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Лин Олсън (НътшелHiFI DIYаудио) За Филтрите, Хода На Мембраната И Изкривяванията

Лин Олсън (Lynn Olson) е един от двамата съавтори на сайта: http://www.nutshellhifi.com/ и един от най-уважаваните членове на най-големия и най-добрия (не винаги е едно и също) форум за направи си сам озвучаване в света: http://www.diyaudio.com/ .

По линия на международното сътрудничество на Пенкилер с хайфисти от останалият свят, господин Олсън ми разреши да публикувам в Пенкилер в превод няколко негови мнения относно връзката между хода на мембраната, честотата на разделяне, резонансната честота на говорителя и изкривяванията. Първо ще ги кача в оригинал, а постепенно и ще ги преведа на Български език.

Ето първото негово мнение, което представлява цяла изчерпателна статия по въпроса със съскащите, фъфлещи и шущящи пищялки (високочестотни говорители). Това е мнение № 17 от ето тази тема: http://www.diyaudio.com/forums/multi-way/190663-can-you-have-sparkling-treble-but-without-sibilance.html

Директна връзка: http://www.diyaudio.com/forums/multi-way/190663-can-you-have-sparkling-treble-but-without-sibilance-2.html#post2604196

Мнението е по темата за т.н. феномен "sibilance", което в превод означава съскане, шуштене, дращене, кривене или иначе казано звука на някои високочестотни говорители, който се отличава с привидна яркост, но всъщност има шуштене на вокалите при произнасяне на буквите "с", "з", "ш" и други подобни звуци.

Нищо никога не е просто. За да обясниш едно нещо трябва да знаеш много други неща и да познаваш взаимодействията между всички явления. Точно поради тази причина, както и защото господин Олсън обича да се аргументира изчерпателно и не се задоволява с изявления съдържащи само извода, но не и аргументите, се е получила статия от която могат да бъдат научени много други неща. По тези причини и сметнах, че ще е хубаво да я имаме тук преведена.

Цитат
A common but unsuspected cause of sibilance is crossing the tweeter too low, or using a shallow-slope crossover. Many designers - unfortunately, a lot of them in the high-end biz - forget that direct-radiator drivers increase excursion at a rate of 12 dB/octave. Thus, it takes a 12 dB/octave highpass filter to merely keep excursion constant in the frequency range between nominal crossover and the Fs of the tweeter.

For example, if the tweeter has a typical Fs of 700 Hz, and the intended crossover is 2.8 kHz (again, typical), it takes a 12 dB/oct electroacoustical filter to merely keep excursion constant in the very critical 700 Hz ~ 2.8 kHz range. Part of the reason that this range is so critical is that audibility of distortion is at a maximum in the 1~5 kHz region. (Perception of distortion similar to, but not quite the same as, the Fletcher-Munson curve.)

Staying with the same example, if the electroacoustical filter is 1st-order (6 dB/octave), then excursion actually increases from 2.8 kHz on down, until 700 Hz is reached. Below 700 Hz, the excursion finally starts to decrease, but not very fast, only 6 dB/octave. This is troublesome because the maximum spectral energy of many recordings is around 300~500 Hz, so energy from this range can crossmodulate with the tweeter output.

This is why auditioning with little-girl-with-a-guitar program material and a full choral piece sound different. The LGWAG is spectrally sparse, and there isn't as much chance the tweeter will be struggling with IM distortion. Throw a dense, high-powered spectrum at the loudspeaker, though, and the tweeter will start to scream - and it is very audible on massed chorus as complete breakup.

At any rate, regardless of distortion of a particular tweeter (none of them are free of IM distortion), crossovers matter. Many designers want to take the tweeter as low as possible because the polar pattern is prettier and certainly measures nicer, but the inevitable price to be paid is more IM distortion resulting from increased excursion (the linear region is most tweeters is less than 1mm). Choosing a crossover is a difficult tradeoff between narrowing of the vertical polar pattern, IM distortion from out-of-band excursion, and how close the designer wants to approach the region of midbass driver breakup. The tradeoff is made more difficult when a rigid-cone (Kevlar, metal, ceramic, etc.) midbass driver is chosen, because the onset of breakup commonly falls in the 3~5 kHz region, right where the ear is most sensitive to distortion.

As you can see, the worst possible solution is a 1st-order crossover combined with a midbass driver that has a severe breakup region (Kevlar drivers, I'm looking at you). The 1st-order crossover fails to control out-of-band excursion, so program material in the 700 Hz-2.8 kHz region results in IM distortion in the tweeter's working range, while plenty of midbass breakup in the 3~5 kHz range gets through as well. And midbass breakup sounds the same as a bad tweeter, since the distortion and resonances fall in the same frequency range.

As a side note, most transistor amplifiers (including very expensive high-end products) go from Class A operation to Class AB around 1 watt. Feedback helps, but cannot fully overcome the two-to-one shift in transconductace as the AB region is traversed. In addition, thermal tracking is typically several seconds to a minute late (depending on the thermal mass of the heatsink and location of bias sensor), so the correct AB bias point is actually several seconds behind the program material. There are various sliding bias-tricks available (which avoid complete turnoff and associated switching transition), but they are all several seconds late. The more output transistors, the more AB transitions there are, since it is impossible to have transistors exactly match the switching transition - in production, they are matched for beta (current gain), but not usually for other parameters. Change the die temperature a bit, and the careful hand-matching goes away.

To recap, if you want lots of sibilance, use a midbass driver with severe breakup in the 3~5 kHz region (this is usually obvious from unsmoothed FR curves), pick a tweeter with limited excursion capability (not always spec'ed), select a 1st-order crossover at a low crossover frequency, and use an amplifier with a very large heatsink, many transistors, and somewhat unstable Class AB biasing (thermal overshoot). That should do the trick. Plenty of distortion from many different sources, even though the overall FR curves may look harmless.

Очаквайте превода.

Разрешението за публикуване е за цялата поредица от мнения на Лин Олсън в темата.

Поздрави!