Feedback at 2.5 kHz Three Hours Before Doors
A 600-seat college auditorium hosts a 90-minute cultural night. The PA is a 12-channel analog mixer feeding two powered tops and two 12-inch wedges. The stage is 9 m wide, and the vocalist walks to the downstage edge, about 1.5 m in front of the centre wedge. In rehearsal, with the wireless handheld's channel gain set so peaks hit +6 on the channel meter and the master at 0 dB, a sustained 2.5 kHz ring builds whenever the singer faces the centre wedge. The technician's fix so far is to pull the master fader to −12 dB, which makes the vocal disappear in the back rows. The other inputs are two lavaliers for comperes, a keyboard DI and a laptop. Doors open in three hours, and there is no graphic EQ on the wedges.
1. Explain why pulling the master to −12 dB does not improve gain before feedback. Give an ordered gain-structure and ring-out procedure for the next three hours.
Read the worked answer
The master fader is inside the feedback loop. Lowering it by 12 dB lowers the singer at the microphone by 12 dB and lowers the level at which the loop becomes unstable by 12 dB, so the ratio of wanted vocal to feedback threshold is essentially unchanged. You have bought no extra gain before feedback; you have only made the show 12 dB quieter. The same applies if you lower the master and raise the channel gain to compensate.
Procedure:
- Set the wireless receiver and channel gain so the singer's peaks sit around −18 to −12 dBFS (about 0 VU), not +6.
- High-pass the vocal at 100 Hz and set the compressor gently (about 3:1, 3–6 dB gain reduction on peaks).
- With wedges off, set the vocal at a normal FOH level and the master at its nominal 0 dB mark.
- Bring up one wedge send slowly until the 2.5 kHz ring just starts; back off 3 dB. If a parametric EQ is available on that send, cut 3–6 dB at 2.5 kHz with Q ≈ 4–6. Repeat for other ring frequencies, then leave 6 dB of headroom below the ring threshold.
- If the only EQ is the channel EQ, remember it will also dull the FOH vocal; prefer physical repositioning unless the monitor send is post-EQ and you accept that trade-off.
- Mark the settings and do a final walk-test with the singer at the downstage edge.
2. Explain the 2.5 kHz ring using wavelength and polar pattern, and propose two physical changes that do not rely on EQ.
Read the worked answer
At 2.5 kHz the wavelength is 343/2500 ≈ 0.137 m, about 14 cm. That is close to the distances between the mic capsule, the singer's mouth and the wedge grille, so the path is short and efficient. 2–5 kHz is also the presence region of most vocal mics and the intelligibility band of the voice, so it is usually the first band to ring. A cardioid has its deepest null at 180°, but a wedge at the singer's feet, or off to the side, may sit 45–120° off-axis where rejection is much less than the brochure figure suggests.
Physical change 1: move the centre wedge so it is directly behind the mic's rear null and angled up at the mouth, not firing across the mic grille. Increase the mic-to-wedge distance where possible.
Physical change 2: switch to a supercardioid handheld if available and place the wedge in its 126° null, keeping in mind that a supercardioid has a rear lobe, so do not put the wedge directly behind it. Alternatively, lower the wedge level and add a second wedge further downstage so the singer does not need the centre wedge loud. Both changes alter the acoustic loop rather than just trimming a frequency.
3. Build the input list and monitor plan for the five sources, and state the two instructions you give the comperes.
Read the worked answer
Input list and processing:
- Ch 1: wireless handheld vocal. HPF 100 Hz, gentle compression, vocal in both wedges.
- Ch 2–3: comperes' lavaliers. HPF 120 Hz, low gain, muted when not speaking, light compression; compere mix in the downstage wedges only.
- Ch 4: keyboard DI. Pad engaged if needed, HPF 60 Hz, FOH only, no wedge send.
- Ch 5: laptop DI. −10 dB pad, watch for ground-loop hum, FOH only.
Keep the vocal and comperes in the wedges; keep keyboard and laptop out of the wedges to reduce stage level and feedback risk. Put no reverb in the monitors. Use groups if the mixer has them so compere mics can be muted together. Ring out the handheld and each lavalier separately, because their polar patterns and positions differ.
Two instructions for the comperes: (1) mute your lavalier when you are not speaking and keep it about 15–20 cm below the chin, not buried in clothing. (2) Never cup the grille of a handheld or point it at a wedge; if you hear a ring, step back from the wedge and let the engineer mute the channel.