Passive Radiator

[MusicStudent1]

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?

[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.

[MusicStudent1]

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.

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.

[Simon Barden]

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.



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.

[JimC]

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.

[Simon Barden]

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.

[MusicStudent1]

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.

[Simon Barden]

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.

[MusicStudent1]

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!

[PJSprog]

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.