Story of Horn
Improving transduction efficiency by using horn
Horns have been used to amplify the sound generated by a membrane since the days of mechanical gramophones.
The small membrane, excited by record player stylus, was not able to provide the sound suitable for a home environment of normal size.
Therefore engineers turned to the use of horns to amplify the faint sound of the membrane.
Looking at the picture below, you can see that the system was already using the compression chamber to increase the transduction efficiency.
Mid-high frequency loudspeaker
It is a diagram of a dome loudspeaker for mid-high frequency, suitable for midrange and tweeter. At the moment we don’t say the why of a convex-shaped dome membrane since it would be off-topic. Such loudspeakers have good frequency response, good angular dispersion and good power handling. However, they have not good transduction efficiency and are therefore not suitable for constructing speaker systems with high efficiency.
Horn loaded dome speaker
Like the gramophone, also a dome electrodynamic transducer can be loaded with a horn to increase its efficiency.
Efficiency may further be increased thanks to a compression chamber placed between membrane and horn throat. In this way, however, there would be a significant phase cancellation with relative worsening of frequency response, because of different sound paths coming from different distances between each emission point of membrane and horn mouth.
The Phase-Plug was invented and developed in order to solve the problem of cancellation due to phase displacement of sound waves within the compression chamber.
These devices become now an integral part of the compression chamber, forcing the sound waves emitted from different parts of the membrane to run through a precise path, such as to minimize the effects of phase cancellation.
Even if the Phase-Plug had solved some of the problems of interfacing between membrane and horn, performance of these devices was not yet satisfying.
In the ’30s, Western Electric, the company at the forefront of the transducers, studied and produced the famous driver 594A:
This driver can be considered the first modern driver. Nowadays, new drivers are still designed on the basis of the model 594A.
About today’s drivers for Hi-Fi systems
Today’s drivers are designed to withstand the enormous power of live concerts and to have the maximum possible efficiency at expense of response regularity, distortion and sound coloration.
Sensitivity of these devices, ranging from 105 dB/W/m to 115 dB/W/m, is really too much for Hi-Fi usage, because driver should be coupled to direct-radiating woofer with a sensitivity of no more than 98-100 dB/W/m. Therefore it is necessary the use of a resistive attenuator in order to make the output level suitable for the woofer level.
Because of these characteristics they are not suitable for Hi-Fi systems, where regularity of frequency response and low distortion are the main aspects to consider.
Even in recording studios, where the use of compression driver was a rule, in recent years these devices have been neglected in favor of horn-loaded systems without compression chamber.
Horn system without compression chamber
Since the membrane is directly facing the radiation throat, it is very important to study the coupling between these two elements that interact so closely with one another.
This system has sensitivity up to 98-100 dB/W/m, clearly insufficient for a professional application but very good for Hi-Fi use, allowing to avoid the presence of attenuation resistive networks which are bad for micro and macro sound dynamics.
Compared to compression drivers, they suffer far less problems with response regularity and with distortion, thanks to the absence of the compression chambers.
In traditional horn, the directivity varies with frequency, so that we have, on one hand, an almost perfectly omnidirectional sound emission at low frequency, and, on the other hand, a very directive sound emission at high frequency.
Constant directivity horn, instead, allows you to have a frequency response that is almost constant in all directions covered by the features of the horn itself.
A device with a constant directivity feature has got a tonally balanced emission across the whole angle of coverage, so that the sound coming to you from the reflecting walls is almost the same of the sound heard in front direction.
In a multi-way system, directivity shouldn’t vary, neither by step nor in other manner, when going from one speaker to another. In a three-way speaker, for example, woofer and midrange should have the same directivity at the crossover frequency and so should be for midrange and tweeter.
In all the cases, this result can only be obtained by using an acoustic impedance adapter, that is to say a horn, since membranes sizes are obviously very different.
Horn loaded compression drivers have a sensitivity from 110 to 115 dB SPL with 1 watt input, so they should be attenuated at least 10 dB in a home Hi-Fi system. Unfortunately, any passive attenuation network eats away part of the micro and macro sound dynamics.
In Casta speakers, driver membrane is directly coupled to the horn throat, in order to get the right sensitivity without any compression chamber that can produce distortion.
In the ’40s, it was usual to utilize AlNiCo magnets, and many builders have continued to use this material for a long time, despite the advent of ferrite.
In the ’70s, finally, one of the raw materials of AlNiCo, cobalt, became to cost so much to prevent in facts the industrial use of this type of magnet.
Thanks to magnetic circuit, voice coil of AlNiCo speaker has a dynamic behavior which is perfectly symmetrical when moving in and out, and this allows it to interface with the amplifier in the correct way.
An acoustic system with closed box (pneumatic suspension) may be appreciated for the absence of colouring.
The sound of a speaker is defined as “coloured” when it is somehow altered by spurious frequencies that are not present in the original sound.
Sometimes these shades may appear falsely pleasing to the untrained ear, but in the long run they will annoy you since they give the sound an articial connotation.
Pneumatic suspension speakers can spread a clean, neutral, dry sound, and are able to return in a realistic way the most difficult parts of a music program, such as violent orchestral deadlift, frequencies emitted by electric bass, definition of the male voice.
Our loudspeakers are designed to give the maximum performance in a normal home environment with no special acoustical treatments. Then we consider the ease of installation as a fundamental feature.
Pneumatic suspension load of woofer is designed to have the maximum linearity at low frequencies with the speaker placed at a distance from back wall between 30 and 100 cm; so you don’t need to put your speakers in the middle of the room and this is surely a great advantage for aesthetics as well as usability of the room itself.
Constant directivity of middle-high speakers firstly reduces the influence of side walls, ceiling and floor, secondly let you free to angle the speakers towards the listening position to optimize the sound image, and finally makes the optimal listening area larger than it would be otherwise.
Casta always designs speakers taking into account that they could also be driven by tube amplifiers with high output impedance (low damping factor): this aim is obtained thanks to an impedance which is never low and remains almost constant on the whole frequency range.
This important feature allows you to avoid the deterioration of the frequency response of the amp-speaker system.
Any amplifier will then be able to interface correctly with Casta speakers.