The science of acoustic absorbers

absorber

The science of acoustic absorbers:

⦁ Why is it worth designing the sound first of all?
⦁ Solution to the problem of acoustics

⦁ How to determine sound absorbers
⦁ Comparing different bass traps

In this overview we summarise how the broadband acoustic absorbers operate and how to choose one the most up to the job. Furthermore, we give basic guidance where to use them and how to compare them with other sound absorbing products available on the market.

This webpage does not want to make you become an expert, we just try to provide general information on the science in question and its intended application. Read our posts and visit our new products on our site. Search for any acoustic items, and find the right device for you to create the terms and conditions of ideal acoustics.

Why Should We Design Sound?

Sound design plays a crucial role in enhancing clarity and ensuring that communication is audible and intelligible in any environment.

Put simply: we need to tame acoustic chaos so that messages can be heard clearly. In a church, the main challenge is making the spoken word understandable. In an airport, it’s about clearly conveying critical flight information.

In a factory, safety warnings must be heard by all employees. And in public spaces like hotel lobbies or restaurants, reducing echo makes communication between staff and guests more comfortable and effective.

In an audio studio the control of acoustics allows us the same way to create a predictable result in order to record quality sounds.

Without acoustic design

Without acoustic design the sound bounces from the walls, floor and ceiling and reaches the point where the room is not able to dampen and diffuse energy.

For example, there is a big difference between a teacher talking silently in a classroom and somebody shouting in a room full of excited children. If they exceed the natural threshold of noise absorption of the room, the conversation and communication will not be clear and audible, thus the room’s acoustic design require much more attention.

The excess of noises generates an effect called “ear fatigue”, when we must highly concentrate while listening, and we must talk louder in order to hear and overcome the other competing sounds.

The bouncing sounds compete the rest:

The acoustic absorbing panels control the echo:

We call these reflected sounds reflections. These can be of primary or secondary reflections, which are reflected from the near surfaces, or secondary reflections, which generate an echoing field. Usually the sound pressure or the direction of the reflected sound must be captured or diverted by mounting acoustic panels on the wall or hanging them from the ceiling.

By adding sound  panels you can easily modify the echoes of the sound’s area, which results in a comfortable and relatively effective communication.

The generic types of echo are as follows:

Direct sound

The direct or initial sound is the sound coming from the mouth of an individual, from a device by which the music is played, or from the loudspeakers. This sound is usually the most important.

The reflections occur when the sound reverberates on the near walls. As they usually arrive some seconds after the direct sound, they may disturb the so-called phase interruption or comb filtering, and may make the understanding of human voice more difficult.

The control of the primary reflections is usually the target of the first action plan. Decreasing the reverberation period usually depends on the sound absorption of the room. The more broadband acoustic absorbers you mount the more energy they will dampen.

Rattling Echoes Without Broadband Acoustic Absorbers

If you clap your hands in an empty room, you’ll likely hear fluttering echoes bouncing between the walls, ceiling, and floor. These echoes are mainly caused by parallel wall surfaces, which reflect sound back and forth and allow persistent reverberation to linger.

Reducing flutter echo is simple when you install broadband acoustic absorbers on opposing parallel walls. This setup interrupts the echo path, preventing the sound from continuously bouncing and extending the reverberation.

We specifically refer to broadband absorbers, because hand claps typically produce sounds above 1000 Hz, and these frequencies can only be effectively and evenly dampened with broad-spectrum absorption panels. However, it’s important to note that lower frequencies still require specialized treatment, as they are harder to control with standard panels.

---

Secondary Reflections and Echoes

If you’ve ever visited an old church, you’ve probably heard long echoes that seem to float through the air. Before the invention of modern loudspeaker systems, these buildings were intentionally designed to use natural reflections from hard surfaces to project speech and music effectively throughout the space.

This reverberation is especially impactful when listening to Gregorian chants, choirs, or classical music, where the prolonged echo adds richness and dimension. In such cases, reverberation becomes a tool—not a problem.

Today, however, controlling reverberation time is key to tailoring a room’s acoustics for modern usage. By increasing the number of sound-absorbing panels, you can significantly enhance a room’s sound absorption capacity, reducing excessive energy and achieving better balance.

---

Sound Absorption with Acoustic Panels

When you play music at high volume and place your hand on a loudspeaker, the floor, nearby furniture, or even the window, you’ll feel the vibrations. This is because sound energy travels through solids and fluids as vibrational energy. When this energy sets the material in motion, it inevitably results in the production of heat.

In essence, sound absorption is an energy transformation process. Scientifically, this phenomenon is known as thermodynamic transfer.

---

The Importance of Material Choice

When sound penetrates a broadband acoustic absorber, the fibres within the acoustic rock wool begin to vibrate. This internal movement again generates thermodynamic transfer, turning sound energy into heat.

Due to the high-density fibrous structure, sound waves are significantly reduced as they pass through, resulting in effective echo control and a noticeable reduction in reverberation—especially when broadband panels are used.

---

Low-Frequency Absorption with Acoustic Panels

In acoustics, low frequencies (bass) are the hardest to manage because of their long wavelengths. Without sufficient density and mass, bass frequencies will easily penetrate most materials.

Producing bass requires more energy—like comparing an elephant’s tail to a mouse’s—and once that energy is released, it’s much harder to stop, much like trying to halt a freight train versus a bicycle.

High frequencies, with their shorter wavelengths and lower energy, are easier to absorb using lightweight foam. However, bass tones pass through these materials almost unaffected.

---

The Simplest Way to Dampen Low Frequencies

The most effective way to absorb low frequencies is by increasing the thickness and density of the acoustic panels. A general estimate of the necessary thickness can be made using the quarter wavelength rule, but real-world testing often reveals surprising deviations.

If the material is not dense enough, a large portion of bass energy will pass through. This is the main drawback of low-density foams—they are ineffective for bass absorption. Conversely, overly dense traps can cause high-frequency reflections, creating new acoustic issues.

That’s why balanced acoustic elements are designed to absorb both low and high frequencies, ensuring even, natural-sounding results across the spectrum.

---

Why Bass Contains More Energy

Bass frequencies carry more energy and require more effort to manage—but with the right combination of material density, structure, and placement, they can be effectively controlled.

These two charts compare the low and high frequencies with the same amplitude. Take into account that the longer low frequencies contain more energy as it is shown in the yellow area. As there is more energy in the sound wave it will be more difficult to attenuate or regulate the bass.

Calculation of quarter wavelength

We call “calculation of quarter wavelength” the mathematics used for prediction of the low frequency performance of elements, where the thickness of the panel equals to the 1/4 of the wavelength of the lowest frequency, plus the factor of the angle of incidence. The thickness of the panel plays an important role.

The solution to acoustics issues

The actual process can be simplified in four steps:

⦁ Specify the scope of the problem.

⦁ Choose the appropriate acoustic element to solve the problem.
⦁ Estimate your available budget.
⦁ Install the sound absorbing device in the strategically most important areas for a maximum efficiency.

First you should find the place in the room where the sound is distorted by identifying the frequency that created it. In other words, you should consider what frequencies you try to dampen before you simply mount some bass traps on the walls and expect them to operate the way you wanted.

For example, it is very important to balance the sound absorption of the whole sound range in a studio, in order to obtain optimum sound recordings also using different audio systems. In this case the goal is to create a neutral listening environment.

The desired linearity is important in the room also in case of home theatres where you should ensure that all central channels that are in dialogue be crystal clear.

In a classroom, an auditorium or an office, the human voices are transmitted, therefore the acoustic design must take into account also the original sound.

Human voice:

The next charts show the measurements of sound of a typical human voice, and show how the sound energy changes depending on loudness. You will notice that by the increase of sound level the energy will increase in the middle band.

The chart shows the range of a typical male voice that keeps the most of the energy in the middle range between 400Hz and 1000Hz and the harmonics extend to 3500Hz.

If we take a closer look at the chart, we can see that the most energy of human voice is between 300Hz and 1500Hz.

This is why it is important to choose the appropriate panels for the task, and opt for the one, which actually operates within this range, such as the broadband acoustic panel.

If only the sound of speech is needed to be designed, we do not advise to choose the leather membrane that dampens the sound only under 600Hz, since the higher frequencies would freely flutter in the room.

Similarly, it would be a wrong decision to choose an acoustic foam, because these attenuate the sound only above 800Hz.

In this case the best choice is to install broadband bass traps:

In the case of absorbers usually the sound absorption coefficient determines the selection of the appropriate sound absorption panel. The specification shows that if the value of sound absorption is 1,0 in a given frequency, the acoustic panel will be able to dampen the sound 100% in the given frequency. The value 0.5 means absorption of 50%.

---

About Acoustic Foams

Broadband absorbers are made of high-density acoustic wool with a density of 90 kg/m³, while most foam panels are constructed from low-density polyethylene, typically around 10–15 kg/m³. To create artistic or decorative designs, large portions of foam are often cut away, further reducing their effective density due to the resulting air gaps.

As a result, the final density of many acoustic foam panels is often barely above 8 kg/m³. Compared to broadband panels—whose density is more than ten times higher—it’s clear why polyethylene foam is ineffective for bass absorption.

Don’t be misled by claims suggesting that acoustic foam or XPS panels can dampen bass, even when accompanied by so-called bass trap test results. Physics cannot be bypassed, no matter how polished the marketing or publicity may be.

---

The Real Priorities in Acoustic Design

From my perspective, the most important aspects of acoustic treatment are sound absorption and noise reduction. These are critical if you want to enjoy a quiet and peaceful environment, whether by day or night.

To achieve this, sound insulation must be properly addressed. The uncontrolled movement of sound through air must be restricted using the right materials and treatment strategies. Only then can you create a truly balanced and comfortable acoustic space.

Let’s Move On: Comparing Sound Energy and Panel Performance

When we take a closer look and compare sound energy absorption between different acoustic materials, the difference becomes striking. Broadband acoustic panels can absorb up to 95% of sound energy at frequencies as low as 100 Hz, while most foam panels only reach that level of absorption at 1000 Hz or higher.

This clearly highlights the importance of targeting the right frequency range with the right materials. (Many popular brands offer a variety of acoustic products—before making a purchase, always review the technical specifications, materials used, and the weight of the product!)

---

Choosing the Right Bass Trap for Your Needs

The type of frequency or noise you wish to reduce will determine which kind of bass trap or acoustic panel you need.

• Thinner panels (typically 6–11 cm) are suitable for mid and high frequencies
• Thicker panels (usually 11–13–20 cm) are needed to attenuate low frequencies

Selecting the appropriate panel thickness and density ensures that your acoustic treatment is both effective and efficient, giving you the best results based on your room’s unique challenges.

Selection Criteria for Acoustic Absorbers

Choosing the right thickness and type of acoustic absorber depends on the frequency range you need to manage and the function of the space. Here’s a general guide:

1. 6 cm panels – Ideal for high-frequency issues in offices, restaurants, or for taming flutter echoes in recording studios.
2. 11 cm panels – Suitable for rooms where music is played and for controlling primary reflections across a broad frequency range.
3. 13–20–30–40 cm panels – Recommended for bass absorption, particularly in areas where standard broadband panels are ineffective at reducing low frequencies.

---

Example 1: High-Frequency Echo in an Office

Imagine a dot matrix printer generating an irritating high-frequency echo as it reflects off nearby walls. In this case, a 6 cm broadband absorber would be the best choice, as it efficiently targets the higher frequency range causing the issue.

---

Example 2: Multipurpose Hall on a Budget

Consider a multifunctional living room or hall used for both dance classes and community meetings, with limited acoustic budget. 11 cm broadband absorbers are a smart choice here—they offer excellent wide-band sound absorption and are cost-effective, making them ideal for controlling musical reflections and maintaining speech clarity.

---

How to Design Sound Absorbers Effectively

If you place just one small panel in a high school classroom, you likely won’t notice any change. Install thousands on every surface, and the room may become acoustically “dead”. Most successful designs find a balance between these two extremes.

To achieve this balance, professionals follow the reverberation curve—a model that helps determine the ideal reverberation time for a given space.

As you increase the surface coverage or the number of absorbers, the efficiency of sound absorption also increases—but gradually. This is why strategic placement and calculated quantity are key to achieving natural, balanced acoustics.

Reaching the Threshold: From Echo to Balance

As more acoustic panels are added to a space, the room transitions from a cave-like, echoing chamber into a comfortable, acoustically balanced environment. However, after reaching a certain point, the benefits of adding more panels begin to diminish—additional treatment yields only minimal improvements.

This indicates that the room has reached the threshold of over-dampening, where the acoustics can start to feel unnaturally muted.

---

No Fixed Rules – It’s All About the Space and Purpose

There is no universal rule for how many sound panels should be installed to achieve ideal acoustic performance. The right solution depends on room size, intended use, and acoustic goals.

For human speech, where maximum clarity is essential—such as in offices, classrooms, or meeting rooms—acoustic engineers typically aim for a reverberation time below 1 second. In larger spaces, slightly longer reverberation may still be acceptable.

In contrast, concert halls built for classical music are designed to embrace a longer reverberation time, as it allows musical instruments to blend and resonate, enhancing the overall atmosphere and emotional impact.

---

Acoustics and Aesthetics Go Hand in Hand

Acoustic treatment doesn’t have to compromise the visual appeal of a space. On the contrary—well-designed acoustic panels can elevate the interior with stylish materials, textures, and custom designs. The better the acoustic solution, the more visually pleasing and functional the space becomes.

The reverberation time curve:

Reverberation period in halls of different sizes

How Much Sound Absorption Do You Really Need?

The extent of wall-mounted sound absorbers depends on the purpose of the room, practical considerations, and personal preferences. For example, in a recording studio, you can design your own creative space to achieve optimal sound quality and an enjoyable working environment.

On the other hand, in a church, where spoken word and live music performances alternate, larger and more efficient sound absorption elements are recommended. Start with 10% wall coverage, gradually increasing to 20%. If you’re still not satisfied, simply add more—it’s truly that simple.

---

Comparing Different Acoustic Absorbers

When optimizing room acoustics, it’s common to manage sound reflections by installing a combination of diffusers and absorbers on both walls and ceilings. The key is placement and panel type, as different products excel in different areas.

There are many types of acoustic panels on the market, each with its unique strengths and limitations. To make your selection easier, we’ve reviewed and compared our product range so you can choose the best solution for your space—with confidence and clarity.

And don’t hesitate to reach out for expert advice—we’re happy to help you design the perfect studio, church, or home environment tailored to your acoustic needs.

Written by Róbert Polgár
If you have any comments, please contact the author.