Range of human hearing, Interference (three ways), Relationship of frequency and size. Lots of different investigations with sound are possible. Here are a few that I do as labs or demonstrations with my high school physics classes.
Step 1: Stuff You Need
Step 2: Range of Human Hearing
You can use Audacity or a free oscilloscope program, Visual Analyser for this. Both will work as function generators. In Audacity just go to the "Generate" menu and select "Tone". Fill in the frequency you want.
In Visual Analyser you simply click the "Wave" button above the scope window. Set the desired frequency, click apply, then click "ON". Notice the right and left channels are controlled independently unless you click the "Interlock" check box. You can also generate a sweep that will move through a whole range of frequencies. You can change the options for this in the "Sweep" tab.
Then go back to the main window, turn the scope on, and change the input in Visual Analyser from "Microphone" to your output (note, on my computer it is "Stereo Mix", but this depends on your computer). Then you can watch the Sweep in the FFT window, while listening.
The normal range of human hearing is 20 Hz to 20,000 Hz. Your sound card may not really be able to reach these limits however. As you vary the frequency you will find that the sounds in the middle seem louder. We tend to hear better in the middle of our range and not as well at the extremes. How high up can you hear? If you can't hear it (and it's still less than 20,00 Hz) ask a teenager before you blame your sound card.
When you get to the really high frequencies you'll find teenagers tend to wince while adults will hardly hear anything. Just crank the frequency up to around 17,000 or 18,000 Hz and you'll have the teen repeller. As we age our hearing deteriorates. We loose the extremes of our range first.
Step 3: Interference
Again use either Audacity or Visual Analyser to generate a function. Make a 400 Hz tone and set the balance so it only comes out of one speaker. Plug one ear and move your head around. You should find loud spots and dead spots.
Why do you have to plug one ear? Evolution has equipped you with two sound detectors. They are conveniently separated by a short distance (this allows you to determine the location of a sound). If you don't plug one ear you might never be able to find the dead spots as one ear will be in the spot and one ear will be outside of the spot.
The sound from your speaker is reflecting around the room. The reflected sound will interfere with other reflections or the sound coming directly from your speaker. In the dead spots you have destructive interference. You have a compression meeting a rarefaction (think crest and trough), the net effect is that the sound is canceled out. In the loud spots you have constructive interference.
Note: Unfortunately you can't use this to map out all the dead spots in your room and reposition your speakers accordingly. The dead spots will move depending on the frequency of the sound. Different frequencies have different wavelengths, so their compressions and rarefactions are in different places.
Here are a couple of links about interference (in case you don't have a physics book handy):
Step 4: Interference - Two Sources
Use Audacity or Visual Analyser to make a sound. 400 Hz works pretty well. This time you want both speakers to be generating a sound. Again, you'll need to plug one ear.
In Audacity you can use the same tone you generated earlier, this time leave the balance slider in the middle. In Visual Analyser just be sure to "Enable" both channels.
Stand a short distance in front of the speakers and have a friend move one forward slowly. At some point most of the sound should go away. This happens when the speaker has moved 1/2 a wavelength forward.
If you're lazy and don't want to move the speakers you can change the [http://en.wikipedia.org/wiki/Phase_(waves) phase of the wave] instead.
In Audacity you'll need to add a second audio track. Select "New Audio Track" from the Project menu and generate the same tone in this track as you did in the first track. Slide the slider for one track all the way to the right and the other all the way to the left. Then zoom way in and select the "Time Shift Tool". Grab one of your waves and slide it until a crest meets a trough.
In Visual Analyser all you need to do is open the wave generator, make sure both speakers are making the same tone, type 180 into the "Phase" box, then click "Apply".
Again, this works better with one ear. Try to place your ear an equal distance from both speakers and point your ear directly between your two speakers.
Step 5: Interference - Beats
This time you will generate two slightly different frequencies, one from each speaker.
1. Select "Tone" from the "Generate" menu. Set the frequency to 440 Hz. Then drag the balance slider all the way to the right.
2. Select "New Audio Track" from the "Project" menu
3. Select "Tone" from the "Generate" menu. Set the frequency to 444 Hz. Then drag the balance slider all the way to the left.
In Visual Analyser all you need to do is go to the "wave" generator and set the frequencies of the Left and Right channels to be slightly different (don't forget to click apply).
You should hear a warballing sound. Those warbales are "Beats". If you have a 4 Hz difference in the two frequency you hear four beats every second. If it were 10 Hz you'd hear ten beats. Since the waves have slightly different lengths there will be points where they line up such that we have constructive interference and other points where they are out of phase and we have complete destructive interference.
You could do this with both speakers making the two tones, but it is more compelling when each speaker makes a single tone. I point both speakers at my class and then alternately turn each off to show that each speaker is making a single, constant tone.
Step 6: Relationship Between Size and Frequency
You'll need Visual Analyser and your water bottle for this part.
1. Drink some of the water from the bottle.
2. Blow across the top of the bottle. If you can't get a good tone try drinking some more water. If you finish the bottle before getting a good tone, you'll need to go to the bathroom and then get a different brand of water bottle and try again.
3. Open the frequency meter in Visual Analyser. Click the "Freq. Meter" tab. The Frequency Meter in Visual Analyser will report the single loudest frequency. If you look in the FFT window you will see this peak and any others that may also be being produced. You'll also have to turn the scope on. The On-Off is in the top left corner of the window.
4. Blow across the bottle and note the frequency.
5. Measure the length of the air column and record.
6. Drink some of the water and repeat.
7. Graph frequency vs. height of the air column.
Note: You can also do this with a wine glass, but rather than blow you drag your finger around the top. You will find exactly the opposite relationship.
In both cases you're finding a natural frequency your object will vibrate at. In the case of the water bottle your object is the air. For the wine glass it's the water. Don't believe me? Fill your wine glass to very close to the brim, wet your finger and drag it around the mouth of the glass till it sings. While it's singing look closely at the water, you can see the waves!!! The instant you stop the waves go away.