RT60 measurement

[Samazar]

How can one measure room reverb time in REW (RT60). Do I have to record an impulse and open it in REW or can REW handle it directly via a umik1 mic ?

 

[Wayne A. Pflughaupt]

Hopefully John can verify, but I’m pretty sure REW can handle it with the mic (any mic can be used for acoustics measurements by the way, no calibration required). You don’t have to provide an external signal source.

Regards,
Wayne

 

[John Mulcahy]

Sure, just make a measurement with REW and look at the RT60 graph.

 

[Samazar]

Hi John/Wayne, thank you for your response. Do I take a sine wave measurement at 75dB for REW to generate the RT60 graph or do I need an impulse like using 2 planks of wood to generate the sound ?

Also, with the latest version of REW is it possible for me to use the HDMI port in my MacBook Pro to connect to my processor and select individual channels within the 7.1 channels ?

 

[Samazar]

How do I interpret the RT60 graph, is it the average "Topt" value that I should be looking at ? The value are different along the single octave bands.

 

[John Mulcahy]

You use the Measure button on REW to take a response measurement and REW does the rest, including extracting the impulse response and calculating the RT60 values. You can select individual channels on a Macbook provided the HDMI output is configured for multichannel in the Mac's Audio/Midi settings.

 

[Samazar]

Fantastic! - I've just discovered the data panel which presents information in a neat table. What should I be looking at or doing to get the RT60 of a room ?

 

[John Mulcahy]

Not sure I follow, that data is the RT60 of the room. RT60 varies with frequency.

 

[John Mulcahy]

If you want a single figure you can get that from the data panel of the Filtered IR graph (with no filter applied).

 

[Samazar]

Hi John, I'm learning how to use REW. Love the interface and how it works. Any references that show how to configure the Mac's Audio/Midi setting for REW ?

 

[John Mulcahy]

Sure, in the help at http://www.roomeqwizard.com/betahelp/help_en-GB/html/calsoundcard.html#multichannel

 

[Matthew J Poes]

If you post your results we can help you interpret them as well.

There are some issues with RT60 in small spaces so it may be helpful to look at both the Topt and EDT. EDT is another measure of decay but is not the same as RT60. Because energy decays so quickly in a small room, EDT may be more informative.

 

[Samazar]

John and Matthew, thank you very much.

Matthew, I will post my results and your help will be appreciated.

 

[Matthew J Poes]

John and Matthew, thank you very much.

Matthew, I will post my results and your help will be appreciated.

If you type @Matthew J Poes I will be sure to see it.

 

[Samazar]

By the way, is it best to use the center channel in conjunction with the subwoofer for the RT60 measurement ? The mic would be placed in the main listening position at ear level. Alternatively, I could take 4 readings around the listening area and average them. I guess spatial averaging enables better measurement repeatability.

 

[Matthew J Poes]

By the way, is it best to use the center channel in conjunction with the subwoofer for the RT60 measurement ? The mic would be placed in the main listening position at ear level. Alternatively, I could take 4 readings around the listening area and average them. I guess spatial averaging enables better measurement repeatability.

You can't get RT60 or decay from spatial averaging or from RTA measurements. You need to use impulse measurements. You can take a number of these, time align them, and then vector average them if you want to look at the average RT60 and Waterfall. That is a little complicated for what you are trying to do.

You only need the center (or any main speaker) if you want to look at RT60 since that is really only valid above where the subwoofer operates.

 

[Samazar]

Sure, in the help at http://www.roomeqwizard.com/betahelp/help_en-GB/html/calsoundcard.html#multichannel

Hi John, I've finally managed to get all my tools and connections in order:-

1. I am using REW V5.19 Beta 8 (Running JRE 1.8.0_102 64-bit on OS x 10.10.5.

2. I am using the Umik-1 USB mic.

My MacBook Pro Retina 15-inch has an HDMI port and I am using a long HDMI cable to connect my laptop to my processor. The processor is recognised and shows up in the Audio Midi Utility. HDMI device shows up in the list of devices to the left. However, the Master Volume control along with level controls for each channel (value and db fields and Mute check-box) are all greyed out. The same is the case when I choose the HDMI option under "Aggregate Device". I have included an image for your reference.

I need to be able to pick/mute channels and play REW test tones to them. I don't understand, what I am I doing wrong ?

 

[EarlK]

If your setup is anything like the above pic then unplug the HDMI cable going to the TV, ( then power down everything ) and turn back on the AVR and the Mac Mini ( starting with the AVR ).

If that doesn't fix/activate the greyed out section ( in the Audio Devices panel ) then I'm out of ideas.

 

[John Mulcahy]

What is the aggregate device for? Not sure why you wouldn't just use the HDMI output directly.

It doesn't matter whether the volume controls in Audio Midi setup are active or not, they aren't used by REW. Just select the individual channels in REW.

 

[Samazar]

Thanks John, I appreciate your help.

 

[louisa.chanx]

Hi,

I'm new with REW and I got similar question about RT60 measurement.
List the equipment first:
Soundcard: Asus laptop built-in (Conexant)/
An active speaker connects to Asus laptop 3.5mm output/
Umik-1 mic connect to Asus laptop USB port

Question about RT60 measurement:
1) Do I need to calibrate umik-1 microphone before the measurement? I have my owned SPL meter, but the SPL value must include A-weighting or C-weighting or Z-weighting, can i use this meter?
2) From discussion above, it looks like I just need to start a frequency response measurement then REW will calculate RT60 for me, right? Why RT60 can be measured by this way?
3) How to calculate RT60 from T20, T30 and EDT?

Look forward to hear back, thank you.

Regards,
Louisa

 

[John Mulcahy]

The UMIK-1 comes with a calibration file that includes a sensitivity figure, if you load the file in the REW mic/meter preferences and select the UMIK as the input device and input REW will show a calibrated SPL figure. REW generally prompts you for that if it sees a UMIK connected when it starts up and will take the necessary steps itself.

RT60 is calculated from the impulse response, the impulse response is calculated from the measurement.

T20, T30 and REW's Topt are all RT60 estimates based on different parts of the Schroeder integral derived from the impulse response. There is more on that in the help. EDT is the Early Decay Time, it is not an RT60 estimate but rather a figure for how quickly the sound initially decays.

 

[louisa.chanx]

Hi,

One more question -- How detail of RT60 that REW can tell?
Some DUTs have unstable results when doing acoustic test in our lab, we suspect that might be DUT is sensitive with reverberation. I hope I can see the RT60 is not so stable in DUT point and more stable in other points or more stable if i put some absorb material right next to the DUT.

Regards,
Louisa

 

[John Mulcahy]

The Filtered IR graph shows the Schroeder integral and has a data panel that includes a Curvature figure, both help in evaluating the quality of the RT60 measurement.

 

[louisa.chanx]

Thank you, John. I can get lab RT60 now and the value looks reasonable.