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IssuesFrequency ResponseIt is technically difficult to make a single driver that covers the full audible frequency spectrum. Manufacturers succeed to a greater or lesser degree. What are the issues with particular designs? Lowther speakers are well known for being both very detailed, responsive, sensitive and also for being lean in the bass and forward (shouting) in the mid treble region. The Lowther drivers naturally provide these characteristics and it is up to the cabinet designer to try and accomodate them so as to produce a speaker with relatively flat frequency response. The older Lowther cabinet designs (such as the Acousta) do not address these issues as well as newer designs (such as the Beauhornand the Carfrae). The Lowther drivers have a broad frequency peak from about 1 kHz to 10 kHz. Cabinet designs need to increase the output below 1 kHz in order to match seamlessly with the frequency peak. Most designers do this by employing a rear-loaded bass horn. Unfortunately, most build them too small (in order to keep the speaker size down) and have too many sharp folds (ditto). This limits bass output and creates a ragged bass frequency response. Also, the bass horn is crossed in at about 300 Hz, too low to help the 300 Hz to 1 kHz region. The solutions are a big, non-folded horn for bass, and a front horn (or waveguide) to raise the 300 Hz to 1 kHz region. Carfrae makes the biggest rear horn, and along with the uncompromised Hedlund horn are probably the leaders in bass output and quality. The Beauhorn speaker incorporates a front horn, specifically to raise the midrange, and is one of few designs to use a front horn in combination with the rear bass horn. Reports are that the Beauhorn sounds "warmer", fuller, and less analytical than say the Carfrae. The German Jericho Horn (with Fostex drivers) uses a RLC Equaliser to flatten the Midrange Hump. This kind of RLC Filter could be adapted for any type of full-range driver and the specific filter will work with the Fostex FE208 Sigma in all kinds of Horns (like a Lowther Acousta). More interesting, if one where to put the FE208 Sigma (or the smaller FE168 Sigma) into a small sealed box of about 12 liters and use this filter with a slightly increased resistor, the resultant speaker will be pretty flat between 120Hz and 18kHz. It could look like the Yamamura Ariadne, a small box full-range speaker. With 120 Hz as cross over frequency, a (say) 8ft 8" PVC pipe with a downward firing 8" paper cone driver could make a very nice fitting Transmission Line subwoofer to be driven from a meaty solid state amp. Power and SensitivityThe sensitivity of a speaker (often expressed as a "dB" figure) is how loud it will play for a given power input. The usual (and proper) sensitivity rating is given as so many dB at one meter distance with 2.83 V playing into 8 Ohms (dB/2.83 V/1 m). 2.83 V into 8 Ohms is exactly 1 Watt of power, so sometimes sensitivity is given as dB/1W/1m. The slashes "/" don't mean "divide by", they are just separators, you can also write dB at 1W at 1m, or dB, 2.83V, 1m, etc. Single driver speakers are often more efficient than multiple drivers because of the lack of crossover. A passive crossover eats up power (how much depends on the complexity and value of the crossover components). The driver sensitivity will usually not be the final sensitivity of your overall speaker system (including enclosure). Often enclosures will add some sensitivity to the loudspeaker. See a very good description of speaker sensitivity and required amplifier power at Welborne Labs (near the bottom after Recommended Speakers). Another list of recommended speakers is by Gordon Rankin at Wavelength Audio. Top
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