Feedback with delayed audio ?

[Externet]

Hi.
In what conditions can a plain public addressing system with internal delayed audio circuitry produce feedback howling ?

 

[KlausST]

Hi,

as always: feedback condition is:
* Gain >= 1
* Phase shift 180° (to an inverting input) or 0°/360° to a non inverting input.

Klaus

 

[BradtheRad]

I guess the aim is to prevent feedback howling by delaying the amplified waveform long enough so it's detached from the live waveform?

I'll describe my experience with a reel-to-reel tape recorder. It has 3 heads. The playback head is separate from the recording head, about one inch distance. This allows me to compare the taped audio right after it's recorded. In other words, an echo. The longest delay available is when running at 1+7/8 inches per second. The delay is half a second. The repeat can be dubbed onto my audio (a feature known as 'sound-on-sound').

So does this setup make feedback impossible? I find I can get feedback even with 1/2 second delay, when the mic is placed near the speaker-box. The echoes build over a few passes across the tape heads, if the volume is loud enough. The howling gets equally annoying as that from a live PA system.

 

[Externet]

Thanks, Brad. Yes, in your proper wording, can some delay detach the output signal from the input ?
Tape recorders not involved. Undesired reverberation, echo, feedback for plain voice amplification.

 

[FvM]

can some delay detach the output signal from the input ?

Basically not. You achieve feedback condition at a certain gain, with or without delay. Assuming a flat frequency response of the PA system, you get many possible frequencies that fulfill the feedback phase condition of n*360° phase shift.

A small variable delay can be nevertheless used to shift phase and suppress feedback due to specific room or speaker resonances.

The main purpose of (digital) delay lines in PA systems is equalization of sound travel time for distributed speaker systems.

 

[crutschow]

The main purpose of (digital) delay lines in PA systems is equalization of sound travel time for distributed speaker systems.

This is particularly required for a large outdoor venue like the Hollywood bowl, where there are speakers near the back of the seating area.

--- Updated Jan 27, 2024 ---

Hi.
In what conditions can a plain public addressing system with internal delayed audio circuitry produce feedback howling ?

If a continuous sound is longer than the delay, then feedback is possible.

 

[D.A.(Tony)Stewart]

The best way to prevent feedback is to use two in differential mode for near -field pickup only.

Cheap electrets have this built in with the rear open, but some cancel better than others.
Beam-shaping the wide angle cardioid types is another way with a cone point towards sound you want and away from those you don't want to re-amplify.

Although Shure and others had parametric notch and phase shift filters to attenuate a limited number of resonances