Understanding sound absorption

 Sound Absorption: A Key Element in Acoustic Design

In our previous articles, we explored the acoustic characteristics of different spaces, specifically delving into the distinction between reverberation and echo. The reverberation time of a space is closely tied to the sound absorption properties of its materials. In simple terms, a space adorned with carpets and curtains will exhibit less reverberation compared to one with tiles and concrete surfaces. This is because materials like curtains and carpets are more effective in absorbing sound than hard surfaces like tiles or concrete.

Defining Sound Absorption

Technically, sound absorption refers to the conversion of acoustic energy into heat. As we know, energy is neither created nor destroyed; it simply transforms. When sound waves encounter a surface like a carpet, their energy doesn't vanish but rather transforms into another type of energy—in this case, heat. However, it's crucial to note that the power of this energy is quite low. To put it into perspective, the energy generated by an entire stadium cheering for a goal would barely be sufficient to heat a cup of coffee. Absorption is just one of three phenomena that occur when a sound wave encounters a different medium; in general, three phenomena take place:

1. Reflection: Some of the energy reflects back.

2. Absorption: Some of the energy is absorbed by the material.

3. Transmission: Some of the energy passes through the material.

For example, in the illustration above, sound energy emanates from a source (S). While in the air, a portion of it gets absorbed (E). Upon reaching a wall or obstacle, some of the energy reflects (A), and another portion transmits and gets absorbed in the material (F and H).

Measuring Absorption

Each material has the ability to absorb a varying amount of sound energy, a property measured by the absorption coefficient. The absorption coefficient is a number indicating the percentage of energy absorbed by a material, and it has no units. A coefficient of absorption equal to 1 implies 100% absorption with no reflection, while a coefficient of 0.25 means 25% absorption and 75% reflection. Generally, porous materials tend to have higher absorption coefficients than hard materials. For instance, curtains have a higher absorption coefficient than concrete.

It's important to note that the absorption coefficient is not constant across all frequencies. It changes with frequency, leading to tables displaying absorption coefficients at different frequencies.

In the image, various absorption coefficients are represented by the Greek letter alpha (α). As observed, gypsum board absorbs lower frequencies better than higher frequencies. Understanding these coefficients is crucial for planning the acoustic treatment of a space, and comprehensive tables exist to guide the selection of materials commonly used in construction.

It's also worth noting that different materials can yield similar acoustic effects. Believing in a "magic solution" is a misconception; achieving the desired acoustic outcome often requires a well-thought-out combination of both absorbent and non-absorbent materials.

Practical Solutions

Sound absorption finds practical applications in various scenarios. For instance, when recording, portable acoustic panels can assist in controlling sound. These panels, crafted from foams with high absorption coefficients, prove to be excellent choices for home studios lacking additional acoustic treatments.

When aiming to acoustically treat a room, the recommended approach involves using acoustic panels like these. In larger spaces, covering a greater surface area becomes essential, necessitating the use of larger panels.

It's crucial to emphasize that the quantity of acoustic material or the surface area to be covered should be determined by an acoustic expert. A thorough study of the room's acoustic characteristics is essential for satisfactory solutions. Filling every wall and ceiling with acoustic material is a mistake, as excessive absorption can lead to a sound that is overly dull and lifeless.

If you want to delve deeper into the world of sound, consider exploring my Introduction to Audio Engineering course, where you can learn the fundamentals of this captivating field!

Also, you can also expand your knowledge of Applied Acoustics by taking my Acoustics online course. More information and free lessons are available here.