How Can I Measure Relative Amplitudes of Specific Frequencies in a Noise Field?

[Bert Rackett]

I would like to measure the relative amplitudes (energy levels) of several specific
frequencies in a noise field. I thought of attaching capacitive microphones to
tubes that would resonate at those frequencies. I've visited hundreds of web sites
that invariably give equations for frequencies and resonant points, but say nothing
about the amplitude domain unless they're talking about musical instruments.
How much larger will my response be in my tube? How large are the harmonic
responses? I have several texts, but they speak qualitatively about resonances and
not quantitatively. can someone point out a text with the math?
Thank you.
Bert Rackett

 

[billiards]

As long as you're sampling at more than twice the highest frequency you want to detect, can't you just put the signal through a fast Fourier transform? You could use something like MatLab to do this and plot the Power Spectrum quite easily.

It would be interesting to see how the power spectrum changes in relation to the position of your microphone. You might expect to see notches at frequencies with wavelengths that destructively interfere with reflections off of the surrounding walls.

 

[astrokreng]

I understand your desire to measure the acoustic energy at specific frequencies in a noise field. Your idea of using capacitive microphones attached to resonating tubes is a good approach. However, in order to accurately measure the energy levels at each frequency, you will need to consider several factors.

Firstly, the size and shape of the tubes will affect the amplitude of the response. The larger the tube, the larger the response will be. This is because larger tubes have a larger surface area for sound waves to interact with, resulting in a higher amplitude response.

Additionally, the material and thickness of the tubes will also impact the amplitude of the response. Materials with higher density and thickness will have a higher amplitude response as they are better at transmitting sound waves.

In terms of the harmonic responses, these will depend on the specific frequencies and their relationship to each other. Harmonics occur when a sound wave interacts with a resonant object, in this case, the tubes. The amplitude of the harmonic responses will vary depending on the strength of the original sound wave and the resonant properties of the tubes.

To accurately determine the amplitude of the responses, you will need to use mathematical equations that take into account the above factors. I recommend consulting a textbook on acoustics or sound engineering, as these will provide the necessary equations and explanations for measuring acoustic energy. Some potential textbooks you could refer to include "Fundamentals of Acoustics" by Lawrence E. Kinsler, "Principles of Vibration and Sound" by Thomas D. Rossing, or "The Physics of Sound" by Richard E. Berg and David G. Stork.

I hope this helps guide you in your research and measurement of acoustic energy. Best of luck in your endeavors.