Introduction to Room Acoustics

The sections ‘Architectural Acoustics’ and ‘Room Acoustics’ deal with two completely different attributes of a building.

BUILDING ACOUSTICS

In building acoustics, the focus is on how much noise is transmitted from one room to another. In practice, this usually concerns adjacent or vertically connected rooms. If the building components involved (walls, ceilings, doors, windows) are not appropriately dimensioned or correctly installed, disturbances may occur within the building. For example, people may complain that conversations can be overheard from neighbouring rooms or that sounds from other areas intrude too strongly into their own space. Requirements for sound insulation in buildings are regulated legally by DIN 4109 “Sound insulation in buildings”, and also generally expected by clients – even in the absence of explicit contractual agreements.
In contrast, while there are well-established standards for room acoustics, such as DIN 18041 “Acoustic quality in rooms” or VDI 2569 “Sound insulation and acoustic design in offices”, these are advisory rather than legally binding. Thus, a contractual agreement should always include room acoustic considerations, as there is no statutory entitlement to appropriate room acoustics.

ROOM ACOUSTICS

Room acoustics always considers a room as a standalone entity: an office, call centre, meeting room, classroom, swimming pool, or concert hall. The focus lies on the acoustic conditions within the room itself, which are primarily determined by the surfaces and furnishings present.

We perceive sound events – such as noise, speech or music – as louder or quieter depending on their sound pressure level. Our perception of sound begins at around 20 dB (e.g. a ticking clock), while a whispered conversation typically reaches about 30 dB.

The key parameter used to describe noise exposure at a workplace is the so-called rating level (Lr), which is derived from the measured equivalent continuous sound level, adjusted by additions or subtractions depending on the characteristics and duration of the noise. Experience shows that from a rating level of 55 dB onwards, concentrated mental work becomes significantly more difficult. Therefore, sound levels in an office should ideally remain below this threshold to provide all employees with suitable working conditions.

During a normal conversation, we are exposed to a sound pressure level of around 60 dB. From a rating level of 80 dB, official guidelines for hearing protection start to apply, and beyond 100 dB, even relatively short exposure can cause irreversible hearing damage. Such levels may occur near pneumatic drills or jet engines – and sometimes even in nightclubs or at rock concerts (see Figure 1).

In addition to sound pressure level, the frequency composition (or spectrum) of sound is especially important. The human ear typically perceives frequencies between approx. 20 Hz and 20,000 Hz. Music covers almost this entire range, whereas speech is largely limited to the range between 250 Hz and 2,000 Hz – where our hearing is most sensitive.

As a result of our ears’ varying sensitivity across frequencies, key acoustic parameters such as reverberation time, sound pressure level, and the sound absorption coefficient of a material are always specified in relation to frequency.

Technical standards also reflect the human hearing range. Building acoustics, concerned with sound transmission between rooms, typically considers frequencies from 100 Hz to 3,150 Hz. Room acoustic guidelines, on the other hand, usually refer to the range from 100 Hz to 5,000 Hz.

The way a room sounds is primarily determined by its reverberation time. In simple terms, this is the time it takes for a sound to become inaudible after the source has stopped. Reverberation time is typically measured across the frequency range of 100 Hz to 5,000 Hz. The longest reverberation times, five seconds or more, are found in churches; the shortest, around 0.3 seconds or less, in recording studios or listening booths. The optimal reverberation time for a room depends on its size and use.

Our perception naturally expects longer reverberation in larger rooms than in smaller ones. If room size, use and reverberation time are not well-matched, users often feel disturbed and complaints are common. A room with too long a reverberation time may be described as echoey or boomy, whereas one with too short a reverberation time may be described as dry or muffled.

DIN 18041 addresses this by recommending that the ideal reverberation time increases with the volume of the room, depending on its intended use.

DIN 18041 provides guidance on reverberation times based on a room’s size and purpose.
For uses such as teaching and communication, the shortest reverberation times are recommended. Insufficient absorption – and therefore overly long reverberation – impairs speech intelligibility, which usually leads to speakers raising their voices. For spoken performances or music, longer reverberation times are recommended (see Figure 2).
Example: A 250 m² meeting room should have a reverberation time of 0.6 seconds.

Measuring reverberation time can reveal existing shortcomings objectively and provides a solid basis for improvement proposals. Based on the results, detailed design suggestions can be made, including suitable materials and the required surface area for optimal acoustic performance. The costs of measurement are often offset by more accurate specification of surface materials – avoiding over-engineering – and offer the assurance that any measures taken will actually be effective.

Although there is a wide range of sound-absorbing materials available, they can all be traced back to two physical mechanisms: porous absorbers and resonant (or panel) absorbers.

POROUS ABSORBERS

Porous absorbers, such as mineral wool or foam, allow incoming sound to penetrate the material. The sound energy is converted into heat through friction within the pores, reducing the amount of sound reflected back.

RESONANT ABSORBERS

Resonant or panel absorbers are excited into vibration by incident sound – for example, a wooden or metal panel. This vibrational energy is also dissipated as heat through friction.
Porous absorbers are typically effective at mid to high frequencies with common thicknesses (10–20 cm). Resonant absorbers are usually tuned to low frequencies.
There are also hybrid types that combine both mechanisms, offering the advantage of broadband absorption across a wide frequency range.

The acoustic effectiveness of a material (or object) is described by its sound absorption coefficient. This value ranges from 0 (no absorption, e.g. a concrete wall) to 1 (complete absorption, e.g. wall surfaces in a recording studio). The absorption coefficient is highly frequency-dependent and should ideally be described not as a single number but as a set of values across the frequency range.

According to DIN EN ISO 354, the absorption coefficient is measured from 100 Hz to 5,000 Hz and denoted as αs. This results in values such as α₁₀₀, α₁₂₅, α₁₆₀, … , α₄₀₀₀ , α₅₀₀₀, as shown in the curve in Figure 3. Each sound absorber thus has its own absorption “fingerprint”.