Controlled Power Response: Its Importance in Sound Reinforcement System Design
This technical note explores the concept of controlled power response in sound reinforcement systems, detailing its significance for system design and performance. It examines the evolution of loudspeaker hardware, the role of equalization, and compares the performance characteristics of different horn and driver designs.
Introduction
The field of sound reinforcement has evolved significantly, moving from early hardware designs originating in the 1930s, characterized by radial horns, multicellular horns, and vented horn LF enclosures, to modern systems. These early systems, integrated into motion picture sound, often prioritized efficiency over power bandwidth, resulting in good on-axis performance but a roll-off at high frequencies off-axis and a rapid low-frequency roll-off below 60 Hz. The widespread adoption of broadband equalization in the late 1960s, while improving many systems, also highlighted fundamental issues in existing designs.
An Analysis of Two Horns
A key distinction between older and newer sound reinforcement hardware can be observed by comparing the JBL 2350 radial horn with the JBL 2360 Constant-Coverage Bi-Radial™ horn. Both are rated for 90-by-40 degrees coverage, but their performance differs significantly. Understanding horn and driver matching begins with examining the driver itself, typically measured using a Plane Wave Tube (PWT). The PWT provides smooth loading, yielding a measure of the driver's total acoustical power output versus frequency.
Figure 1. PWT Measurement of HF Drivers
The JBL 2445 driver's PWT curve shows a peak response between 500 Hz and 2000 Hz, followed by a characteristic 6 dB/octave roll-off above approximately 3500 Hz. This inherent roll-off is common in current HF compression drivers for wide-range applications.
Figure 2. 2445 Driver Mounted on 2350 Radial Horn
When the JBL 2445 driver is mounted on the 2350 horn, the on-axis response (0 degrees) is nearly flat up to 10-12 kHz. However, off-axis responses (15, 30, 45 degrees) exhibit a severe roll-off above 6 kHz. This indicates that the horn "beams" on axis, meaning its directivity increases with frequency.
Figure 3. Summation (A + B) of PWT Response of 2445 and DI of 2350 Horn On Axis
This figure demonstrates the summation of the 2445 driver's PWT curve (A) and the 2350 horn's Directivity Index (DI) (B). The combined curve (A+B) closely approximates the 0-degree response curve shown in Figure 2, illustrating how axial response is influenced by both the driver and the horn's DI.
Figure 4. 2445 Driver Mounted on 2360 Bi-Radial Horn
In contrast, the JBL 2445 driver mounted on the 2360 horn shows no significant beaming on axis. The on- and off-axis response curves are nearly parallel, closely mirroring the driver's PWT response across the frequency spectrum.
Figure 5. As in Figure 4, But Equalized
With the JBL 2360 horn, it is possible to equalize the driver's response by boosting it at 6 dB/octave above 3.5 kHz. This equalization results in a relatively flat response for all on- and off-axis positions, as depicted in this graph.
Figure 6. DI for 2360 Bi-Radial Horn
The Directivity Index (DI) for the JBL 2360 Bi-Radial horn is shown to be relatively flat. This characteristic indicates minimal high-frequency narrowing, allowing for consistent coverage.
Power Response: A Definition
Power response measures the total acoustical power output of a horn-driver combination or a full-range system as a function of frequency. While achieving perfectly flat power response across the entire band is challenging, it can be maintained within +/- 2 dB from 250 Hz to 10 kHz. Flat on-axis response is critical for the quality of the first arrival sound, while power response influences the nature of reverberant sound in enclosed spaces. For optimal naturalness, both axial and power responses should be flat or parallel up to about 8 kHz. When a system is equalized for flat on-axis response, its power response is inversely proportional to the system's DI.
Low-Frequency Enclosures
Similar to HF horns, LF systems achieve smoothest power response when their on- and off-axis curves are parallel. The JBL 4508 LF system, shown in Figure 7, exhibits smooth horizontal response up to 500 Hz. However, its vertical response shows a fall-off beginning around 200 Hz, which aligns with the vertical coverage of a 90-by-40 Bi-Radial horn if the crossover is set at 500 Hz.
Figure 7. Response, Horizontal (A) and Vertical (B), of 4508
Figure 7 presents the horizontal (A) and vertical (B) frequency responses of the JBL 4508 LF system at 0, 15, 30, and 45 degrees. The horizontal plane shows smooth response, while the vertical plane indicates a fall-off starting around 200 Hz.
Figure 8. Response, Horizontal (A) and Vertical (B), of 4550
The JBL 4550 system's horizontal and vertical responses are shown in Figure 8. Both sets of curves indicate a rapid decrease in total power output above 200 Hz, despite the 0-degree response remaining high until about 700 Hz.
Interaction With The Acoustical Environment
Properly designed full-range systems offer two primary benefits: listeners on- and off-axis perceive smooth first arrival sound, and all listeners experience a smooth reverberant spectrum influenced by room acoustics. This often reduces the need for extensive broadband equalization.
Figure 9. Unequalized Response of Old-type Theater System (Dolby Data)
This graph illustrates the unequalized power response of an older theater loudspeaker system, characterized by a "camel back" shape, necessitating significant equalization to achieve a desired contour.
Figure 10. Response, A, of Academy Theater, When HF and LF Elements are Set for Flat Power Response. At B, HF System has Been Raised 3dB.
Figure 10A displays the response of a JBL system in the Goldwyn Theater, adjusted for flat power response, showing +/- 6 dB variation from 40 Hz to 8 kHz. Figure 10B shows the impact of a +3 dB adjustment to the HF section.
Figure 11. DI Plots of JBL 4430 Monitor (A) and UREI 813 Monitor (B).
Figure 11 compares the Directivity Index (DI) of the JBL 4430 monitor (smooth within +/- 2 dB from 400 Hz to 16 kHz) with the UREI 813 monitor (showing a +/- 4 dB variation over the same range).
Figure 12. Power Response Correction in Networks; Passive (A) and Active (B)
Figure 12 shows network configurations for power response correction. Figure 12A depicts a passive network with LF loss and HF boost. Figure 12B illustrates an active system setup using a 5234A unit for HF power response boosting.
System Implementation
JBL's 3100-series dividing networks have been updated to provide HF power response boosting, as shown in Figure 12A. Careful system planning is required to prevent exceeding driver power ratings. Bi-amping setups utilize plug-in cards for the 5234A for this purpose.
The degree of one-third octave equalization needed to match desired response contours is often minimal when systems are designed with appropriate DI characteristics. In studio environments, where the ratio of direct to reflected sound is critical, the reflected sound spectrum tends to follow the inverse of the loudspeaker's DI curve. Monitors like the JBL 4430, with smooth DI, contribute to a more predictable and manageable reflected sound spectrum compared to monitors with less consistent DI, such as the UREI 813.
General Recommendations
- Unless specific sonic requirements dictate otherwise, simple ported enclosures are generally more cost-effective and perform better for LF systems than older vented horn designs.
- While large Bi-Radial horns offer excellent axial and power response, Flat Front Bi-Radial™ horns are suitable for many applications, providing comparable horizontal coverage with only minor reductions in vertical pattern control.
- Most stock JBL system designs exhibit strong axial and power response. Models 4660 and 4671 are particularly recommended for speech-only applications.
References
- J. Eargle, J. Bonner, and D. Ross, "The Academy's New State-of-the-art Loudspeaker System," Journal SMPTE, Vol. 94, No. 6 (June 1985)
- M. Engebretson and J. Eargle, "Cinema Sound Reproduction Systems," Journal SMPTE, Vol. 91, No. 11 (November 1982)
- D. Smith, D. Keele, and J. Eargle, "Improvements in Monitor Loudspeaker Systems," Journal AES, Vol. 31, No. 6 (June 1983)
