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Sound Equipment - Loudspeakers
Loudspeaker systems
There are a large number of "power ratings" for loudspeaker systems, each has its own merits.
AES
Audio Engineering Society - the equipment being tested is subjected to a broadband pink noise signal. This type of signal contains equal energy in each frequency band over the specified frequency range. Testing using such a signal is more reliable when compared with a single frequency test (RMS) as it is testing over the whole frequency range at once.
IEC
This test procedure produces similar results to the AES method - filtered noise is used with a Crest Factor (see below) of 6dB (i.e. the peaks are 4 times higher than the average level) over long test periods. The AES rating has replaced the IEC rating and both are generally accepted as being the most reliable methods of testing.
Program Power
This rating reflects the music or program material which will be replayed through the system. The program rating is 3dB above or twice the AES rating.
RMS
Root Mean Square - this test can be deceptive, particularly for high power applications as the test is carried out at just one frequency at a time. This does not put a realistic demand on the driver compared to real life use and it is therefore very difficult to compare an RMS rating to a pink noise test. A 100 Watt "RMS" amplifier can produce a 100 Watt sine-wave into its load. With music, the total actual power would be less. With a square-wave, it would be more.
Peak Handling
The peak rating can also be misleading. The period of time the equipment is exposed to the peak signal is important - it may handle a peak of 1000W but it may burn out if exposed for more than 5 milliseconds. This can be a useful guide, but should always be accompanied with a noise test rating. The peak handling of a driver may be extracted from both the AES and IEC ratings - see Crest Factor below.
Crest Factor
The Crest Factor of a test signal is the ratio in dB between the volume of the average level and the volume of the peaks. If the Crest Factor is known for a test signal, e.g. 6dB for the IEC rating, then the peak handling can be calculated by doubling the noise rating for every 3dB of Crest Factor, e.g. an IEC rating of 200W will have a peak handling of 800W for a 6dB Crest Factor.
Power Compression
The Power Handling is not the final word in how loud a system performs. The voice coil of loudspeakers will heat up when a signal is applied to the input signals. The amplifier is not as efficient at delivering power to a hot coil and will therefore start to compensate by pushing more current around the coil. The coil heats up more and the circle of power compression begins. This translates to a drop in sound output when the speaker is operated at near maximum power input.
DIN Power
The DIN 45000 defines different methods to measure power, depending on the device under test. Power measurement of an amplifier requires that it is properly terminated by Ohmic resistances of nominal value both at input and output.
The continuous power is measured when the amplifer is supplied by its normal power supply. It must then be able to deliver the rated power at 1 kHz for at least 10 minutes while the maximum THD does not exceed 1%.
To measure the peak power the normal power supply is replaced by a regulated power supply and the time for delivering the power is reduced. Thus, higher values for peak power are obtained. You may skip measuring the peak power by simply multiplying the continuous power by 1.1. The power bandwidth is defined as the frequency range for which 1/2 of the rated continuous power can be obtained.
DIN 45 500, CNF 97-330, EIA RS-426 and the encompassing IEC 268-5 specify not pink noise, but pink noise filtered by a filter that provides significant attenuation in the low and high frequency region of the spectrum to more closely model the long-term spectral distribution of music. Pink noise itself does not accomplish this.
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