Speakers, Amps, Impedance & Power… Causing fear and confusion for some, Steve from AC-ET endeavours to make this popular topic a little clearer.
Speakers, Amps, Impedance & Power…
Low Impedance Speaker Systems
This article doesn’t refer to 100v systems (sometimes referred to as constant voltage systems) as the rules for those work differently. Here we discuss low impedance systems.
Impedance (measured in Ohms/ Ω) is resistance to electrical AC current. A power amplifier outputs an electrical current through a speaker cable and the speaker offers a resistance to that current.
The power rating of most amplifiers is often quoted at a load of 4 Ohms*. For example, “1000w per channel into 4 Ohms”. If a speaker rated at 4 Ohms is then connected to that amp, the amp will supply 1000 watts of power to that speaker.
What then happens when we plug an 8 Ohm speaker into that same amp? In short, that speaker “resists” more of the current and the amp isn’t able to supply as much power to the speaker. Depending on the design of the amp, the power may drop to around 750w into an 8 Ohm load.
Using a 16 Ohm speaker will drop the power further again but the amp will still operate fine with no issues just at a lower power.
Let’s use a silly analogy to try and describe what’s going on…
Imagine you are trying to run through a pool of treacle (yum!). You are the current and the treacle is the speaker or impedance. The thicker the treacle or the higher it’s impedance the more it resists your power to run through it.
Water would have less resistance (a lower impedance) and you’d be able to run through it much faster. You’d have more power.
Power amplifiers require a minimum impedance load to operate safely. Anything less than the minimum means the amp doesn’t have enough resistance at the end of the speaker cable to stop it pushing out more power than it was designed for.
If you have an amplifier rated at 1000w into a minimum 4 Ohm load and you were to put a 2 Ohm speaker on the amp, it will produce more than 1000 watts of power. This can over-heat the amp causing it damage. Most amps have “over-heating protection” and will cut out prior to any damage being done but you should always check the spec of an amplifier before using it.
It’s all fairly straightforward for a single speaker but what if you have multiple speakers on a single amplifier channel? How does that effect the total load? It all depends how you have the speakers wired…
If speakers are wired in series you simply add the impedance of all the speakers together. For example, 2 x 4 Ohm speakers in series would have a total impedance load of 8 Ohms. 4 x 8 Ohm speakers in series would total 32 Ohms.
Most often, speakers will be wired in parallel. An amplifier with multiple speaker sockets on each channel, running a cable from each socket to each individual speaker is wired in parallel.
If you go from the amp to the first speaker and then daisy chain speakers together via the sockets on the speaker themselves, they are also wired in parallel. These two methods are the most popular ways to link your speakers to your amp.
How to work out the total impedance?
It’s far easier if the speakers are the same spec as you can then halve the impedance of the speaker, for example…
2 x 8 Ohms speakers in parallel would have a total load of 4 Ohms
4 x 16 Ohms speakers in parallel would have a total load of 4 Ohms
It’s confusing when there’s multiple speakers at difference impedances, for example…
1 x 4 ohms speaker and 1 x 8 Ohm speaker. Use the equation below to work this out (imp = speaker impedance):
- Divide 1 by 4 = 0.25
- Divide 1 by 8 = 0.125
- Add those numbers together, 0.25 + 0.125 = 0.375
- Then divide 1 by 0.375 = 2.66 Ohms (recurring). This is the total impedance of 1 x 4 Ohms speaker and 1 x 8 Ohms speaker wired in parallel.
Use same equation and method for however many speakers you use, for example:
Imp1, Imp2, Imp3, Imp 4 etc.
Divide the number 1 by the impedance of each speaker
Sum the results and divide the number 1 by that summed figure. That’s your total impedance load on your amp.
To summarise, all low impedance amplifiers have a minimum impedance load they will safely run at. The more speakers we add to an amp channel in parallel, the lower the total impedance on the amp. We have to be aware of this when specifying any speaker system for an installation or live use.
Multiple speaker applications
If multiple speakers are required for good coverage in a large area, you’re better using 16 Ohm speakers rather than 4 Ohm as you’ll be able to safely run more speakers per amp channel. Otherwise you’ll have to specify a larger quantity of amps which obviously increases the cost to the client.
Another option is to use a power amplifier that can safely run at low loads of 2 Ohms. Always thoroughly research your 2 Ohm amplifier in case they’re prone to overheating. There are plenty of excellent brands including Nexo & Crown, to name just a few. The Nexo NXAMP or Crown CDi/ DCi series of amplifiers are often used in speaker system installations where running at 2 Ohms is required.
With the flick of a switch, many power amps can be “bridged”. This effectively joins two channels of the amplifier together, doubling the power.
Bridging doubles the minimum impedance the amp will safely work at. An amp that will reliably work at 2 Ohms in standard mode will require a minimum load of 4 Ohms in bridged mode. Again, always do your research, this is only a guide.
Correctly matching speakers with amplifiers
There are 3 figures mainly used to rate the power handling of a speaker. These may vary depending on the manufacturer.
- Continuous power
The continuous power rating tells us how much power (rated in watts) the speaker will handle ‘continuously’ over a long period of time. This will usually be tested with white noise for a period of at least 2 hours.
- RMS Power
RMS is an average of what power the speaker will handle and is more of an indication of what the speaker will handle in the real world. Live music, recorded music and speech is constantly changing in volume. It is not considered continuous.
- Peak Power
The Peak power rating is how much power the speaker will handle for a very short burst of time – usually only a few milliseconds.
With most speakers the RMS rating is twice the continuous power rating and the peak power rating is around 3 times the continuous power rating. Be sure to research your speaker to confirm that the advertised speaker rating is factually correct.
Matching your speaker and amp
In the hands of an experienced and disciplined sound engineer your amp can be 3 to 4 times the continuous power rating of your speaker. This enables the amp to cleanly provide the peak power handling of the speaker without clipping or distortion. This amount of power should only be used for headroom and not volume! With headroom in reserve, the system will be clean, punchy and very dynamic but any misuse of this power may blow the speakers.
If the amplifier is being used by someone with less experience, another option would be to use an amp somewhere between the continuous and RMS power rating. A limiter can then be used to catch and squash the peaks, producing a less dynamic sound at higher volumes but overall protecting the system.
As mentioned earlier, 100v or constant voltage systems work quite differently and will be discussed in a future article.
The above is advice only. You should always research your audio system before purchasing/ installing. For help specifying your system, contact the audio sales team.
*Written for AC-ET by Steve Eaton | Senior Audio Sales Executive.
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