How to solve spatial acoustic problems with audio technology

Can EQ really solve room acoustic problems?

First, let’s think about this: Are the acoustic problems we face limited by the performance of the speaker system, or are they due to defects in the indoor architectural acoustics, or both?

Let's first look at the acoustics of a typical large music performance space and how it affects the tuning of a large indoor space. For readers who have not received architectural acoustics training, advanced PA system design, or have no basic knowledge of physics, we will try to explain the definition of the sound field in simple language while explaining how to optimize the sound field of the performance space by setting the correct equalization for the sound system.

Acoustics/sound field definition

There is no doubt that equalizing a speaker setup is beneficial; we'll discuss this later. But first, we need to start with the concept that there are different acoustic areas (or fields, as they are called) within a large performance space, and some of the fields overlap. For decades, many of us have been a little confused by the several definitions of sound fields, which are somewhat synonymous; in addition, some magazine articles and websites have confused or omitted some of the different sound fields. So, let's first establish these definitions.

Near Field

The sound field/area closest to the sound source is called the near field, and it is a space where the sound pressure is not proportional to the speed of acoustic particles - it has higher direct sound levels than the far field. Sound system equalization is very effective mainly in this direct sound area. While EQ is effective in small spaces with very short reverberation times (such as control rooms and home theaters), the role of speaker EQ in large performance venues with significant reverberation times is not necessarily the case.

As acoustic consultant Neil Thompson Shade explains, “A sound source in a space will have a near field close to the source, then transition to a far field. As a rule of thumb, the transition distance from near field to far field is three to five times the size of the largest sound source. The near field is very unstable, with sound pressure levels varying significantly with position and angle relative to the source. Any attempt to balance the near field is futile, as it is not uniform and not representative of the audience’s experience. What if you add equalization to a two-way loudspeaker? In the near field, the response is dominated by either the tweeter or the bass horn, depending on the proximity of the microphone.”

Free field

The official definition of free field is "the area where sound propagates without being affected by any form of obstacles." The free field we often come across refers to the outdoors, and the direct field refers to the indoors (such as an anechoic room).

The terms "free field" and "direct field" are somewhat synonymous. Another similar term, which is used less frequently but is synonymous with the other two, is "open field."

Critical Distance ​

The critical distance is the radial distance from the sound source (loudspeaker) where the direct field and the reverberant field are at equal levels.

Figure 1 graphically shows how the sound pressure level of the direct field decreases as it propagates from the direct field to the reverberant field.

Figure 2: The inverse square law shows that sound pressure is inversely proportional to the square of the distance from the sound source, with the sound pressure level decreasing by -6 dB for every doubling of the distance from the sound source.

The inverse square law tells us that if there is no echo or reverberation, there will be a 6 dB drop in sound pressure for every doubling of the distance from the sound source. As shown in Figure 2, suppose we measure the sound pressure level of a continuous audio source at some reference distance "d" and detect an SPL value p1. Now, if we move to a distance twice "d", we will detect a new pressure value p2, which will be half of p1. This process can be continued indefinitely, with the sound pressure halving for every doubling of the distance.

Acoustically reducing reverberation will move the critical distance further from the sound source. Of course, the goal of large line array loudspeakers is to extend the critical distance farther into the reverberant space—thus fitting more people in the direct field.

Far Field

The far field is the sound zone that exists at a certain distance beyond the near field. It contains the free field and the reverberant field, which are separated by the critical distance. In the reverberant field, despite the existence of the free field, the reverberation dominates the sound heard by the audience, and the sound pressure of the free field continues to decrease. The system equalization in the reverberant field has less impact on the listener's perception (compared to the free field), where the reverberant sound pressure is higher than the direct sound pressure as the distance from the sound source increases.

Direct/Reverb Ratio

Note that while the reverberation sound pressure level varies by less than 3 dB throughout the reverberant field, the direct sound continues to drop due to the inverse square law. For speech and fast-paced music, a higher direct/reverberation ratio is preferred - as in a typical free field (while a lower direct/reverberation ratio, where the reverberation sound is louder than the direct sound).

Speaker Equalization

While adding EQ to the speakers to get a more linear/flat frequency response is good for everyone in a large music space (or conference hall), it can have a detrimental effect on people in a free-field environment if you try to reduce the bad room acoustics in a reverberant field. These free-field listeners will hardly hear the acoustic anomalies and reverberation at the rear of the venue. Therefore, trying to use system EQ to cancel out the reverberant sound of the room acoustics will deteriorate the sound quality in the free-field. Of course, gain before feedback can be improved by using parametric/notch EQs/filters.

Balance/optimize large space sound field

Managers of many churches, conference rooms, and concert halls have long added acoustic panels to their spaces to reduce excessive echo and reverberation. However, when too much sound-absorbing material is used—such as products that effectively absorb high-frequency sounds (such as foam and fiberglass, as shown by the black dashed line in Figure 3)—it can result in an acoustically “dead” space with no sense of ambience. In fact, the acoustics of large spaces can be adjusted using tuned absorption devices.

While there are some high-Q (narrowband) absorbers available (which can act like parametric/notch EQs/filters), the fact is that since the 1990s, when products like RPG's BAD (Binary Amplitude Diffuser) panels became available, such low-Q acoustic panels (which sometimes also include some diffusion properties) have been very effective for "lightweight" rooms - meaning rooms built with framing members and drywall walls (compared to large halls built of concrete or stone).

Figure 3: Absorption efficiency of RPG’s BAD (Binary Amplitude Diffuser) panel at different frequencies compared to 1-inch fiberglass (dashed line)

This lightweight room (with its plasterboard and frame construction) will contain a lot of low frequencies and will usually have significant high frequency absorption, leaving a long reverberation time in the mid-bass region. This enhanced reverberation results in the sound being heard in the reverberant field - beyond the free field (or direct sound) - which is very detrimental to rock music and speech. Here we can see that the 2" BAD A-mount Panel in Figure 3 provides the highest absorption primarily in the mid-bass region (300-600 Hz), which is exactly what is needed for the acoustic optimization of a lightweight room!