Grundlagen der Bauakustik

As the custom installation industry grows, good acoustic design is becoming more important to architects and building users. Interior room acoustics is a key part of architectural acoustics. It covers room shape and volume, the best reverberation time, frequency response, sound-absorbing materials, reflective surfaces, and the control of early sound reflections. These choices help create a room that sounds clear, balanced, and comfortable.

Decibel

A decibel (dB) is one tenth of a bel. It is commonly used to describe sound level. A decibel does not describe sound by itself. It describes the ratio between two energy levels. The human ear responds to sound on a logarithmic scale. Using decibels makes these large changes easier to express.

Here are a few useful points. A change of about 1 dB may be the smallest change that a person can notice in normal conditions. Doubling speaker power usually creates an increase of about 3 dB. A 6 dB increase needs about four times the amplifier power. Perceived loudness also depends on the listener, the room, and the type of sound.

Frequency

Sound begins with vibration. Frequency, shown by the symbol f, is the number of vibrations that occur each second. The unit is hertz (Hz). The human ear cannot hear every frequency. Most people can hear sounds from about 20 Hz to 20,000 Hz.

Sounds below 20 Hz are called infrasound. Sounds above 20,000 Hz are called ultrasound. People cannot normally hear these sounds. Human hearing is most sensitive between about 100 Hz and 3,150 Hz. In architectural acoustics, sounds at about 200–300 Hz or lower are usually called low-frequency sound. Sounds from about 500–1,000 Hz are mid-frequency sound. Sounds from about 2,000–4,000 Hz or higher are high-frequency sound.

Sound Absorption

Sound absorption happens when a material reduces sound reflection. When a sound wave reaches a soft or porous material, part of its energy enters the material and changes into a small amount of heat. Less energy returns to the room as reflected sound.

Different materials absorb sound at different levels. Their performance depends on the material, thickness, installation method, and sound frequency. Acoustic panels, fabric-wrapped absorbers, carpets, and other soft surfaces can help reduce reflections and improve room acoustics.

Sound Absorption Coefficient

The sound absorption coefficient shows how much sound energy a material absorbs when a sound wave hits its surface. It is usually written as a value from 0 to 1. A value of 0 means that the surface reflects almost all of the sound. A value of 1 means that the surface absorbs all of the sound in the test condition.

When the absorption coefficient is multiplied by the surface area, the result is the equivalent absorption area. This value is measured in sabins. The unit is named after Wallace Sabine, a Harvard professor and acoustic researcher. The coefficient changes with frequency because materials absorb low, mid, and high frequencies in different ways.

Acoustic designers use absorption data to estimate reverberation time, including RT60. Many building materials have tested absorption coefficients. These test results help designers choose acoustic materials for offices, meeting rooms, studios, classrooms, and other spaces.

Anechoic

Anechoic means without echo or sound reflection. A true anechoic room is designed to absorb almost all sound reflections. An open outdoor space can seem close to anechoic, but the ground and nearby objects still reflect sound.

A completely reflection-free environment is very difficult to create. No practical material absorbs every frequency perfectly. It is easier to control high-frequency sound than low-frequency sound. This is because low-frequency waves are long. For example, a 100 Hz sound wave is about 11 feet (3.4 meters) long. Effective low-frequency absorption may require thick materials or a large air space behind the absorber.

For this reason, a large room and a large amount of acoustic material may be needed to create a near-anechoic environment. In most building projects, designers aim for controlled reflections rather than a completely anechoic room.

Bass Trap

Every enclosed room has natural resonance frequencies. These frequencies depend on the room dimensions. Room size affects how low-frequency sound builds up, changes, or disappears in different areas.
A bass trap is a low-frequency sound absorber. It helps reduce standing waves and excessive bass energy in a room. Bass traps are often installed along walls or in corners. They can make the listening area sound more accurate and balanced.

Bass traps come in many shapes and sizes. Their performance depends on their design, thickness, position, and installation. A well-planned bass trap can improve room acoustics in recording studios, home theaters, offices, and other listening spaces.