RT60 and your listening room

One of the more interesting acoustics discussions that I haven't seen to date is the measurement of the contributor's room RT60...reverberation time, typically stated for -60 dB decay from the initial pulse. I really haven't seen posts of measured RT60 curves for listening rooms on audio forums, in general. This would be a good place to post them it seems. Statistics would be difficult to gather since the plots are not scalar, but a collection of plots in this thread might be useful to others.

I once did a study of listening room sizes (reported here and here) and found that there seemed to be a correlation of where you live geographically to the room size (no surprise there), but for the survey respondents there seems to be a larger size than I would have guessed (ave. length - 21.1 ft, ave. width - 18 ft, ave. height - 9 ft). This indicated to me that many that have smaller listening rooms weren't reporting on the survey.

One of the more interesting comments that stimulated this thread on RT60 was from REW help on the subject:

For domestic listening rooms and recording studios with volumes of less than 50 cubic metres (1,800 cubic feet) the recommended RT60 value is 0.3 s. For larger rooms, up to 200 cubic metres (7,000 cubic feet) the recommendation is 0.4 to 0.6 s. In both cases the value should be fairly uniform across the frequency range, though it will typically tend to increase at lower frequencies.
In the spirit of jumping into this, here is a typical RT60 plot taken from a REW measurement in my listening room (approximately at the listening position):

Chris A's listening room RT60 plot.jpg

I consider my listening room to be just a little bit "live" vs. what you would experience in a typical home theater room, but much less live than many lofts and hard surfaced dens and living rooms without any treatments. The room volume is 152 m^3 (5500 ft^3).

Chris
 
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By the way, there are other reasons to post this information.

For instance, I find that many preamp/processors turn on artificial reverb as default condition. This usually shows up as a rise in RT60 with frequency from 5-20 kHz, instead of a drooping slope curve, as shown above. This can also be seen in the impulse spectrograms at greater than 25ms delay from the impulse zero time, but I usually am not looking out at those kind of delays when looking at impulses, choosing instead to look at plots of less than 15ms duration from impulse, so the RT60 curve is a quick check to see if someone's pre/pro is "adding a little something" that's not actually desired.

Chris
 
Living in a cave isn't all bad, it seems. :) I'd take that ETC in my room.

I bet your cave probably could give George Massenburg's Blackbird studio room a run for its money...

BlackbirdAvid.jpg



Of course, there is the problem of dusting all that. I'm sure the local fire marshal was probably interested in the anti-flame properties of that, too.



Perhaps the total lifecycle requirements at Blackbird weren't really thought out.



Chris
 
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The "name" RT60 is not applicable in domestic rooms as one cannot reach a diffuse sound or have reverberation in it. (The RT60 concept is based on a diffused sound in the room. This can only be reached in very large rooms where the lowest mode is below hearing treshold, so concert halls and larger.) As a domestic room is dominated by modes a more appropriate term is T60 and decay, instead of RT60 and reverberation. The decay time varies quite a lot depending on the frequency of interest. The decay for different frequency can be shown i REW if one uses the filter function in REWs ETC-diagrams. Check the 3rd diagram in this post: Gearslutz Pro Audio Community - View Single Post - Quantifiable Indicators of a dead room 63 Hz has a much loooonger decay time than 250 Hz.
 
If you took the time to read the first post, you would see that the Topt, EDT, T20, T30, and Ts curves are posted above. RT60 is a term that many people identify with. You can call it what you want. I've found that the measurements in-room available from tools like REW actually do help with diagnosing issues and seeing trends across home-sized (small) acoustic rooms.

Could you post yours?

I do humbly request that the first posts in this thread by participants also be accompanied by their own listening room "RT60"-like measurements (room decay times). After that, then commentary posts on content are okay.

It helps if the measurements are in a like format--like REW provides, but something is better than nothing at all.

Chris
 
I know ”RT60” is very often used instead of the more proper ”T60”. But … going back a 100 years or so on how ”RT60” came to be established in acoustics, a requirement of the room and the RT60-use was that the room had a diffused sound wherever you measured it, showing the same value = without any modes. Small rooms have modes, so RT60 isn’t correct to use, even if many places do use it. In the diagram in post #1, the REW-program correctly uses EDT / Early Decay Time, T20 and T30. The curves are a bit of a roller coaster as it is measured in a small room. If the microphone was moved a couple of feet the curves would show different results, again because it is a small room with modes. The curves would also show different results if the microphone was fixed and a larger piece of furniture was moved from one place to another. (Happened to me when I moved my previous speakers which are W 570 cm/ 22,5 ” x H 109 cm / 23”)

If one ticks the box ”Schroeder integral” in REW’s ETC-diagram, you will see (well you can calculate) easily your rooms average decay time over the frequency range. The integral = the black line in the diagrams below, starts measuring from -5 dB = EDT. The integral curve’s inclination shows the decay time for the room / measurement. If it is a more or less straight line = fine, you can get your result anywhere from the Schroeder integral curve. Check the time at say -30 dB, check the time also at -60 dB. What is then the time for a 30 dB decay from -30 dB to -60 dB? Multiply that with 2 and you have your T60 average over the frequency range. My T60 turned out to be 240 ms.

The diagrams are old and with my previous Altec 604-8K speakers. The first diagram show some unwanted too strong early reflections within 20 ms before the room was fixed. The second diagram after the unwanted too strong early reflections were fixed acoustically and replaced by some wanted strong ones later in time, around 28 ms. (The reflection around 14,5 ms in both diagrams turned out to be a 2-way reflection sidewall-microphone stand-microphone.) The 3rd diagram is the exact same measurement as the 2nd diagram, only with a longer time axis and show a pretty even decay after the wanted reflections with no serious bumps = good and that the decay time is 240 ms.)

post-398-0-09712200-1409755902_thumb.jpg


post-398-0-27860800-1409765095_thumb.jpg


post-398-0-03137100-1409765145_thumb.jpg
 
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Could you also post the "RT60" plot window from REW for comparison?

Here is my equivalent filtered IR plot with Schroeder Integral applied:

K-402-MEH Impulse with Schroder Integral Filtered IR.jpg

I'm not sure that this is really useful in my case. All like plots in my room look like this one. The rise in reflection energy at ~50 ms corresponds to the position of a bar rising to belly button height in the middle of the room (the room's total depth is 12 metres). I don't think that it's anywhere near the "near field" to acoustically treat.

However, the family of curves under the "RT60" button (in REW) are much more useful in understanding nearfield effects, I find.

Chris
 
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Chris, my listening room is fairly live too for a 30' x 16' x 8' room. Some pics in this article.

There are 4 x 4' x 2' x 2" cloth covered Roxul panels on the back wall. 2 more of those wide band absorber panels on the walls to the left of the speakers. Quiet curtains on covering the windows on the front wall and a couple of bass traps covering a gas fireplace cavity.

I used to be an acoustic manufacturer of bass traps, wide band absorber panels and diffusers a long time ago.

RT60 and the other views for the left speaker https://www.diyaudio.com/forums/attachment.php?attachmentid=705396&stc=1&d=1537748834

Left and right overlay Topt: https://www.diyaudio.com/forums/attachment.php?attachmentid=705396&stc=1&d=1537748834

Reasonably smooth response from 225 Hz on up... Love that diyAudio has an acoustics forum!
 

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No T60 data, sorry. I took ARTA's advice and used T30, as they say that T60 is difficult to get clean.
What this does point out is the large reduction in the sheetrock wall Ping! echo around 1600 Hz. Very audible and very annoying without treatment. I later found another ping up above my speakers and treated those parts of the wall. I couldn't hear it directly, but reducing did make the room a little smoother.
 
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Hi Pano,
This 'ping' where treated within sidewall location?
With sheetrock / plasterboard i've experienced some 'honkyness/ping' from ceiling too. A nice place to use diffusors ( the frequency usually make them easy to place as they don't need to be to big in depth).
 
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Yes all treatment in this case was on the walls. The floor was thick Berber carpet on a thick pad. The ceiling was wood, and at 3.3 meters. I dabbled a little with ceiling treatments, but never found what I wanted. The Altec horns didn't hit the ceiling too much. I was lucky to have a little extra height and the wood. Ceilings can be a big problem.