Or: The carpet is a poor absorber!
Anyone wanting to improve their room acoustics in the studio intuitively has the right thought at first: “I need soft material in the room that swallows the sound.“
In the next step, however, it comes down to choosing the right material for the room and its usage. Materials that are too thin, such as convoluted (egg-crate) foam, large-area carpets, or thin curtains, are problematic. After introducing such materials into a room, acoustic disappointment often follows: the reverberation in the high frequencies is now overly dampened, but the rest of the frequency spectrum hasn’t improved at all. Booming in the bass range and obtrusive-sounding mids now dominate the room’s sound profile.
Poor acoustic solutions: carpet, thin curtains, acoustic foam
The problem caused by absorbers that are too thin can be found in the frequency-dependent absorption coefficient of sound absorbers. In this post, you will learn what that means, why broadband absorption is the decisive factor for professional sound in the studio, and how you can noticeably increase the precision of your audio productions in your rooms with targeted sound absorption.
The theory: what is the absorption coefficient α?
The absorption coefficient α (Alpha) is the unit of measurement for the effectiveness of an acoustic element. It is a factor and has no unit of measurement, so it applies independently of the absorber’s surface area and describes the ratio of incident to absorbed sound energy on a scale of 0 to 1:
- α = 0: Complete reflection (the sound is completely bounced back, much like from a hard concrete wall).
- α = 1: Complete absorption (the sound energy is completely swallowed).
This value is specified for various frequency ranges in professional acoustic solutions.
A material can, for example, absorb sound very well at high frequencies (α = 1), but have a weak or no effect at lower frequencies. This frequency-dependent consideration is one of the most important criteria when planning good room acoustics and selecting absorbers.
The risk of narrow-band absorption
Due to physics, thin acoustic elements only absorb high frequencies. If a room is equipped exclusively with such materials, you are – subjectively speaking – only stripping it of its brilliance. This means the high frequencies are heavily dampened, but the energy (the reverberation) in the mids and basses remains unchanged. The result is an unnatural sound profile that seems both dull and obtrusive at the same time, making reliable decisions during mixing and mastering extremely difficult.
It is comparable to the acoustics in normal living spaces where carpets and curtains shape the sound. With absorbers that are too thin, it sounds at best more like a normal living room than a professional recording studio with continuous volume levels and workflows that require uncompromised listening.
Therefore, it is always advisable to rely on the right products and consult professionals on larger projects.
Porous absorbers: energy conversion through friction
To effectively dampen sound, we at HOFA-Akustik rely on so-called porous absorbers. For context: A porous absorber consists of open-pored material and – put simply – a defined resistance that counteracts the sound energy and slows it down.
When the sound wave hits the absorber, the air molecules must vibrate through the fine pores of the material. This creates friction, which converts the kinetic energy of the particles into heat energy. Which frequencies an absorber can “slow down” depends on the material thickness. Only a sufficient thickness of the absorber makes it possible to effectively capture the larger wavelengths of lower tones.
Cross-section of wall and absorber with incoming sound
A short excursion into physics: wavelength and absorber thickness
Wavelength is a physical property of sound. Every frequency has its own wavelength. It can be calculated by dividing the speed of sound by the frequency whose wavelength you want to determine. Examples:
- 10,000 Hz has a wavelength of 343 m/s / 10,000 Hz = 0.0343 m = 3.43 cm
- 1,000 Hz has a wavelength of 343 m/s / 1,000 Hz = 0.343 m = 34.3 cm
- 100 Hz has a wavelength of 343 m/s / 100 Hz = 3.43 m = 343 cm
In theory (according to the textbook), an absorber should have a thickness of about one-quarter of the wavelength of the lowest frequency you want to absorb. (Broken down further, this is related to the difference between particle velocity and sound pressure, but we will skip this in this blog post for the sake of simplicity).
A quarter of the wavelength means that to completely absorb sound with a frequency of 1,000 Hz, an absorber would thus need to be 0.343 m / 4 = 8.5 cm thick. However, this only applies to sound coming directly from the front and being reflected back exactly once.
Outside the textbook, in a real room, the goal is not to almost completely eliminate perpendicularly incident sound energy. Instead, the plan is to reduce the sound energy reflected from the wall or ceiling in such a way that the balance at the listening position between direct sound from the loudspeakers and sound reflections improves enough for you to work well and securely on your audio, while simultaneously reducing the reverberation time to a studio-suitable level.
This is because, on the one hand, most of the sound travels a longer distance within the absorber than the absorber is thick. Additionally, not only the direct sound flows through the absorber, but also sound that has already been reflected one or more times in the room. It is therefore always advisable to consider the effect of absorbers on reverberation; so to speak, on sound coming from all directions.
Thus, for example, 10 cm thick absorbers are very well suited to improve the sound in the room on a broadband scale (over a very wide frequency range). From 200 Hz upwards in the frequency spectrum, they unfold a very good effect in the room, even though a quarter of the wavelength of 200 Hz is actually 343 m/s / 200 Hz / 4 = approx. 43 cm.
The HOFA Absorber Natural can serve as an example here. It is 10 cm thick and shows the following effect in its acoustic measurement:
By the way: An absorber does not have to be filled for its complete thickness; rather, the thickness is primarily about keeping the surface of the absorber far enough away from the sound-reflecting surface behind it. Simply put, it doesn’t matter whether an absorber is 10 cm thick, or 5 cm thick with a 5 cm gap behind it. However, this system is only applicable to a limited extent. A 5 cm thick absorber does not automatically become a bass absorber if hung 50 cm away from the wall or ceiling. Nevertheless, this principle can be utilized – for example, with our acoustic sails for walls and ceilings, which can be mounted with a distance from the wall or suspended from the ceiling.
A little bit more physics:
The effectiveness of an absorber does not just depend on its thickness. Additional factors include so-called porosity, density, and the previously mentioned resistance that counteracts and slows down the sound energy. If the resistance is too low or too high, the absorber is not effective. This is one of the main reasons why building acoustic modules is best left to the professionals.
In summary, all of this means that a professional acoustic module should work evenly over the widest possible frequency spectrum. The ultimate goal in the room is a linear reverberation time across the entire audible frequency range so that the room’s sound remains natural.
HOFA Absorbers: precision for studio
Our classic HOFA Absorbers (Studio-Line) are developed exactly for these requirements. Due to their design, they work effectively from 200 Hz. They eliminate annoying flutter echoes and make the localizability in the stereo field more precise without making the sound profile seem muffled.
The perfect duo: absorbers & basstraps
Although HOFA Absorbers work very broadly with their thickness of 10 cm, they should be combined with HOFA Basstraps for high demands on room acoustics:
- Absorbers as well as ceiling and wall sails provide transparency, clear mids, and defined localization.
- Bass traps are placed in the corners of the room, for example, to control the bass foundation and reduce the effects of standing waves (room modes).
Only through this coordinated combination does a controlled acoustic environment emerge in which you can fully focus on the essentials during recording, mixing & mastering.
Optimal studio acoustics with bass traps, absorbers, diffusers and acoustic curtains
Ceiling and wall sails: acoustics and design combined
In rooms where flexibility and aesthetics play a major role, HOFA Ceiling Sails and Wall Sails (Style-Line) are the ideal choice.
Ceiling sails are particularly effective for controlling vertical reflections on the ceiling or between the floor and ceiling – a critical point in almost every room. Thanks to the associated ceiling mounting sets, a ceiling distance can be adjusted. This makes the sound absorption effect even broader.
Wall sails offer large-scale, broadband damping in an elegant design that integrates seamlessly into modern work environments or high-end studios. With the corresponding wall mounting sets, the effect in lower frequencies is additionally reinforced by the mounting distance to the wall.
Conclusion: think about acoustics holistically
Good room acoustics are no coincidence, but the result of physically sound planning. Those who rely on broadband solutions create a reliable working foundation. Our HOFA modules are coordinated to complement each other—for a sound that convinces in every frequency range.
Discover our modules in the shop or let our experts advise you to find the right combination for your project.
