Understanding SPL with no room treatment, and first reflection treatment

[bommai]

Hello,

My son and I are new to REW. I have played around with it before but not in a serious way. My son is doing a science project on the whole room treatment issue (for his 8th grade science research class). I am attaching the data file to this post. His project scope is to see how treating first reflection points help with sound quality. We have a center channel speaker that we are trying to test with. We are using a 2014 Mac Mini running Big Sur OS and latest version of REW (5.20 beta 61) connected via HDMI to a Denon x4300h receiver. We are using a JBL 530 book shelf speaker as a center channel on a stand. We are using a Umik-1 as microphone with the calibration file loaded. We are using Measure and setting the signal level as 80dB (by adjusting the volume on the denon). Then start measurement. We are using Pure Direct on the Denon so it does not perform any bass management or any eq (I hope that is a correct assumption). We took 10 measurements back to back with a frequency sweep of 80-20kHz. We did an average after that. We saved off that result. Then we repeated that by adding a 2" compressed fiberglass panel treatment to the left and right wall where the first reflection point is for the center channel speaker. We also repeated with 4" compressed fiberglass panels instead of 2". Then we used 2" mineral wool (rockwool) panels, then 4" rockwool panels, Then the loose fiberglass batting as treatment. We are limiting our project to treating first reflection points. We did not treat the ceiling because it was too hard to do! We thought something would look better with the treatment but as you can see from the attached data files, nothing looks that great. He is having trouble writing his conclusion for his project. Yikes. Any ideas.

To make matters worse, I accidentally broke the UMik-1 by trying to reseat a loose tip and the wire came apart Luckily we have already collected the data but we won't be able to collect anymore!

Thanks!

 

[sam_adams]

Since you averaged the responses in the posted .mdat file, there are no impulse responses to determine the room reflections from. If you have the un-averaged responses in another .mdat, they would be of the most help.

 

[EarlK]

Yes, Yikes!

What was the "Abstract" given out in this assignment ?

At this point it would seem that what you have proved is that room treatment doesn't work ( when looking strictly at the Frequency Responses posted ).

Since we know that conclusion is quite wrong we ( you ) are left wondering why ( the false conclusion ) ?

The simple answer is insufficient square footage ( in coverage ). IOW; You didn't cover enough reflective surface ( +/or the correct ones ) to make a measurable difference .

It also seems that you focused on density ( volume vs area ).

If the project is due tomorrow , well, that's the conclusion one can use for your failure .

NOW:

Unfortunately by averaging all your individual measurements you threw away the means/info ( IR > Impulse ) that might have helped extract some data to salvage the project ( IMHO ).
Do you still have the individual non-averaged files ??

Had you asked for some guidance previously to this, I would have suggested re-creating the seminal ( 1985 ) work of D.R. Jones, W. L. Marten and G. S. Kendall ( called ) " Optimizing Control Rooms for Stereo Imagery" .

Here's a link to William L. Martens Acoustics, asn.au pdf
- It's a bit of a ( 2016 ) summary to the original 1985 ASA paper.

Here's a pic ( taken from the linked to pdf ) that shows the initial response and the final response after adding selected Sonex ( foam ) panels to critical points within a small room.
- Pay attention to where the single speaker was placed for this test and also try to get some insights as to where ( + why ) the authors decided to place the panels in the specific//sequential locations.

This smoothing of the frequency response was achieved by covering only 20% of the rooms total reflective surfaces ( which includes floor + roof ).

Location, location, location is the mantra to take away from this exercise.

If there's time > fix the mic and recreate this work > one can even put the absorbing panels in other ( wrong ) areas to see their compromised effect.

I believe the sonex panels used were 2'x2' .

 

[bommai]

I have the raw data as well I will post this soon. Thank you very much for your suggestions

 

[sam_adams]

There's some interesting information from Bob Gold on RT60 calculations and absorbers on his site.

 

[bommai]

The purpose of the project was to determine which material provided the best sound quality improvement for first reflection. We used a mirror to determine where the first reflection was for the single speaker under test. We use 2" OC 703, 4" OC 703, 2" Mineral wool, 4" mineral wool, and also the fluffy batting. We covered 2' W x 4' H area on the left and right. We did not do the ceiling. The following files contain 10 samples each. Just in case you find something that makes sense. In the mean time, my son and I will read up on the material posted above. We are really really thankful for all the help you are providing.

 

[EarlK]

EDIT

 

[DanDan]

The Ceiling and Floor and perhaps a Desk? are all delivering Early Reflections from multiple sounces.
A 2'x4' area each side, maybe relatively far from speaker and listener, is unlikely to cause a big change.
If you want to clearly see echoes being neutralised I suggest focus on one wall, or the floor. If you want to demonstrate the sonic change, play some music and record.

 

[bommai]

The Ceiling and Floor and perhaps a Desk? are all delivering Early Reflections from multiple sounces.
A 2'x4' area each side, maybe relatively far from speaker and listener, is unlikely to cause a big change.
If you want to clearly see echoes being neutralised I suggest focus on one wall, or the floor. If you want to demonstrate the sonic change, play some music and record.

For the purpose of this science experiment, we chose to place a book shelf speaker where a center channel speaker will typically go and tested only that speaker using REW. The width of my room is 15'. I am about 12 feet from the center channel speaker. I have a 8' ceiling. My floor is carpet. We determined early reflection point using the mirror method on the wall. We were going to take more readings but our mic is broken ! I have ordered another one from Amazon but it won't be here until wednesday.

 

[jtalden]

There are several ways to see the impact. One visual idea is along these lines:

The circled horizontal line at 7.36 ms in the first spectrogram below is the reflection of one of the walls with no absorbent. The second spectrogram shows a greatly reduced sound energy at this same time due to the acoustic treatment on the wall.

[I suspect the line at 8.4 ms in the first chart may be the other side wall as the speaker may be off center that much? There is some impact there as well, but not as much. The other horizontal lines are from other room surfaces and are thus not changed significantly.]

The difference in distance of the reflected sound off the wall vs. the direct sound from the speaker (for the 7.36 ms reflection) will be about 8.3 ft. If your physical measurement of the difference checks out then this helps confirm the analysis. The attached mdat file contains the measurement used in this example.

Lower frequencies are not impacted very much as the absorption is most effective at the higher frequencies.

 

[EarlK]

The purpose of the project was to determine which material provided the "best sound quality improvement for first reflection."

Let's assume the above quote also translates into most attenuation of total ( direct + reflected ) energy.

One could do more in-depth comparisons <> of the type that jtalden has done for two scenarios looking to see the reduction in reflected energy <> and expand the comparisons to all absorption types.

Moving on to what I did ( looking at a reduction of total energy ).

First Conclusion ( from the data I analyzed ) :

- There's little to no advantage in using the 10cm versions of the 3 product types ( though a 5cm Fluffy Batten style fiberglass was never studied ).
- Here are the pics that led to my conclusion:

Second Conclusion :

- There's always some advantage gained by using any type of the 3 absorption types ( here I had to include the 10cm Fluffy Fiberglass along with the other 2 , 5cm types ).
- Here are the pics that led to my conclusion:

Third Conclusion ( what's best ?? is inclonclusive ) :

- All 3 product types offer slightly different HF profiles ( or flatness above 3K )
- The 10cm "Fluffy Batten" attenuates the most. Whether that makes it the "best" can be debated.
- The 5cm Mineral Wool example attenuates the least ( in all frequency areas ).
- The 5cm Compressed Fiberglass ( Owens Corning 703?? ) is somewhat in between the preceding two examples ( it's not quite as flat above 3K ).
- Here are the pics that led to my "non-conclusion":

The expanded db scales shows that there is indeed acoustic treatment happening with all the different types of absorption.

It's a subjective call ( that includes costing, availability, ease-of-use, and environmental // health questions ) as to which is the "best" to utilize in this project ( at that location ).



PS: Here's a clarity C50 >< honestly this doesn't tell me much >< though "Purple" is more consistently higher than the rest ( higher is better ).

.

 

[bommai]

Everyone that has helped us so far - a big thanks. I think if we can surmise that any treatment is better than no treatment, that would make a case. We are learning Just wish we had more time

 

[DanDan]

Different graphs settings might reveal more. i.e. a smaller vertical range and less smoothing.
HF effects will not be different between 5cm and 10cm. One would want to look further down the spectrum.
FR is one way. Filtered ETCs might be interesting here.