Beginner’s Guide To PA Systems, Part 3: Amps & Speakers

Last updated 27 March, 2018

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A PA power amplifer
Very different to the type of amplifier you may have in your home stereo, PA power amplifiers are designed to run at high levels for long periods of time.

In the first part of this four-part series we looked at the basic elements you need to know about when it comes to DJ PA systems. In the second part, we carried on laying the groundwork by answering some common newbie questions. Now, we’re ready to look more closely at the two major components of any PA system: Amplifiers (“amps”) and speakers. Today we’ll look in-depth at both, and discuss the crucial matter of speaker placement, before next time moving on to cables, setting up and sound checking.

Your essential guide to amplifiers

Simply put, an amplifier (in our case we speak of a “power amplifier”) is needed to get the line output signal of the mixer or DJ controller to a level where it can drive your speakers to the volume you need. It is vital that the amplifier can supply sufficient power for your speakers (say 500W), but your speakers also need to be able to handle this amount of power. So if you have the amplifier with 500W but your speakers are only made for 250W, they will die on you rather soon. So I always recommend to shop for amplifier and speakers together. Alternatively, you could go for active speakers. These have the power amplifier built in and the manufacturer has already done the job of matching the amplifier(s) to the speaker(s), so you no longer need to worry about it.

Now, a PA amplifier is very different from a hi-fi amplifier. Power amplifiers are made so they can handle their maximum power output for an long period of time. They often have serious heat sinks and fans built in, that might worry you as these could sound loud in the shop. But think that you will play in front of 100 people at a decent volume and trust me: You won’t hear the fans!

The first thing you should look at are the basic specifications of the amplifier, and especially at which temperatures it can operate. In venues in hotter parts of the world it can get quite hot even in climate controlled rooms and you do not want your amp to die on you in such circumstances. So be sure to consider whether the amp you’re looking at can operate in such conditions. Next, I would always advise to get amps with protective circuits. These should cover thermal protection (so the amp will shut down rather than die if it gets too hot), and short circuit protection (in a hectic set-up, a short circuit can always happen, and again you do not want your amp to simply die from it).

Take good care that you buy a decent amplifier. The worst thing that can happen is that you buy a cheap amplifier that can’t handle the heat and its thermal protection keeps cutting out every five minutes. This will drive you crazy, trust me – I know this from experience!

Amplifier testing procedure

Give a prospective amplifier a listen at low volume. Now, if you hear some noise from the fans, that is alright, but you should not hear too much static from the speakers. The sound should be clear and flat. Next, play a tune that you know very well and that has a broad spectrum and then turn up the volume to “as loud as they will allow you to in the shop”. Even at the high volume the sound should be as close to the same as at low volumes, just obviously much louder. There should not be loss in frequency response.

Some cheap PA amplifiers tend to “forget” to amplify the mids when they go louder, and in the end your PA might sound like a tin can with a bass drum. That should not happen. As a test sound I mostly use the WAV file of the THX test sound you hear in cinemas. You can download that all over the internet. Also some rather “chill” music like deep house or Balearic can be revealing.

The lowdown on speakers

Speakers are of course the final link in the chain. Speakers are crucial as they are the thing that actually converts the electrical signal from your amplifier into moving air, aka sound waves. Now why am I stressing moving air? Because it’s crucial to full understand that sound is simply waves in the air that are made by moving the air. Think about this closely, and you start to realise why speakers are the way they are.

Cerwin Vega P
PA speakers. As the final link in the chain, speakers play a massive part in any successful PA set-up.

First thing to understand is that the higher the frequency – up to 20,000Hz (20kHz) is the hearing range – the shorter its wavelength, which means a smaller driver will be sufficient to move the air loud enough. A small driver is also needed as a big driver would be simply too slow. Now if we go to lower signals (down to about 20 to 50Hz is as low as you can hear or feel), we find that the driver needs to be bigger in order to move sufficient air at the longer wavelengths. Simply put, this is why a bass speaker or a subwoofer needs to be considerably larger than higher frequency speakers.

However, the science doesn’t stop there. In addition, the cabinet (aka the case) plays a serious role in this, as the driver’s membrane is not only moving air when it moves out, but also inside the cabinet. This again is especially important with bass speakers and subwoofers. They can include porting in the cabinet (bass reflex) to move air out of the cabinet, and the inside of the speakers can be designed like wind tunnels to move the right amount of air.

One interesting point to note is that for sounds below about 100Hz, you cannot determine where the sound is coming from. That’s why there’s no need for stereo with subwoofers – and why typical computer gaming speakers (and some small PAs) have two mid/top speakers but only one “subwoofer”.

Practical tips for buying speakers

Now that was a lot of information. So practically, what does it mean for you when you’re making your buying choices? Really, it’s just important to remember that there are no miracles in the world of speakers. Specifically:

1. Size matters

Forget all the marketing talk. Truth is, if you want to move enough air, you need big enough speakers, or more speakers. There is a reason why an open-air Tiesto stadium show will have speaker towers the size of small houses – Tiesto wants to reach 50,000 people, and he knows you need lots of big speakers to do that.

Some speaker systems nowadays are sold on the premise that by using small and light fancy speakers that deliver only 500W, everything will be great. But you know the science now, and therefore you’re smart enough to instantly see that this is a trade-off. Truth is, you will need a subwoofer for such systems. Those that come with subwoofers tend to be engineered so that more of the frequencies are handled by the subwoofer (in order to allow the use of those small, light satellite speakers), which is not always a good thing, especially if your venue is full of people; as we’ll learn later, the ideal place for the bulk of the frequencies to be delivered is above head height.

Bottom line: If you have larger cabinets (aka more volume) and bigger driver membranes, they will amplify the air better, period. Now while we can argue that there is a limit on size needed for higher frequencies, for lower ones a subwoofer with a bigger membrane and a bigger cabinet with more intelligently placed portings will always sound better.

2. The weight of the speaker and the build quality of the cabinet are important

A good speaker will be heavy. The cabinet will be made out of wood (or in higher-end models out of “compressed wood” or “iron wood”, which is basically wood grain mixed with special epoxy and possibly carbon fibre to reinforce the cabinet). Again if we think back to moving air, you can see why this is good.

If the speaker is heavy enough, it won’t jump around on the floor or cause other vibrations that may interference with the purity of the sound. If the cabinet is not constructed well, it will vibrate on its own and in worst cases could cancel out whole frequency bands or even cause serious feedback loops. This is why it’s important when buying to take care that the cabinet is constructed well.

3. The quality and durability of the drivers is crucial

The driver membranes should be made out of decent material. Higher-end membranes are generally made from Kevlar, carbon fibre or other similar material. PA speaker drivers need to resist considerably more stress than your hi-fi at home, and move a lot more air, so their construction is crucial. If the drivers on speakers you’re considering feel like they’re made from paper (which would be acceptable for many home systems), think twice.

Other things to consider…

If you’re looking at active speakers, check that that they have enough inputs for your set-up (and any predicted future needs). If they are not active, consider buying speakers with Speakon sockets, as well as XLR and TRS. And do make sure they have mounting holes for speaker stands!

Finally, a word on “ohms”. Speakers come usually in either 4, 8 or 16 ohm variants. The only thing you need to take care of is that this number matches your amplifier. Most amplifiers have a switch to set the output to the right ohm for your speakers and you should look for that. Also the wattage on the amplifier always state at which ohm it delivers that power. So if you have a power amplifier that delivers 500W at 8 ohm, but you speakers can take 500W and are 4 ohm, you will probably damage your speakers.

Again, if you buy active speakers (with amplifiers built in), that matching has already been done for you and you needn’t worry about this.

The art and science of speaker placement

In venues in the past, I have seen sound engineers using diagrams and doing number-crunching beyond what I thought possible, and the whole thing still ended up sound bad. Now while there’s undoubtedly a place for science and measurement in installations, for the likes of us with our portable PA systems, some simple rules of thumb are probably going to be more useful than dry theory about wave dispersion theory and such things.

Experience and working with various sound guys (particularly, a house engineer at a large concert venue who imparted the following knowledge to me 20 years ago) means I can share with you you how I go about speaker placement. And while I have no formal qualifications in this area, I do know what sounds good. So here goes:

PA speaker placement
The simplest layout for a four-speaker system has your speakers (and associated subwoofers) wide apart pointing in to the room, with the mid/top speaker on a pole slightly above crowd head height, and the subwoofers on the floor.

Firstly, place your speakers as far left and right from your audience as you can go, pointing in to the room slightly. (For example, the corners of the stage or near the side walls of the venue.) Secondly, always put the speakers on speaker mounts where the lower edge of the speakers should be a little above the audience’s head height. And thirdly, always leave at least 30cm up to 1m “breathing room” from walls and ceiling. And that’s it!

Again, this makes total sense if you think again about moving air. If you do not mount the speakers above the audience, you will blow all the moving air into the first row of people, who will be deaf soon, but the sound will not carry to the back of the venue. If you put the speakers too close together you will have to take care that they do not angle against each other and do not cancel themselves out in some places. And if you leave them no breathing room (aka room to “suck” air in), the speakers will not be able to move the air efficiently enough and you might get excessive coloration of the sound from the wall due to wavelength interference.

Subwoofer placement

Now some speaker sets (especially active sets) come with a pair of subwoofers matched to the speaker pair. These are simple to place; you simply leave them on the floor below the main speakers. In most cases you can put the main speakers on top of the subwoofers and use a simple connector pole (or extension pole) to get them high enough in the air. Indeed, most PA subwoofers are made that they can safely be placed on the floor and even use the floor as a resonating membrane.

With a single subwoofer, I tend to place it in the middle between the main speakers. This is a compromise as you might have issues with “sound arrival time” for some people who are sensitive to such things. So ideally, consider getting subwoofers paired with speakers unless it’s for a very small PA system (below 500-800W), where in most cases it won’t matter.

You may now be scratching your head, thinking that I previously said it is important to move the air over people’s heads! While this is true for speakers, for subwoofers that only transmit frequencies in the very low range (below 100Hz) the placement on the floor is OK as these long waves will “bend” and reflect around the people better and still distribute themselves throughout the venue. By sitting on the floor and using the floor to help with resonance, they can even amplify the bass experience.

Next time…

Next time, we’ll move on to showing you all you need to know about cables, before running through how to conduct a PA sound check.

Check out the other parts in this series:

Do you have any questions or queries regarding buying PA system parts, or matching amplifiers and speakers? Got an issue with speaker placement you feel we can help with? feel free to ask in the comments below.

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