Passive Radiator

I’ve got an idea I’m going to try out soon.

I’ve got a tiny 15w Behringer combo practice bass amp with an open back. I don’t understand why it’s not sealed. It seems it’d have a way better bass response if it were.

I’m thinking about building a custom speaker cabinet to be powered by this small amp. (I’ll just disconnect the built in speaker and use the leads to power the external cab) It’ll be 100% sealed, filled with polyfil and possibly will contain a passive radiator.

I’m just wondering why the passive radiators aren’t used in small bass amps. They sure make little Bluetooth speakers sound good, why not our amps?

This will be a simple box made of wood or MDF. If it doesn’t work as a bass cab, I can just use the enclosure for another project.

Passive radiator yes? Or no? If no, why not?

There have been such things as passive radiator bass cabs, but the word on the talkbass forum seems to be that latest design ported enclosures work better.

Bass cab design is a world in itself with a lot of science involved.

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Originally Posted by JimC

There have been such things as passive radiator bass cabs, but the word on the talkbass forum seems to be that latest design ported enclosures work better.

Bass cab design is a world in itself with a lot of science involved.

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Yes, I recall asking about it on talkbass and got flamed, lol! I don’t remember getting a straight answer - taboo subject for some reason! I think I just need to do it and see how it sounds. It is just for a little practice amp.

From my Speaker Design Cookbook, passive radiators need to be used with a driver with a high cone mass, which can be achieved by adding mass to the speaker cone, if it doesn't have sufficient mass already. Passive radiators will extend the low frequency response (at the expense of losing high end from it), but high mass speakers are less efficient, so you lose maybe 3dB - 6dB of overall volume. So to get the same output level, you'll need at least double the amplifier power. You don't get something for nothing, so you're trading more low end for less output.

I'd guess that commercially available bass speakers tend not to be high mass (Qms is the parameter to look for) and this should be in the 7 to 10 region for a passive radiator to work properly. Looking through the Eminence bass speakers, there are a few with a Qms in this region, though not many, and most are in the 4 to 5 region. The high Qms speakers by nature have lower efficiencies than the lower Qms speakers, and the passive radiator would drop this efficiency even more, so they would need to be paired with much larger amplifiers to achieve similar outputs.

You need to remove a lot of material from the speaker enclosure to install a passive radiator, which then makes the enclosure weaker, so it needs more strengthening. Which then puts cost and complexity up. Porting a bass enclosure is a lot easier, and cheaper.

You get a few passive radiator designs in hi-fi and monitor speakers, where stage volume levels aren't required and mid an/or HF drivers can add in the higher frequencies the passive radiator reduces. You lose some of the drawbacks of ported design like port noise at high volumes, but you do gain a few others like increased group delay (frequencies being produced over a period of time rather than all at the same moment).

I'm just reporting stuff from the book, and have no real understanding of these passive radiator systems, so can't currently enlighten you any further.

BTW, the current Bugera BT108 has a vented back, rather than fully open one, so I'd imagine that it is acting as a basic port. The port size probably needs to be quite large to get the right response, and it looks larger because it's a small enclosure.

Attachment 37190

If you took that back off, then you'd get much less low end from the unit and you'd probably quickly destroy the speaker if playing at loud volumes as that bit of wood will still help reduce speaker excursion..

Remember that it's a product built to a very low budget, so you'll always get compromises with something like that.

It's something I meant to add earlier, but I wasn't getting a good connection to the forum. Yes. Simon is right, it is possible to design a bass cab with a very short but large port on the rear that looks for all the world like an open back. It doesn't behave like one though.
It's almost a rule of bass cabs that speaker area is the key to volume or sound. If you
a) have the space and
b) don't have to transport it
then the most cost effective way to get more out of that amp is to pick up an old school heavy bass cab, at least one 15 or two tens, and plumb it into that. In many areas things like old school 4x10s are available on the secondhand market for less than the cost of a single new driver, and you'll be amazed how much more a little low power amp will deliver through a big cab.

I’d guess that an 8” speaker will struggle to put out any fundamental frequencies of the first few notes on a bass, even with a ported design (or even a passive radiator) so I’m guessing the gap is probably more to open up the sound a bit and stop the small cab from sounding too boxy, whilst still providing some resistance to air movement and cutting down on low-frequency speaker excursion.

Interesting info gents, thank you!

Probably what’s going to prevent me from doing it is finding the right speaker. I’d like to go really small, like a 6” bass speaker. That may not even exist. An 8” Bass guitar speaker would need at least a 10” radiator because the radiators are supposed to be bigger than the driver.

I just might really go cheap and make a big MDF box for the existing speaker in my combo amp. I’ll remove the stock speaker and might just put it in a big sealed enclosure without the radiator.

The last thing I need start is another project but the way these things are built bugs me. I’m almost certain I can make that little rig sound better with the speaker properly installed in a big solid box. I could experiment with sealed vs ported, too.

You’d probably have to look at a PA speaker at 8” and certainly 6”. I had a look yesterday and neither Celestion nor Eminence do dedicated bass speakers smaller than 10”, so I suspect that will be the case for most manufacturers. However, a speaker is a speaker and there normally isn’t a huge difference between PA and Bass drivers in the 10” to 15” range anyway, though the PA ones are probably slightly more neutral in their voicing.

Let’s break some rules!

What if I got a 4” or a 6” full range speaker and used it. As long as I don’t overpower it and burn the voice coil, I bet that would work fine. Keeping in mind - this is a practice amp setup. Deep bass, inexpensive, innovative, and small.

I actually did that in the mid ‘80s, except it was a bunch of 6” full range speakers that I used with my Crate guitar amp. I built a cabinet out of plywood and wired the 8 ohm speakers together so they were 4 ohms the same as my existing 12” speaker. (I did something like that, I can’t quite remember.). Ahh, the 1980s....good times!!! The Radio Shack full range drivers sounded a lot better than the 12” “Instrument” speaker that came with my amp. I even played with a band at a house party with that homemade rig, it was great!

A bass would be a little harder on the speaker, but if it were in a sealed enclosure and wasn’t overpowered, that should work. I must confess, I’ve never used a full range speaker with a bass...they do work for guitars.

I need to get some free time and just build the damn thing and see how it sounds!

Phil Jones makes some wonderful bass cabinets using smaller drivers like you're suggesting. One difference is that the drivers he uses are uniquely voiced to his specifications, and the enclosures are designed around them. His stuff is also quite expensive.

Those small bass amps were never meant to produce solid low end. They are made to provide a little volume for your bedroom practice sessions. You are not likely to find one that is a tone monster. I built a lot of those little buggers in my 10 years at St. Louis Music (Crate and Ampeg). Even the little Ampeg amps really don't sound all that great. They are limited by size and power.

Bass frequencies require air movement, and that requires power. Even if you use the chassis to power a larger cabinet, while the tone will definitely get fatter, you still won't have enough power to move the air necessary to get solid fundamental tones at any kind of volume.

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Originally Posted by PJSprog

Those small bass amps were never meant to produce solid low end. They are made to provide a little volume for your bedroom practice sessions. You are not likely to find one that is a tone monster. I built a lot of those little buggers in my 10 years at St. Louis Music (Crate and Ampeg). Even the little Ampeg amps really don't sound all that great. They are limited by size and power.

Bass frequencies require air movement, and that requires power. Even if you use the chassis to power a larger cabinet, while the tone will definitely get fatter, you still won't have enough power to move the air necessary to get solid fundamental tones at any kind of volume.

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No way - you built Crate amps?! I have very fond memories of Crate. For me, they were very ‘80s. They were in all the guitar shops, they were pretty loud (good for playing Quiet Riot songs with, lol) and they were fairly inexpensive.

You are correct about everything you stated. This will just be a low cost project for practice and maybe recording.

Last year, I built a wired stereo enclosure for my iPad out of a $20.00 wine box and used two $4.00 speakers and a $20.00 amp. I plugged in my iPad and played some subwoofer bass demos...you wouldn’t believe the bass response. Too loud, and the voice coils will burn out but at a low volume, the bass is ridiculous. The box is sealed and the little 4” speakers don’t move that much but somehow the bass is there! (Just not very loud).

That little project sort of inspired me. Heck, I could just plug my bass into it and see how it sounds, come to think of it but I’d rather make the next one a tad bigger and power it with my bass amp.
https://youtu.be/h4bYGBB9sNA

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Originally Posted by MusicStudent1

No way - you built Crate amps?! I have very fond memories of Crate. For me, they were very ‘80s. They were in all the guitar shops, they were pretty loud (good for playing Quiet Riot songs with, lol) and they were fairly inexpensive.

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Yeah, it was a fun time in my life. I started out working on the assembly line in the CAT Department (Chassis Assembly & Testing). We put the assembled pc boards, transformers, and wiring together in the chassis, and tested them. Spent some time making wire harnesses and testing and grading tubes. I eventually became a packer, then a sound tester (coolest job on the planet, getting paid to sit in a sound booth testing amps all day), and then line leader / electronics tester / technician. The last four year there I was in Quality Assurance.

Most of the Crate line of stuff was indeed inexpensive. It was from the start geared towards the beginners market. Those little 15w GX-15 combos? We made in the neighborhood of 25,000 of those things a year. Some of the latter days tube amps were not as inexpensive, though, like the Blue Voodoo series or the Vintage Club series. The VC-5212 combo is still my favorite Crate amp, and I still have a VC-120H head ... which I no longer use since I bought a Helix.

When we closed that facility, the last thing we ran on the production line was a limited reissue of the original CR-1 combos ... the ones that looked like an actual crate. I tried to get my hands on one, but they were all made for the sales staff and certain retailers who'd been with us from the beginning. It was pretty neat to build and test those.

Lots of great memories from that place. It closed in 2003, but a bunch of us still get together every year and have a little reunion. Good people who had fun making music equipment.

Quote:

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Originally Posted by PJSprog

Lots of great memories from that place. It closed in 2003, but a bunch of us still get together every year and have a little reunion. Good people who had fun making music equipment.

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Thanks for your post! You can tell the group the next time you’re together that I was a huge fan of Crate amps. I was just a kid and was trading stuff, changing guitars, etc, etc. I regret trading in my Crate gear.

One guy I played with in the 1980s had one of those combo amps and it sounded great to me. I think it was a 20 watt amp but it could’ve been a 15. We played some simple Ramones songs together, he was using a Gibson single cutawaygguitar called “The Paul” with the Crate. His tone was outstanding.

I sometimes think in the quest to get the perfect sound, perfect guitar, etc. we don’t realize we had it right all along!

I would stay away from passive radiators. My understanding is that they do essentially the same thing as the port on a reflex cab. They are not necessarily any louder or better. I agree with Jim that there is a fair amount of science that goes into designing a good bass cab--which is not to say that every cab out there is a good one....

If you are building it's a good idea to "tune" your cab to the speaker you will be putting into it. There are a number of calculators for this on the internet, such as the one below.

http://www.mh-audio.nl/Calculators/R...alculator.html

I haven't done it for a while, but there is also some free software you to help you design dimensions. I can look for it if you are interested.

I think Simon is right that there there isn't necessarily much difference between the woofer in a PA and a woofer in a bass cab. Lots of Eminence speakers are used in both. That said, if you are bulding a cab you want to get one with published specs so you can tune your cab to the speaker you want to use.

Another thing to watch out for is that small and cheap speakers may have (a) a low power rating, and (b) too short "excursion" (a.k.a xmax--the distance the voice coil can travel to move the cone) for bass.

Think of it this way. Bass sound waves are much bigger than guitar sound waves. To produce the low E on a bass you have to move a lot more air than you do for a low E on a guitar. All things being equal a small cone will move less air than a big one. Higher Xmax will help a bit since it will allow the cone to move farther. Exceed the Xmax by turning up the volume too high, and the voice coil can jump out of it's spider or housing. That's never happened too me, but what has is a melted down voice coil. This happens because speakers have a maximum volume that they can produce. However when the speaker reaches that point, I might still want more volume--more power to a speaker that has reached it's maximum volume results in heat rather than sound...and, in my experience, also a bad smell then silence.