3 friends build speakers.
#1 builds a sealed 2-way, 250mmWx300mmDx500mm tall. The instructions say line the back with 3" of fiberglass, so he does.
#2 says, "Your box has back waves bouncing around inside." So #2 builds a box 250mmW and 500mm tall, but he increases the depth to 600mm, and inserts 600mm of fiberglass. He says, "This will kill the back wave".
#3 builds the same box as #2 (250W x 600D x 500 tall) and inserts the 600mm of fiberglass, but he inserts wire mesh to hold the batting and leaves the wooden back off the cabinet.
That's right ..... no back. Just fiberglass batting, 600mm thick.
Question: is there merit to the design of #3?
Thanks. Jim
#1 builds a sealed 2-way, 250mmWx300mmDx500mm tall. The instructions say line the back with 3" of fiberglass, so he does.
#2 says, "Your box has back waves bouncing around inside." So #2 builds a box 250mmW and 500mm tall, but he increases the depth to 600mm, and inserts 600mm of fiberglass. He says, "This will kill the back wave".
#3 builds the same box as #2 (250W x 600D x 500 tall) and inserts the 600mm of fiberglass, but he inserts wire mesh to hold the batting and leaves the wooden back off the cabinet.
That's right ..... no back. Just fiberglass batting, 600mm thick.
Question: is there merit to the design of #3?
Thanks. Jim
I guess you're figuring that opening the rear of the box will take away the reflections, but I'd have to disagree. The sound can still react to jumping out of the back of the box. The effect may be like an open ended transmission line.
On the other hand, the damping material is going to have a significant effect, except at lower frequencies.
On the other hand, the damping material is going to have a significant effect, except at lower frequencies.
That's what I needed to know. The lower frequencies were the ones that needed to be absorbed the most. Thank you. Jim
There is merit to making an open backed box, but there are many issues that come with it.
First off builder #3 will notice that much (maybe most) of the bass is gone. This is due to front to back cancellation. Bass will reduce at a rate of about -9db per octave starting from a frequency corresponding to roughly 13500/(2*cabinet depth + cabinet width in inches).
Secondly there will be a resonance at approximately 13500/(4*cabinet depth in inches). It could be quite strong.
But these problem can be mitigated by using the correct drivers, driver placement, and crossover. This almost never happens by accident. So I would say that if this is just haphazardly done (as is my interpretation from reading the original post) builder #3 would be disappointed in his result.
Or he might love it because it is his baby ;-)
First off builder #3 will notice that much (maybe most) of the bass is gone. This is due to front to back cancellation. Bass will reduce at a rate of about -9db per octave starting from a frequency corresponding to roughly 13500/(2*cabinet depth + cabinet width in inches).
Secondly there will be a resonance at approximately 13500/(4*cabinet depth in inches). It could be quite strong.
But these problem can be mitigated by using the correct drivers, driver placement, and crossover. This almost never happens by accident. So I would say that if this is just haphazardly done (as is my interpretation from reading the original post) builder #3 would be disappointed in his result.
Or he might love it because it is his baby ;-)
The bigger issue going open back with a 2-way is that you are going to lose LF extension. And not just a little. Attached shows the diffraction effects of a front baffle of your dimensions, one with a back baffle (red) and one without (green). You are losing everything below about 300Hz at a significant rate with the back open.
Attachments
Too little information to the point of being a being a useless question. What's the driver? You describe three very different boxes and are are asking which is better.
So which is better: a plane, a train, or an automobile?
-b
"Too little information" is deliberate. I basically wanted to know the merit (or lack of) between the 3 design types .... but especially #3. I had thought that an extremely thick layer of fiberglass at the rear of the open box would be sufficient to kill the rear wave. The speaker would act as a monopole in radiation characteristics, but the pressure buildup characteristics inside the box MIGHT make it act like an aperiodic design. I didn't know. I still don't know about that part, but if that extreme amount of stuffing isn't enough to completely absorb the back wave at low frequencies, I'll go no further. I'll stop right here. Thanks. 🙂 Jim
If you wanted to try it and if the driver wasn't going down below 60Hz? 80Hz?, results would depend on what type of insulation you used. The very dense, rigid, 6lb/ft^3 rockwool panels are about the best at absorbing LF 's. 600mm of that might result in absorption similar to a 3/4" piece of wood at frequencies down to those specified above. But I haven't tried that much before or seen anyone else show what the results would be. I do use it inside regular boxes though.
I want to absorb the low-frequency back wave so that the speaker doesn't have the typical dipole cancellation. I want to omit the back panel to see WHETHER OR NOT the bass driver will behave in a manner similar to open-baffle response (no pressure build-up) or aperiodic response (most benign pressure build-up).
But like I said ..... if that much absorbent material won't kill the back wave at low frequencies, I'll just stop here.
Thank you, everyone, for your help. Jim
But like I said ..... if that much absorbent material won't kill the back wave at low frequencies, I'll just stop here.
Thank you, everyone, for your help. Jim
You don't realize that you don't even provide enough detail to discuss the merits (tradeoffs) of ANY design.
Look again at my question and try and understand it: you need to transport "something" but I'm not going to provide you with the little detail of what you are transporting. So which is better: a plane, a train or an automobile?
* to transport lunch to a friend nearby?
* to transport 1000 tons of coal across the country?
* to transport yourself on a vacation to Hawaii?
You don't provide enough detail to even discuss the question, much less answer it.
Your first two examples are both sealed boxes. They are definitely not different "design types". One may sound good, one may sound like crap. They both may sound like crap. That is dependent on (among other things) the driver, the frequency range you are trying to reproduce and the environment your are putting it in. Same for the open back design.
If you want an intelligent conversation then pose an intelligent question.
-bill