Regenerative Electroacoustical Feedback EqualizationTuning out regenerative feedback is a simple procedure that is now possible using a common test tool.
Feb 20, 1997 12:00 PM,
By Paul D. HendersonHenderson is a 15-year-old home-schooled student who plans to pursue a career i
Electroacoustic feedback suppression is the key to increasing sound-reinforcement system performance. Electroacoustic feedback is caused by excessive acoustic gain at one or more frequencies, resulting in regenerative feedback looping through audio the system.
Without complete acoustical isolation between the loudspeaker and microphone transducers, the system’s electrical transfer function must be modified to compensate for nonlinear available energy-gain vs. frequency (EFC). This modification involves the insertion of minimum phase electronic filters into the system’s electrical signal path to correct any minimum-phase amplitude anomalies in the EFC loop-path transfer function,i.e., the combination of transducer, electrical and environmental responsecharacteristics.
These filters constitute the entire range of equalization devices, including common critical bandwidth, 1/3-octave equalizers adjustable to provide both band rejection and bandpass filtering characteristics. Ideally, this filter set would be tuned to introduce an inverse EQ curve into the feedback loop path. In unequalized form, a system would reach maximum acoustical gain as one or more frequencies approach the feedback stability margin (FSM), approximately 6 dB under unity gain, which is the point at which the system goes into regenerative looping, or acoustical feedback. Upon the application of an inverse curve to this loop path, with average electrical insertion loss per filter recovered in a gain block of the signal path, all frequencies can then reach the FSM simultaneously. This permits maximum acoustical power transfer across the system, effectively increasing potential acoustic gain before feedback.
To calibrate this electronic filter set to compensate for EFC transfer function nonlinearity, the system must undergo a tuning procedure that allows the location of potential feedback frequencies. This is normally a final stage of the original system installation process, after all calibration, driver synchronization and testing procedures are complete. Also, curve-shaping tonal equalization procedures are normally done before feedback equalization, as minimal deviation from optimal audience-area response characteristics can often be achieved.
The following procedure describes the use of the Fluke ScopeMeter in regenerative feedback tuning, which uses its frequency counting capability to measure accurately the primary feedback frequency with resolution to four numeric digits. This method is useful in tuning both fixed center frequency filter sets and parametric equalizers and can be beneficial in performing feedback equalization without high-resolution transfer function measurement equipment.
The setupFigure 1 shows an equalization setup diagram. This setup description is greatly simplified. The system microphone is placed according to the position of normal use and is connected to a mixing console input. Depending on the specific application, several microphones may be tuned simultaneously, further optimizing the system’s stability. The console input’s level control will be used to bring the system into feedback during the tuning.
An excitation source is connected to another mixer input to provide a signal source, which will accurately force the system into feedback. In most cases, this is a 20 Hz to 20 kHz pink noise source; however, rapid sine-wave sweep generators or controllable program sources can also be useful. A console output feeds the equalizer under tuning, which then feeds the system power amplifiers and loudspeakers. The entire electrical and acoustical structure should be configured to reflect normal operating conditions.
The mixing console also feeds the ScopeMeter’s input A, whether by an isolated console output (as shown in the diagram) or by directly connecting to the main console output. The instrument is then manually configured into the meter mode and is set to perform a frequency (Hz) measurement. To make alignment easier, dB, AC Vrms and other measurement types can also be selected.
The meter mode also contains a versatile waveform display to permit signal monitoring. See Figure 2 for a useful configuration. The setup data can be stored in the ScopeMeter’s memory for simplistic recalls in the future.
Tuning the systemTo tune the system, the pink noise excitation source is activated, and its level is brought up to present an average listening level in the audience area. The feedback loop is then induced by bringing the microphone input level up until self-sustaining feedback begins. The excitation signal is removed from the system, and the ScopeMeter displays the primary feedback frequency. The corrective equalizers are adjusted to present rejection with filter center frequency tuned to the feedback primary frequency and magnitude depth adjusted until the feedback loop no longer self-sustains. In the case of fixed-center frequency filters, combining characteristics are used by attenuating several bands at once. This procedure is repeated until the acoustic gain capability of the system is acceptable or until reaching maximum deviation from EFC response flatness.
Often, in large systems, this procedure is used to tune one master filter set for maximum performance with a variety of system microphones. Also, individual filters can be inserted to affect only one microphone or microphone group; they are then tuned to present maximum effectiveness. Many applications exist for this procedure, as it can be used to fine-tune extremely narrow- band transfer function anomalies with accuracy.
Other usesThe Fluke Series 11 Scope-Meter is also a useful tool in many other professional audio applications. Although this article primarily deals with a meter mode application, the scope mode makes possible high sample rate, dual-channel waveform acquisition, with full cursor measurement capability. Other applications include service of system components, including loudspeakers, mixing consoles, signal processors, power amplifiers and other acoustical transducers and electronic devices.
The use of the ScopeMeter in alignment of electrical power transfer between devices is simple with the dBm scale (with selectable input impedances). Detecting ground loops and parasitic oscillation is also possible with its waveform acquisition and display capability. The new ScopeMeter-B series adds integrated video signal triggering along with several other improvements over its predecessor. The use of the Fluke ScopeMeter is useful in many applications, providing accuracy and broad measurement capability while delivering all qualities in a compact, logical, user-friendly package, capable of replacing a variety of specialty instruments.