Measuring standing waves - dad's studio treatment masquerading as a 6th-grade science project ;)

[Bobo Fret]

Thanks (and apologies) in advance guys and gals cuz this is a dry-ass read:

So my 11-year-old was tasked with coming up with ideas for a science fair, and we have some ideas, but his teacher is nudging him towards something math/physics-related. I thought it might be interesting (and practical for my recording space) to demonstrate the presence and then (hopefully) the absence of acoustical standing waves pre/post room treatment. I've been reading up on it a bit learning about room modes and such, and I see how you can calculate standing waves and predict potential problem resonant frequencies based on the dimensions of my physical space, but how do you actually measure/visualize/demonstrate a standing wave? Or even differentiate it from a non-standing wave? A microphone and....some kind of app/software? Oscilloscope? Watt Dabney's Inverted Firkin? Is it something I can detect with just the waveforms recorded in my DAW?

Assuming this is possible, what kind of polar pattern mic would you use? Where would you place it. My space is not a perfect rectangle.

 

[Tone_Terrific]

Interesting. I would explore laying out a grid pattern on the floor and using calibrated mics to map out peaks and valleys of the levels using test signals, at all the intersections.
I have no idea how it is properly done.

 

[MikeMcK]

Interesting. I would explore laying out a grid pattern on the floor and using calibrated mics to map out peaks and valleys of the levels using test signals, at all the intersections.
I have no idea how it is properly done.

The grid thing could work, but remember, there's no way to catch a single reflection. What you'd actually capture would the sum of all waves (source plus any reflections) at that point in space. IOW, in a reflective room you might find a spot where a specific frequency seems "dead" due to a reflection arriving at that point out of phase with the source signal. Even that is much simpler than real life, because at every point on the grid you'd be capturing all the waves, reflected from every reflective surface.

The mic would probably be a reference microphone. Theoretically (and remember, real-life engineering is never as simple as the theory) you could get a data set by generating a white noise signal into the room, capturing response by a reference mic at many points of the "grid", and saving the data as frequency data... I know there are VSTs that do Fourier Transforms (FFTs) to convert time-domain data (like what you see on the tracks of a DAW) into frequency plots.

If this is even feasible, it might be simpler to pick a couple of frequencies (or maybe just one) and capture them one at a time instead of the white noise signal. In that case you'd generate a sine wave instead of the noise profile. For example, if you generated 110 Hz signal you could probably get some useful data by spacing your grid at 1 ft. intervals. Even then, that would be 200 measurements for a 10' x 20' room... that's a big job already. For higher frequencies the grid would need to be smaller. And the number of measurements increases quickly... to go from a grid with 12" spacing (200 measurements) to 4" spacing, you now need 1,800 measurements.

 

[Terry McInturff]

You'll need a calibrated mic such as the one sold by Dayton. Look into the free download Room EQ Wizard and you'll be off to the races, Dad!

 

[Bobo Fret]

Wow! Plenty to get me started. Now I need to learn some stuff...

Thanks fellas!

 

[Bobo Fret]

An yeah, my hope was to try to isolate the experiment to bass frequency standing waves basically using just bass traps in a room. Maybe frequencies in the 40-100Hz range.

 

[sws1]

Put out a test tone at a certain frequency. And then measure at key spots.
You can figure out which frequencies to show peaks and dips by using the dimensions of your room

ex: based in the length of my room, I know 31hz is a bad frequency. Sure enough, if u walk to the center of the room, that signal is gone. But at the front and back wall, it’s extremely prominent.

 

[Bobo Fret]

Thanks!

Actually, that is related to one of the questions I had...

So do I actually NEED the dimensions of my room? I see a number of frequency analyzer apps/software have areas to input room dimensions to help with calculations. I'm not sure if it's required, or if it's just helpful. But what if my room is irregular in shape? Do I even need this information, or can I simply measure the wave amplitudes at different locations?

 

[John Quinn]

Here is a particularly good analysis of room modes and standing waves. It's complicated - but also easy to understand.

 

[sws1]

Use this website.

Plug in your dimensions. Not only lists the frequencies where the nodes are. But the 3D graph shows you where in the room those frequencies go up and down. Use that to pick out some places to put a mic.

Beyond the 3rd or 4th order node, it's a bit harder to hear the change. But for 1st, 2nd, and 3rd, it's very easy to hear sound go up and down.

amroc - THE Room Mode Calculator

HTML5 room mode calculator. Read and hear the tune of axial, tangential and oblique modes. Scientific sources included.

amcoustics.com

 

[MikeMcK]

Thanks!

Actually, that is related to one of the questions I had...

So do I actually NEED the dimensions of my room? I see a number of frequency analyzer apps/software have areas to input room dimensions to help with calculations. I'm not sure if it's required, or if it's just helpful. But what if my room is irregular in shape? Do I even need this information, or can I simply measure the wave amplitudes at different locations?

You can just take the measurements (again, the smaller the grid spacing, the more work but the more accurate your data will be). Software can calculate the expected theoretical response, which is going to be useful as a comparison to your measurements.

I've never used software like this but again thinking in terms of the theory, there will be differences between measured and calculated data that depends on a lot of things... this topic goes deep. For example, I don't know how you account for reflection coefficients (basically "how reflective is that surface, because if it's concrete it will reflect differently than a thickly carpeted sheet rock wall?") but that would be an interesting part of analyzing the data.

You could narrow the whole thing down to picking one or two frequencies and measuring with and without bass traps. Picking two frequencies with and without traps means measuring the grid four times.

Edit: P.S. - yes, an irregularly shaped room is definitely a factor... no clue whether there's software to account for that. But it's common... one of the things you notice about rooms designed for recording is that there are pretty much never any two parallel surfaces for this reason, and if there have to be, one of them will have acoustic tiling to scatter those reflections.

 

[Bobo Fret]

Thanks!

I found the amroc Room Calculator already, and that was sort of the basis of questioning how to deal with an irregularly shaped room.

The grid idea sounds like the way to go. I recognize that there are a number of variables that go into analyzing/treating a room, and this definitely interests me, particularly as this is a brand new studio space for me, but for the purposes of executing a not too overly complicated experiment with numerous variables (ie. wall materials, bass trap effectiveness made with this vs. that material, etc., etc.), I think just a simple pre/post bass trap measurement of a couple frequencies should put us where we need to be.

Cheers

 

[Tone_Terrific]

, I think just a simple pre/post bass trap measurement of a couple frequencies should put us where we need to be.

So my 11-year-old was tasked with coming up with ideas for a science fair,

If you explain and demonstrate the notion of standing waves within a room and how the room itself and furniture location can affect what we actually hear, throw in some equations and software math, I think you will have enough for an 11 year old to tackle.