Audio

A Question Of Balance: Good Acoustics Need A Balanced Approach

Figure 1. Unbalanced auditorium RT characteristic and target curve.

Part of my job is to review the work of others or proposals from contractors and installers. This can be quite educational, as well very frustrating. The abundance of acoustic prediction programs has led to a new wave of “pseudoacousticians.” These are people who have bought a program and then, with the use of its “help files” and possibly a quick internet search, produce acoustic models and designs in-house rather than getting an experienced acoustician on board.

Now, I have nothing against others doing the work I do, it’s just that the resulting mess often brings the profession into disrepute. The approach can also relate to acoustic measurements, but predictions are where things can really go awry. A recent job I was involved with highlights the problem. The project was a prestigious conference center. The original design employed virtually all hard (i.e., acoustically reflecting) surfaces and the resulting reverberation times were horrendous, a problem first spotted by a tendering sound systems contractor who modeled some of the spaces and raised the alarm.

The design engineering consultants then carried out their own assessment. Whether they had purchased the acoustic prediction program before this project or especially for it, I do not know but, although highly intelligent people, their lack of acoustic experience shone through. Having determined from the program’s help pages that the Reverberation Time (RT) should be around one second for the main auditorium, a reasonable model was built and basic materials employed. After some “tuning,” a final proposal was submitted.

A while later, I was appointed to review the acoustic and sound system designs for the project to ensure that all would be well. It wasn’t, as even an initial glance at the design proposals showed, and an immediate warning email was sent to my client. There were several issues that had to be dealt with. The first was the reverberation time. As noted, the engineers had determined that the mid-frequency RT should be about one second (at 500Hz) and not too much away from this at 1kHz…absolutely fine!

However, their total lack of understanding and experience meant that they didn’t consider the RT at any other frequency. So, having pretty much achieved the one second mid-frequency target, they decided “job done” and submitted their recommendations. The resultant RT curve is shown in Figure 1. As you can see, the bass frequencies were almost completely uncontrolled with an RT of three seconds at 250Hz, and 3.7 seconds at 125Hz. I can only assume that the help notes of this particular program did not discuss the need to balance the RT characteristic.

The curve shows there to be a total disaster waiting to be built. To put this into context, I have added my target curve as the dotted red line. Although some bass rise is permissible, this has to be limited to about 25% for a speech auditorium or space where amplified music is to occur. (Concert Halls for orchestral music can have a greater increase: up to 50%, but these are generally poor venues for amplified sound unless additional mitigating measures are taken.)

Figure 2. Some typical sound absorption coefficients.
Figure 2. Some typical sound absorption coefficients.

The problem that was not appreciated (apart from the need to balance the RT) was the way in which materials absorb. Most dissipative, porous absorbers, such as carpeting, drapes and upholstery, only absorb sound at mid and high frequencies. Structural elements, such as gyprock walls and ceilings and glazing, absorb only at low frequencies, but are not efficient absorbers and so an imbalance can easily occur. Figure 2 shows the sound absorption characteristics of some typical auditorium and room components.

The effect of the frequency dependence of sound absorption can be seen in Figure 3, which shows the RT characteristics for three untreated/poorly treated rooms.

Figure 3. RT characteristics of some untreated/poorly treated rooms.
Figure 3. RT characteristics of some untreated/poorly treated rooms.

Balancing the RT can be a tricky job and often some compromise has to be made. Figure 4 shows how the low-frequency reverberation time of a large (3000-seat) venue was tuned so amplified music could be appropriately catered to. The blue curve achieved the 1.1 second mid-frequency target that I had set, whereas the optimized lower red curve achieved the required low-frequency control.

Figure 4. RT tuning to optimize bass control.
Figure 4. RT tuning to optimize bass control.

Acoustics, therefore, is not just about numbers but understanding what the numbers mean and how to apply them. Interestingly, the project I mentioned at the start of this piece also featured a dedicated cinema. I can only assume that the program help files do not mention a cinema because an RT of 1.4 seconds was decreed as being desirable…just over double what it should be!

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