Introduction to Acoustics

Transcription

1 Introduction to Acoustics Absorption Have you walked into a racquetball court and heard the echo as the ball strikes the wall? Or have you been in the country with snow on the ground and noticed how quiet it is. The difference in the sound between these to places is due to the degree that sound is absorbed. In the case of the racquetball court, the noise of the ball striking the wall travels to another wall and bounces off of it, travels to another wall and bounces off of it and so on. The echo is caused by the sound continually whizzing past your ear due to reflecting off another wall. In the case of being in a field with snow, your voice either continues to propagate down the field never striking anything, or strikes the snow and disappears. Whether a sound when striking an object reflects back or disappears is due to the objects sound absorption. The racquetball court typically is made of concrete painted walls. Concrete painted walls have very low sound absorption and hence, reflect. Five inches of fluffy snow, however, absorbs the sound when it strikes it. Some materials allow sound to easily enter it. These materials are called porous. Acoustic porous materials can have a porosity greater than 90%. Porosity is the amount of volume that is just air. Common sound absorption materials are open cell foam and polyester fiber. Sound absorption is an energy conversion process. The kinetic energy of the sound (air) is converted to heat energy when the sound strikes the cells or fibers. Hence, the sound disappears after striking the material due to its conversion into heat. We know that most sounds contain many different pitches or frequencies. A bass guitar plays lowfrequency sounds while a violin plays high-frequency sounds. Low-frequency sounds have long wavelengths and high-frequency sounds have short wavelengths. A wavelength can be visualized when going to a beach. The wavelength is the distance from one wave crest to the next wave crest. In the air, a long wavelength propagates in all directions easily. That is why a single subwoofer is typically needed to fill a listening space due to the fact that subwoofers are not directional in nature. In contrast, a tweeter that emits high-frequency sounds is very directional. Reproduction of high-frequency sounds requires at least two carefully placed tweeters to produce a good stereo image of the sound. Lowfrequency sound passes through materials much easier than high-frequency sounds. That's why one can hear the constant thud of a subwoofer through the wall of the room next door. Because sound passes through materials differently at different frequencies, the sound absorption will typically change with frequency. Besides the sound absorption changing with frequency, it also changes with the thickness of the material. For the same materials, thin sections will not absorb as much low-frequency sound as will thicker pieces. Figure 1 provides absorption curves for the same material at three different thickness. The ordinate (left vertical axis) contains the absorption (alpha). The absorption can go from 0 (no absorption) to 1 (all sound is absorbed). The abscissa (horizontal axis) provides the frequency at which the absorption was measured.

2 1 - Figure 1. Affect of thickness on absorption One may be wondering how sound absorption is measured. There are two main methods, one use a special room called a reverberation room and the other uses a special tube called an impedance tube. The reverberation room allows the sound to strike the material from all directions and hence is called random incidence. The impedance tubes have the sound strike the material straight on and are called normal incidence. Table 1 compares the two methods. 2 - Table 1. Comparison between Random Incidence and Normal Incidence Absorption The Technicon impedance tube is shown below in figure 2:

3 Figure 2. Impedance Tube for Sound Absorption The impedance tube consists of a speaker, tube, two microphones and material sample holder. A special sound called white noise is generated in the speaker. The white noise is composed of sound contributions from all frequency bands in the audible range. The sound travels straight down the tube and strikes the material. Some of the sound is absorbed and some are reflected back. The two microphones measure the reflected sound. From the two microphone's signals, the sound absorption can be calculated. From figure 1, one sees that one way of increasing the low-frequency absorption is by using a thicker material. Absorption of a non-faced porous material is called resistive. Placing a film on the surface of the material can increase the low-frequency absorption. This type of absorption is called reactive. Figure 3 compares non-faced foam with foam with an aluminized polyester face.

4 3 - Figure 3. Faced versus non-faced foam for same thickness Figure 3 shows typical changes when placing a film or facing on the top surface of a porous material. The change in the sound absorption for the faced material is an increase at the lower frequency absorption and a decrease in absorption at the higher frequencies. This is due to the film now acting as a spring-mass resonator for the low-frequency peak. At the higher frequencies, the face causes the sound to reflect. Knowing how different faces can change the absorption of a material, Technicon can design the best sound absorber for our customer's noise problem. Sound Transmission Loss The second technique to reduce noise is sound transmission loss. Sound Transmission Loss is abbreviated as STL. As the name sound transmission loss implies sound is blocked from transmitting through one space to another. For example, a wall in a movie theater separating one theater from the other requires high sound transmission loss to keep from hearing the gunfire, explosions, and crashes from the adjacent theater's action movie. There are a variety of ways of producing sound transmission loss. One way is distance. We all know that noisy machines are louder at close distances versus when farther away. When designing a school, for example, distance plays a good common sense noise control solution many times. A poor school design

5 would place a gymnasium or cafeteria next to a library. However, in regards to machinery, distance, as a noise control solution is rarely practical. Sound is lazy and hence will always take the easiest path to get from point A to point B. The easiest path for sound to travel is a clear unobstructed path. If you can visually see the noisy machine, there is nothing obstructing or reducing the noise from the maximum amount that you could receive. Hence, it is important not to only place something in between you and the noisy machine, but also to make sure that there is no place for that sneaky noise to slip through holes and cracks. A solution for blocking the noise from getting to you would be to place the machine in an enclosure. A well-designed enclosure will always make sound work hard to get to your ears; hence its essential to not have any holes, gaps or cracks. If ventilation is required for combustion intake and exhaust or cooling purposes, a sound silencer or labyrinth is necessary. The labyrinth lined with sound absorptive material forces the sound to strike the sound absorptive material hence causing the sound to be reduced significantly by the time it exists. Now that the enclosure is pretty well sealed, sound can still reach you by transmitting through the enclosure walls. This is called airborne sound transmission. There are ways of increasing this sound transmission loss. The first is mass. Mass can be increased by using a thicker wall of the same material or use a more dense material. There is a relationship between sound transmission loss and weight of the barrier and this is called the mass law. The mass law states that for every doubling of the weight of the material, one can expect a 6 db increase in the transmission loss. In addition to the mass having an effect on the transmission loss, the frequency of the sound also has a similar effect. A doubling of the frequency will create a 6-dB increase in the transmission loss. Figure 4 shows the effect of mass and frequency on sound transmission loss.

6 4 - Figure 4. Mass and Frequency dependence on Transmission Loss One sees that the cost and weight of the wall must double for each 6 db incremental improvement in transmission loss. If there are two walls and they are separated by a significant distance, each wall transmission loss is additive. This additive effect is shown in figure 5.

7 5 - Figure 5. Comparison between double and single wall for same total weight. One can see from figure 5 that by adding a second wall and large air gap between the walls, the transmission loss can be dramatically increased. Double wall construction is encountered in many everyday systems. For example, in homes, drywall, stud construction is made of two sheets of drywall separated by the studs. In the real world, the air gap between walls will be relatively small. In machinery enclosures, the gap may range from 1/4" to 2". The greater the gap, the more the walls behave as two separate walls instead of a single wall comprising of both walls total weight. In real-world double wall systems, a double wall resonance in the airspace occurs which reduces the transmission loss. To reduce the detrimental effect of the double wall resonance, a foam or fiber is placed in the air gap between walls. Technicon offers a variety of barrier composites. Barrier composites create a double wall system when attached to the interior wall of a machinery enclosure. Vibration Damping

8 Many time's large panels vibrate. The cause of vibration may be an engine, generator, compressor, road irregularities in vehicles or a boat hull slapping against the waves. When this occurs, the panel vibrates as a large speaker cone creating noise. This type of noise is called structural borne noise. The greater the panels move, the more air is displaced, the louder the sound. The vibration creates resonance in the panel causing it to move. Resonance is a natural phenomenon that can cause a mechanical structure to vibrate violently if the panel is excited at the same frequency that a resonance occurs. To keep a panel from resonating violently, the panel must be damped. Steel or fiberglass panels by themselves are stiff and elastic, causing them to vibrate a long time when struck. Hence, steel and fiberglass have very little damping. Hence, to damp the panels to reduce resonant vibrations, damping must be applied to the surface of the panels. Damping materials can be applied as a trawled on a compound or as sheets with an adhesive backing. The key feature of a damping material is that they are viscoelastic. Viscoelastic materials require energy to be extended and compressed. This is exactly where we want the vibration energy from the panels to go, into the viscoelastic materials and then dissipated as heat. The amount of damping necessary to effectively reduce the panel vibrations is dependent on the panel thickness and material type. If one overdamps the panel, this adds extra cost and weight, hence, there is an optimal amount of damping for a given panel material and thickness. There are test standards to measure the damping loss factor of the damping material. The test standard performed at Technicon is ASTM E756. A picture of the test fixture for measuring the damping loss factor is given in figure 6. Figure 6. Damping Test Rig with Sample

9 The damping test rig consists of a bar made from the panel material with the damping material bonded to it. Like the impedance tube, a white noise signal is fed to a vibration exciter at the top of the bar. This causes the bar to resonate. At the bottom of the bar, a vibration pickup measures the bar s resonance. The transfer function is defined as the output vibration response of the bar divided by the input vibration excitation is calculated. This is shown in figure Figure 7. Transfer Function of Damping Bar The peaks in figure 7 are excited resonance in the bar. The broader the resonance peaks, the greater the damping. The composite damping loss factor is calculated by the formula: Composite Loss Factor = [fh(-3 db)- fl(-3 db)]/fr Where: fr = Frequency at resonant peak fh(-3 db) = Higher Frequency at -3 db below resonant peak fl(-3 db) = Low Frequency at -3 db below resonant peak.

10 7 - Acoustics Lab for Reverberation Testing See how we can assist you with your Acoustic and Thermal needs sales@tcnind.com