In the Kingfisher2, I replaced the direct radiating 8″ Silver Flute W20RC38-08 with my Albatross, a 10″ bandpass subwoofer based on the Dayton DA270. With this upgrade, the midrange clarity and bass definition improved tremendously.
Bandpass vs Direct Radiating
Fig 1 – Black plot = Albatross Bandpass Sub. Brown plot = Silver Flute W20RC38-08
Fig 1 shows the difference between the low pass slopes of the Silver Flute W20RC38-08 and the Albatross bandpass subwoofer. Measurements were made with the 250Hz low pass filter attached.
It is obvious from these measurements that there is a substantial amount of the Silver Flute W20RC38 bleeding into the Fountek FE85 midrange. The result of this harmful mix is that blurness or veil in the vocals. The more the bass woofer is kept out of the midrange, the clearer the midrange will be.
Fountek FE85 Bandpass Response
Fig 2 – Fountek FE85 with Bandpass Network (250Hz HP, 2.5kHz LP)
Now that the bass is taken care of, it’s time to have a look at the critical midrange. Fig 2 shows the bandpass response of the Fountek FE85. It starts with a 250Hz high pass that effectively blocks the bass from entering the FE85. At the treble end is a 2.5kHz low pass for handing over to the tweeter. In addition, I had to install a Baffle Step Compensation network to tame the rising upper midrange.
Fountek FE85 ToneBurst Energy Storage
Fig 3 – Fountek FE85 with Bandpass Network
Fig 3 is the Stored Energy of the Fountek FE85 with it’s bandpass network. It is generally very clean except for some stored energy from 1kHz~4kHz. It is possible that these light blue slices are sound waves diffracting at the edge of the enclosure.
Kingfisher2 Frequency Response
Fig 4 – Summed Response of Albatross Sub, Fountek FE85 and Peerless DX20BF00-04
Fig 4 is the summed response of the Albatross bandpass subwoofer with the Fountek FE85 and the Peerless DX20BF00-04. Measurements are smoothed at 1/24th octave for easier viewing. Disregard the huge notch at 150Hz. It’s caused by a floor bounce anomaly in my lab.
What is interesting is the response below 100Hz. This is where the Albatross bandpass subwoofer comes into play. The bass response is flatter whereas the Silver Flute W20RC38-08 is peaky. Both measurements were taken in the same room. I did not apply any special bass treatment like bass traps for the bandpass measurements.
Kingfisher2 Cumulative Spectral Decay
Fig 5 – Kingfisher2 Waterfall
Fig 5 is a highly magnified view (t=1.00 ms) of the Kingfisher2 waterfall. As in the previous version, the Kingfisher2 shows negligible artifacts from 4kHz upwards.
Fig 6 – Kingfisher2 Spectrogram
The Spectrogram in Fig 6 shows some burst of energy from 1kHz~2kHz. This is consistent with the stored energy shown in Fig 3.
It is not often the directivity of loudspeakers are revealed. In the Kingfisher2, I decided to plot the horizontal and vertical directivity to give readers an idea of the dispersion patterns. My design goal is to achieve +/- 25° in the horizontal plane and +/-15° for the vertical.
Kingfisher2 Horizontal Directivity
Fig 7 – Kingfisher2 Horizontal Polar Responses
In order to generate the polar plots, I took six measurements of the Kingfisher2 at angles of 15° each (Fig 7). The Black plot at the top is the one on tweeter axis and the lowest plot is 75° off-axis. With these plots, OmniMic then generates the Polar display (Fig 8).
Kingfisher2 Horizontal Directivity
Fig 8 – Kingfisher2 Horizontal Polar Plot
My frequency bandwidth of interest is from 1kHz~20kHz. Reason being that it gets omni directional below 1kHz.
We can see from the polar plot that most of the energy (Red-0dB) is within a +/-25° window. Frequencies above 10kHz falls off gradually to -10dB. This is the natural response of the Peerless DX20BF00-04 tweeter. If I don’t want this fall-off, I will need to apply an EQ network. That will extend the Red to 20kHz.
What this Horizontal Directivity plot tells us is when we are seated with our ears at the same height as the tweeters, moving left 25° or right 25° results in no change in the music. Even moving as far out as 50°, we will still hear almost the same sound. The difference is in the treble. At 6kHz and above, they get softer. Instruments like cymbals are softer but the vocals are still heard at the same intensity.
Now, what happens when we squat or stand up. For this, we need to look at how the speaker behaves in the vertical plane.
Kingfisher2 Vertical Directivity
For the vertical dispersion, twelve measurements were made at 15° intervals, 0~75° and 0~minus 75°. This will give a total coverage of 150° in the vertical plane. I spliced the two plots on the 0 axis and merged them to one for easier viewing.
Kingfisher2 Vertical Directivity
Fig 9 – Kingfisher2 Vertical Polar Plot
Unlike the horizontal polar plot, the vertical dispersion is not symmetrical. When we are standing up (0~75°), there is one green patch stretching from 2.5kHz~3.5kHz. This indicates that there’s a null at these frequencies. This null is recorded in the dispersion plots at about 2.7kHz (Fig 10).
Kingfisher2 Dispersion Plots (0~75°)
Fig 10 – Kingfisher2 Dispersion plots at 15° intervals
If we are squatting (0~minus 75°), the intensity at about 2kHz is the strongest. This indicates peaking with a downward tilt. So, what is happening?
Fig 11 – Sound propagation Pattern
Fig 11 is a good example of what the Kingfisher2 radiation pattern looks like in the vertical plane. Around the crossover frequency, the sound is tilting downwards. This is caused by a combination of factors. Most obvious is the vertical displacement of the acoustic centers of the tweeter and the midrange. This is compounded by another displacement on the Z-axis.
One solution in arresting this downward tilt is to align the acoustic centers of the tweeter and the midrange. This is commonly done by adding a step on the front baffle for the midrange driver. Another way is to tilt the top of the front baffle backwards.
Both these methods involve re-working a rectangle box. There is an easier way. Simply invert the box so that the tweeter is now below the midrange driver. With this inversion, the crossover lobe will now be shooting upwards towards the ceiling instead of the floor. If the tweeters are slightly lower than your ears in your seating position, which is quite common, it will work out just fine.
Looking at Fig 9 again, we can see the Kingfisher2 doesn’t quite meet my vertical polar target of +/-15°. However, this is a not really an issue for home use. Most of the time, we are seated with the loudspeakers in front. We won’t be standing or squatting.
I am not suggesting that dispersion properties of a loudspeaker are irrelevant. No. It depends on how the speakers are used. For example, the horizontal and vertical directivity are vital when the loudspeakers are used in a lecture hall. Students must be able to hear the lecturer clearly when seated anywhere.
The Kingfisher2 is quite an easy load for modern day power amplifiers. Generally, it’s 8Ω. When the tweeter comes in, it drops to 3.5Ω at 3.5kHz. For those who are uncomfortable with this, I believe there is an 8Ω version of the DX20BF00 that will be available soon. Needless to say, the crossover will have to be modified for this change.
The electrical phase of the Kingfisher2 is quite impressive. It doesn’t deviate more than +/-15º from 200Hz~20kHz. Below 200Hz, the deviation is +/-25º.
Kingfisher2 Sound Quality
The Kingfisher2 is the kind of sound that I strive for in all my designs. It exhibits an ultra clear midrange, a quality which I place a very high value on. I find that if the midrange has excellent clarity, one need not play loud to make out the vocals or instruments.
The bass character of the Albatross bandpass subwoofer is totally different from the direct radiating Silver Flute W20RC38-08. Bass is now tighter resulting in better articulation.
As for the treble, I’m still astounded by the quality from a $10 Peerless DX20BF00-04 tweeter. I guarantee you will not cringe when the highs come on.
Note: Unless otherwise stated, all measurements were made with the mic at 36 ins, tweeter axis. Impulse Window=5ms. No smoothing applied.
June 5, 2019Projects