Why horns? (cont.)
Very interesting article by Rod Elliott (ESP) on Public Address Systems for Music Applications ->
http://sound.westhost.com/articles/pa.htm
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Let's look at a simple example. Two systems are set up side-by-side. One has an overall sensitivity of 100dB/W/m, meaning that an input of 1W will give an SPL (sound pressure level) of 100dB at 1 metre distance. Power rating is 100W maximum continuous average for this first example.
The second system is rated at 90dB/W/m sensitivity, but has a power rating of 1,000W - also continuous average. The question is ... which will be louder?
In theory, both will be exactly the same - they will be capable of 120dB SPL at 1 metre with full rated power. In reality, the 100W box will be somewhere between 3 and 6dB louder than the 1kW system, because at such a low average power there will be little power compression in the loudspeaker(s). All loudspeakers have to dissipate nearly every Watt from the amplifier as heat, which means that the voicecoil gets very hot indeed at sustained high power. For those that can (allegedly) handle 1kW, this is the same amount of heat as you'll get from a 1,000W radiant heater. As voicecoil temperature goes up, so does the resistance of the voicecoil itself, which increases the impedance, which in turn reduces the power obtained from the amplifier.
If only 3dB is lost to power compression, the amp power needs to be doubled (2kW) to restore the balance, but this makes the voicecoil get even hotter. This vicious circle continues until either the speaker fails or the amps have no more to give ... often both. A 90dB/W/m loudspeaker has an overall efficiency of about 0.62%, so with 1kW going in, only 6.2W comes out as sound - the remaining 993.8W is converted to heat. The 100W speaker (100dB/W/m) has an efficiency of roughly 6.2%, so 6.2W emerges as sound, and only 93.8W is wasted as heat. It is far easier to remove 94W of heat from a loudspeaker than it is to remove 994W - this should be immediately obvious. In both cases, the acoustic power is 6.2W, but it will only take a short time before the 1kW system shows power compression and reduces its output. Power compression figures for high powered loudspeakers range from around 4dB (very good) to as much as 7dB or perhaps more - this is not at all good.
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Almost all large concerts now are using line arrays for the PA system. These have become very popular, and even very small ones are available for smaller venues. While I know that many people will disagree,
I consider the line array to be an unmitigated disaster in most cases. Those that I've heard all sound (often radically) different from each other, but they all share one thing - they generally sound bloody awful. Coupled with bizarre thinking about how they should be set up in the first place, the only ones I've heard so far that sounded even passable were in relatively small clusters (4 per side), and were situated high above the stage area. Contrast this with the glowing comments you may see elsewhere - a lot of people think that the line array is the best thing since sliced bread, and will wax lyrical about how they have solved all PA problems.
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The new systems that emerged in the 70s typically used fully horn-loaded designs. W-bins were adapted from movie theatre designs for the bottom end - the "adaptation" was to make them small enough to move around, but that reduced their bass response. The W-bin was a folded horn bass speaker, and was much loved by a lot of bands because of the great chest compression it produced with the kick drum. Most struggled to reproduce anything below around 70Hz at significant sound levels, but this was better (louder) than anyone had heard before. There were various horn loaded boxes used for midrange, but like the folded horn W-bin, most were adapted from (mainly) Altec Voice of the Theaterâ„¢ designs, as well as various JBL and RCA theatre systems. In many ways this was unfortunate, because the old theatre systems were engineered mainly for speech clarity in a theatre, and were based on the most efficient loudspeakers available. Power (and audio bandwidth) was severely limited, so the loudspeaker had to make up for the lack of electrical power. Fidelity was usually pretty woeful in theatres at that time (and in many cases still is), but if the audience could understand the dialogue then that was the best that could be hoped for.
Many quite large concerts (such as Woodstock in 1969) used remarkably little power. The Woodstock system is said to have used 10 x McIntosh MI-350 mono valve amps - a total of 3,500W. There is some disagreement on this, and surprisingly little real information. Many of the early (even quite large) systems only used about 1500W in total, and this was generally far more than could be obtained economically from any valve amps that were available at the time. Relatively large transistor amps became common in the late 60s (the Crown DC300/DC300A was rated at 150W into 8 ohms, but gave closer to 200W). These were only possible after various semiconductor manufacturers had perfected power transistors that were capable of handling significant voltage and current. It wasn't until around 1970 that these new devices became available at prices that mere mortals could afford, but once their price came down to something tolerable, things changed forever. Before this, the only (cheap) readily available high power transistor was the venerable 2N3055. In the early 70s, I built guitar amps and (mainly column) PA systems, and the only high-power transistor I could get at the time was the Solitron 97SE113 - now long gone, but not forgotten. The release of the Crown DC300 power amp in 1967 (closely followed by the DC300A,
Phase Linear 200, 400 & 700 and a few others) signalled a new opportunity - plentiful power.
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