0.5 Microphone fft lab

Microphone fft using national instruments low cost data acquisition

Introduction

Whenever a sound is made, a pressure wave travels through a medium (such as air) and vibrates our eardrums.This same principle can be used to convert sound information into an electrical form so that the experimenters can visualize,interpret, and analyze the sound information.

In addition to visualizing the amplitude of a sound wave (electrically) over time, we can also look at thefrequency content of the sound signal. Simply put, we can use the Fast Fourier Transform (FFT) algorithm to look at how much of eachfrequency the sound signal contains. More information on the FFT will be provided in the theory section.

During this exercise, the experimenters will use a microphone element to convert a sound wave into an electricalsignal. This signal will be then digitized using a Low Cost USB DAQ device. Finally, the Signal Express application will be used toquickly visualize the time domain sound signal as well as compute its Fast Fourier Transform for viewing in the frequencydomain.

Pre-lab assignment

1) Find a microphone element datasheet by searching the internet for “electret condenser microphone element.”Most detailed datasheets should show the frequency response curve of the element. This curve shows how much of each sound frequencymakes it through to the electrical signal produced. What does this curve look like for the element you found?

2) What frequency response would be ideal for a microphone element to have? Would a flat curve be advantageous?What about a curve that rolls off at 5 Hz? Hint: first determine which frequencies are audible to the human ear.

3) Write a short summary about how the electret microphone works. You should be able to find a variety ofsources online; Wikipedia is a good starting point.

4) Become familiar with the National Instruments USB 6008 and 6009 data acquisition devices. Thesedatasheets are available at www.ni.com .

Theory

Electret microphone elements

One way to convert sound pressure waves into an electrical signal is using an electret microphone element. Apicture of such an element is shown below:

Inside the electret microphone element, a dielectric material is made to hold a permanent charge. When theelement vibrates, the internal capacitance changes and an electrical signal is produced. A variety of additional components inside the microphone element act as a small output amplifier.

The fast fourier transform (fft)

It is very common in science and engineering to view a signal’s amplitude vs. time. For example, imagine that adoctor is watching a patient’s heartbeat on an electrical device. He might see peaks in the heartbeat signal every 1 second if thepatient’s heart beats 60 times a minute.