Understanding Watts Ohms and Hertz
A quick guide to watts, ohms, and hertz for home audio systems...
Buying a stereo system can be a little confusing for those who have no idea what watts, ohms, and Hz are, and at the end of the day getting a sound out of a new stereo isn’t a difficult task, so why should you worry?
Well as with all things its better to have some kind of knowledge about what you’re buying in order to get the best deal suited to your needs. That doesn’t mean you need a degree in sound engineering to make a smart audiophile purchase, simply knowing a little bit about audio ratings will ensure you don’t get fooled by the confusing barrage of specifications that come with every product.
Understanding Audio Specifications
Understanding Audio Specifications
Watts are a measurement of power e.g. how loud an amp can drive your speakers, however is these ratings are generally rubbish. The problem is that every amp is rated different, for example a cheap amp rated 100 watts may produce 100 watts at a certain frequency but only manage to put out a small percentage of that when running multiple speakers playing actual music.
Another confusing point is that the difference between a 100 watt and a 105 watt amp is negligible, in fact if it’s a unit from the same manufacturer it’s probably the same amp inside, just rated differently. Basically, for a listener to appreciate the difference in volume, the wattage needs to almost double.
Of course, another important point to remember here is that watts don’t directly relate to the quality of the sound. Two amps from different manufacturers that put out similar watts will sound the same in volume but they’re sure to actually sound different.
Ohms measures the resistance the electrical signal faces as it travels from the amp to the speakers, the higher the ohm rating, the harder the speaker is to drive.
When a manufacturer gives a watt rating, its always related to a specific ohm resistance, e.g. 100 watts @ 8 ohms = the amp will produce 100 watts when the resistance is at 8 ohms. If the resistance is reduced to 4 ohms, the amp will put out 200 watts.
This calculation differs depending on how your speakers are wired – in series or parallel – fortunately most home audio systems don’t require such elaborate setup, we won’t go too in depth here but the basic rule that speakers wired in series increase resistance (ohms) and decrease total output (watts), and speakers wired in a parallel set up reduce resistance (ohms) and increase total output (watts).
The most important thing is that amp and speaker watt and ohms ratings need to match up. Since home audio system are usually pre bundled you shouldn’t have to worry about this, but it does illustrate why plugging two speakers into one output on your amp is not a good idea.
Speakers will always come with max watt rating, however this should be treated as a minimum watt rating since your far more likely to damage a speaker when you give it too few watts and try to crank up the volume then you are if you plug them into a powerful amp.
A speaker with a max rating of 50 watt won’t blow up on an amp designed to put out a 1000 watt, but it certainly won’t do your huge sound justice. As mentioned before, the important thing to remember when pairing speakers with your amp is that the watts and ohms ratings must match.
The frequency response of a speaker measures the total range of sound it can reproduce. Most speakers and headphones will claim a frequency response of 20 Hz to 20,000 Hz (low end sub bass to high end treble) but again in reality this is isn’t the whole truth.
Speakers claiming to reproduce sounds from 20 Hz up to 20,000 Hz may indeed to that, but sounds at the lower end of the scale take much more power to reproduce and therefore are often much quieter, sometimes even inaudible.
This is why high-end studio monitors with a totally flat response (meaning all frequencies a reproduced equally) are so expensive.
Aside from studio monitors and subwoofers, which usually come with a more truthful frequency response ratings starting around 40 Hz (club speakers generally won’t go mush lower than 30 Hz), most other speakers ratings are useless indicators of sound reproduction. The only real way to tell is to use test tones and spectrum analyzers to hear and see exactly what frequencies are reproduced and at what volume.
Fortunately most modern home audio systems come with a subwoofer to handle the power hungry low-end frequencies and this not only extends the frequency range below that of traditional speakers, it elevates the strain on them making the whole system more efficient.
Nevertheless, cheap systems with a sub will still only produce frequencies starting at 60 Hz, but that’s good enough for most listening purposes. If you listen to bass heavy music however, you may require a dedicated subwoofer capable of reproducing sounds as low as 35 Hz.
- Geoffrey Morrison: Myths, Marketing, and Misdirection: Home audio edition. Cnet, 02/24/2012.