Baffle Step Loss Question

dynamo

Hi guys, I have a pretty basic question.. I know the longer waves of lower frequencies start to wrap around and roll off, typically up to -6db by the time you get under 100hz. I also know this can be mitigated somewhat by boundary reinforcement if you place the woofer low on the cab right above the floor.

Here is my question. If I had a band pass loaded woofer and the port exiting under the cab or in front but at ground level, will I still be -6 db from the modeled 2pi numbers or will I get some gain back from the floor? Thanks!

kenrhodes

In theory the port being the output should benefit from being "loaded". This goes for the input and output of the port. Putting the port in the corner of a box vs the center of the baffle should make it so that you can shorten the port. Modeling this can be very hard to do and is often best done by trial and error. Being that these are low frequencies the room itself will have a huge affect on the overall output.
In my living room I placed the same subwoofer in 3 different locations and played the same test tones. Between the three locations there were 15 db difference from the highest and lowest output levels. Two of the locations were in the typical stereo locations and there was a 3db boost on the left side vs the right. Point being that low frequency is so room dominant that what works in one room may not work in another. You may want to build a quick and dirty box and play around in your room to see what will work best.
I have all but given up on good low frequency extension without equalization. It seems that the best asset we have for low frequency is EQ/DSP. So looking at your power handling and x-max is essential.

dynamo

Thanks. I’m just playing with some modeling of a four way passive speaker with a passive woofer (not sub). I’m toying with the idea of going 4th order bandpass for the woofer section to control the excursion below tuning and simplify the crossover. So I’m curious how the output level of the woofer(port) (regardless of room location) would be compared to a standard ported or sealed configuration in the same speaker in the same location. Hopefully that makes sense?

I definitely concur on room placement being key for bass, and that typical stereo speaker location is not usually ideal. This is just the bottom end of a passive multiway speaker so room location is what it is despite if the woofer is sealed, ported, bp4, etc.

rjj45

I built a "bitching" sub with a 15 inch Ultimax sub and a 500 w amp. For several months, I did not like the sub, it did not sound "authoritative" or "tight" - then I flipped the polarity 180 degrees and suddenly, it was awesome. A big lesson for me.

jr@mac

@rjj45 said:
I built a "bitching" sub with a 15 inch Ultimax sub and a 500 w amp. For several months, I did not like the sub, it did not sound "authoritative" or "tight" - then I flipped the polarity 180 degrees and suddenly, it was awesome. A big lesson for me.

That is usually measurable - will show as a null similar to what happens when you flip polarity on the tweeter in an LR4 crossover.

kenrhodes

The port being loaded against the floor should help with some losses.

dynamo

Cool, thanks. Ken I think your most recent comment is mostly what I’m after.. basically if boundaries will reinforce output from a port the same as it would for output from the cone of a woofer in the same location.

jhollander

The Bagby boundary reinforcement program can give you more details and some nice graphs. http://audio.claub.net/software/jbabgy/BDBS.html

PWRRYD

I believe they will dynamo.

a4eaudio

@kenrhodes said:
The port being loaded against the floor should help with some losses.

Ken, where did you get that image, that is pretty slick?

kenrhodes

https://blogs.qsc.com/live-sound/practical-tips-on-loudspeaker-placement-for-live-sound-part-i/

dynamo

Thanks gents!

4thtry

You could combine Ken's chart for boundary reinforcement above with a baffle step chart for your baffle to see the combined effect. This is what Jeff's BDBS spreadsheet does, as noted by John. However, the accuracy of this information, at any given listening position in the room, is somewhat sketchy at best, at least based on my testing. This is known as the "Allison Effect". A 4 to 5dB dip in the power response from about 80Hz up to 250Hz is created when you move a speaker well out into the room, well away from all boundaries, including the floor. Moving a speaker closer to a boundary will help to fill in this power response dip.

4thtry

@dynamo said:

Here is my question. If I had a band pass loaded woofer and the port exiting under the cab or in front but at ground level, will I still be -6 db from the modeled 2pi numbers or will I get some gain back from the floor? Thanks!

I think the answer to your original question depends heavily on the crossover frequency you choose between the BP woofer and midrange/midbass driver. The Allison room boundary gain decreases with rising frquency and is almost completely gone above 500Hz. So if you place your xover at 400Hz you may recover some of the baffle step loss in the 100 to 500Hz region by placing your BP woofer near the floor. Just how much you recover varies with the distance to the floor. If, however, you cross over at 100Hz, things change significantly. Placing the BP woofer near the floor will have little to no effect on the baffle step loss. You can place the BP woofer port almost anywhere and it will make no difference.