號角喇叭的理論－stein大小聲

號角喇叭作用是匹配負載阻抗，提高功率轉換的效率。

好像真空管功率放大器的輸出阻抗高（例如＝ 1k 歐母），而喇叭的負載阻抗低（例如 = 8 歐）

最好就接上一個輸出變壓器 output transformer（初級 2k歐，次級 8歐阻抗）, 給真空功率管一個 2k歐的負載。

勝經本handbook上 Section 4 有論述：

號角喇叭有三個難題：

例如下文的計算例子，12吋的喇叭需要一隻開口直徑 2825 mm 接近三公尺，筒身長度延伸 3750 mm 接近四公尺。

佔據的空間太龐大，連帶的造價也高。

因此用在大禮堂、戲劇院、陣前喊話是很適當，擺在豪宅客廳也夠氣派。

放在公寓小房裡，是 "西施效顰"了。

號筒內部駐波，standing wave 引起的 "音染"，俗話說的 "銅罐仔聲" "隧道聲"。

筒身越長，駐波的量越多。

提高筒身的吸音，相對的也殺掉高頻，無法兼顧。

號筒材料本身的自然共鳴。

按照 F= (1/2*pie)*square root of ( Stiffness/Mass)

選擇適當的材料，提高剛性 stiffness，降低質量mas，就可以提高共鳴頻率到耳聞之外的超高頻。如金屬薄皮。

同理，降低剛性 stiffness，提高質量mas 就可以降低共鳴頻率到耳聞之外的超低頻。如鋼筋水泥。

但是看來只能擇一而行。

總之，號筒不是好惹的。

因之市場看到的號筒大半用在中高頻。
就算加州恐龍，也只用來管低頻而已。他的客廳裡，另外有配合的中、高音喇叭音箱。

取自 wiki 的動作圖

A horn loudspeaker is a loudspeaker or loudspeaker element which uses a horn to increase the overall efficiency of the driving element(s). A common form (right) consists of a small compression driver which produces sound waves with a small metal diaphragm vibrated by an electromagnet, attached to a horn, a flaring duct to conduct the sound waves to the open air. Another type is a woofer driver mounted in a loudspeaker enclosure which is divided by internal partitions to form a zigzag flaring duct which functions as a horn; this type is called a folded horn speaker. The horn serves to improve the coupling efficiency between the speaker driver and the air. The horn can be thought of as an "acoustic transformer" that provides impedance matching between the relatively dense diaphragm material and the less-dense air. The result is greater acoustic output power from a given driver.^[1]

The narrow part of the horn next to the driver is called the "throat" and the large part farthest away from the driver is called the "mouth".^[1] The angular coverage (radiation pattern) of the horn is determined by the shape and flare of the mouth. Unlike cone speakers, horn speakers usually have rectangular apertures, with the width tailored for proper horizontal coverage angle, and height tailored for proper vertical coverage angle. A major problem of horn speakers is that the radiation pattern varies with frequency; high frequency sound tends be emitted in narrow beams with poor off-axis performance.^[2] Significant improvements have been made, beginning with the "constant directivity" horn invented in 1975 by Don Keele.

The main advantage of horn loudspeakers is they are more efficient; they can typically produce 10 times (10 dB) more sound power than a cone speaker from a given amplifier output. Therefore horns are widely used in public address systems, megaphones, and sound systems for large venues like theaters, auditoriums, and sports stadiums. Their disadvantage is that their frequency response is more uneven because of resonance peaks, and horns have a cutoff frequency below which their response drops off. To achieve adequate response at bass frequencies horn speakers must be very large and cumbersome, so they are more often used for midrange and high frequencies. The first practical loudspeakers, introduced around the turn of the 20th century, were horn speakers. Due to the development in recent decades of more efficient cone loudspeakers, which have a flatter frequency response, use of horn speakers in high fidelity audio systems has declined.

取材日本的梁式教科書上有簡單佴要的說明