Betsy K in sealed box
I've been looking at the boxes with the Betsy K drivers for months, and decided it's finally time to get them into some boxes as I had promised Paul when I got them.
The boxes in question are my old converted Speakerlab boxes which have had a pair of the Altec 755C drivers in them for many years. Oldtimers may remember them from the coverage of NYNoise 2001. (Almost 10 years ago???) Here is the Enjoy The Music coverage of that event: Enjoy the Music.com New York Noise Show In the top picture they are the black ones - the right one is third from the right. (Yes, that's me on the right, with the estimable Jeremy Epstein of Brooklyn, NY.) My speakers are the only ones with grill cloths - you will see why in a bit, but I can assure you it's not because I'm a meticulous craftsman.
I'm going to do this report in chunks, so I guess that's the end of the historical part. More to come shortly.
Here is the bedroom system those speakers reside in. (You can see the left grill cloth attachment has not done well in the past 10 years.) It mostly does video duty with the Toshiba TV, Tivo Premiere, Sonic Impact T-amp and the Altecs. There's also a Panasonic DVD player (whichever was the one to get in 2000) and a Mac mini. Music comes out of the Mac through the USB output into a Hagtech HagUSB (put together from the kit, of course) into a Lite DAC-AH. The signals go through a very high-end Radio Shack switcher into the Sonic Impact.
Well, my efforts to attach the pictures failed, so instead I will provide the link to picasaweb album which shows the bedroom stereo, the speakerlab with the Altec, the BetsyK which replaced it, and the BetsyK in place. Picasa Web Albums - Peter - Speakerlab 75...
Before I put the Betsy K drivers in I pulled out my trusty Radio Shack sound level meter and my Stereophile Test CD 2 and did measurements of frequency response. At first I had the switch on the "A weighting", and didn't like how it drooped in the bass, but when I realized my mistake I switched to "C weighting". then after I swapped the drivers I did it again (C weighting only), and then graphed it out. A scan of the graph is attached below. The 755C is in blue and the Betsy K in red. (I left the data points from the A weighting in pencil for comparison.)
I don't put too much stock in these measurements - if I understand correctly, the A and C weighting should be about the same over 1k, but they aren't. Also, sometimes I had to change the range on the meter when it went up or down 10dB, and since it's not exactly a precision instrument I'm not confident it doesn't vary with that change. I just put the meter on a stool in front of one of the speakers, about 3 feet away, but had both speakers going. I also left the grill cloths on. So take them with a big chunk of salt. (I just took a look at the graph on the Wild Burro site and at least I see similar characteristics: Wild Burro Audio Labs - Fullrange Speakers - The Betsy & Betsy-K Drivers , so I guess it's not TOO bad.)
That being said, the differences are interesting. I guess the bass rolloff they both show is a function of the narrow baffle design - maybe I'll try a BSC sometime.
The treble rolls off on both, but the Betsy K rolls off much less - there's lots more there over 10k. (Yeah, I know I'm 50 years old, but I still can hear most of the way up there.) I have a pair of the Planet 10 phase plugs, but I wanted to try the drivers stock first. They also have the same droop over 100Hz - I don't know if that's also an enclosure thing or something else.
The other interesting thing is the 5 or so dB difference in the 1k to 8k range (except the Betsy K graph shows a peak about 4k). When I finally get some access to be bedroom for music listening that's one area I'm curious to listen for.
If anyone has any thoughts/suggestions/ideas (or can get those pictures to post here), please feel free to pipe up.
More developments to follow.
Thanks for posting your project! Its a little scary to be compared to the 755, but it sounds like they are doing alright. It is tough to eliminate the need for BSC with many boxes, especially narrow ones. Do you listen to them off axis a bit? It sounds like you measured dead on? It is quite possible that you'll get less treble off axis, and then the whole thing will look flatter. The box looks decently sized, so I presume that 80hz bump has something to do with room/placement. It could be room modes contributing to that hole at 200-300hz, but I think it is more likely that we're seeing baffle step followed by a bump below 100hz. Since you are using a computer, you might be able to implement some form of digital BSC, at least to see how it sounds.
Wild Burro Audio Labs - DIY Full Range Speakers
the Radio Shack spl meters are for information only, not for serious measurement
if you search web, you will find info about both of them, analogous and digital and the corrections you need to apply
there are circuit modifications for both available somewhere on the web, the analogous is better, its hi fr response can be corrected quite well
digital one has much poorer electronics and the modifications in upper reagon are limited, it will never be acurate
low fr are rolled of on both meters, can be corrected by simple cap replacements
for more serious measurement check out Holm Impulse, free software
your low frequency response is different from Pauls, maybe its a box and room effect, because Betsy is flat to 50Hz and rolls of smoothly, mine measurement on Betsy WOW is shown here
offcourse high fr part must be different
That graph looks, to me, like it could be perfectly true in the bass. I don't think one can get to 50hz with the BetsyK in a sealed box. That extension looks about right for that size, and as far as room modes go, that bump isn't that bad. I've seen +/- 20db!
Wild Burro Audio Labs - DIY Full Range Speakers
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