Sound waves can be described using several key terms, like speed, intensity, frequency and pitch.
Speed of Sound
The speed of a sound wave depends on its medium. As you learned on the previous page, sound waves are created by compressing the medium, then allowing it to expand. In some substances, the molecules are easier to "squish" than others; therefore, compressions and rarefactions are made more efficiently. When one of these substances serves as your medium, the sound waves will be able to travel faster. In addition to the medium, the temperature also affects the speed of sound waves. When the temperature of a substance is higher, its particles are moving faster, which allows the vibrations to move more quickly through the medium. Below is a table which shows the speed of sound through various substances at various temperatures.
Intensity of Sound
The intensity of a sound wave is really just its loudness. Sound intensity is measured in decibels (dB). We could think of intensity as the "strength" of the compressions and rarefactions. A large speaker will have the capability of squeezing more air into a smaller space than a small speaker could. Stronger compressions and rarefactions will eventually push on your ear drum with more force and make the sound louder. Our ears are sensitive to loud sounds. We are often exposed to dangerously intense sounds on a regular basis, which could lead to long-term hearing damage. You should never be exposed to sounds above 120 dB without ear protection. Repeated exposure to sounds above 80 dB can also cause hearing loss. Below is a table of intensities of various sounds.
Frequency of Sound
The frequency of a sound wave is the number of cycles, or waves, it makes each second. Each compression could be considered to be one cycle. Frequency is measured in units of Hertz. If a sound source makes 5 compressions every second, its frequency is 5 Hz. The frequency of a sound wave affects how it sounds. Check out the following demonstration to see how.
In this demonstration, I will use a motor to spin a wooden disk. The apparatus is pictured to the left. You can click on the picture to see a larger view. The disk has three rings of holes drilled through it. As the disk spins, I will use a straw to blow air through the holes in the disk. You will be able to hear the sound created by each of the three rings on the disk. Listen closely to figure out how the hole spacing affects the sound that is created.
First, I will blow air through the outside ring of holes in the disk as it spins. Notice in the photo to the left, the outside ring has holes that are equally-spaced. You can click on the image to enlarge it. You can see from the ruler that the holes are approximately 2 inches apart. Click the play button underneath the photo to hear the results.
Next, I will blow air through the middle ring of holes in the disk as it spins. You can see from the photo that the middle ring has holes that are equally-spaced, but only about one-half an inch apart. Click the play button underneath the photo to hear the results. How is it different from the sound that you heard before?
Finally, I will blow air through the inside ring of holes in the disk. If you look carefully, you'll notice that the holes in this ring are not equally-spaced. Some are very far apart, while others are close together. Compare the sound that this ring generates to the other sounds you've heard. What makes this sound different?
As you could hear in the demonstration, the hole spacing really affected the resulting sound. When the holes were close together, the frequency (the number of wave cycles each second) was higher. This resulted in a higher pitched sound. This sound waves had more compressions each second. It made your eardrums vibrate back and forth more quickly. We can conclude that the frequency of a sound determines its pitch. A higher frequency equals a higher pitched sound.
The last audio clip may have been confusing. In fact, this last sound didn't make any particular pitch at all. Because the holes of the inside ring of the disk were unequally-spaced, the frequency was not consistent. This jumbled mix of frequencies did not produce a clear pitch, or tone. When the frequency isn't steady, we just make noise.
Humans can hear a large range of frequencies, from 20 Hz to 20,000 Hz. Unfortunately, age and hearing damage can prevent us from hearing the entire audio spectrum. In fact, most people begin to lose the ability to hear the extremes of this range at around 25 years of age. Many animals can hear sounds outside of this range. Frequencies above the audio spectrum are called ultrasonic, while frequencies below the audio spectrum are called infrasonic.
Play the audio below to hear sounds of different intensity and frequency. You can also determine if you're able to hear the entire audio spectrum.