Measuring modes

[Manslick]

so you pick a room, enter the dimensions in a mode calculator and the results show the room sucks. standing waves across the board.

you add some absorbent which changes the room.

what do you use to actually measure the room?

to be continued...

 

[Massive Master]

A measurement mic, SPL meter and spectrum analyzer (Room EQ Wizard is a fave).

 

[Chili]

behringer has a measuring mic for about $60. I read it is actually pretty good considering it's a behringer.

 

[Manslick]

thanks guys.
i was just reading about REW and it turns out i can use my trusty RS meter for a mic. i found an 1/8" to 1/4" plug. i even have a tripod too, so i spose i'm good to go.
i'm already pretty sure i have a nasty mode at 123 hz, simply based on the fact that the room really energizes at that frequency when i play a B on the guitar.
so knowing that, how do i tame it?

 

[jhbrandt]

Okay guys.. hang on! Not so fast...

Masnslick, you said, "...enter the dimensions in a mode calculator and the results show the room sucks. standing waves across the board." Emphasis ADDED.

Yes! Every single enclosure WILL HAVE STANDING WAVES. This is NOT a bad thing. They SUPPORT the sound in the room. Without them, you might as well be mixing in a field of flowers.

Now, when standing waves begin to create huge peaks and nulls it becomes a problem. That's why we need treatment so that these 'things' don't go on 'sloshing around' for ages. You want a couple of good bounces before they are absorbed. That's enough to give the sound IN the enclosure some support. Enclosures are rated by their volume and absorption to give a 'Schroeder frequency'. This is where sound begins to act like waves. Above the Schroeder frequency, sound will behave both like a wave and a ray.. this is called the transition region and above this is the Ray region.
All must be taken into account. Remember that all these things work together as a system.
Ideal absorption for a room to be used as a control room can be calculation using the following formula:
0.25*((room volume in cubic meters/100)^(1/3))
This is done for you on my room mode calculator spreadsheets - on my publications page. If you have difficulty or don't understand, please ask questions here or start a question thread so that everyone can benefit.

Cheers,
John

 

[Manslick]

hi John, as i have learned this year, acoustics is complicated. I’m reading the book The Master Handbook of Acoustics and it has me wanting to know more, specifically what’s going on in my room, a room I use for recording and monitoring.
I don’t remember offhand which site I entered my room dimensions, but it was near to top of a google search on measuring room modes. Maybe it was yours. But the results displayed a piano keyboard with green bars on every key. Then for fun I entered the dimensions on Abbey Road Studio 2. No green bars except for the very low end. I know this is just funning around. So if the first results weren’t depressing enough I had to torture myself a bit more by looking at Abbey Road.
I have no delusions of turning my room into Abbey road. I actually want to see what happens measuring the room acoustics; exactly where are the “bad guys” and what can be done. Right now I am enamored by the Helmholtz resonator. Can something as simple as a pipe tame a 123 hz standing wave? Maybe I should have mentioned at the beginning that I am interested in improving the low frequency response.
I’m not in a hurry.

 

[jhbrandt]

Understood.
Pressure traps are great when you get them to work properly. You can tune them like a missile to target certain frequencies, however in practice, I have not found high 'Q' traps very useful. - Same as high 'Q' equalizers. The most 'musical' EQ will have a 'Q' of .7 or lower.
So, I'd recommend beginning by treating the whole room to see where the response stands after that. If you get the LF tamed / under control, you've won 99% of the battle.

Cheers,
John

 

[Manslick]

You're right of course and the Q did make me realize that I'd need a tuned trap for every problem, which would make for an interesting space.
Right now it's an experiment and this is the classroom and you guys are the teacher.
My first assignment is to measure the room.
Thanks

 

[Chili]

If possible, can you post up before and after charts of the room's response? I very interested in how you make out. I understand this is a long term project.

 

[Manslick]

i already have (6) 2'x4'x4" rigid panels in the room and a floor to ceiling ~7'x2'x 6" thick panel angled in one corner. the room is much deader that it was of course. after reading the book chapters on diffusion etc, i got to thinking i went way overboard with the absorbent. i found it kind of ironic that spaciousness is achieved indoors thru reverberation and that outdoors in a free field, is not spacious. i mean, who thinks up this stuff. but it makes sense. so now i'm kind of lost, no i am lost. like what is the right amount of adsorbent in this room? i know i need some, because with none, my recordings sound like they were recorded in a basement, i get too much of the room. but then too much takes away the spaciousness. so somewhere in between there must be the happy medium. then there is the low frequency issue which i know is a problem.
I may just take it all out and start over.

right now i am reading the lit on REW and i am concerned about connecting my interface output to its input. isnt this like no no number 1? it says it needs to calibrate the card but i fear i may ruin it. this bad boy cost me a lot. its the propellerhead balance unit and i do not want to hurt it.

so ye its a long term project.

 

[Chili]

i am concerned about connecting my interface output to its input. isnt this like no no number 1?

How bad could it be????

 

[Manslick]

the guys at propellerhead helped me and i got my interface calibrated. right now i have the meter on the tripod and connected to the interface. i am now worried about feedback when i turn this on. talk about crossing streams. this one appears to be a gulf.

 

[Manslick]

also i made a scale drawing of the room so you can see the geometry.

 

[Manslick]

that was nerve wracking.

here is the SPL+Phase and the waterfall graph.

what does it mean?

 

[jhbrandt]

Manslick,

What room is this? MIXING or TRACKING?

For Either room, the decay is great. You need the trapping or the LF resonances will be out of control.

To keep mid & HF energy in the room, I recommend that you 'slat' the panels vertically. Maintain spacing of about 70% slat to 30% open absorption. This will provide more 'balance'.

You are on the right track. However, treatment in a tracking room can differ greatly. Rule of thumb for tracking spaces is; Never let untreated surfaces face each other. - For a control room Symmetry is critical. Tracking rooms are best Asymmetrical and asymmetrically treated.

Cheers,
John

 

[Manslick]

John, the room is both a tracking and mixing room. yes, the LF resonances are my focus. can you elaborate on the slat panels?
thanks

 

[GIK Acoustics]

that was nerve wracking.

here is the SPL+Phase and the waterfall graph.

what does it mean?

The waterfall graphs show the decay of frequencies over time. So it's sort of like seeing the frequency response, but you get to see which notes 'ring out' longer than others, etc. You can check a pretty detailed explanation of Waterfall graphs that I wrote on our site here: Understanding Decay Time and Waterfall Graphs - GIK Acoustics
Waterfall graph looks not that bad, though we're only seeing about 30-40dB down - might want to look down to about 60dB in the scale to see some more prominent room modes. The modes in the room under 100 Hz would be best suited for thicker corner traps - regular panels 5-8" thick will cover the rest fine.
You've also got some dips around the 150 Hz region which is normally due to speaker placement / SBIR. Could you elaborate on the monitor's distances from the walls? (front/side/floor/ceiling) ?
John's suggestion was to use wood (or similar) to affix in front of the absorption. This helps reflect some of the highs so you aren't deadening the room too much. And it helps mix the reflections in the room, leaving you with mostly the direct sound and some ambiance, instead of direct sound with a ton of specular reflections. This was the goal we were working towards when we designed the Scatter Plate - you can get an idea of that here: GIK Acoustics Scatter Plate

 

[Manslick]

thanks GIK i was hoping to find a place that explained the graph. post 13 above has a scaled drawing of the room.
yes i was not sure what part of the graph was important to post.
i attached another capture. so if i read this correctly, there is 71.1 dB @ 1.317 Khz for 400ms.?
ill check out those scatter plates too.
appreciate the feedback.

 

[GIK Acoustics]

thanks GIK i was hoping to find a place that explained the graph. post 13 above has a scaled drawing of the room.
yes i was not sure what part of the graph was important to post.
i attached another capture. so if i read this correctly, there is 71.1 dB @ 1.317 Khz for 400ms.?
ill check out those scatter plates too.
appreciate the feedback.

That looks a bit odd; typically no sounds should rise back in volume level. Was this measurement made with multiple sweeps or did it just go once? Try increasing the number of sweeps and make sure your speakers are pretty loud when you do the measurements to avoid the noisefloor messing with your decay measurements.

 

[Manslick]

it was one sweep and it was loud. ill try again tomorrow. right now fireworks are booming all around.

oh i should mention there is a drum set and acoustic guitars in the room. the effects from these i do not know.