How to Avoid/Reduce Feedback in Sound Systems

Principal cause of acoustic feedback

Most, if not all, of us have experienced hearing that piercing whistle from sound systems. What causes it and what can we do to minimize or eliminate it?

There are several types of feedback but in this case, the proper name is acoustic feedback, the type of feedback that you can hear. In simple terms, feedback happens when the sound from the speakers enters the microphone/s, goes through the mixers and amplifiers, out through the speakers and back into the microphone/s. In the case of guitars, the sound from the speakers make the guitar strings vibrate, which goes through the sound system, out the speakers, and vibrates the strings again. So, essentially, the sound is going 'round and 'round the sound system.

Feedback may be caused by microphones, guitar pickups, and other sound system inputs along with speakers. They can range in frequencies from a low rumble to an ear-splitting shriek.

So, how can you prevent your sound system from going into feedback?

Factors that can trigger feedback and solutions you can apply:

1 – System volume. The louder the sound, the more likely the sound will reach the microphone.

Solution: Reduce the volume and/or speak closer to the microphone so you can lower the volume. You can also use noise-cancelling microphones.

2 – Speaker to mic proximity. The closer the microphone is to the speaker, the more likely that feedback will occur.

Solution: Get the speakers as far away from the microphones as practically possible.

3 – Microphone is pointing at the speaker (the sound has a direct path toward the microphone).

Solution: Orient the speakers so they are pointing away from the microphones. It's easier to do by using unidirectional (a.k.a. cardiod) microphones.

speaker vs mic placement

4 – Room has a lot of reflective surfaces (even if the microphone is not pointing at the speaker, sound from the speaker can reach the microphone from reflections from the walls, floors, and ceilings).

Solution: Cover the walls, ceilings, and floors with sound absorbing materials. You can also install slats on the walls to break up reflections. Upholstered chairs will also help. If the chair is unoccupied, the upholstery will absorb sound. If the chair is occupied, the person sitting on it will absorb sound.

sound absorbers or sound diffusers on walls

5 – Room has parallel walls and parallel floor and ceiling. Sounds that bounce back and forth between parallel walls create "standing waves" that take longer to die down.

Solution: Same solution as #4. You can also build the room so that it doesn't have parallel walls, ceilings, and floors.

non-parallel walls (with sound proofing/diffusers)

Other solutions:

1 - Install "phase delay" equipment. These devices will shift the phase of the incoming sounds so that they will not generate "constructive interference" or sound that adds on to itself.

2 - Use a "cut-only" equalizer and a spectrum analyzer to reduce the frequencies where the feedback occurs. These are equalizers that don't have plus adjustments, only minus adjustments (the 0 dB is at the top of the sliders) and have bandwidths of 1/3-octave. When you increase the system volume and feedback starts to occur, you check the analyzer to find the frequency of the feedback and reduce the level of that frequency. Keep increasing the level and cutting the frequencies of the feedback until you attain your target system volume.

cut-only equalizer (A.K.A. room equalizer)

You can also use equalizers with cut and boost controls. The difference is, cut-only equalizers usually have a larger cut range than cut and boost types.

Urei brand room equalizer

Note: these days you can download a spectrum analyzer app to your phone. It should be accurate enough for general sound system adjustment.

Another Fake Proposal

email supposedly from Kindle Direct Publishing

I received another email with a proposal to convert my book into a movie. This time, it's from Kindle Direct Publishing. It's obviously fake because the sending address is amazon_submissions_at_gmail_com. Amazon using a gmail address?

Furthermore, my book is hardly a bestseller. If it was a bestseller, I'd understand if someone would contact me with a proposal such as this but it's nowhere near being even moderate seller.

Lastly, if some big movie production outfit really wanted to convert my book to a movie, I don't think they will ask me to submit a screenplay. Of course, they would offer to make the screenplay for me, which means I will have to pay for it. I would think a real proposal would be free.

If you're also a budding author and received something like this, don't believe it right away. Look at the sending address. If it's a gmail, yahoo, or other free, public email service, don't believe it.

Design Flub Fixed

 

Pinout diagram of Zilog Z80 microprocessor

Back around 1989, I received a request to fix an infusion pump, a device that allows medical staff to pump very small, and precise, amounts of fluids and medicines into patients.

I was told it would turn on but wouldn't start pumping. The display showed rubbish and pushing buttons didn't produce any response.

I opened the thing and quickly discovered a Z80 microprocessor, a computer chip that I had been studying for some time with the intention of building my own computer. I was very familiar with it.

From the description of the problem, it seemed as if the unit was not starting up properly. Opening the unit and checking pin 26 (the RESET pin), I found...nothing, the pin was floating (unconnected to anything).

The chip needs the RESET pin to be held at 0 volts for about a split-second after power-on to start the program at the beginning. If the chip is not reset, the program may start at a random line and the unit will likely hang or run erroneously.

A resistor, diode, and capacitor usually sufficed to do the job of resetting the chip but, in this case, there was nothing. I told the owner that I needed to buy the parts and would return the next day.

At home, I checked the Z80 user manual. While it explained what the reset pin was for and how to use it, the manual didn't say it could be left unconnected. Unfortunately, it also didn't say that a reset circuit was required. Anyway, I already had a course of action and I'd know after I put in the parts whether the unit would function or not.

The next day, I brought the parts, a soldering iron, and some soldering lead to the clients 's office and soldered in the parts. These was no provision on the board for them but there was enough space for me to put them in without difficulty. Once finished, I crossed my fingers and turned on the power.

Yes! The infusion pump's display gave an initial display and waited for an input. Someone set it up, inputted some parameters and pressed start. It worked! A few more tests and we declared the unit fit for use.

While basking in the thrill of a successful repair, I was concerned about the seeming design flaw in a commercial medical product. I told the client that they should inform the manufacturer about what we had found so they can recall the units already sold and have them updated. I went home fully expecting to soon receive a call from some engineer wanting more details. Unfortunately, there was none. They had either understood what the problem was and implemented the fix or they had discovered the issue themselves and were already putting out a recall order.

Anyway, this was just one of those repair jobs that sticks in my memory due to its unusual nature.