The ways to eliminate unwanted or undesired noise is a problem that arises in our everyday lives. For instance, the pesky reflex sound from speakers has plagued man since the invention of the speaker. Reflex sound is the sound that comes off the back of you speaker. The bass can be redirected and used but the higher frequencies are just a nuisance. Or how to get rid of that unwanted road noise; who doesn’t want a noise-free car?

To correct this problem you must first understand the physics of sound. Sound is the product of air disturbances that vibrate to create waves¹. Waves of sound vary with the frequencies in which they are transmitted. Unlike light waves sound waves can travel through opaque objects. The distance the sound travels is affected by the density and thickness of the object and the intensity and frequency of the sound.

At lower frequencies (a.k.a. bass) sound travels in larger wavelengths while at higher frequencies (a.k.a. treble) sound travels in smaller, more condensed wavelengths. Sound travels at a constant speed of 343 m/s or 767 mph. The sound’s wavelength varies with the frequency transmitted as shown by the following equation³:

V=¦ l (Speed of Sound = Frequency X Wavelength)

Example: Low frequency 343 m/s = 100 Hz X 3.43 m

High frequency 343 m/s = 20000 Hz X .01715 m

As shown by the examples above, the given low frequency of 100 Hz has a wavelength of 3.43 m (comparable to the size of a car). The given high frequency sound of 20,000 Hz has a wavelength of approximately .02 m.

Sound waves are longitudinal¹ meaning that they vibrate and travel in the same direction. The density of the object or material through which the sound is traveling affects the wave’s intensity. The more dense the object the less intense the output sound. For example, you will not hear a person talk through an inch of drywall. On the other hand, it is likely that you will hear them through a wall of foam padding an inch thick. This concept is present in speakers and the enclosures in which they’re held. Speaker cases are designed to direct the sound to be as effective as possible thereby eliminating any undertone. Undertone can be described as unwanted speaker noise that could create destructive interference. This is one reason why speakers are placed in cases (this does not include bass). Any uncontrolled noise, like the sound coming off of the back of the speaker, is dampened by the case. This is to prevent the destructive interference that would occur if the case were not present.

The four materials most typically used in speaker case construction are as follows: particleboard, Styrofoam, foam padding, corkboard. But, which of these substances will dampen best under different conditions? To find the conditions under which each material performs its best we will first place a speaker in a soundproof container (sound will only be allowed to pass through the front of the speaker). The transmitted intensity of the sound coming from the speaker will be kept at a constant throughout the experiment. Varying frequencies will be used to check the effectiveness of the materials. The final intensity, that which will come out the back of the material, will be recorded. The thicknesses of the different materials will be recorded. It is to our belief that particleboard will not be successful at dampening the bass so much as it will the treble, which we think will be muffled greatly. The cork shouldn’t mute out the treble as efficiently as the particleboard, but will somewhat dampen the bass. The styrofoam is expected to be substandard in dampening treble, but will adequately muffle the bass. The foam padding, we imagine, will dampen both sufficiently. We anticipate that as the material used gets less dense, the bass absorption will be greater with the treble absorption decreasing.
 
 

Decibel Meter                                 Function Generator

3-way Speaker                                 Particleboard

Styrofoam                                         Foam padding

Cork Boarding                                 Blankets

Amplifier
 
 

First the construction of a box to house the speaker was needed which was built out of particleboard. The inner dimensions of the box were 37cm X 37cm X 37cm. In order to allow sound to only escape through the top we filled the box with heavy blankets and ran the speaker wire through its side. When placing the speaker in the box it was flush with its top. Next we attached an arm to the side of the box to hold the decibel meter one meter above the speaker. To complete the setup we ran the function generator through the amplifier and to the speaker. Doing this we were able to put out a stable frequency.

The experiment took place in a garage that was about 10 x 8 x 7 meters. The apparatus was placed in the center. Both people were seated about 2 meters away in different locations. The positions of the two people, with respect to the setup, remained constant throughout the experiment. One person recorded the readings given off by the decibel meter while the other controlled the frequency output and the material placement. In conducting the experiment we first took an initial reading at 50 Hz. We then placed a sheet of particleboard over the opening of the box, covering the speaker. We took another decibel reading at this frequency. We continued to add up to four layers of particleboard, taking readings after each addition. We repeated this for the foam padding, corkboard, and styrofoam at the same frequency. This procedure was performed using varying frequencies.

Hearing 24,000 Hz at 105.4 dbs will make more than the neighborhood dogs go nuts. Good thing We had ear plugs. As you can see from the charts, particleboard was the best at dampening the sound overall. What do ya know, you can also build speaker cases out of it. Particleboard dampened treble more than it did bass. Four sheets of particleboard at 200 Hz brought down the sound intensity 25%, to 71 dbs. At 5000 Hz, the decibels dropped 48%, to 55.7 dbs. The styrofoam did a fairly consistent but not really significant job. The frequency at which it performed its best was at about 5000 Hz. Here the intensity level dropped 14% with four sheets. With each additional layer of foam padding, the decibel level dropped at a uniform rate. The foam did best at 200 Hz, where a dampening of 15% was experienced. Corkboard caused the sound intensity to change at inconsistent rates. The damping was fairly small until 10000 Hz. At 10000 Hz a dramatic 63% drop was observed!

As we had anticipated, particleboard did better with the higher frequencies. It dampened better at these frequencies than all the others. We thought styrofoam was going to dampen treble more than bass. But this was not at all the case. It ended up dampening the mid-range frequencies better than it did at the extremes. The foam padding was expected to be decent overall, but it dampened the high bass frequencies best. Corkboard, we assumed would perform poorly at muffling bass, in which we were correct. But it did do well with the treble, which we were not expecting. At around the 10000 Hz range the cork did extraordinarily well.

If you look at the data table you can tell that there is a big difference when you compare the densities. With the general data particleboard comes out as the best over all. When the densities are compared the particleboard it didn’t perform well. The new sound dampening champion is corkboard. If you look at the data you would see that corkboard is by far the best way to dampen you sound. The problem with corkboard, styrofoam, and foam padding is you can’t build a speaker enclosure with them. Particleboard is widely used for speaker enclosures and the others are used as liners or bafflers.

The experiment could be taken a step further in many different directions. Someone could focus specifically on high, middle, or low range frequencies or even include different materials in the testing. Different aspects of speaker systems such as amplification of bass without the use of any additional stereo equipment could also be addressed. There is a multitude of potential sound experiments covering issued not examined in our project.

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