- Loudness in Decibels (dBs)
This is a measure of loudness and is important for choosing speakers, especially if you like to listen at a high volume. Something to remember about decibels, is that for every 10 decibel rise the noise is twice as loud- so small increases in decibel levels= a big impact on the ear.
- Power in Watts (W)
A watt is a measure of electrical power. The output of an amplifier is measured in watts. All loudspeakers have a maximum number of watts that they can cope with and the manufacturer will tell you what this is.
It is important to make sure that your amp does not put out more power than your speakers can handle or they can be damaged.
Usually, the manufacturers give 2 figures for power for both amplifiers and loudspeakers
RMS = the power an amplifier can put out over a long period
Peak = the power an amplifier can put out in short bursts.
Nominal power= what a speaker can handle long term without being damaged
Peak power= what a speaker can handle in short bursts without being damaged
- Speaker Sensitivity
Very good speakers are more sensitive than mid-quality speakers and can deliver a lot of sound with only a little power coming from the amplifier. Mid-priced speakers need more power to give you the same volume.
Speaker sensitivity is expressed in terms of the number of decibels (dB) of sound pressure level (SPL) per 1 Watt of amplifier power measured at 1 meter from the speaker. Usually manufacturers drop the ‘SPL/W/M’ and just say ‘db’.
Most loudspeakers have sensitivities in the range of 85 to 91 dB. So a sensitivity of less than 85dB is not so hot. 130 Decibel concerts can heard up to fifteen miles away.
How Watts Translate into Loudness
A pair of reasonably sensitive speakers delivers the following decibel levels at a distance of about one meter.
2 Watts = 93 decibels
4 Watts = 96 decibels
8 Watts = 99 decibels
16 Watts = 102 decibels
32 Watts = 105 decibels
64 Watts = 108 decibels
128 Watts = 111 decibels
256 Watts = 114 decibels
512 Watts= 117 decibels
1024 Watts = 120 decibels
10db = normal human breathing
60db = normal human conversation
110db = power saw, car horn, shouting in ear,
120db = jet aircraft close by, emergency vehicle siren, rock concert
If you like really loud music, you are looking for something between 85 dBs and 110 dBs. Anything louder than this and you will be destroying your hearing. So 120 watts is about the most you should ever need for mid-price speakers in the home.
When you move up the price range and get to the super-efficient speakers, then the power needed to get a big volume goes right down.
The amp in this public address system, circa 1920, produced only 10 watts!
- Total Harmonic Distortion (TDH)
This is a measure of how faithfully speakers translate what is on a disc or hard drive into sound. The lower the figure, the less distortion there is. Usually the values are between 0.05% and 0.08% THD for a quality “clean” system but any figure below 0.1% THD is pretty good.
This tells you how much current a speaker will draw. 8 ohms is standard. 4 ohms is very good and usually a lot more expensive. If you are buying 4 ohms speakers you need a very good amplifier to get the most out of them.
This is a measure of what a system can deliver in short bursts. A large headroom figure is important if you have a home cinema system and want to get a jolt from the explosions in action movies.