2ch listening room - first measurements and no treatment

[fuji]

Hi everyone!

I'd like to share my measurements of my untreated 2ch listening room - it's time to start listening to gear, not the room. Room size is far from perfect - square 4,5x4,5m. Listening spot 80cm from back wall. The biggest issue here is reverb time - around 700ms. Caused by large window and empty concrete walls. Could you please help me understand measurements and RT60 Decay graph? I've read that RT60 itself is not a good measurement for small rooms reverb time and RT60 Decay is preferred; however, it is more complicated

 

[Cbru]

What are you using to do measurements? REW outputs are easier to read.

I’m not an expert, fair warning.

You seem to have fairly consistently long decay times, with a big bump maybe a room mode, around 40 hz. It’s actually theoretically nice that it’s evenly long decay (by frequency) but this is way too long and I’m assuming doesn’t make for good listening.

There are a number of things to do; not sure how much time you’ve spend positioning your speakers — this is importing regardless of first-order acoustic treatment (first reflection points and overall decay time).

Based on my understanding and experience, I would start with broadband absorbers. You have a lot of long resonance, and the suboptimal square shape. (I’m assuming your ceiling is not 4.5m, and hoping it’s not 2.25m). You will want to try to uniformly reduce the overall reverberation time to a lower value (opinions vary on the optimal, but start with some treatment then measure again and listen).

You will inevitably need more than you think. I would start with 6-8 broadband absorbers. You may want to consider diffusion panels (aka scatter plates) on the front of absorber panels to avoid over deadening the room, but this is probably not going to be an issue for your first pass. Broadband means thick. Ones that I’ve used with success are 6” thick or more. (May also be called a ‘bass trap’ but ensure its absorbing in the mid and treble range too — most do)

I would start with sidewall reflection points (as determined optically between your seated ear position and speaker tweeter position, use a laser pointer and a mirror), ceiling reflection point (if you’re feeling brave but it’s worth it), and wall behind the listening position. Using a laser pointer find where the tweeters hit the back wall, and place absorber there.

In this way you are mitigating the first / worst reflections, and also reducing the overall reverb time.

By using broadband absorbers you are treating the bass as well as mid and treble. This is important BECAUSE materially absorbing the bass energy in a room requires lots and lots of treatment, which has to be thick. If you start with only midrange absorption, you will be leaving the long bass tail untreated while reducing the mid / treble reverb.

Honestly this is pretty generic advice. If you are able to measure with REW (follow the guidelines about graph limits posted on other threads) there may be more to say. But given what you’ve identified about your room you should do a ‘first pass’ mitigation treatment, measure again, listen again, reposition your speakers, measure again, then we can go through more detailed options.

 

[Keith_W]

You are looking at the wrong graph here to determine RT60. Nevertheless, we can still gain useful information.

1. Unless you measured your noise floor with an SPL meter, REW has no way of knowing what the noise floor is. Nevertheless, human eyes can tell you the noise floor. In your graph above, you can see that the sound stops decaying after a certain point. This is your noise floor, it is -36dB below the main signal. This does NOT mean that your noise floor is 36dB above 0dB ... who knows what that is if you don't have an SPL meter.

2. The RT60 = "reverberant time for sound to decay by 60dB after the EDT". The RT60 has no meaning in small room acoustics because (1) sound does not form reverberant fields at low frequencies, they form room modes, and (2) the noise floor in typical listening rooms is too high for sound to decay by 60dB. In your listening room, we can see that the noise floor is about -36dB to the main signal.

We can still estimate the RT60 by extrapolation. The "EDT" (Early Decay Time) is the initial decay before the first reflection. By convention this is 5ms. So we can see that at 5ms, the sound has decayed by -4dB. The time to decay to -24dB (i.e. 20dB decay) is 150ms. Since sound decays at a linear rate, the estimated RT60 is 450ms. This is known as the T20 - or "time for sound to decay by 20dB after the EDT, extrapolated to 60dB".

In reality, the RT60 target is frequency dependent, room size dependent, and application dependent. It looks something like this:

Look at the green lines only, ignore the other lines. The green lines show the RT60 tolerance target for a particular room which I grabbed from an online RT60 calculator. Where those green lines should sit depends on the variables mentioned.

Your listening room is divided into zones. I have calculated the zones and drawn them in for you. I have assumed a room height of 2.5m so the estimated room volume is 50m3. I used the T20 calculated before of 0.45s. Read more about the Schroder frequency here.

In the pressure zone, wavelengths are too long to form room modes. They pressurize the room instead (assuming your room is sealed and there are no open doors which will throw off the calculation). In the modal zone, long wavelengths form typical peaks and dips that we see in any room. In the diffuse field, sound behaves less like waves and more like rays. Above the transition zone, the loudspeaker dictates the sound. Below the transition zone, it is the loudspeaker and room combined.

I strongly suggest that you buy Dr. Toole's book and read it. He points out that the domestic RT60 target can usually be achieved with normal room furniture and without any room treatment. Remember that the target has upper and lower limits - where that limit lies for YOU depends on your preference and your views on direct vs. reflected sound. It is surprisingly controversial. Everybody agrees that too many reflections = loss of clarity, too few reflections = unpleasant loss of ambience and "dead" sound. But where those limits should be ... well, do a search on ASR and read those long threads.