For me, to reach a live like sound it means i need great dynamics, high sensitivity and big radiation area. Many of my projects start with these aspects in mind and it is clear now that all these are related to efficiency.
Efficiency is a power ratio and in acoustics it is acoustic power delivered vs electric power consumed. There is some debate on how important this parameter is (many manufacturers don’t even publish it), because most audio amplifiers are designed as voltage sources and not power sources. Since impedance of a loudspeaker varies with frequency we can say that the power drawn from the amplifier will vary with frequency.
In the below simulation you can see this variation.
Modeled impedance curve of high efficiency woofer
Calculated input power with constant voltage source having the above impedance as load
You can easily see how power drawn in the low frequencies drops pretty much this causes sensitivity to drop but not because of low efficiency in the low register but because of how amplifiers are made.
A very important aspect i found to be true is to use high power amplifiers with high efficiency loudspeakers. These amplifiers will most likely have their output stages powered by high voltage and this allows for high voltage swings necessary when driving the high impedance at high levels.
Horns are by far the most efficient sound reproduction systems but since they are passive components they get very large at low frequencies. Because of size and complexity a large number of systems were designed using a direct radiator for low frequencies and horn loading for medium and high frequencies. But can a direct radiator reach a good enough efficiency?
It really depends on its components. For high efficiency you will need a big motor (strong magnetic flux), you will need a light moving assembly (light cones and voice coil) and you will need a big radiating area (large cone diameter).
It is my belief that for good bass from direct radiators one needs at least a 12 inch cone. For domestic use i like the 15 inch cone and there are many great drivers at this diameter that have proven their worth over the years.
At these diameters most drivers one can find come from the professional audio sector. There are many very good designs on the market, it can be hard to find something that can totally please me but given the fact that i like to modify the drivers a new project begun.
First thoughts were to make a standard 2 way system using the 500Hz sectoral horns i had in stock. For this purpose i build a couple of test boxes but first i will introduce the drivers.
First 15 incher was 6523C having a 3 inch voice coil and while the motor provided a good flux density in the gap, the thickness of the top plate was about 10mm. Not the thinnest but the greatest either. The structure is very simple with straight poles and no faraday rings. On the mechanical side again it revealed this was not a woofer for this project for the filter needed alot of optimization to make it work good with a xover point around 1kHz.
Here is the impedance model for this woofer:
The second 15 incher is based on the 8512F which i modified in various ways. Originally this woofer is designed compact bandpass pro applications, the suspension system is pretty stiff and it needs alot of power to get it moving. So first i changed its spider and outer suspension.
The unit has a tremendous motor designed to match JBL 2226H with a 220mm ceramic magnet ring and 12mm thick top plate. A 4 inch voice coil, a T shaped pole piece makes this 15 incher a real beast.
The first compression driver tested was a 1″ exit with Ti diaphragm and Ti suspension. I later replaced the suspension with mylar one. I had a sheet of mylar and cut out a ring to form the suspension. This improved on the transition region between the woofer and the CD as i will later show.
Phase plug is after the old Tannoy Monitor Gold and should provide a better low frequency response.
Below you can see the model for the 15 incher and the bass reflex box. The box is tuned lower as the xover will bring the second peak down a bit.
Here are some of the first ideas for the enclosure:
I will end here the first article and will continue soon.
Thank you for visiting,