2 Way Loudspeaker System

This loudspeaker system is based on loudspeaker units from Vifa.

My cousin, Jens recently moved into a new apartment, which had a living room that was too small for his 3 way KEF loudspeakers.
He asked if I would design two 2 way loudspeaker systems that could be hung on the wall, because it would save space.
He still wanted a good sound, and was willing too spend 4000 dkr for the system alltogether.

Loudspeaker System features:

I used two different kinds of filters: A 3rd order Butterworth filter crossing at aproximately 2kHz, zobel networks for impedance linearization and a notch filter for smoothing impedance peak of the bass. The other filter was a first/second order serial filter with impedance smoothing circuitry for the bass and L-pad attenuation for the tweeter
The enclosure was made in 22mm MDF, shaped like a four-sided pyramid with an internal volume of ca. 27 liters. Rockwool was used for dampening the enclosure. .

The choice of loudspeakers

I am a big fan of danish loudspeaker drivers. Vifa, Dynaudio and Scanspeak loudspeakers are in my opinion characterized by a very clean sound. Unfortunatly, Dynaudio drivers are no longer available for the amateur diy people, and Scanspeak are quite expensive, so these were left out.
The choice fell on Vifa PL18WO09-08 which I found a test on performed by Linkwitz. The test was performed using multitone test signals. I found that it was good for implementation in a 2 way system due to the wide frequency range. The THD measures are quite good although it was not the best in the test, but considering the test participants which are all high priced drivers, I think it did pretty good.

I used Vifa PL18WO09-08 which is a 18 cm woofer/midrange loudspeaker with quite low THD and a relatively constant frequency magnitude response untill approximately 3kHz.

The tweeter, PL27TG05-06 is also from the Premium Line series at Vifa. It is characterized by low THD measures and a relatively constant frequency magnitude response from 1-20kHz.

The Enclosure

It was decided to use the 4-sided pyramid design for this loudspeaker enclousure aswell as for the three way project, because it minimizes the amount of standing waves due to the few amount of parallel surfaces. The top and bottom of the enclosure are the only surfaces where room modes may appear, but in order to avoid this possible deterioration, the enclosure was filled with dense rockwool. PL18 has an acoustic equivalent volume of 24 liters and therefore, an inner cavity of approximately this volume will give a critical damping. A volume of 27 liters (+ the virtually increased volume due to the rockwool will overdampen the acoustical system slightly). Below, you can see plots of the mathematical models that I have used for simulating the response of the system, in the on-axis position.

Simulated acoustical response of PL18 mounted in a sealed enclosure of 30 liters.

PL18 in 30 liter enclosure

Simulated electrical response of PL18 mounted in a sealed enclosure of 30 liters.

Simulated response of PL27

Simulated acoustical response of PL27.

simulated response of PL27

Simulated electrical response of PL27.

In order to meet the demand about the enclosure volume of 27 liters, I used the enclosure illustrated on the schematic below.

Schematic of the enclosure

Schematic drawing of the enclosure.
A decent apperance was nescessary because the loudspeaker systems were to be located in a living room and therefore the enclosure was made by a professional carpenter. The total price was 1500 dkr, which I think was a very fair price given the amount of work. (angled corners, assembly, milled down holes for the loudspeakers and the spray painting job and including all materials)

The complete system

The final result!.

The Filter Network

First, the third order Butterworth filter was implemented. It consisted of an RLC resonator for smoothing the bass resonance frequency region, a Zobel was implemented for both the bass and the tweeter. Besides attenuation of about 3 dBV was included in the tweeter network. The designed/implemented network is illustrated below.

The third order filter network

Filter diagram of the third order parallel filter network.
This filter network was modelled using Matlab and the it proved to be beneficial to reverse the polarity of the tweeter. The electrical and acoustical response is modelled below.

Acoustical response of parallel filter.

Electrical response of the parallel filter.

After having listened to this design for some time, we agreed that the sound was very nice and had very detailed. However, the sound was a little thin for certain genres of music; simply because the tweeter was too loud relative to the bass. It is not apparent on the above simulated responses, but in that respect, notice that these responses are based on relatively simple models, without baffle considerations.
Furthermore, the models are based on perfectly time-aligned arrangement of the tweeter and bass driver, which is ofcourse only reality at a certain position in front of the speakers. In the meantime, I had experimented with serial networks and learned that they are very good for certain designs, involving low Q loudspeaker drivers.
Below, you can see the series network with impedance smoothing circuitry and the simulated acoustical and electrical response.

Electrical diagram of the serial network.

Electrical response of the serial network.

Acoustical response of the serial network. This design differs to some extent from the parallel solution. The drivers are in phase, thus resembling the Linkwitz-Riley filter solution, the filter cuttoff is set at 2500Hz and the tweeter output is attenuated. The in-phase response improves the off-axis response, providing a broader area in which the stereo-perspective is maintained. The crossover frequency is a little higher, but really does'nt change anything. The increased attenuation of the tweeter is made in order to include some sort of baffle step compensation.

It's not a baffle step compensation based on the dimensions of the baffle, but one thing is for sure: the serial network sounds better and requires fewer components. The downside to the solution is that the out of passband attenuation is smaller, whereby the THD is probably larger than the parallel network solution.