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Osprey-II Revisited I designed the Osprey-II with one objective in mind and that is for best Vocal Clarity. It is meant for use in Colleges, Universities and Live Bands. It is even good enough for Home Theater and HiFi. Since the Osprey-II is mainly for Voice Reinforcement, there’s no necessity to produce low bass. A compact closed box of 27 liters is sufficient. It is small and light enough to be mounted on a tripod. For live musical performances, it can sit on top of a bass bin. This will extend the low frequencies to 50Hz. Working with the Eminence Beta 10CX is not without difficulties. The cone breaks up at about 2kHz. This results in a serious of peaks, the first being the most offending.
The Black plot in Fig 1 is the RAW response of the 10CX woofer. The largest peak is at 2.5kHz. To ensure that this peak does not interfere with the summed response, I rolled it off with a 18dB/oct low pass at 1.5kHz (Blue plot). I will leave it as such for the time being. If the peak is still a problem, I will have no choice but to apply a notch filter.
Fig 2 shows the Selenium D220ti with a high pass filter of 1.5kHz (Red plot). The two drivers cross acoustically at close to 1.5kHz, which is a good sign. What is disconcerting with the Selenium D220ti compression driver is the series of notches. The first and deepest notch centers at about 6.7kHz. Subsequent but smaller notches are seen at 12kHz and 15kHz. I believe these are caused by sound bouncing off the walls of the 10CX horn.
Fig 3 is the summed response (Black plot) of the Eminence 10CX with the Selenium D220ti. The two drivers sum smoothly. We can clearly see the effects of the 2.5kHz peak of the 10CX cone breakup. It created a notch at 2.5kHz. The only solution to avoid this notch is to use a LCR network to remove the 2.5kHz peak.
Fig 4 is the final frequency response of the Osprey-II. It doesn’t look very pretty like HiFi drivers but the notches are quite benign. I would be very concerned if the situation is reversed, that is instead of notches, they end up as peaks.
To check on the time alignment of the 10CX woofer and the D220ti, I rewired the compression driver back to absolute phase. This resulted in a nice, deep notch at about 1.5kHz. This indicates the Osprey-II is very close to being Time Aligned. All it takes is for the 10CX woofer to be delayed by a few milliseconds and the two drivers will be time aligned. But in it’s current form, it’s perfectly usable. This is one of the main attractions of using a coaxial. There is marginal delay in the two drivers. Another huge advantage is that both drivers lie on the same z-axis. Since there is no vertical displacement (y-axis), the issues associated with a horn on top of a woofer doesn’t exist.
Before I touch on the step response of the Osprey-II, let’s have a look at the individual drivers first. The plot on the left is the step response of the 10CX woofer with a 18dB/oct low pass filter. For a 10″ woofer, it’s actually quite fast. I’ve seen worse. The plot on the right is of the Selenium D220ti with a 12dB/oct high pass filter. Note that the selenium is wired in reversed phase.
Fig 7 is the step response of the Osprey-II. Bear in mind that this step response is with the 10CX woofer and the Selenium D220ti together as one speaker, with the crossover network in place. The rising edge is a bit jagged but the Osprey-II still produces a fast transient. What surprises me is the decay. The Osprey-II decays quicker than the 10CX woofer alone. Compare it with the 10CX step response in Fig 6 and the difference is obvious.
The Waterfall plot of the Osprey-II (Fig 8) shows the treble artifacts exhibiting longer decay when compared to HiFi tweeters. Whether this is an issued is left to be seen.
The Toneburst Energy Storage in Fig 9 is another presentation of the decay. In this plot, the stored energy starts at 2kHz and goes all the way up to 20kHz. From 10kHz onwards, there’s more stored energy.
The Spectrogram in Fig 10 gives a clearer picture of the treble decay. Fortunately, they are very short, lasting not more than 1.5 msec. What all these measurements indicate is the horn rings. Another term for this is coloration.
Fig 11 is the Harmonic Distortion of the Osprey-II at 85dB. This is reflective of HiFi playback. Generally, 2nd (Red) and 3rd (Violet) harmonics are about -55dB below the fundamental. No anomalies are recorded.
When the SPL is increased to 90dB, the harmonic distortion increases accordingly. From 1.5kHz onwards, it is the 2nd harmonics that dominate which is to our advantage as even harmonics are more pleasant to the ears.
Lastly, Fig 13 is the impedance of the Osprey-II. The lowest is 7Ω at 200Hz and 1kHz. The electrical phase is only +30° at 2kHz. Overall, the Osprey-II is a friendly load for power amplifiers. Osprey-II Sound Quality One of the first albums I used to test the 10CX horn is Acoustic Highway by Craig Chaquico. In this album, the Washburn acoustic guitar went through electronic processing for it’s distinctive sound. It doesn’t sound like a normal acoustic guitar. Neither does it sound like an electric guitar. It’s somewhere in between. If one listens carefully, the harmonics in the notes is at another level (Return of the Eagle). To top it off, the mix is superb. The layering is so well executed that the artist is never buried in the mix. Craig’s guitar stands out, regardless of which track. Diana Krall is next. In “Stop this World”, the sibilance sounded just right. If the crossover is bad, it will sound like a jet engine. In the next track, the Girl in the Other Room, you can tell that she was close miked during recording. Same for Stronger Woman (Jewel). For male vocals, I had on The Very Thought of You (Nat King Cole). This is a 1958 recording. Simply amazing that it sounds so good. And this was done way before all the digital technology we have today. Even earlier is Innamorata (Dean Martin-1956). UFO Tofu (Bela Fleck and the Flecktones) is noted for fast transients and wide dynamic range. In Nemo’s Dream, the band sounded very tight. Their timing was impeccable. What is eerie is the first track, The West Country. There is an ominous tak, tak, tak in the background that I never noticed before. Having auditioned the Osprey-II over a few days, I honestly cannot hear the ringing in the horn. But it’s not the ringing that I’m concerned about. It’s the dreaded honking and harshness that some horn speakers exhibit. The Osprey-II is free of those flaws. I made sure of that. She may be meant for live applications but I designed her at HiFi level. She is as smooth as can be. Clear as crystal. Absolutely no muddines in the mids. The Eminence Beta 10CX coaxial is meant to be an affordable 10″ coax. To be able to squeeze this sound quality out of it without costing an arm and a leg is an achievement in itself. For prettier horn measurements, one would have to spend 3~4 times more. Even then, whether it’s audible is another matter. Osprey-II Bass Reflex by Bruce Hatch Note: Unless otherwise stated, all measurements were made with the mic at 36 ins, tweeter axis. Impulse Window=5ms. No smoothing applied. |
July 17, 2019PRO DRIVERS, Projects
Fig 1 – Black plot = RAW. Blue plot = 18dB Low Pass. Nearfield below 500Hz.
Fig 2 – Blue plot = 10CX with Low Pass. Red plot = Selenium D220ti with High Pass
Fig 3 – Summed Response with 10CX Low Pass and Selenium D220ti High Pass
Fig 4 – Osprey-II Frequency Response
Fig 5 – Null Response
Fig 6 – 10CX woofer and Selenium D220ti Step Response
Fig 7 – Step Response of Osprey-II
Fig 8 – Waterfall of Osprey-II
Fig 9 – Toneburst Energy Storage
Fig 10 – Spectrogram
Fig 11 – Harmonic Distortion at 85dB
Fig 12 – Harmonic Distortion at 90dB
Fig 13 – Osprey-II Impedance