Home/Pro 8" 2-way: RS28A and B&C 8BG51

[davepellegrene]

Re: Home/Pro 8" 2-way: RS28A and B&C 8BG51

Yes, 0-80 in approximately ten degree steps.

I was curious because of the way the top end holds as the rest of the response drops off as you get to 30 degrees off axis. It must have to either do with the round guide or the phase plug. I notice the same thing on my measurements. Most tweeters do the opposite.

[buzzforb]

Re: Home/Pro 8" 2-way: RS28A and B&C 8BG51

What considerations should be payed to physically aligning the acoustic centers of the drivers, especially when you consider that the XO will have an affect on the outcome. Is there any good reading on the subject, preferably with pictures

[davepellegrene]

Re: Home/Pro 8" 2-way: RS28A and B&C 8BG51

What considerations should be payed to physically aligning the acoustic centers of the drivers, especially when you consider that the XO will have an affect on the outcome. Is there any good reading on the subject, preferably with pictures

Using the waveguide Pete is using it would be best to rear mount the B&C driver. The tweeter roughly sits back 2" in the guide. The B&C roughly has a 1-1/4" back set and rear mounting it in a 3/4" baffle would line the tweeter and woofer pretty closely. Doing this though you could not use Pete's crossover. You would need to measure each driver separately, Usually from the tweeter axis, without moving the mic. Jeff B. Has a right up explaining how to do this and load it into PCD. He also loads both drivers measured together to adjust the offsets.
You can only use PCD on axis if you are using a waveguide. It doesn't have the ability to adjust for the waveguide off axis.
I'll look for his paper and post it if I can find it.

Dave

[davepellegrene]

Re: Home/Pro 8" 2-way: RS28A and B&C 8BG51

http://techtalk.parts-express.com/sh...coustic-Offset

[Pete Schumacher]

Re: Home/Pro 8" 2-way: RS28A and B&C 8BG51

Using the waveguide Pete is using it would be best to rear mount the B&C driver. The tweeter roughly sits back 2" in the guide. The B&C roughly has a 1-1/4" back set and rear mounting it in a 3/4" baffle would line the tweeter and woofer pretty closely. Doing this though you could not use Pete's crossover. You would need to measure each driver separately, Usually from the tweeter axis, without moving the mic. Jeff B. Has a right up explaining how to do this and load it into PCD. He also loads both drivers measured together to adjust the offsets.
You can only use PCD on axis if you are using a waveguide. It doesn't have the ability to adjust for the waveguide off axis.
I'll look for his paper and post it if I can find it.

Dave

You could adjust the acoustic offset closer to 0 by doing what you say, rear mounting the woofer in a recess cut into the back of the baffle. As it stands right now, the 1000Hz acoustic center of the woofer is 1cm forward of the tweeter. If I were to move the woofer to the back of the baffle it would get it right where it needs to be. But the crossover wouldn't have to change from where it is right now. Moving the woofer back results in small fractions of a dB difference in the range from 1000 - 3000.

[davepellegrene]

Re: Home/Pro 8" 2-way: RS28A and B&C 8BG51

You could adjust the acoustic offset closer to 0 by doing what you say, rear mounting the woofer in a recess cut into the back of the baffle. As it stands right now, the 1000Hz acoustic center of the woofer is 1cm forward of the tweeter. If I were to move the woofer to the back of the baffle it would get it right where it needs to be. But the crossover wouldn't have to change from where it is right now. Moving the woofer back results in small fractions of a dB difference in the range from 1000 - 3000.

[ATTACH=CONFIG]43041[/ATTACH]

Does it effect the phase much?

[Pete Schumacher]

Re: Home/Pro 8" 2-way: RS28A and B&C 8BG51

Does it effect the phase much?

Minor. The reverse phase null goes from -15dB at 1000Hz to -12dB.

Because the crossover is at 1000Hz, roughly 33cm wavelength, the minor changes to the offset from where it currently sits will have negligible impact to the crossover, akin to the different ear heights of those who sit and listen in the same chair.

[davepellegrene]

Re: Home/Pro 8" 2-way: RS28A and B&C 8BG51

Hey Pete,
What's your measuring distance and gating?

Dave

[Pete Schumacher]

Re: Home/Pro 8" 2-way: RS28A and B&C 8BG51

Hey Pete,
What's your measuring distance and gating?

Dave

Everything I've posted so far is ungated, smoothed to 1/6th octave. My room is quite lively with an audible echo decay.

The off axis plots were 1m distance but I did use some panels to help a little.

I'll set up a test with the tweeter wired out of phase to show the reverse null on the current crossover. Maybe later tonight.

[davepellegrene]

Re: Home/Pro 8" 2-way: RS28A and B&C 8BG51

Everything I've posted so far is ungated, smoothed to 1/6th octave. My room is quite lively with an audible echo decay.

The off axis plots were 1m distance but I did use some panels to help a little.

I'll set up a test with the tweeter wired out of phase to show the reverse null on the current crossover. Maybe later tonight.

I figured the dip at 700hz had to do with the room and also the humps from 3-7 kHz. I was just currious how you measured and that makes sense.

Dave

[Pete Schumacher]

Re: Home/Pro 8" 2-way: RS28A and B&C 8BG51

I figured the dip at 700hz had to do with the room and also the humps from 3-7 kHz. I was just currious how you measured and that makes sense.

Dave

Thanks for pointing out my warts . . . ;)

My living room could really use some serious treatments. Probably the best thing I could do is hang some heavy drapes, but the room just gets too dark with them. Wife wouldn't be too happy with that. The only other possibility is adding quite a few absorptive panels. But the biggest problem is the windows, which have their own self resonance. It's really obvious when some engine is running outside, lawnmower, trimmer, truck, and you can hear the sound of the windows being excited. The only cure for that is the drapes. When you compare the nearfield measurements to the 1m distance, that's got "room" written all over it.

[davepellegrene]

Re: Home/Pro 8" 2-way: RS28A and B&C 8BG51

Thanks for pointing out my warts . . . ;)

My living room could really use some serious treatments. Probably the best thing I could do is hang some heavy drapes, but the room just gets too dark with them. Wife wouldn't be too happy with that. The only other possibility is adding quite a few absorptive panels. But the biggest problem is the windows, which have their own self resonance. It's really obvious when some engine is running outside, lawnmower, trimmer, truck, and you can hear the sound of the windows being excited. The only cure for that is the drapes. When you compare the nearfield measurements to the 1m distance, that's got "room" written all over it.

Is there any reason you don't gate your measurements? I always use 5ms at 1 meter and that's good down to about 300Hz. It's pretty hard for me to get a good clear measurement doing 0-90 degrees in my room any other way.


Dave

[Pete Schumacher]

Re: Home/Pro 8" 2-way: RS28A and B&C 8BG51

Is there any reason you don't gate your measurements? I always use 5ms at 1 meter and that's good down to about 300Hz. It's pretty hard for me to get a good clear measurement doing 0-90 degrees in my room any other way.


Dave

I'm kind of addicted to seeing the relative bass levels. But all I'm really looking at is room modes.

Here's a response I just took. I had some OC703 panels laying around. I put two on the floor between the speaker and mic and one on each side of the speaker half way to the mic. It improves things somewhat. You can see gated 1m and ungated/smoothed 1/6th.

[davepellegrene]

Re: Home/Pro 8" 2-way: RS28A and B&C 8BG51

I'm kind of addicted to seeing the relative bass levels. But all I'm really looking at is room modes.

Here's a response I just took. I had some OC703 panels laying around. I put two on the floor between the speaker and mic and one on each side of the speaker half way to the mic. It improves things somewhat. You can see gated 1m and ungated/smoothed 1/6th.

Do you think the dip at 700 Hz is still from the room even in the gated measurements? I was looking back on post #265 at your close mic measurements and I don't see anything in the tweeter response that could be causing a suck out but I'm not quite sure why the tweeter drops about 15 Dbs from the crossover then flattens out until about 200 Hz then drops off. Is that your noise floor? Looking at the woofer response after crossover and tweeter response after the crossover is what my noise floor looks like when my furnace is running.

Not trying to point out your warts. I'm just trying to interpret the graphs. Sometimes it's hard to tell when your not taking the measurements. It's been a will since I've used HOLMImpulse and if I remember you can slide the gating back and forth. I think if the gating is moved higher the dip would disappear. Which would indicate it is the room.

Dave

[Jeff B.]

Re: Home/Pro 8" 2-way: RS28A and B&C 8BG51

Do you think the dip at 700 Hz is still from the room even in the gated measurements? I was looking back on post #265 at your close mic measurements and I don't see anything in the tweeter response that could be causing a suck out but I'm not quite sure why the tweeter drops about 15 Dbs from the crossover then flattens out until about 200 Hz then drops off. Is that your noise floor? Looking at the woofer response after crossover and tweeter response after the crossover is what my noise floor looks like when my furnace is running.

Not trying to point out your warts. I'm just trying to interpret the graphs. Sometimes it's hard to tell when your not taking the measurements. It's been a will since I've used HOLMImpulse and if I remember you can slide the gating back and forth. I think if the gating is moved higher the dip would disappear. Which would indicate it is the room.

Dave

For what it's worth, at last week's Loudspeaker Design Workshop I gave a presentation on how to get quasi-anechoic frequency response in your room that is completely accurate down to 10Hz with no room interactions at all. It requires taking two measurements that I defined precisely how they are to be taken, and putting them together using a tool that Charlie and I put together. I stated that the conventional method of splicing does not result in an accurate frequency response, and graphs like the one pictured in Joe D's book on Measuring Loudspeakers is incorrect, but I showed that my method produces a correct frequency response that you can verify fairly easily. I need to write this process up in a paper. This new method is one of the special treats I gave the group last week. It was the first time this has been presented anywhere, and I have never seen this methodology presented anywhere I have ever read, online or in a book, so I think what I have is a new twist on the subject.

Jeff B.