Introduction
Have you ever seen all those sliders and buttons on fancy sound equipment? Equalizers like the one shown at right allow sound engineers to individually adjust specific frequencies of sound. Much like the treble and bass dials on a car stereo, these sliders can turn up the amplitude of low or high frequency sounds. As a result, the engineers can customize the sounds exactly how they want.

Sound engineers work on television shows and movies. They work in recording studios and concert halls. They're the ones who master all of this complex equipment to make singers, actors, TV anchors, and orchestras all sound their very best.

In this lesson, we will learn about the frequency spectrum of sound. In the process, we will see how musical instruments and, in fact every human voice, has its own combination of frequencies that makes it sound unique. 


Lesson

• To learn a little more about what sound engineers (or "audio engineers) do, watch this brief "Day in the Life" video. (http://youtu.be/9yRA8K0pY1Y)
  
  
• When an instrument makes a sound, the sound wave itself is not usually a perfect sine wave. Instead, it has lots of wiggles and bumps in the wave. For example, watch this video, which shows the sound waves of a guitar playing a classical song. (http://youtu.be/nSqMx3CtOV8)
  
  
• The wiggles and bumps of the sound wave come from the fact that there is more than one frequency of sound being created by the instrument at one time. In the last lesson, we learned that an instrument has one resonant frequency for a given fingering, which is why the instrument plays a particular note. This is certainly true, but in addition to the natural frequency, the instrument also produces sounds at other frequencies, which give the instrument its characteristic sound.
  
  
• Visit this sample simulation. It will time-out after about one minute, but you can refresh the page to load it again. (http://www.edumedia-sciences.com/en/a533-spectral-analysis)
  
  
• Using the simulation, play the same note with the keyboard set to piano, sine wave, square wave, and violin. Compare the changing sound as well as the shape of the sound wave on the o'scope. You can see how the extra bumps make the violin and piano sound more warm and interesting. In music, the character of the sound is called "timbre" (pronounced tam-ber). In this instance, we can see on the o'scope that the piano and violin have different timbre. In fact, even two violins playing the exact same note will have a different timbre - it depends on many factors including the way the instrument is played by the musician.
  
  
• Now return to the simulation and check the box "Frequency Domain Plot". Set the keyboard to "sine" and play a few notes. You will see that the graph on the right shows a vertical line corresponding to the frequency of the wave. We call this graph the "Frequency Spectrum."
  
  
• Explore the frequency spectrum of the sound wave generated by the piano, square wave, and violin. You can see that each of these instruments has more than one frequency, which is why it sounds different from the sine wave. They each have own unique spectrum, which is like a fingerprint illustrating the timbre of the instrument.
  
  
• Now this is just a simulation, real instruments (and especially real songs with lots of instruments) have hundreds of frequencies working together to make up the sound. We often like to visualize the frequency spectrum using what are known as "spectrum analyzers." You have probably seen some of these before. Check out a couple of the YouTube videos below to see very cool homemade spectrum analyzers.
• An LED-powered spectrum analyzer (http://youtu.be/LKSYwUlggRQ)
• An artistic display using plastic beads (http://youtu.be/b4HtUwAkVDg)
• A spectrum analyzer-like device using fire! (http://youtu.be/HpovwbPGEoo)
  
  
• Equalizers (like the one shown above or at right) allow you to turn up some frequencies and turn others down. In this way, you can manipulate the frequency spectrum of a recording or performance. This is part of what sound engineers do in their work. 
  
  
• Let's take a little time to examine the spectrum of our own voices and investigate some of the applications of this type of frequency analysis. To complete the following investigations, you will need:
• Audacity: This is a powerful and free program available for Windows or Mac. (http://audacity.sourceforge.net/download/)
• A microphone (either built-in or plugged-in)
  
  
• Once Audacity is installed and you have a microphone setup, record your voice talking or singing by click on the red record button. Stop the recording and play it back.
  
  
• You can view the frequency spectrum of your recording by selecting the menu option Analze ? Plot Spectrum. As you can see, there are LOTS of frequencies in this recording.
  
  
• Create a new recording in which you also include some silence (at least a second or so). Listen to the recording. In particular, listen for the noise in the silence - you should hear a high-pitch hissing in the background. Audacity is able to remove this noise from your recording. Click and drag across the silence to highlight that part of the recording. Then select Effect ? Noise Removal, and click on the button Get Noise Profile. This will measure the frequency spectrum of the background noise. Then, highlight the whole recording and again select Effect ? Noise Removal, and this time click the OK button at the bottom of the screen. This will remove the frequencies associated with the noise from the whole recording.
  
  
• Experiment with this noise removal feature by comparing the frequency spectrum of your recording before and after the noise is removed. Do you see a difference? See if you reproduce the noise removal process by using the built-in equalizer in Audacity (Effect ? Equalization). 

Assignment
Create a sound file (.wav) that contains your raw voice, you voice with noise removed, and you voice with the bass frequencies boosted. This can be accomplished with the built-in equalizer, or with the other effects built in to Audacity.

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