In this blog post I test Altec 288-8G Compression Driver with Altec 805B Multicell.
I borrowed the horn from my brother.
Specification
Compression Driver
- 2.80" Diameter Voice Coil (71mm)
- Aluminum diaphragm
- 7.1" outside diameter for compression driver body
- 29.3 lbs compression driver weight (13.3 Kg)
- Alnico magnet
Horn
- 40 x 80 coverage pattern
- 500Hz Cutoff frequency (Fc)
- 62cm wide x 34cm tall x 55cm deep
Measurements
Frequency Response
I measured the horn with a 2m mic distance. Above 1kHz my measurement is gated to 4ms using 1/24 octave smoothing. Below 1kHz the repones is ungated and 1/3 octave smoothing applied. Generally we see a reasonably smooth response with a midrange emphasis centered at about 2kHz. The horn loads strongly down to 400Hz with very little first order reflection near the the horn's cutoff frequency which is published as 500Hz. The diaphragm is breakup free until 13.5kHz.
Below is the horizontal off-axis colored polar map using the same 2m mic distance.
Characteristics
- 90 degree coverage (500Hz-1.5kHz)
- Widens to 120 degrees (1.5kHz-17kHz)
- High frequency output drops when measuring directly in front of each cell
Time Domain
Conducted in the near field using a 20cm mic distance.
The burst decay shows some stored energy in the treble region.
Distortion
I measured distortion with the 20cm mic distance. SPL level was set at 85dB and 95dB referenced at 1m. I set a fixed resistor L-pad using a 20w 7.5ohm and 1.5ohm (R1 & R2) which provides about -15dB attenuation. This brought the amplifier up out of its noise floor (Hypex FA500). Above 1kHz we see H3 and H4 at very low levels while H2 dominates. H3 for the 95dB signal is only 0.015% for the 1kHz region.
Multitone is shown below for the same 85dB and 95dB test signal levels.
The 85dB test signal shows 70dB dynamic range for the 1.5kHz region reducing to 60dB for the 10kHz region.
The 95dB test signa is unchanged for the midrange but we see 10kHz reduce to 50dB dynamic range.
Contour Circuit
The horn has a strong midrange presence along with some coloration near the horn's cutoff frequency. So I decided to develop a basic high pass filter along with a contour circuit. I settled on a LR3 high pass filter to eliminate the coloration in the 500Hz region. I then implemented an parallel LCR contour to bring down the 2kHz region. The raw response is shown in red, the blue response is with the LCR, and the green response is with the high pass filter. 
Subjective Listening
With contour circuit No.2567 in place, I've summarized my listening impressions.
- Spatial and enveloping treble due to 120 degree wide coverage
- Despite the class D amplifier, the sound had some typical H2 smoothness.
- Missing upper treble detail
- Some horn coloration across its bandwidth
- Strong presence on vocals (voice modulation easy to follow)
- Some masking of soundstage depth
Overall the 288-8G with the 805B horn offers an entertaining and enjoyable listening experience if you can get past the slight horn coloration which masks overall clarity. A super tweeter may offer some resolution improvement for the upper treble, but the coloration through the midrange would still exist. For example the burst decay is shown below with the contour circuit in place. Perhaps there are methods to improve this which is beyond the scope of this post.
