Notes on Audio equipment
For Home, Mobile or Computer Use
- Impedance is measured in Ohms, and is a measurement of resistance. A 4 ohm power resistor across the Positive (+) and Negative (-) terminals of a speaker or speaker system will make 8 ohm speakers or speaker systems 4 ohms. This is a cheap way of usually roughly doubling the output of an amplifier. The power resistor should be about twice the RMS rating of the amp, however utilizing one with the RMS rating equal to that of the amp will sometimes suffice if the amp is not "cranked". (Of course the amp has to be capable of driving a 4 ohm load, of which most are. Amps with a 4 ohm rating of course are capable of driving a 4 ohm load).
- "RMS" = Root Mean Squared. Depending on its use, this is either the AVERAGE output of an amp (in watts), or the AVERAGE power handling of a speaker (in watts). Peak values are much higher. In amps or receivers, this means the continuous average output level. In speakers, this means the continuous average input level.
- "THD" = Total Harmonic Distortion, the lower the better. Less than .3% in amplification, and less than 10% in speakers, is considered "inaudible", and good. Since speakers are electro-MECHANICAL devices, their moving parts of course cause more distortion, however our ears are more or less more "accustomed" to this type of distortion than that of amplification distortion.
- "S/N Ratio" = Signal to Noise Ratio, the higher the better. Anything above 65db is considered "high fidelity", and good.
A word on speaker power ratings...Don't be mislead by these, they can be rather confusing. Generally, it's not actually the power than can damage or blow a speaker or individual driver. Sure, it CAN, but usually it's the distorted waveform of the amplifier that "clips", and when this happens distortion becomes infinite 100%, and it is this clipped waveform that will damage or blow a driver. You can actually have a low power cheap amp that will blow a speaker BEFORE it would be damaged by a high power amp! This is because a low powered or cheap amp will either have more distortion, and/or it will reach it's clipped output much sooner than that of a higher powered or more expensive amp!
Also, don't be concerned with having a ~1000 watt amp, and using speakers that may be only rated at 50 watts RMS or peak. These power ratings can be misleading to the novice. Any speaker can safely be used with any amp (as long as impedance is not any issue), what is important is HOW you USE the amp! Depending on program material, 1 watt continuous (RMS) output can be extremely loud! It could be peaking at a couple of hundred watts! This may not be very likely, but nonetheless possible. By the same token, ~30 continuous RMS input or output can be rather quiet! Again this all depends on the type of program material. In our general day-to-day listening of music, we may usually hear less than 1-2 watts continuous. It's best to use some kind of power output meter at the amp level (many amps have these and if not they can be purchased separately), to determine what exactly the output may be.
As for the individual drivers inside speakers and their power ratings....Woofers are going to have a much higher power rating that midranges and tweeters. This is simply due to the fact that the overwhelming bulk of power goes to the bass or low frequencies. This is why midranges and tweeters are only rated for a fraction for what a woofer or subwoofer will be rated. Typically a tweeter will be rated from 5-50 watts, with 50 watts being rather uncommon and usually for extreme power applications. If you were to lower the bass on an amp or receiver down to zero, or if you have an equalizer and you lower all the low frequencies down to zero then checked a power meter, you would see the power meter go down to nearly zero watts! The massive bulk of power is delivered to the low frequencies because they simply require much more power to reproduce due to their longer wavelengths of low frequencies. This is why speakers have "crossover networks". This is a filtration circuit of capacitors and coils/inductors that route the appropriate frequencies to the appropriate drivers. Sometimes resistors are also used for tweaking. It routes low frequencies to the woofer and high frequencies to the tweeter, (and in the case of 3-way or higher systems it routes the mid frequencies to the midrange driver). From its filtration, it prevents low frequencies from going to the tweeters (or midranges), for if not for this, the tweeters would blow from the massive amount of cone excursion needed to reproduce such low long frequency waveforms. They also generally prevent the higher frequencies from going to the woofer (and midrange where applicable) because this can "muddy" up the sound and cause loss of clarity and detail.
Each speaker driver is best suited for its own range of frequency reproduction due to its design. A typical home 3-way speaker with a 10" woofer may have for example crossover frequencies of 1000hz and 5000hz; whereas the woofer will begin a "cut-off slope" of X dB per octave at 1000hz, the midrange will begin its FULL operation at 5000hz (and PARTIAL operation as determined by the slope of crossover at X + 1000hz, and where the tweeter then starts its FULL operation at 5000hz (with PARTIAL operation as determined by the slope of crossover at X - 5000hz. A graphical representation of this crossover network slope can be seen here. In that example, this is a 3-way network with the crossover frequencies at 300hz and 3000hz. Note the slopes in that example, where the midrange (blue) begins its operation at 100hz (but it's 40dB down), but it's fully operational from 300hz through 3000hz, at where the tweeter (red) begins its full operation. Note how the tweeter is 40dB down at 1000hz. The points at which these lines cross is the crossover point, and note how in this example they are 5dB down, this is called a 5dB down insertion point, typically they are 3dB. For that particular example, this would be called a 3-way 300hz 3000hz crossover network.
The amount of slope is determined by the amount of components in the crossover network. For example, the simplest network is 6dB/octave, which means at the crossover point the drivers are 6dB down less than their normal operating frequencies. In a 2-way system, if a tweeter crosses over at 3000hz, it will be 6db down at 3000hz, but 12dB down at 1500hz (one octave less), 18dB down at 750hz (two octaves less), and so on. This is achieved by simply placing a capacitor of certain value (determined by the impedance and frequency desired), in series with the + terminal of the tweeter, and an inductor or coil in series with the + terminal of the woofer (determined by the impedance and frequency desired). Note how the capacitors block the low frequencies, and the coils block high frequencies. In a 3-way system with a midrange, the midrange driver would use both capacitors and coils because it is a "bandpass" filter. (With more expensive 12dB or 18dB/octave crossovers, more capacitors and coils are used in a series-parallel configuration).
So now you now a little something about why tweeters and midranges do not have high power ratings. ;-)
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