How to choice acoustic panels

Building interiors – from the point of view of the needs of acoustic adaptation – can be divided into two main groups:

I – production, office, service rooms, etc.
II – home rooms, entertainment, educational purposes, etc.

The basic task in the design of rooms from the first group is to reduce the noise level, both from external and internal sources, in order to improve working conditions and increase speech intelligibility. This is achieved by the appropriate design of anti-noise protection, the shape of the room and the use of sound-absorbing systems.

The rooms of the second group – which will interest us more – are divided into:
– interiors for direct listening,
– interiors for recording and playing speech and music.

To ensure correct acoustic conditions in a room in this group, we must consider three issues:

1. Ensuring a low level of interference – means to properly design the insulation from external interference, as well as to silence the internal sources of noise.
2. Ensuring proper reverberation conditions – means to properly design and perform acoustic adaptation.
3. Ensuring proper uniformity of the acoustic field – that is, we are again bowing to proper acoustic adaptation.

ASSESSMENT OF THE QUALITY OF THE ROOM

It sounds maybe a bit strange that we are talking about the quality of the room when it is not yet there, but it is important to set some rules at the beginning, according to which our room will be assessed, in order to adopt the appropriate assumptions and carry out the design process .

The criteria for assessing acoustic quality can be divided into subjective and objective ones. In the case of the former, we rely on the opinions of listeners, performers and other specialists (smaller or larger) in the field of interior acoustics. Unfortunately, these opinions for one room can be very different, so you can – for example, after appropriate statistical treatment – treat it as an addition to objective criteria.

An objective assessment can be made at the design stage – eg on the basis of calculations and computer simulations, or acoustic measurements carried out on scale models – as well as on the basis of measurements made during and after the project implementation. The architect and designer of interior acoustics are primarily interested in the acoustic parameters of the room, which can be used in designing and which have a decisive influence on the acoustic quality, i.e. for a sufficiently low level of sound coming from external interference, intelligibility of the verbal text and musical material, uniformity of the sound system or the lack of an echo.

In the case of large concert halls, obtaining good intelligibility usually does not go hand in hand with obtaining high sound qualities, because good intelligibility is obtained with a relatively short reverberation time, while ensuring proper full sound of music requires a longer reverberation time. Hence the necessity to choose the compromise value of the reverberation time.

Very important for the sound qualities of the room is the difference in the time of direct and reflected sound to the listener in the audience, especially the first reflection. The fact that the reflected energy reaches a point in the form of a scattered sound is also of great importance, or whether the sound wave has a specific direction from which it comes. Reflections of the sound wave coming to a given point can be divided into two groups:
– reflections reaching the listener with a delay lower than the critical one, causing the increase of the direct sound intensity,
– reflections coming with a delay greater than the critical one.

Critical delay is the difference in the time of arrival of the reflected signal in relation to the direct one, which may cause an echo and resulting deterioration in intelligibility.

 

The time delay in the investigation of the first reflection has been the subject of many studies. It was found that the value of the critical time delay between direct and reflected sound depends, among others, on since reverberation, as well as the type of audio signal (speech, music), and for music in addition to its type.

 

In the case of listening to music, the noise causes masking of individual elements of the song, deterioration of its sound, making it difficult or even impossible for the listeners to properly receive musical impressions. Noise also causes nervousness of performers, which is the cause of unjustified increase in the volume of performed music parts, thus disturbing the musical balance of the song.

In studio (recording, radio, television) rooms, in contrast to the rooms intended for direct listening to music, the sound receiver is a microphone. The human ear, I hope, everyone knows, responds less to sounds in the small and large range, and strongly in the medium frequency range – hence the sound pressure level is often expressed in accordance with weighted curve (A or C), more suited to the sensations of human hearing. The microphone, on the other hand, carries approximately the same sounds (depending on its transmission characteristics) in the wide frequency band. Hence, the characteristics of the permissible sound level in studio rooms more restrict the noise in the low frequency band.

SHAPE AND VOLUME OF ROOM

The shape and volume of the room have a direct impact on the distribution of acoustic energy and room sound.

For radio and television studios, the optimal proportions – depending on the volume of the studio – can be read from the diagram in figure 4. It shows that for small studies, with a volume of 80 to 300 m3, the most favorable proportions of dimensions (height x width x debt) are 1: 1.25: 1.6, for medium (from 300 to 800 m3) 1: 1.6: 2.5, and for low (V = 800-3.000 m3) or long (V = 3,000-8,000 m3) the optimal ratio is 1: 1.25: 3.2. Acceptance of tolerance ± 5%, often used in practice, does not give noticeable differences in the dispersion of sound energy.

 

RESPONSE TIME AND UNIFORMITY

The reverberation time is an acoustic parameter of the interior, which can be designed and obtained with sufficient accuracy. For each room, depending on its volume and application – will it be the room where the majority will be speech (auditorium, lecture and theatrical rooms), or music (chamber, concert and opera halls) and depending on the type of music – you can choose the so-called the optimal range of reverberation time values ​​and its optimal characteristics.

 

Figure 5 shows the dependence of the optimal reverberation time on the type of artistic production and the volume of the room, for the frequency of 512 Hz. Also, the type of music presented is governed by its own laws – classical music (eg Mozart or Bethoven) or contemporary (eg Stravinsky, Penderecki) require slightly different conditions, while others are romantic (eg Brahms). In the first case, the optimal reverberation time in the 500-1.000 Hz band for large concert halls (3,000-1,000,000 m3) is about 1.5 seconds, while romantic music needs more “breath” in the form of a larger reverberation, around 2 seconds. The compromise output, for different types of music, is about 1.7 seconds.

For smaller rooms with a volume of 1,000-8,000 m3, and concert halls with a volume of up to 20,000 m3, the optimal reverberation time in the 500-1.000 Hz band (according to Bruckmayer), in packed rooms is shaped as in the adjacent table. In turn, the recommended course of reverberation time characteristics as a function of frequency, depending on the purpose of the room and the type of music, according to various researchers in this subject, is presented in Figure 6.

 

The recommended reverberation time and deviations from this value (percentage and time), frequency function, for studies for speech and spelling registration, studies for recording music, dubbing studios and photo halls, are shown in figure 7. In contrast, for cinema halls the recommended time value is the reverberation for 500 Hz is within the range of 0.9-1.2 s, depending on the room volume.