"So how do you know what mouth size to use? It depends on the conditions your horn will be working in. If your horn is designed to hang 1000 feet above a cornfield, then it will effectively be radiating into full space. Leach's model assumes that the mouth of the horn will be infinite in this case. Of course you cannot build a horn with an infinitely large mouth, but a mouth that has a circumference equal to the wavelength of the lowest frequency to be reproduced has been agreed on as being close enough to infinite. If you put the horn on the ground, then it will be radiating into half space. The ground provides an acoustic boundary. In this case, a mouth with half the area of the full space mouth will be close enough to an infinite mouth. The area can again be cut in half for a horn placed on the ground and against a wall. If the mouth is placed at the junction of two walls and a floor, then the mouth area can be 1/8 the size of the full space mouth and still be considered large enough. However, many people make the mouths of their horns smaller than this so that they can be stuffed into aesthetically pleasing boxes. In this case, the efficiency of the low frequency end of the horn's bandwidth will typically decrease, and the response will have dips and peaks. This is the case most of the time, but not always."
U kratko, ja to shvaćam da kod konstrukcije horne ne treba samo uzimati omjer grla ili drivera i usta, već je bitna i najniža prijenosna frekvencija, kako za određivanje dužine horne, tako i za određivanje veličine usta.
Tekst je od od ovdje;
https://www.jhsaudio.com/design.html