Are your sonic issues due to loudspeaker system performance, room acoustics — or both?
This month, let’s take a look at the acoustical fields in a typical large music performance space (or meeting hall) and how that affects the approach of actually tuning a large room. For the readers that have not been trained in architectural acoustics, advanced sound/P.A. system design, or physics, we will endeavor to bust the myth that a performance space/room can be properly tuned by equalizing a sound system while clarifying acoustical field definitions.
Acoustical/Sound Field Definitions
Of course, equalizing loudspeakers is beneficial; we will get into that a little bit later. But first, we need to start with the concept that there are different acoustical zones — known as fields — within a large performance space (and some of the fields overlap). For decades, many of us have been a bit confused about the several acoustical field definitions, as some of them are somewhat synonymous; also, some trade magazine articles and web sites have confused or omitted some of the different sound fields. So, let’s get to the bottom of this.
The Near Field
The closest acoustical field/zone to a sound source is known as the near field — a space where the sound pressure and acoustic particle velocity are not in phase — and it has a higher direct-sound level than the far field. A near field is related to products known as near field monitors. They are only really effective within close proximity of the compact monitor speakers (where the direct field is dominant). It is primarily in this direct field that sound system or loudspeaker equalization can be highly effective. While EQ is effective in small rooms with very-low reverberation times — such as control rooms and home theaters — loudspeaker EQ is different in a large performance venue with a typical/significant reverberation time.
As acoustical consultant Neil Thompson Shade explained, “A sound source in a room will have a near field close to the source that transitions into the far field. As rule-of-thumb, the transition from near to far field is at a distance three to five times the largest source dimension. The near field is very unstable and sound level changes significantly with position and angle relative to the sound source. Any attempt at equalizing the near field is futile since this sound field is not homogeneous and not representative of what the audience experiences. Think of trying to equalize a two-way loudspeaker. In the near field, the response will be controlled by the tweeter or woofer depending upon the proximity of the microphone.”
Free Field
The official ISO and short definition for the free field is “a region in space where sound may propagate free from any form of obstruction.” I have usually seen free field referred to outdoors, and direct field referred to indoors (such as in anechoic chambers).
The terms “free field” and “direct field” are somewhat synonymous. Another similar term is “open field,” which is less frequently used, but synonymous with the other two.
Critical Distance
Critical distance is a radial distance away from a sound source (loudspeaker) where the direct field and reverberant fields are equal level. Fig. 1 shows graphically how the direct field sound level drops as it spreads from the direct field to the reverberant and far field within a large performance space/venue.
The inverse square law teaches us that for every doubling of the distance between a sound source and the recipient of the sound, a -6 dB drop will occur, if there are no echoes or reverberation. As shown in Fig. 2, let’s say we measure the sound pressure level of a continuous audio source at some reference distance “d” and detect an SPL value of p1. Now, if we move to a distance that is twice “d,” we will detect a new pressure value, p2, which will be one-half of p1. This process may be carried out indefinitely, with each doubling of distance creating a halving of pressure.
Lowering the reverberation level/time acoustically will move critical distance further away from the sound source. Of course, the goal of large line-arrays and other high-Q (more tightly focused dispersion) loudspeakers is to extend the critical distance away from the loudspeaker and further into a reverberant space — to include far more people in the direct field — thus minimizing the level (effects of) the reverberant-sound as heard by a smaller area of the distant audience.
The Far Field
The far field is an acoustical zone that exists at a distance beyond the near field. It contains both the free field and the reverberant field, which are separated by the critical distance. In the reverberant field, reverberation controls what the audience hears despite the presence of the free field, which continues to decrease in level. System equalization in a reverberant field has less effect on what is perceived by the listeners (than in the free field), Here, the sound of the room’s acoustics is a higher level than the direct sound as the distance from the source increases.
Direct-to-Reverberant Ratio
Note that while the reverberant sound level varies by less than 3 dB throughout a reverberant field, the direct sound level continues to drop due to inverse square law. For speech and rapid tempo music, a higher/positive direct-to-reverberant ratio is preferred — as is provided in a typical free field — providing a more dry/direct/intimate sound, as compared to the lower direct to reverberant ratio in a typical reverberant field (where reverberant sound is greater level that the direct sound). For more detail on ideal acoustics for rock music and speech, see the May 2021 issue of FRONT of HOUSE.
Equalizing Loudspeakers
While equalizing loudspeakers for a more linear/flat frequency-response is beneficial to everybody in a large listening/music space (or meeting hall), trying to counteract the sound of room acoustics in the reverberant field will not be beneficial to people in the free field (i.e., in the “money” seats up front), as these free field listeners are not hearing nearly as much of the acoustical anomalies and reverberation, heard in the back of the room. So, attempting to use system EQ to counteract the sound of room acoustics worsens the sound quality in the free field. But, of course, gain-before-feedback can be improved through the use of parametric/notch equalizer/filters, although, this has little to do with the acoustics.
Equalizing/Tuning Large Room Acoustics
Managers of many churches, meeting and music halls have long been adding acoustic absorption panels to their spaces to reduce excessive echoes and/or reverberation. But when too much generic acoustic absorption is used — such as products that are efficient at absorbing sound in the high frequencies (such as foam and fiberglass, as indicated by the dashed black line in Fig. 3), it can lead to an acoustically “dead” space, without ambience. Actually tuning the acoustics of a large room can be done using tuned absorption devices within a large auditorium.
While there are some high-Q (narrow-band) absorbers available (that can be used much like a parametric/notch equalizer/filter), I have found that, since the 1990’s, when products like RPG’s BAD (Binary Amplitude Diffusor) panels in Fig. 3 have been available, such low-Q acoustical panels (sometimes incorporating some diffusive properties as well) can be quite effective for acoustically equalizing/tuning what I call “lightweight” rooms — meaning rooms that are built with framing members and sheetrock walls (as compared to massive halls built from concrete or stone).
Such lightweight rooms (with sheetrock and frame construction) leak low-frequencies out of a large auditorium/room and typically have significant high-frequency absorption, leaving an elevated reverberation time in the mid-bass region. This elevated reverberation dominates what is heard in the reverberant field — beyond the free field (or direct sound) — that is not beneficial for rock music and speech. Note in Fig. 3 how the 2” BAD A-mount acoustical panels provide the highest acoustic absorption primarily in the mid-bass region (300-600 Hz), which is exactly what we need for acoustically tuning lightweight rooms!
For an overview on auditorium (large room) acoustics, see our Architectural Acoustics 101 article (FOH, Feb. 2021). For a more in-depth look at acoustics and sound fundamentals/theory, see our reprint of the late John Eargle’s text and excellent graphics in the March 2020 and April 2020 issues of FOH.
David K. Kennedy operates David Kennedy Associates, consulting on the design of architectural acoustics and live-sound systems, along with contract applications engineering and market research for loudspeaker manufacturers. He has designed hundreds of auditorium and HOW sound systems. Visit him at www.d-k-a.com.
