One of the biggest problems young bands have when starting out is feedback. During rehearsals or whilst playing their first few gigs in pubs or small venues the backline amps are ear splittingly loud but no one can hear the vocals. The engineer is desperately trying to get more but he can’t because the pa is on the verge of feeding back.

This article attempts to explain in simple terms what feedback is, what causes it and how to get rid of it for good.

Feedback is normally heard during a live performance of music or spoken word. It is usually a high pitched squeak which rings out at tremendous volume causing members of the audience to clutch their ears in pain and flee from the venue if it doesn’t stop. 

Feedback is caused by the sound from the PA system over-spilling or feeding back through into microphones, instrument pick-ups or record deck needles and then being amplified again and again through the PA system, looping round and round becoming increasingly loud. At worst it can break loudspeakers and can ruin a live performance if it happens too often.

There are a number of ways to avoid it or eliminate it completely.

Speaker Placement:

If you point a mic at a speaker it’s likely to cause feedback. Make sure that the front of house speakers are at least a few feet in front of any on stage mics. Don’t ever walk in front of the house speakers whilst holding the mic. Face the speakers directly away from the band towards the audience. Obviously monitor speakers have to be pointing back at the band but we will cover this later. It’s a bad idea to place speaker stacks on the stage. You can get low frequency feedback from sound travelling through the floor of the stage from speakers and looping back through drum mics etc. Put the stacks on the floor in front of the stage so that they are not touching the stage at all.

Microphone Choice:

There are generally two types of microphone: dynamic and condenser.

Condenser mics pick up any ambient sound in a room. They don’t need to be near a sound source to pick it up. This makes condensers notoriously bad when it comes to feedback – if they are set with even a modest amount of gain on the desk they will feedback. In live sound applications these types of mics are most commonly used as overhead mics for the drum kit. They need very little gain and are usually just there to reinforce the sound of crashes and ride cymbals etc.

Dynamic mics have a limited area around them that is sensitive to sound. This area is often referred to as the cardioid pattern because of its heart like shape. These mics are ideally suited for vocalists where the singer will put the mic on or next to their lips. Dynamic mics do not pick up other on stage sound so the performance happening through the mic can be isolated in the mixing desk enabling a brighter, cleaner mix. The other advantage of dynamic mics is that they have high gain before feedback characteristics. In other words you can boost the gain on the vocals without causing feedback. If you go too far then feedback will happen but not as much as with a condenser mic.

Mixer Settings

When sound checking mics as a general rule don’t have the gain on the channel at any more than 11 o’clock. If you are not getting enough vocal in the mix and the output volume (Channel Fader) is set at full then either your vocalist is not projecting very well; the mic is too far from their mouth or the pa system is just not keeping up with the backline amps. Try turning the amps down or get a bigger pa.

The Room.

Some rooms are particularly bad for feedback. One club I know has a floor to ceiling mirror at the back of the stage! Sound travels in waves like light so sound coming out of the monitors bounces off the mirror and straight back into the vocal mics causing major feedback problems.  Even if it’s a shiny wall at the back of the stage this can make feedback worse. The solution is to paint it black or drape material to dampen the acoustic reflections.         

Monitors

Monitors go against all the common sense ideas about reducing feedback. They are pointing straight back at the band and generally get pushed harder and harder throughout the gig at the request of the lead singer who insist they can’t hear themselves. If you have followed the other rules and you still have feedback then 9 times out if 10 it’s the monitors.

EQ

Luckily there is an effective solution but it does involve buying (ideally 31 band) graphic EQs for every channel of monitor you will be using and for the left and right of the FOH (Front Of House) mix. A standard gig might have 4 monitor mixes and FOH so you would need 6 channels of EQ. They normally come in pairs. Some people think graphic EQs are simply for tweaking the overall sound of the PA system to suit a room, which is one use, but in fact their main application is in eliminating feedback.

When you get a feedback squeak it’s always at a particular frequency or at a number of different frequencies. This is generally governed by the acoustic properties of the room so it is different depending on which venue you are in. A graphic EQ splits the signal into 31 frequency bands from 20 Hz up to 20 kHz. If you know which frequency the feedback is occurring at then by notching that frequency down on the EQ you can stop it in its tracks. Experienced engineers can hear what frequency feedback is occurring at from having done so many gigs. Beginners can buy graphic EQs with a feedback detection system. This shows you what frequency feedback is occurring at with a little LED signal above the frequency range in question on the EQ.

Ringing out

Obviously it’s best if you can sort out any potential feedback issues before the audience arrive at the gig…this is how it’s done:

Set up the stage with all the mics you will be using and do a quick sound check and get rough levels for monitors.  Now ask the rest of the band to go outside and get some fresh air for 5 minutes while you ‘ring out’ the room.

Start with monitor mix one. Turn down the master faders for the other auxiliary sends.  If you already have feedback in mix one then eliminate the troublesome frequency or frequencies on the corresponding EQ channel. Note: don’t take out too much of any particular frequency or the monitor will start to sound rubbish – just notch it down enough so that the feedback stops. Now gradually turn up the master fader for Auxiliary 1. If you get more feedback on other frequencies then notch these out. You may need to adjust for the initial problem frequency more if it has reared its head again. Turn the auxilary master up a touch more and repeat the process. You don’t need to turn the auxiliary master right up because you will never be doing that during the gig. You just want to make sure you have room to manoeuvre in case the lead vocalist is wanting more and more in his monitor.

Now turn the auxiliary master back down to where you had it during sound check. Repeat this process for all other monitor mixes and for front of house.  

Now you have rung out the room. There should be much less chance of any feedback during the gig and you will be able to get far more out of the monitors and FOH.

If you drop a stone into a pond it creates waves of water which move out from the impact. When you clap your hands you create waves of air which move out. When these waves hit your ear drum the energy is transferred to the cochlea in your inner ear where it is converted into nerve impulses which you perceive as a sound, “clap�

The pitch of a sound, (how low or high it is) is governed by the frequency of its waves. Low pitch sounds have long drawn out waves whereas high pitch sounds have short bunched up waves.  Frequency is given in Hz which is a measure of cycles or oscillations per second. 1 Hz is exactly one complete wave from peak - trough - peak per second.

The lowest frequency sound Humans can hear is around 20Hz. This means we can just about perceive waves of air vibrating past our ears at anything above 20 oscillations per second. These are the kind of deep rumbling sounds made by earthquakes and distant huge explosions or a space rocket launch for example.

If you attach a loudspeaker to a signal generator and get it to produce a note at around 20Hz you can literally watch the cone driver moving back and forth creating the sound wave. As you increase the pitch it becomes more and more of a blur until you can’t really see the movement (although you can certainly hear the note being produced.)

The cone driver has a metallic coil like a bracelet attached to its back – this fits snugly into a grove within a chunky circular magnet. When a current is applied to the coil it can move back and forth within the magnet. This is essentially all that is needed to create the oscillations that move the air to create sound.

For a sound to be audible it needs to push enough air against your ear drum.  This is known as sound pressure. Because long waves are relatively infrequent they need to be big waves if you are going to hear them. The size or depth of the wave is known as its amplitude; literally the distance from peak to trough. It takes a huge amount of amplifier power and a big cone driver to produce the colossal tsunami sized waves needed to register deep bass notes on your ear drum. This is why you can usually feel the bass more than you can hear it.

Bass loudspeaker drivers are 21, 18 or 15 inches in diameter. They need lots of amplification power in order to shove enough air to produce the long waves of bass that you can hear and feel.

Most bass drivers cover the frequency range from around 35Hz up to around 500 Hz. Producing frequencies of more than 500 Hz is impossible for drivers this size because they are simply too bulky to oscillate much faster than 500 times a second.

Producing very high frequencies is done with compression drivers.  A compression driver is a very small driver with a diameter of 1, 1.4 or 2 inches. These can oscillate at incredible rates, up to about 20,000 times a second. Compression drivers can cater for all the higher frequencies, usually 16 kHz down to about 2 kHz.

A quality loudspeaker should accurately reproduce the signal coming into it by ensuring a comprehensive coverage of all audible frequencies of sound. Average Humans can hear sound waves with frequencies between 35 and 18 kHz.  

Budget speakers comprise a 15� or 12� bass driver with a 1� or 2� compression driver. This is ok but the system completely lacks anything in the mid-range. There is a gap in most budget speakers between about 800 Hz and 2 kHz.

This is ok for most modern dance music because it has very little going on in the mid-range. It’s all bass lines and kick drums with snares and hi-hat. When it comes to classical or vocal rich music and in live sound applications you really miss the coverage of the mid-range frequencies.

In addition to the bass driver and compression driver a professional quality loudspeaker will be fitted with a mid-range driver. These can range in diameter from 4� – 10� but are usually around 6.5�. This fills the gap between 800Hz and 2 kHz and makes a huge difference to the detail and clarity of sound reproduction.      

This article aims to provide the complete beginner with a basic understanding of loudspeakers & amplification equipment. In addition to power ratings and how ohms (impedance) affect this, the reader will attain a basic grasp of amp bridging and how and why to allow “head room�. The main objective is to provide the layman with knowledge of how to match up speakers with amps in any pa / sound equipment set up.

A loudspeaker is a fairly simple piece of equipment. It’s a wooden box with a piece of hardware known as a “driver” screwed into it. The driver is made up of two ring magnets one within another, attached to a cone (the familiar round black thing with a bump in the middle). When power is supplied to the outer ring the inner magnet moves back and forth which in turn moves the cone creating sound waves that humans can hear. The faster the cone moves back and forth the higher pitched the sound that is heard.

All loudspeaker drivers have power ratings which are given in watts. This can be confusing because the wattage can be quoted in three different ways:

RMS continuous power, program power and peak power.

Peak power is the maximum a loudspeaker can handle for a very short time (feedback spikes etc.)

Program power is a relatively safe range but you wouldn’t want to run into this for too long.

RMS continuous or AES is the actual power the speaker driver can supposedly handle continuously without overheating.

Program Power is double the RMS and Peak is double the program power.

Whenever you are sizing up a pa speaker always look for the RMS or AES continuous power rating.

A high power pa speaker might be rated like this:

450 Watts RMS, 900 Watts Program, 1800 Watts Peak.

Anyone who knows their stuff and isn’t trying to con you will always quote the RMS or AES rating.

Most pa speakers are rated at either 4 or 8 ohms.

Amplifiers will usually handle loads of 4 ohms or more. Some good quality amps can handle loads of 2 ohms.

Amp manufacturers provide power ratings for their products at 4, 8 and sometimes 2 ohms. The amount of power an amp will supply depends on the speakers “ohms” that you have hooked up to it.

A 4 ohm speaker would get (very approximately) twice the power that an 8 ohm speaker would get from the same amp.

Loudspeakers have an input on the back and an output so that you can link speakers together (sometimes called daisy chaining). If you “daisy chain� two speakers together and connect them up to an amp you effectively halve the load (ohms). From the amps point of view, two 8 ohm speakers linked together is the same load as one 4 ohm speaker.

Don’t worry if you’re already confused.

Let’s look at an example:

The Peavey PV 2600 amp is fairly common. It has two channels A & B. Each channel is rated at 900 watts @ 4 ohms and 550 watts @ 8 ohms.

In this example we are just using channel A.

If we hook up a 4 ohm speaker to channel A on the PV 2600 it would get 900 watts. If we plug in an 8 ohm speaker it would get 550 watts.

Now, if we “daisy chain” two 8ohm speakers together then the amps would share the 900 watts between the two speakers so they would kick out 450 watts each.

Although the power output is the same - this arrangement would actually sound “louder” than one 900 watt 4ohm speaker.

Only a top quality professional 18� Sub driver rated at 4 ohms could comfortably handle 900 watts RMS.

If we were to daisy chain four of the 8 ohm speakers onto channel A of the Peavey PV 2600 then this would be equivalent to a load of 2 ohms. The amp cannot handle this and would very quickly overheat and break down. 

Some stage monitors like the “galaxy hot spot” are rated at 16 ohms. This means that you could “daisy chainâ€� four of them together and connect them to channel A on the PV 2600 because this would be equivalent to a 4 ohm load on the amp. 900 watts divided amongst four speakers equals 225 watts per speaker - this is just about right for galaxy hot spots which are rated at 200 watts RMS.

Touring companies or large scale pa hire companies will choose very high power, good quality amps that can handle loads down to 2 ohms.

As another example the Peavey CS4000 amp can handle a load of 2 ohms. This particular model supplies 2000 watts @ 2 ohms 1350 watts @ 4 ohms and 800 watts @ 8 ohms. This means that if you “daisy chain� four 8 ohm speakers onto a CS4000 you get 2000 watts shared amongst four speakers, 500 watts each.

It’s not usually a good idea to run amps at 2ohms as they get very hot. It’s a bit like buying a new car and thrashing it around town. It won’t last very long. It’s the same with amps - running them cool at 8 ohms is the best way to protect them and make sure they last.

Sub woofers always need the most power and you can buy drivers like the Eminence Kilomax series or the PD 2150 that will happily handle 1000 watts RMS all day long.

To drive these you either need a bad ass amplifier which will cost you big bucks or you can do what is known as bridging. This is where you take an amp like the Peavey PV1500 which is rated at 500 watts @ 4 ohms and 300 watts @ 8 ohms and combine channels A + B. This is done by pushing a switch on the back of the unit. You then, effectively, have four times as much power output. (It’s actually double but you have also halved the impedance (ohms).)

So you only have one channel (A), but it is chucking out 1000 watts into an 8 ohm speaker (amp thinks it’s 4 ohms) and 1500 watts into 4 ohm speaker (amp think it’s 2 ohms). You could plug in one 1000 watt 8 ohm speaker and drive it perfectly or you could “daisy chainâ€� two 750 watt 8 ohm speakers and drive both of them safely. 

Don’t ever hook up a 4 ohm speaker onto a bridged amp. I have never seen a 4 ohm speaker that handles 1500 watts! Don’t ever “daisy chainâ€� two 4 ohm speakers onto a bridged amp as this would be equivalent a load of 1 ohm and the amp would probably melt through the floor!

The 15″ / 12″ or 10″ main driver in your speaker can vibrate back and forth producing most sounds from very low frequencies 35 Hz up to about 2000 Hz or 2 kHz. Hz is simply a measure of how fast the cone is moving back and forth. Humans can hear from about 40 Hz up to about 18 kHz (18,000 Hz).

Full range speakers always have a compression driver or “tweeterâ€� to fill in the gap from 2 kHz - 18 kHz. These are like mini versions of the main driver usually 1″ in diameter. These can produce all the higher pitched sounds you need so that when combined with the bass driver the speaker cabinet is truly “full range”.

Full range speakers provide all the detail in sound reproduction so it’s best that they are kept “comfortable”. They need to be running cool but with a bit more power than they need. This is called “head roomâ€�. It’s like cruising in a Ferrari at 70 mph, the engine is “purringâ€� with plenty of power left.  You would use an amp that is approximately 20 % more powerful than the RMS rating on the speaker but it’s very important NOT to crank the mixer to the max. 

Don’t go “daisy chainingâ€� all your full range cabs so that the amp runs at 2 ohms. For the best sound quality you need to run the sound system cool. So you want the amps at 8 ohms (usually one speaker on each channel), with 20 % more amp power than the RMS rating on the loudspeakers.

The subs provide all the “thud” in a big sound system and need lots of power to move the big 18″ or 15″ drivers. It’s ok to run these at 2 ohms in fact you may have to get enough power out of your amps.

Hopefully you now have a basic appreciation of how sound amplification works and you understand which speakers go with which amps. You also know how to safely bridge your amplifier and you understand about allowing head room in a full range system.

Ben Igoe (16/05/07)