# RT30 issue (help me to understand please)



## will050488 (Nov 20, 2012)

Hello!

I made some impulse responses with some balloons in a concert room. I have edit the wave file. Each impulse during 500ms (nothing was present on the waveform after this time). I have normalise the files to get the maximum of the impulse to 0dBFS.

I have import these impulses in REW and this is the impulse response I get in the "Filtered IR".
We can see that the T30 that is indicated is 0.741s. We can see that after 200ms, there is no sound louder than -30dB, so 2 questions :

1) Why the T30 indicated is 0.741s while there is no sound louder than -30dB after 200ms ?
2) How REW can found a TR30 bigger than the time of the impulse file I gave ti him?

Thank you very much to explain me where I'm wrong please!










if the img is not visible you can see it on this link : htp://toufman1.free.fr/impulse%20response.png


----------



## JohnM (Apr 11, 2006)

T30 is the 60dB decay time calculated using the slope of the best-fit line to the Schroeder Integral derived from the impulse response using the part that goes from -5dB to -35dB (the 30 in T30 means it uses a 30dB span of the Schroeder Integral). You can see the Schroeder Integral for your measurement by selecting the Schroeder trace. The meaning of the various decay parameters is explained further in the Filtered IR graph help.


----------



## will050488 (Nov 20, 2012)

Ok, thank you so much for your answer!
With your experience, could you say if the RT60 T30 by the Schroeder Integral is pretty clothe to the real RT60 measure? 

Other question, do you think that my method with balloon explosion is pertinent?
Is there any condition / standard to respect to import an IR measure wave file in REW ?

Thank you for your help

Will


----------



## JohnM (Apr 11, 2006)

The RT60 calculation methods REW uses are the accepted method of determining RT60.

Balloon bursts are one way of getting an impulse response, but using REW's usual sweep measurement method is more accurate and more consistent.

If importing impulse responses it is best if they are normalised and/or use 24 or 32 bit sample length. You will generally get better results by using REW to measure the impulse response itself, however.


----------



## will050488 (Nov 20, 2012)

Thanks for your answer.

I don't want to use the REW's sweep measurement method because I want to know the IR of the Room only, not the IR of the Room AND the speakers !
I want to know what is due to the speakers and what is due to the room. 
If you have a big resonance at a specific frequency, you can equalise that but it will be less efficiency than if you do an acoustic room treatment.

Thanks again for your answer!


----------



## AudiocRaver (Jun 6, 2012)

will050488 said:


> Thanks for your answer.
> 
> I don't want to use the REW's sweep measurement method because I want to know the IR of the Room only, not the IR of the Room AND the speakers !
> I want to know what is due to the speakers and what is due to the room.
> ...


It is a very interesting approach you are taking, looking at the room totally separate from the speakers. If you have time, it would be great if you could update us on the results you get with your room treatments. Best of luck!:sn:


----------



## AudiocRaver (Jun 6, 2012)

Any update for us? We would love to hear how your approach is working and what it is telling you about your room.


----------



## will050488 (Nov 20, 2012)

Sorry for the time before this answer, and please apologise my English (I'm French, but I look myself after!)

I didn't know that my approach have something special. I just try to know what are the problems (if there are) and what is the cause of these problems.

For example (very very basic example) if you have a standing wave problem a 250Hz, if you only do a measure with the speakers, and found that you have a big level at 250Hz at the sweet spot, you can make a correction with an EQ. This solution will probably give you a flat response at this point of the room, but will also make the situation worst in the other points of your place. The good solution would be to make some acoustic corrections and not EQ.

Well, by making measure with and without the speakers I can get the response of the place and the response of the place+speakers. So if I have a bump which is the same on the two curves, I will apply acoustic corrections, and if the bump or hollow is only on the place+speaker curve, I will correct this with the EQ, or I will try to see if it's not a coupling problem.

It's a very basic explanation but representative of the method. In reality, it suppose to make several measures with different balloons and to average the response because a balloon response is not so close to a dirac and so don't have an equal energy at each frequency.

I hope you have understanding my English, and it was helpful for you.

Bests
Will


----------



## JohnM (Apr 11, 2006)

An alternative would be to measure the speaker both in the room and outside.


----------



## will050488 (Nov 20, 2012)

Yes it's also possible in some case but totally impossible in my case (PA system in concert room, and recording studio). Plus, you have to find a place outside with electricity and no building around.... Or have an anechoic room...


----------



## AudiocRaver (Jun 6, 2012)

Don't know how much it matters, but is there a way to determine how close your balloon pop is to an ideal impulse? Just curious.:bigsmile:


----------



## will050488 (Nov 20, 2012)

it's a very interesting question that show us the limit of this practice.
But empirically, I can confirm that this method is not even so uninteresting and give us lots of very helpful informations.

But we have to keep in mind that it's not a perfect impulse and so, not a perfect IR. A way more accurate but most expansive in time and money is to use calibrated speakers, that we have the IR by analyse in anechoic room. Then you use this speaker (when i say speaker here, i also consider the amplifier which have to be the same than in laboratory!) to produce an IR using MLS response for example. Now you have an IR corresponding to the speaker+room. You just have to de-convolute this IR with the speaker's IR and you'll have the real room IR !!

Balloon method is less accurate of course, but if you use your here to verifying the results could be coherent with the reality of what you're hearing and if more of that you use different balloon IR to compare them and make an average, you should have precise idea of the room's problems....

Other things : I think we should also be careful about the level of the measurement when we are doing some measures. Traditional acoustic rules say that the level is not important (except for a signal to noise reason) and that you're room will have the same responses at low or hight level. But there are some researches that not considering sound by the wave model but by a flow (look at the Laurent Millot researches), and in this approach, level could change some things... Plus, I think it's also necessary to make the measurement near to the real level of utilisation of the speakers to have the same response of the amplifier and speakers and the same isophonic response of your ears... 

Well I hope you'll understand something despite of my bad English....


----------



## AudiocRaver (Jun 6, 2012)

Your English is fine.

Thank you for sharing your insights. I find this a very interesting topic.

Certainly you are very busy, but if at some point you have measurement graphs you could add to a post and share with us, we would love to see them.

Again, best of luck!:sn:


----------

