Ask an Acoustic Engineer (me)

[dumdum]

Seems like you have the right ideas. I'm not sure what it would sound like to have the same reverberation time "shape" but shifted down like you show. Jon Whitledge put a spectacular amount of effort into making his sprinter van have a good reverberation time and his project would be a good one to study. Look for his website and the build log where he talks about the binary amplitude diffusors and the numerous bass traps he built for that purpose. He also has lots of graphs and theory to go with it.

In general though, yes it's hard to have a longer RT with a smaller interior volume. I once thought about making the RT longer artificially with some clever electronics like the "stadium" or "hall" modes on home theater receivers but never got around to it.

Perhaps differential rear fill would give a similar effect thinking out loud?

[Jscoyne2]

http://linkwitzlab.com/JAES/jaes_papers76.htm



Okay so this looks super interesting. Can you translate this to um..English? Specifically the part(43) about "the main lobe of the radiation pattern would shift away from the cabinet axis whenever both drivers contribute to the total acoustic output"

The paper from what i can gather is about crossover networks in general but more specifically about how a crossover point and slope, should take into account the distance between drivers as much as it should take into account what the drivers are capable of such as a midbass driver.

I have only really seen this talked about on Diyma in a very small amount in some of the deeper complicated threads. I've come to think from my own experience that the closer two drivers are together. The steeper you can have your crossovers as they will sum really well due to similar reflections. If they are farther, like mid range kicks and high mounted tweeter. Using a 6 or 12db slope helps because...um...idk..it lets the sound kinda..nope..i have no idea..but it seems to work better.

[Justin Zazzi]

This paper was written in the 1970s when analog circuitry was king and DSP was not.

The paper says you can use active (analog) all-pass filters to create a time delay between two drivers, like a tweeter and a woofer, so that they sum together nicely on-axis.

The part about "the main lobe of the radiation pattern would shift away from the cabinet axis whenever both drivers contribute to the total acoustic output" means if two drivers are not the same acoustic difference away from the listener, then they will have a dip in frequency response where both of the drivers play at the same time. For example a woofer and tweeter crossed over at 2khz would have a dip at roughly that same frequency.

All-pass filters are one way to correct for the different acoustic centers that speakers have such as a typical woofer's acoustic center being a little behind the acoustic center of a typical tweeter. Another way to correct for this difference in acoustic distances is to physically mount the tweeter further back relative to the woofer, and this is why you sometimes see speakers have a sloped front face where the tweeter is further away from the listener than the woofer is.

A method we use often is to just add a time delay to the signal on the tweeter, and that will correct for the acoustic center differences. However, adding a full-on DSP is complex or outright impossible in the 1970s and so the active analog all-pass filter was the next best thing since it's very simple and affordable.

example of sloped front face:

borrowed from: https://forums.stevehoffman.tv/threa....581598/page-5

[BigAl205]

I've seen reference to the Whitledge curve, and half-Whitledge...is that associated with JonW, or is it somebody else?

[Justin Zazzi]

Yep. I added the Whitledge curve to my rew tuning companion spreadsheet and thought "that's a bit much but I like the shape of it" so I also added a version that was half for fun. I copied the curve that he posted on his website and I do not claim anything other than trying to replicate the graph he posted.

[BigAl205]

I was just curious

[Grinder]

Can the net cabinet volume required to achieve a given Qtc (.707, for example) be accurately/reliably calculated from nothing more than Fs, Qts, Qes and Vas (as per such online calculators), or is there a more accurate/reliable method to do this - given additional TS parameters, for example?

Do you know of any fundamental reason why Bass Box modeling results in this regard might wildly differ from those of WinISD?



Full disclosure:

The reason I'm asking is due to an as yet unexplained wide disparity between my WinISD modeling results and a fellow CAJ member's Bass Box modeling results, both recently posted on behalf of a third fellow CAJ member, in that member's thread.

[rton20s]

Can the net cabinet volume required to achieve a given Qtc (.707, for example) be accurately/reliably calculated from nothing more than Fs, Qts, Qes and Vas (as per such online calculators), or is there a more accurate/reliable method to do this - given additional TS parameters, for example?

Do you know of any fundamental reason why Bass Box modeling results in this regard might wildly differ from those of WinISD?



Full disclosure:

The reason I'm asking is due to an as yet unexplained wide disparity between my WinISD modeling results and a fellow CAJ member's Bass Box modeling results, both recently posted on behalf of a third fellow CAJ member, in that member's thread.

Someone dun screwed up.

Seriously though, do you have a link to the thread? Which parameters you enter and which you allow WinISD to auto-calculate can make an appreciable difference. That could be at least part of the issue. I would not expect to see "wide disparity" between various modeling softwares given the same exact parameters entered. Especially in the case of a simple sealed enclosure.

Also... not an acoustical engineer.

[Justin Zazzi]

In theory you can use Fs, Qts, and Vas for lots of box designing including finding a Qtc number.

I know BassBoxPro has a couple extra calculations built in like the ability to simulate cabin gain of an arbitrary room, or accommodate box stuffing, or compensate for box volume differences due to driver displacement or internal bracing, or use the measured frequency response of a woofer in addition to the T/S parameters.

I too would like to see the dramatically different results, and also the inputs used (speaker and box parameters) for both programs. It's hard to say much otherwise!

[Grinder]

Sorry, guys. Here's the thread: https://www.caraudiojunkies.com/show...ome-to-)/page5

The issue at hand begins at post #41 (at the top of page 5), and continues here and there through post #72.


<edit> When I model a driver, I enter these TS parameters (Qes, Qms, Fs, Re, Vas, Xmax, BL) in the order shown, allowing WinISD to auto-calculate everything else.

The driver is a Dayton Audio RSS265HO-44 10": https://www.parts-express.com/dayton...oofer--295-463