I've restored a couple of Acoustic 807 loudspeaker for use with keyboards:
The low end comes from a pair of 12" drivers mounted behind a not-exactly-Bessel-curve horn structure and the backs of those drivers communicate with the port at the bottom.
As you might expect, these cabinets don't have much to give you below ~120Hz and whereas I'm running them with a one-third-octave EQ in the signal path, I'm concerned about just trying to compensate with amp power. It has been suggested to me that one way to get roughly an extra octave of bass would be to push the two cabs side-by-side to increase the width of the aperture but I was wondering if perhaps there were another (small-a) acoustic approach.
I've noticed that some models of Altec-Lansing Voice Of The Theater cabs have flat baffles that extend out to the left and right. Do any of you have experience with adding side baffles to extend bass response? I'm envisioning free-standing panels about 12"-14" wide, cut on top at a slant with the lower side adjacent to the cabinets about where the blue panel is. They could sit flat with the front of the cabs, angled so as to extend the angle of the cabs' horn structures, or somewhere in between that and flat to make the overall shape more of a crude Bessel approximation.
The low end comes from a pair of 12" drivers mounted behind a not-exactly-Bessel-curve horn structure and the backs of those drivers communicate with the port at the bottom.
As you might expect, these cabinets don't have much to give you below ~120Hz and whereas I'm running them with a one-third-octave EQ in the signal path, I'm concerned about just trying to compensate with amp power. It has been suggested to me that one way to get roughly an extra octave of bass would be to push the two cabs side-by-side to increase the width of the aperture but I was wondering if perhaps there were another (small-a) acoustic approach.
I've noticed that some models of Altec-Lansing Voice Of The Theater cabs have flat baffles that extend out to the left and right. Do any of you have experience with adding side baffles to extend bass response? I'm envisioning free-standing panels about 12"-14" wide, cut on top at a slant with the lower side adjacent to the cabinets about where the blue panel is. They could sit flat with the front of the cabs, angled so as to extend the angle of the cabs' horn structures, or somewhere in between that and flat to make the overall shape more of a crude Bessel approximation.
Attachments
Many have used added "wings" with electrostatic panel speakers to extend the lows.
https://www.stereophile.com/content/sound-lab-1-electrostatic-loudspeaker-page-3
https://www.stereophile.com/content/sound-lab-1-electrostatic-loudspeaker-page-3
I've built a pair of potato-grade baffles to perform tests with.
I have a venue in mind - hopefully I can get permission to use it - to set up a measurement mic and run it into a spectrum analyzer app on my MacBook, then blow white noise through the speaker and see if adding the side baffles kicks up the low end any. I'll try them both in a plane with the open front of the cabinet and also canted in at an angle so they form more of an extension of the bass horn.
There's a church near here with a large, high-ceilinged, and carpeted fellowship hall that seems like it'd be a great place to run this experiment.
I have a venue in mind - hopefully I can get permission to use it - to set up a measurement mic and run it into a spectrum analyzer app on my MacBook, then blow white noise through the speaker and see if adding the side baffles kicks up the low end any. I'll try them both in a plane with the open front of the cabinet and also canted in at an angle so they form more of an extension of the bass horn.
There's a church near here with a large, high-ceilinged, and carpeted fellowship hall that seems like it'd be a great place to run this experiment.
If using a standard octave binned RTA (like 1/3 octave), pink noise is the desired source. But some apps also do narrow/frequency based binning which does require white noise, so it's a little more confusing than it used to be. More info here if you you're unfamiliar with the differences.
https://www.softdb.com/blog/what-is-white-noise/
"White Noise VS Pink Noise: A Subtle Yet Significant Difference"
It's also worth noting that white noise has more high frequency energy, so it's harder on tweeters.
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Thanks for the linkage. I was going to watch the measurement mic signal in near-real time using the iSpectrum and/or the REW apps on MacOS. iSpectrum only displays the frequency axis linearly.
I don't think I need to crank these up too loudly for the purposes of my testing so I'm not very worried about blowing anything especially since the crossover sends the highest of the highs to the piezos. I'm not sure what the effective crossover freq is but the sense that I get from the crossover design is that Acoustic was relying somewhat on the piezos crossovering themselves.
On this subject, I've been careful to always unwire and rewire the components so as to preserve the phase of the driver wiring but, other than keeping the pair of 12"s and the pair of piezos in phase with each other, do I really even need to care? I've thought about perhaps switching the phase of the mid horn to see if that increases the radiated power within its range. After all, my objective with the baffles is to get as much as I can out of amplifier power without resorting to boosting bands with the EQ and it occurred to me that driver-to-driver phase might make a difference.
I don't think I need to crank these up too loudly for the purposes of my testing so I'm not very worried about blowing anything especially since the crossover sends the highest of the highs to the piezos. I'm not sure what the effective crossover freq is but the sense that I get from the crossover design is that Acoustic was relying somewhat on the piezos crossovering themselves.
On this subject, I've been careful to always unwire and rewire the components so as to preserve the phase of the driver wiring but, other than keeping the pair of 12"s and the pair of piezos in phase with each other, do I really even need to care? I've thought about perhaps switching the phase of the mid horn to see if that increases the radiated power within its range. After all, my objective with the baffles is to get as much as I can out of amplifier power without resorting to boosting bands with the EQ and it occurred to me that driver-to-driver phase might make a difference.
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You want to use pink noise- white noise sounds like hiss, pink noise is filtered white noise to give equal energy per octave.
"Barn doors" won't extend the low end, but will increase the forward output a bit, your "potato-grade baffles" probably around 1.5 dB. Hardly enough to bother with, as they will increase the mid bass more than the bottom.
The big theater "barn doors" can give over 3dB increase- a big deal back when a 50 watt amp could now sound like 100 watts without power compression in the speakers.
I've done quite a bit of testing outdoors to learn the effects of adding wave guides/barn doors:
https://www.diyaudio.com/forums/subwoofers/204472-multiple-cabinet-combined-response.html
https://www.diyaudio.com/community/threads/horn-extender-wave-guide-for-th.184986/
As far as low end, you won't get any below Fb (cabinet tuning) in your cabinets. Boosting below Fb just wastes power and makes the speakers "flap".
At Fb, the cone excursion reduces, above and below it increases.
That said, cabinets of the era of your Acoustic don't have much response at Fb- if it's like the Yamaha 4415H, it will be around -15dB at Fb (the impedance and excursion minima). The speaker cones don't have enough excursion to allow anything near that kind of boost.
Speakers of that era usually had only about 2mm Xmax (linear excursion).
12" drivers capable of 40-1500Hz response with 6mm Xmax, and subwoofers with 30-160 Hz response with 12mm (or more) Xmax are common now.
Doubling Xmax allows an increase of 6dB, a 2x12" sub with long throw drivers about the size of one of your cabinets crossed at 100Hz could give you flat response to below 40Hz with enough output to keep up with the Acoustic at full tilt boogie.
Time for a subwoofer or two
Art
@weltersys Art - I really appreciate this information, thank you. My intended application is keyboards (mostly plugin sources - pianos, electric pianos, many synthesizers, Hammond) and just at low volumes in my practice room and a crude scoop set up on the graphic EQ, the 807s sound pretty good. I've posted in other threads about how one of the original Heppner horns was blown out and has been replaced with a cheap unit that doesn't sound like (or as good as) the Heppner (there's also a thing about the originals being roughly 16-ohm (DC resistance is 15 ohms) and no one-piece replacement horn I can find is available in 16-ohm) but I hope I can EQ out some differences. It sounds like at the very least I should anticipate replacing the original 12"s, which I can do fairly economically with these Goldwoods but I can't find an Xmax spec on them. These GRS speakers can also be had in 4-ohm and have an Xmax of 8.5mm. The original 12"s are 16-ohm in parallel; I haven't found any 16-ohm 12s yet - but I can put those GRSses in series instead.
It sounds like if I want to go forward without carrying any additional equipment (like a sub and a perhaps a separate power amp for it) I really should replace the original 12"s if I want more low end. I'll still run my experiment with the side baffles.
My Denon test CD does have a pink noise track so I'll use that instead of the white noise track.
It sounds like if I want to go forward without carrying any additional equipment (like a sub and a perhaps a separate power amp for it) I really should replace the original 12"s if I want more low end. I'll still run my experiment with the side baffles.
My Denon test CD does have a pink noise track so I'll use that instead of the white noise track.
I've ordered a set of four of these GRS speakers in four-ohm and they're good up to 800Hz. Parts Express has another model, the GRS 12SW-4HE "High Excursion" but it tops out at just 500Hz - I didn't think that was really how I wanted to go. I do not know where the lo/mid crossover point is in the 807 so I'm a little worried that the cab might have a dip but I'll know more when I measure. It's just that after I thought over what Art said, I felt like just EQing more bass into the cabs will only damage the original Heppner 12"s anyway so I'd might as well replace them now instead of later. At low volumes and just running recordings through them (Steve Hackett playing Genesis songs live with lots of Moog Taurus pedals) at low volumes is sounded pretty good with my current EQ settings so I'm feeling pretty positive about what this would be like not having to push the leftward sliders up this much on the EQ.
You can try some things:
Tune the reflex port lower by putting some wood into it making the diameter smaller
Add in line 700 to 1000 microfarad bipolar condenser and see what happens, sometimes you get a lower bass and you filter below port resonance.
Do some damping on all internal surfaces in the boxes well fixed with glue like with 3-5cm thick foam. This helps make the mid frequencies be clearer and less loud - you then hear the bass better
Most effective of all is putting the speaker close to any room boundary - you get more bass for free - better than with any wings! Most effective is corner placement, it's like +10db in the bass
If you tune the port lower you can EQ the box down to 70 or 60 hertz. For two 12 inch this is manageable without problems. Don't forget to put a filter cutting off below port resonance
Against distortion: try the magic and put a 20 watt 1 ohm resistor in line with the box. You get more clear bass (and mids)
Tune the reflex port lower by putting some wood into it making the diameter smaller
Add in line 700 to 1000 microfarad bipolar condenser and see what happens, sometimes you get a lower bass and you filter below port resonance.
Do some damping on all internal surfaces in the boxes well fixed with glue like with 3-5cm thick foam. This helps make the mid frequencies be clearer and less loud - you then hear the bass better
Most effective of all is putting the speaker close to any room boundary - you get more bass for free - better than with any wings! Most effective is corner placement, it's like +10db in the bass
If you tune the port lower you can EQ the box down to 70 or 60 hertz. For two 12 inch this is manageable without problems. Don't forget to put a filter cutting off below port resonance
Against distortion: try the magic and put a 20 watt 1 ohm resistor in line with the box. You get more clear bass (and mids)
The port is an across-the-bottom slot that's 23" wide, 1.5" high, 15" deep at the bottom (all the way to the back wall of the cab), and 10" deep at the top, meaning that there's a 5"x23" opening between that and the "bin," if you will, that holds the two 12"s one right above the other. The wedge-shaped open areas to either side of the cab communicate all the way up and down (25-1/4" high) and there's a divider running horizontally between the 12"s that goes all the way front to back and side to side. At the very least, I can get damping on the back wall and the horizontal divider. So I really can't do much to "tune the port" but I have some ability to add damping; I probably should leave holes in the damping on the back wall where the magnets of the speakers will be because there's not a lot of clearance (the new GRS speakers will fit).
I don't know just what the port or cab frequencies are. One thing I thought of was to take the 12's out of one cab, run pink noise through the other cab (maybe lowpass-filter it - my power amp can do that from 3kHz to 50Hz), use the EQ and measurement mic to get a flat response (with or without highs rolled off), and then roll up the cab with the 12"s removed until the measurement mic is inside the cab - see where a hump forms in the response.
I don't know just what the port or cab frequencies are. One thing I thought of was to take the 12's out of one cab, run pink noise through the other cab (maybe lowpass-filter it - my power amp can do that from 3kHz to 50Hz), use the EQ and measurement mic to get a flat response (with or without highs rolled off), and then roll up the cab with the 12"s removed until the measurement mic is inside the cab - see where a hump forms in the response.
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Okay, here's what's happened. I replaced the two pairs of original Heppner 12"s with the aforementioned GRS speakers. I created a track in Audacity to wig-wag white or pink noise from one side to the other and was immediately hit by how different the midrange horns sounded - something I'd hoped to be able to EQ my way out of (1/3rd-octave two-channel Alesis). I set about revisiting the chore of sourcing replacement 4x10" horns just to get them identical and was prepared to even make new crossovers from scratch when suddenly I remembered I've already been through this!! Early on in this process I tried two different pairs of readily available horns (actually, the only two!) and even took the metal-horned pair to a machine shop to have the long sides of the mounting flange cut down in a mill so they'd fit, but at the time I didn't care for how they sounded compared to my one good Heppner horn with music playing through the cabinets and I was also disturbed by their tremendously lumpy frequency response (including a crazy -45dB-down-from-peak notch at 12.5kHz). Still, if I went with them they would solve the inequality between cabs at a stroke, so I put them in and worked with a measurement mic, the REW app, and the Alesis EQ (albeit at close range in my basement instead of a nice big open room) to try to flatten out response best I could. I wound up with a pretty scattered bunch of sliders on the EQ but with music program they sound pretty decent...but with the keyboards they're better than pretty decent and I'm excited at the prospect of taking at least part of the rig to a big enough room where I can turn stuff up loud.
I didn't just ramp up the lows with the EQ; I followed what REW was telling me so if for instance I were way over my midline at 80Hz I turned that slider down. At least with the GRSses I know I can push a lot more power at the low end than I could before withough damage and I still plan to see if I can get range extension with my batwing side baffles.
I didn't just ramp up the lows with the EQ; I followed what REW was telling me so if for instance I were way over my midline at 80Hz I turned that slider down. At least with the GRSses I know I can push a lot more power at the low end than I could before withough damage and I still plan to see if I can get range extension with my batwing side baffles.
Okay, any of you naysayers who questioned the effectiveness of my "batwing" side baffles will be interested to know that I have finally run far-field tests with a measurement mic and REW and I am now prepared to report that the side baffles have absolutely no effect at all!!! None! It's like they didn't even exist!! I'm so glad I didn't paint them, do any more than glue and tape rigid cardboard angles to them as stands, or make a second pair, and now I've got more for the scrap wood collection!
Pushing the two cabinets together side-by-side, however, did produce a notable bass extension.
A volunteer at a nearby church let me drag all this stuff into their fellowship hall to make the measurements. I had hoped to use this particular hall because it was carpeted but in recent years they had replaced the carpet with tile and I was told that the room became so live that it was unbearable. They tacked drapes to the upper walls and hung a bunch of hospital room curtains from the lean-to ceiling to calm it down.
After using REW's sweep captures to set my 1/3-octave EQ, music blown through them at volume sounded quite good. And replacing those 12"s with modern drivers with much more Xmax made a huge difference. I still need to establish the Fb so I can EQ down everything below that to conserve amp power and preserve voice coils.
With music program the 807s were plenty loud from across the room with the Crown XLS1000 mostly running about 20dB down. I feel like I could run a better (if heavier) amp at considerably less power for my purposes if I wanted.
Pushing the two cabinets together side-by-side, however, did produce a notable bass extension.
A volunteer at a nearby church let me drag all this stuff into their fellowship hall to make the measurements. I had hoped to use this particular hall because it was carpeted but in recent years they had replaced the carpet with tile and I was told that the room became so live that it was unbearable. They tacked drapes to the upper walls and hung a bunch of hospital room curtains from the lean-to ceiling to calm it down.
After using REW's sweep captures to set my 1/3-octave EQ, music blown through them at volume sounded quite good. And replacing those 12"s with modern drivers with much more Xmax made a huge difference. I still need to establish the Fb so I can EQ down everything below that to conserve amp power and preserve voice coils.
With music program the 807s were plenty loud from across the room with the Crown XLS1000 mostly running about 20dB down. I feel like I could run a better (if heavier) amp at considerably less power for my purposes if I wanted.
Did you calculate the frequencies that would correspond to the length (width) of the speaker with the side extensions? I would certainly expect to see a difference, albeit probably in the 100s of hz range and not exactly in the bass region (if my maths is right). If playing 2 cabinets (both powered) I think you should ideally get +6db as you're doubling the area. Baudline (http://www.baudline.com/erik/bass/xmaxer.html) has a handy tool for such calculations.
After seeing almost identical spectra with the side baffles as without, whether they were set flat across the face of the cabinet or swung inward to more or less match the angle of the bass horn surfaces, I didn't bother calculating anything. I think that when the two cabs are pushed together, it isn't so much that you're doubling the area as doubling the width of the aperture - both the bass horn section and the bottom slot port. So depending on what my setup circumstances are when taking the keyboards out, I can push the cabs together and have more fun "stomping" the Moog Taurus pedal plugin or spread the cabs out for room-filling and stereo imaging purposes.