What Kind of Material Reflects Sound?
When we talk about materials that reflect sound, we're talking about materials that are hard and dense. While these types of materials do absorb some sound, they do not absorb much at all. In fact, hard, dense materials tend to absorb more sound than they can transmit. As a result, the best materials for carrying sound are metals like aluminum and hard substances such as diamond. However, there are some exceptions.
As we've established, sound propagates through the air as a longitudinal wave. Just as with light, the law of reflection states that the angle of incidence of a sound wave equals its angle of reflection. Essentially, the sound that gets reflected is a portion of the original sound wave that stays within the room, which the human ear will hear as a prolonged signal. Fortunately, the law of reflection holds true for many materials.
Some materials are better at reflecting sound than others. For example, metals tend to have flat surfaces, which amplify sound waves. The sound absorption coefficient of steel is 0.03, which means it reflects 97% of all sound waves. For this reason, metals are often used to make reflective noise barriers around highways. While these barriers are supposed to reduce noise, they actually amplify it!
Materials that reflect sound are usually porous. They are not dense, which makes them more prone to transmitting sound. Regardless of thickness, porous materials are good for absorbing sound waves. For example, a four-mm-thick brick wall can absorb 30% of low-frequency waves and only 2% of high-frequency sound. A thicker brick wall will absorb 98.3% of the frequencies and will absorb no more than that.
While metals tend to have smooth, polished surfaces, they're not very dense. This means that they can bounce sound waves back in the same way that they traveled. This creates an echo. The best way to diffuse the sound reflecting off of a brick wall is to cover it with carpet. Wooden acoustic panels and curved surfaces can also diffuse sound. The curves in these materials will reflect the wave at a varying angle from the original wave. This is why wooden musical instruments sound so good.
The laws of acoustics explain why certain materials reflect sound. In general, when sound hits a flat surface, the material can reflect the sound in a different way. The angle of the SOUND WAVE matches the angle of the reflecting surface, and the result is a mirror-like image of the stimulus. This principle works only when the WAVELENGTH of the sound wave is smaller than the dimensions of the surface, and the materials that reflect sound do not have to be spherical.