Brüel & Kjær Sound & Vibration Measurement A/S

Noise monitoring solutions for room acoustics - Manufacturing, Other

Room Acoustics concerns the way sound is created, propagated, perceived, measured and modelled inside enclosures. Enclosures can be dwellings, offices, workshops, factory halls, lecture rooms, auditoria, concert halls, transportation terminals, etc. Reverberation Time is the single most important parameter used to describe Room Acoustics, but in addition, parameters describing music quality and speech inteligibility are important.

Sound Level Versus Reverberation Time

Sound level is the most important parameter to measure when solving environmental or occupational noise problems. In the open air (or the anechoic room) it is often all you need to measure. But indoors, the Reverberation Time (RT) affects not only whether you can understand what others say or enjoy music, but also the level and distribution of sound. The problem is usually that the RT is too long, but it may also be too short or not properly balanced over the frequency spectrum.

Getting the RT Right

Reverberation time that is too long will muffle speech sounds so that one sound (or word) cannot be distinguished from another, making it impossible to understand what is being said. This is critical in classrooms, auditoriums, churches, theatres and airport buildings. On the other hand, reverberation time that is too short will make levels too low at a distance, and make the sound too 'dry'. The optimal RT for music depends on the type of music and the volume of the room. Church music requires 2-4 seconds while concert studios need 1-2 seconds. The RT frequency spectrum should be reasonably flat and even. Excessive RT will cause the sound level to rise, causing annoyance or risk of impaired hearing. Typical examples are concrete stairwells in apartment blocks or workplaces with hard reverberant walls.

RT in Practice

Reverberation time is measured using either interrupted noise (pink or white) from a loudspeaker source, or impulsive noise from a starting pistol. It is measured in 1/1- or 1/3-octaves, serially or simultaneously. It is usually averaged over several positions in the room and over several decays in each position. Quite often a wide-band average is calculated by mathematically averaging the RT for a range of frequency bands. For critical applications, the shape of the decay curve is also important. Deviations from the straight line can reveal acoustical defects.

Room Acoustics concern the way sound is created, propagated, perceived, measured and modelled inside enclosures. Enclosures can be dwellings, offices, workshops, factory halls, lecture rooms, auditoria, concert halls, transportation terminals, etc.

One of the most important parameters in Room Acoustics is Reverberation Time. It is a measure for how long the acoustic energy emitted from sound sources inside a room will linger. In regular rooms with an even distribution of sound absorption, reverberation time is also a measure for the amount of absorption present inside the room. The optimal reverberation time depends on the use of the enclosure. For example, for churches and large concert halls longer reverberation times are often desired, since this supports the music. Halls for speech should have a shorter time, since otherwise lingering sound will blend with subsequent spoken words, making it difficult to understand what is said. Factory halls should also have short reverberation time. In this situation the goal is to prevent the accumulation of sound energy. This will help reduce the overall sound pressure level and the noise exposure for all workers in the hall.

Though reverberation time is a very important measure for a room’s acoustical properties, using this parameter alone would not be enough to quantify the quality of halls and spaces for music and speech. Instead, a range of parameters has been developed, which takes the temporal development and direction of incoming reflections at the listener’s position into account. Examples of these are Early Decay Time, Intelligibility and Clarity.

All these parameters are derived directly from, or with the help of, the so-called Room Impulse Response (RIR). It is the acoustical footprint of a room. However, a room does not have just one single room impulse response. The RIR is the history of the direct and reflected sound at one position in a room caused by an impulse emitted from a source at another position in the same room. Thus different receiver and source positions will lead to different RIR in the same room. One consequence of this is that the perception of the same piece of music will be different for different seats in the same concert hall.

Therefore, it doesn't come as a surprise that the room impulse response is central to both, measurement and modelling of room acoustics. Room Acoustic measurements are used to validate new constructions and to troubleshoot existing ones. The Room Acoustic Measurement System DIRAC Type 7841 is the perfect tool to help you keep control over the acoustic design of rooms.