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Anomolies in Science Project (help!)

978 Views 10 Replies 6 Participants Last post by  cmslick3
We've completed our science project (thanks again for the help on the test tone generators!)

Anyway, I think we may have introduced some anomolies by making the sealed box out of plywood.

To give you an idea of the what this looks like and how we proceeded:

1. This is the open box with nothing inside:


2. This is when we put the wood (actually 2 pieces of 3/4" particle board)


3. This is when we put the top on


We repeated this test with a variety of materials and different frequencies (66,125,250,500,1000,2000,4000 & 8000) and these are the results:


From the results, you can see at 2000 hz, all materials actually registered a higher db than with nothing at all! We validated this over a number of tests.

I seem to recall there is something called "harmonic resonance" ?? where the "box" can actually start resonating, creating it's own sound. Does this explain why air has a lower db level than with other materials? My son needs to explain this ;-)

Also - it was curious to see that wood performed worse than cardboard at certain frequencies..I think this may be because of all the layers (8 pieces of cardboard taped together)...or it is another anomolie created by making the box out of wood.

Another question my son had was "why did the decible readings vary so much by frequencies with air"? My limited knowledge says that the speaker's efficiency varies by frequency. I think (again that dangerous word)! this is called frequency response?

Finally, I was trying to find specs on the speaker...this was a Marantz bookshelf speaker, model LS540MX. I couldn't find anything on it using google or Marantz's web site...it only says "6 ohm" on the backside.

In summary, obviously it would have been better to make a box out of cement and perhaps put some sort of insulation around the SPL meter ... but this is a 5th grade science project, so we are doing the best we can.

Any help / comments appreciated!

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Every material in the physical world has a resonant frequency. I'd bet that a knock on the wood with your knuckles would generate a sound of the same frequency where your conducted-audio peak is.
So, let's see. The speaker is outside of the 'box', presumably facing the various materials your son is interested in examining for their ability to attenuate different frequencies. The RS SPL meter is inside the box and can be viewed through an opening in the top lid (is the opening 'open' or is it sealed with plexi? A hypothetical practical goal might be to determine which material is best suited to be used to make a room more soundproof. Is that pretty close to things?

Right now, I've just got two comments. First, the RS SPL meter is really only intended to be used as a means of measuring or estimating SPL from a broad band source. Using it to measure SPL's of individual frequencies and expecting the unit to have a linear response (even with supposed corrections) is asking for a lot. Second, you can get SPL variations if the material you're investigating - cardboard, wood, ceiling tile, etc. doesn't seal with consistency.
1) A nice bit of eye candy would be to plug the meter into a real-time display on a PC. The software is free (e.g. TrueRTA), and all the PC needs is a sound card and a cable.

2) The box is an acoustic disaster. It will have all sorts of effects on the result. Right off the top of my head, the best quick fix is to line it with sound-absorbent foam, and try to stay away from it's resonant frequencies.

3) Explaining the effects of the box on the result is not going to be easy. One approach would be to do it empirically. Run a test without any test material, then subtract that curve from the one you get with test material. Absolute levels will be different, but any chamber resonance should appear on both curves, and subtracting will cancel them out. Understanding the purpose of a control and how to use it may well be the best lesson that could be taught in such a science project.
The intent of the project was to measure transmission loss for various materials. We did use "air" or no materials as our baseline, or control group.

This project really created more "research needed" than real answers because of all the anomolies. He did learn a lot through the various iterations - consistent distance from SPL meter to Speaker, modifying the box to eliminate "gaps" of air-space that would allow sound through, calibrating volume on the speaker so he could repeat the experiment after CD player was turned off, etc.

There is no doubt this "box" is an acoustic disaster - My son took the idea off of a science web site where there was no box - just a speaker and a SPL meter with a "stand" between the meter & speaker to hold the various materials. We opted for the box b/c we didn't think it was wise to have the sound waves bouncing all over the place - not to mention the various background noises in the house. Is it possible that this "open air" approach is better than a box & we made the wrong decision in trying to isolate the SPL meter?

So from the answers, is it called "resonance", "frequency resonance" or "harmonic resonance" when the box actually vibrates?

Thx for the understanding on the SPL meter and it's tendency to measure broad band (also called "white noise"?) and is not particularly suited towards specfic frequencies. Am I wrong in my assumption that speakers themselves often can't produce a consistent linear response? I seem to recall that speakers (especially a cheap one like this one) are more / less efficient at certain frequencies. Does this not play at all in our experiment?

No doubt the pros go about this differently, but for 5th grade it really opened his eyes up & he learned a lot!

Thx again for the responses & I welcome any more clarification to the questions above!
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Well you can repeat the experiment without the box using test tones without a box. You can also run pink noise instead and examine the results both ways. The FR of speakers is always different in a room and the specs you see would be if place in an anechoic chamber.

In the comparison of the results, you can ratio them to 'air' and illustrate to your son the logarithmic nature of dB and how that translates to % reduction.
Free air would be better, but I agree that it would be impractical given the context.

Linearity of the meter and the speaker should be factored out when you subtract the control data.

If I were judging a project where someone did an experiment, and then did a critique of the experiment showing weaknesses and proposing improvements, I'd want to give him the gold medal. That's *real* science. Kinda tough to present that in an eye-catching way, though...
Free air would seem to be better and then the attenuation that you get will be predominantly for the material being tested. You could also put foam, fiberglas or pillows on the sides bottom and top of the box to prevent or diminish unwanted reflections. Keeping in mind that this is a 5th graders project, you aren't trying to get absolute accuracy. What you want is a reasonable look at first order effects. Any sound absorption material will have a set of frequencies that it works well with and it may have frequency ranges that it does not work well with. Keep his focus on the first order effects. If there is an anomaly that you need to explain, Then run an additional experiment to help understand the anomaly. If you think that your test box is the problem with a particular material then run a quick test in free air. Do the results agree or disagree? Take some pictures of the testing done without the box and discuss that as test method validation. The key is keeping it simple. Too many science fair projects turn into a competition of the dads instead of provoking a deep interest in science for the kid. Your role is to ask him questions that help him figure out what is going on. Don't worry about the linearity of the meter. It is an instrument and it is the best one he has and it will do.

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Originally posted by DoyleS
Too many science fair projects turn into a competition of the dads instead of provoking a deep interest in science for the kid. Your role is to ask him questions that help him figure out what is going on. Don't worry about the linearity of the meter. It is an instrument and it is the best one he has and it will do.
My problem is that I know so little. I paid a guy to help me build my HT and while I learned a lot - that was 6 years ago and to be honest, I've forgotten most of it.

I can't answer many of his questions so that is why I am posting here. He's asking great questions and doing all the work, I just feel at a loss for not being able to give him straight answers.

That's why I am asking these questions for him here, because I know you guys know so much.

While people have offered great advice and suggestions, no one has answered his simple questions....or I'm not getting the answers from your responses...sigh

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Why not put him on the phone?
I believe the results you are seeing are correct based on the theory of "bone conduction". Try talking to your son a normal distance apart, now plug your ears and touch your heads together and talk. This is what's known as bone conduction. Same concept that helps some deaf people hear. http://www.entific.com/directBoneConduction.asp So basically what you have done is prove that implants do work.!

I have been through years of problems with my ears and each time I had a hearing test they did one with open air (headphones) and the other with bone conduction. Each time the bone conduction score was much better.

In order to eliminate this from effecting your experiment you need to either suspend the meter via string or fishing line from an un-attached piece of wood (isolating the meter from the box structure)... OR put it on foam and try again.

Hopefuly this will remove as much error as possible from your experiment.
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