Room acoustics

WHEN STUDYING THE ACOUSTICS OF A BUILDING, we need to deal with all the phenomena mentioned in Chapter 2. In an enclosed space, sound waves will be modified by the interaction with the building elements.

5.1.1 Reflected sound and path differences

The use of absorptive materials is concerned with controlling reflected sound within a room. One of the purposes of controlling reflected sound with absorptive materials is to reduce the level of sound that arrives late enough after the direct sound, as this can cause disturbance and reduce speech intelligibility. By absorbing this sound, we reduce its energy and so reduce how far it can propagate or how loud it is perceived.

There are also instances where we want to encourage sound to be reflected off room surfaces, because if it arrives soon enough after the direct sound it can have the perceived effect of making a voice sound louder or clearer. Achieving these effects is dependent on the difference in time and on the difference in distance a sound wave has to travel. If we place a hard reflective surface close to a sound source and we know its position, its size, and the angle at which it is oriented, we can control the direction in which sound coming into contact with the panel will travel.

Figure 5.1 shows sound propagation out from a single source: Figure 5.1a shows the direct sound between source and listener along with the indirect sound being reflected off untreated surfaces allowing for long reverberation times as a result of the difference in paths. Figure 5.1b is after treatment with absorptive materials, to reduce the effects of late reflections of sound, and an acoustic reflector has also been added above the sound source. The angle size and positioning of this panel encourages sound to be reflected, so that it arrives at the listener at the right time to help reinforce the sound level.

5.1  Absorbing and reflecting sound-path differences

By understanding the position, size, and orientation of such reflective panels and the distribution of absorptive panels, we can design for better acoustics.

5.1.2 Optimum path differences

The path difference is linked to the difference in the time it takes for the direct sound to reach the listener, compared to the time taken for the indirect sound to reach the listener. Table 5.1 details optimum path differences and its effect on the listening environment.

As path differences between direct and reverberant sound are a result of sound waves being reflected off room surfaces, it is clear that the size, as well as the shape, of a room has a direct effect on acoustics. Get the room dimensions wrong, or place a reflective surface in the wrong position, and the acoustics will be poor.

Table 5.1  Subjective effect from differences in path lengths

The mathematics used to predict path differences is detailed in Eq. A.8. For acoustically reflective panels, whether they are suspended or are taken to be the rear wall of an assembly room stage, the key property is the panel size. As long as the panel is between two and four times the size of the sound wave enacting upon it, then the angle of the sound wave going into the panel will be equal to the sound wave reflecting off the panel (Figure 2.2).

The most relevant frequencies to speech intelligibility lie between 1000 Hz and 4000 Hz, while the frequencies where the voice carries the greatest energy center around 500 Hz. Therefore the optimum size of reflective panels, for speech, would be between 4.25 ft² and 9 ft² (1.4 m² and 2.8 m²).

IF YOU STAND IN A LARGE ROOM and make a sudden noise, you might hear an echo. The sound you first hear is the sound which has traveled directly from where the noise was created to your ear. The next time you hear the sound you have made is when it has traveled from you to one of the surfaces in the room and bounced back to where you are standing. As this sound has had to travel further, the time it takes for the sound to reach your ear is longer and differences in the timing are perceived as an echo. But, as we mentioned in Chapter 3, our brains have the ability to merge all reflections of sound that arrive at our ears during the first 50 ms, so early reflections won’t be perceived as a distinct echo. Late reflections might not be perceived as echoes if they are not very loud. Instead, we perceive them as a permanence of sound that slowly decays. This is known in acoustics as reverberation.

5.2.1 The importance of reverberation within buildings

If the time delay between the direct sound and the reverberant sound is too long, it can adversely affect how well we hear speech or music, but if we can control and enhance the sounds which arrive soon after the direct sound, the early reflections, this can have the effect of improving sound quality.

Reverberation is present in all rooms of any size and shape. By knowing the shape, size, and materials that are used to create a room, we can define how reverberant that space will be, and so we can design avoiding the sort of problems associated with reverberation. It is often thought that the consideration of room acoustics is something which is limited to the design of music venues; however, if we do not consider reverberation when designing other buildings, then we run the risk of providing educational buildings where teaching becomes difficult, healthcare buildings where patient recovery is hampered, workspaces where people are easily distracted, and commercial spaces which customers do not feel comfortable using.

Through designing the reverberation of a room, we can:

  alter the overall noise level within a space

  make a space more or less suitable for a range of activities from quiet study to live music

  create environments in which listening and understanding are enhanced.

5.2.2 Reverberation time

The phenomenon of reverberation is quantified by reverberation time (RT). It is defined as the time it takes a sound to decay 60 dB after the source has stopped emitting it. Reverberation times are given in seconds and they can be measured, calculated, or simulated. When measuring reverberation times in a room, it is not always possible to obtain a 60 dB decay. This means that, however high the noise floor (background noise) in the room is, the initial level of the sound before it starts decaying needs to be 60 dB above that (Figure 5.2a). For this reason, the measurement of reverberation time is usually extrapolated. In Figure 5.2b, we see that a 30 dB decay is measured from –5 dB to –35 dB below the starting level. If we then multiply this time by 2, it would be the same as measuring the entire 60 dB projected decay (T60). This is referred to as T30 and is equivalent to the calculated RT. T20 and T10 are also commonly used, always subtracting 5 dB from the initial level to start counting down the desired decay.

5.2  Reverberation time

5.2.3 How we control reverberation

To stop sound being reflected, we use materials which are acoustically absorptive (see Section 2.5). They are generally softer material finishes or slim panel systems which may also have perforations or slots in them.

There are also instances where we may want to increase or maintain reverberation. By placing acoustically reflective materials closely behind a sound source, we can produce an effect that reinforces the sound being made. This is because we create an echo, which has a very short time interval between it and the first sound wave. Its close arrival has the effect of amplifying the sound.

Acoustically reflective materials will commonly be heavy and have hard, smooth, even surfaces.

5.2.4 Measuring reverberation

When acousticians measure the reverberation within a room, they will create a sudden impulsive or interruptive sound, often from a starter pistol or a loudspeaker. They will then use a microphone connected to a recording system, such as a sound-level meter, which will measure how long it takes the total noise level within the room to drop by a defined amount, e.g., 30 dB or 60 dB. As time progresses, the noise level reduces at each frequency. Reverberation times can vary from between a few seconds, common in large churches or cathedrals, to a few fractions of a second, common in bedrooms or living rooms. The shorter the time it takes for the sound to reduce, the less reverberant a space is and so the greater the quantity of absorptive finishes. Longer reverberation times usually mean less absorptive materials in a room as well as a larger volume of space being enclosed.

Over the last 100 years, assessments of reverberation times in common spaces have been undertaken to determine the optimum level or reverberation. The optimum reverberation time will be dependent upon the room size and what the room is to be used for, such as speech or music. From these assessments it has been possible to derive good practice guidelines of reverberation times in common spaces.

Appendix B provides more detailed recommendations for specific room types, while Appendix A, Eq. A.25 provides a calculation method that can be used to determine the optimum reverberation time, dependent on room size and whether it is to be used for speech, music or choral music.

5.2.5 Predicting reverberation

The beneficial aspect of having optimum reverberation times is that not only can they be accurately measured in completed buildings, but it is also possible to predict reverberation times with a good degree of accuracy. This means that we can design rooms so that they are suitable for their intended use.

For acousticians, the Sabine equation or the Norris-Eyring equation are familiar design tools used in calculating reverberation times; however, they may be less familiar to architects and designers. An explanation of these two calculations is given in Appendix A (see Eq. A.22 and A.23). For the purposes of basic design, the key elements to be aware of are room volume, the materials chosen as surface finishes, room shape, and the angle of room surfaces.

Taking an atrium space as an example, here we have a space which is likely to be large – therefore reverberation, particularly at lower frequencies, is likely to be high; there will be long reverberation times due to the time it takes for a sound wave to reach a boundary surface and be returned. In addition, atrium spaces are likely to have a lot of glass and even hard floor surfaces. These hard surfaces are efficient at reflecting sound and as a result the sound wave being returned does not lose a lot of energy.

Contrast an atrium with a small bedroom. The distance the sound has to travel from the source until it reaches a boundary surface is small; therefore reverberation times are short. In addition, bedrooms are likely to have carpeted floors, curtains, as well as the soft materials associated with a bed mattress and bedding. These materials are poor at reflecting sound, because they tend to absorb a lot of the energy of the sound wave. As a result there is less sound returned to the listener.

If we want to change the acoustics of an atrium or a bedroom, it is unlikely that we will have the option of reducing or increasing the volume of the space, so we tend to rely on techniques which impact on the strength of a reverberant sound wave, usually through altering absorption or directing the sound waves where we want them to go. It is here the architect or designer has a part to play, as these control methods can have a visual impact on a finished space.

5.2.6 Distribution of sound absorption in a room

Figure 5.3 details some common methods of distribution for absorptive materials.

5.2.7 Room shape and its influence on acoustics

The ideal shape of a room for good acoustics varies with the room function, the capacity requirements, and architectural constraints. As a result it is a complex issue that demands greater depth than can be allowed for in this book. While shoebox-shaped rooms have been found to be good for orchestral music, particular classical choral music may favor a more churchlike space. Whether the development of these styles influenced building design, or the shape of the building influenced the style of the music, is a subject of debate, but what is clear is that some shapes do cause potential acoustic problems that should be known to the designer.

Narrow corridors can result in echoes becoming an issue, as sound is reflected off of two parallel walls. This can be a particular problem in hotels. Good levels of absorptive finishes, plaster or timber moldings on wall surfaces, and fire doors across corridors can help to reduce this effect.

Curved ceiling or curved alcoves and walls can allow for sound to be focused on a particular position, causing issues with sound clarity, particularly in theater or music venues. In addition, echoes are also possible along the curve of large domes (often creating whispering galleries where sound is reflected along a ceiling to the opposite side of a room). The addition of acoustically absorbent linings or suspended acoustic reflector panels can help to reduce focusing. The application of these and plaster or timber moldings, or less reflective wall finishes, can significantly reduce the effect of whispering galleries.

5.3  Distribution of absorptive materials

Parallel walls in music rooms should be avoided. Walls should be offset by a minimum of 5 degrees to avoid the potential for standing waves to occur (areas of high or low sound pressure). This can be achieved by building rooms that are not perfectly square or rectangular, or by applying angled surfaces after completion of the structural room elements (see Figure 5.3).

(see Figure 5.3, page 77)