In college, one of my closest friend’s dad was a physicist who specialized in acoustics – PhD, Professor, consultant for the Air Force – the type of guy that other experts went to when they were stumped. He used to describe acoustics as the “Black Magic of Physics”. The idea he was trying to convey is that acoustics is such a complex field, that it takes a physicist’s physicist to make sense of it – and he, a man distinguished in his field, felt he was just starting to wrap his head around it. I am in no position to argue with one of the worlds experts on the subject. Yet, after working with and teaching acoustics for a number of years, it has become evident that the basics for our applications need not be as frightening, dangerous, and unapproachable as “Black Magic” may imply. In many cases, understanding some fundamental principles allows system designers to approximate the acoustic behavior of an environment, match the right treatment and equipment to the location, and assist in maximizing performance of a system within a given space. Even if someone elects to rely on – or even ignore – the expertise of others, it is important to recognize that acoustic issues, whether simple or complex, impact the performance of all audio systems. By some standards, this accounts for 50% of system performance – intentionally or otherwise. Even modest awareness can be the difference between a satisfied customer and a failed installation. This very scenario came up last week when a system designer called looking to replace a system installed by another company. The audio portion of a VTC system was suffering from feedback issues and the first place they looked to solve the problem was to replace the equipment. In reality, the equipment alone was not the primary culprit.
With this in mind, the room itself is one of the first elements we look at when someone asks for help with an audio system design – either new or retrofit. Taking in to account the room dimensions, layout, construction, HVAC systems, and in many cases room contents, it is helpful to estimate the behavior of the room in order to select the right type and location of equipment. Using a conference room example, the following are some acoustic characteristics to consider and their implications:
- RT60 (Reverb Time – or how long does it take for a sound to decay by a 60 dB). This measurement indicates general intelligibility in a room and can have major implications for speaker and mic placements. Conference rooms need a lower RT60 than a music performance space. Mixed use spaces sometimes benefit from adjustable acoustics with movable panels, or finding a balance between the two. A Sabine analysis, which uses published absorption coefficients for various materials, their surface area, and the volume of the room, can be used to estimate RT60.
- Critical Distance – Close micing sources in a conference room provides the best intelligibility since the level of direct sound from the source will mask background noise and reverberation at the microphone. Close micing also improves available gain before feedback and minimizes system noise caused by additional gain from mic preamps. However, close micing is not always practical due to aesthetic, ergonomic, or budgetary reasons. Critical Distance is the distance from a sound source where the direct sound and reverberant sound are equal. Knowing the critical distance is helpful for mic placement as it is one factor that helps identify how far a mic can be placed from a listener. Too much reverberant sound in the mics and intelligibility suffers. The polar pattern of a microphone also determines how much of the reverberant sound becomes part of the signal. As a general guideline, directional mics should be less than ½ the critical distance to a source. Since omni-directional mics pick up all around – including reflected sounds from the back of the mic, staying within 1/3 of the critical distance can help with intelligibility.
- Signal to Noise – while this is typically a specification associated with equipment itself, it is helpful to assess the environmental background noise in the space that participants may encounter and that the system may need to overcome.
- When determining mic and speaker placements, calculate the proposed distances from talkers and to listeners. This can help gauge effectiveness of various options for mic placement and count, along with potential speaker locations and power requirements. These distances, together with the above measurements, facilitate calculations related to gain before feedback, or PAG-NAG (Potential Acoustic Gain – Needed Acoustic Gain). This is what InfoComm refers to as “Stability” in their CTS-related documentation.
I know the above ramblings don’t unlock any mysteries of acoustics, or begin to scratch the surface, but hopefully they draw attention to some of the factors that should be considered during system designs. There are resources available to make these elements manageable. If there are specific areas about which you are curious, let me know and we can address it in another post or offline.