viewed 1341 times
(As before, the first and last frame are held for a couple of seconds, and the mix position is paused.)

You can clearly see the modal stuff here: any peak (or null) that rings and stays in the same position on the graph, while just changing in amplitude, is a mode. Because modes are standing waves, they will always occur at the same frequency everywhere in the room: they will only change in intensity as you move from place to place. in other words, you are placing yourself at different points on the standing wave, but because the wave is at a fixed frequency, and forms a fixed pattern in the room, you can move to different spots on that wave as you walk around. This is an easy way of determining if a problem in your room is modal, or not modal. Peaks and nulls that stay in the same place are modal. Modes also "ring": they decay slowly. In waterfall plots, they appear as sharp peaks with long tails.

Peaks and nulls that seem to move along the above graph (left to right, or right to left), are not modes. They are phase summations and cancellations, due to some form of reflection or other. If the peak or null moves UP the graph to a higher frequency as the mic moves, then it is getting closer to the surface that is causing the problem. If it moves DOWN to a lower frequency, you are getting further away from the surface. Phase summations and cancellations do NOT ring: there is no decay time for them. They appear as peaks with no tails at all, or valleys.

For an example of a mode, take a close look at 24 Hz. That's the the first axial mode for this room (1.0.0). You can see when the mic is that close to the front and back of the room, there is a peak that rings, but in the middle of the room, it is a null. Clearly, as the mic moves forward it is "walking" along this axial mode, and the pressure peaks are near the front and back walls, with the pressure null in the middle of the room. And if you look closely at 48 Hz, you can see the first harmonic of that mode (2.0.0), which does the opposite: it peaks in the middle of the room, but drops down to a null close to the walls.

For an example of a phase issue, look at the graph around 45 Hz at the beginning of the animation, and you'll see a "hole" or steep dip in the waterfall that slowly moves over to the left, down to lower an lower frequencies, until ir goes completely off the bottom edge by the time the mic gets to the mix position: that is SBIR from the rear wall of the room. There's no ringing associated with it, and because it goes down in frequency as we move towards the front wall, it must be coming from the back wall. There's another similar one that starts at about 180 Hz and also moves down: That's probably part of the same SBIR issue. Since SBIR is a phase cancellation, it also causes comb filtering that continues all the way up the spectrum, right to the top, at multiples of the lowest dip frequency. The 180 Hz hole is the 4th multiple of 45Hz... which implies that there should be another related dip that starts at 90 Hz and also descends... Take a close look, to see if you can find it....

(Don't get confused by the 60 Hz issue that pops up and disappears at random! That's not modal: it is the faulty mic cable that I mentioned before... It looks like it it is modal, because it is a peak that rings and is always at the same frequency, but if you look carefully it does not decay at all: it stays at a constant level.)

Learning to decipher these patterns and clues is the key to understanding the acoustic response of your room, so that you can then treat it properly, and tune it properly. Click here for an example of a high-end room that has been fully treated and tuned:

Of course, because the mic is only moving from back to front along the center-line of the room, and always at the same height, this animated graphs does not tell you much about reflection issues that are occurring between the side walls, or between the floor and ceiling. To get a better idea of what those are doing, you could conduct a similar "walking mic" test at the mix position by moving the mic right from left to right across the room, and also from floor to ceiling going up the room. For precision tuning of the room, I sometimes do that, when the client really wants things to be as good as possible, and to find out where the limits of the "sweet spot" are. It can be interesting to that, but not really necessary, for most home studios.

By doing a test like this in your room, you'll find it easier to figure out what is what. Just looking at the REW results for one single point in the room (the mix position) like we normally do when testing and tuning a room can tell you a lot about the room, but mostly only about that one spot. It still leaves you with some mysteries in identifying many of the issues. Moving the mic around the room in carefully planned patterns with fixed intervals, as in these graphs here, can tell you a LOT more about the room, especially when you step through them, or even animate them, as I have done here.

- Stuart -




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