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

[Brian Steele]

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

Hmm... that 2dB rise @ 4kHz might stand out a bit on certain passages...

[bkeane1259]

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

Pete I have a dumb question regarding your PCD screen shot. Why is there such a large SPL disparity between the individual raw driver responses and overall system response? What happened to the sensitivity? FWIW, this is why I am attending Jeff's PCD speaker design seminar...LOL. So what's going on there?

[Pete Schumacher]

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

Hmm... that 2dB rise @ 4kHz might stand out a bit on certain passages...

Could be. The RS28A has a bit of a rise at that frequency which is built into the tweeter. Only harmonics should be emphasized, and only a little bit at that. The top octave roll off seems to be another feature of the RS28A which according to manufacturers measurements as well as third party, definitely drops off quite a bit above 14KHz. That's a "feature" of 28mm hard domes that stay pistonic over their operating range.

I'll post a series of off axis plots once the baffle is mounted to the cabinet. When you start getting off axis, the high end response actually improves.

[Pete Schumacher]

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

Pete I have a dumb question regarding your PCD screen shot. Why is there such a large SPL disparity between the individual raw driver responses and overall system response? What happened to the sensitivity? FWIW, this is why I am attending Jeff's PCD speaker design seminar...LOL. So what's going on there?

Holm impulse only displays data relative to 0dB (full range). So when exporting FRDs for use in PCD, I first add 99dB and then export. That way, they show up in Jeff's default display range.

These PCD plots are not relative to 1W, thats for sure. The raw response plots were taken with at least 1W to the drivers.

[bkeane1259]

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

Holm impulse only displays data relative to 0dB (full range). So when exporting FRDs for use in PCD, I first add 99dB and then export. That way, they show up in Jeff's default display range.

These PCD plots are not relative to 1W, thats for sure. The raw response plots were taken with at least 1W to the drivers.

OK, I can understand the part about Holm and the response not being SPL accurate, but I guess I was wondering more about the 15db drop in sensitivity at 1.5k. At the end of the day, the raw response IS relative to the final system response, yes? So you're still expecting the final iteration of this to be about 87db? Really showing my noobishness right now…..

[Pete Schumacher]

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

OK, I can understand the part about Holm and the response not being SPL accurate, but I guess I was wondering more about the 15db drop in sensitivity at 1.5k. At the end of the day, the raw response IS relative to the final system response, yes? So you're still expecting the final iteration of this to be about 87db? Really showing my noobishness right now…..

You're referring to the raw tweeter response at 1500Hz being reduced so much? That's a consequence of EQ'ing for the effects of the waveguide. System sensitivity is set by the average level I see back at 200-300Hz on the woofer, where baffle step compensation needs to take place. That just happens to fall near the 80dB line on the plot. After identifying the system sensitivity, I shape the responses to follow that line.

The fact that the waveguide "boosts" the tweeter output so much allows for the use of a low crossover point since the amount of signal you need to hit the baseline sensitivity is now 15dB less power. Using the guide has multiple benefits over simply flush mounting on a flat baffle. The smooth curvature of this guide act like a huge roundover and reduce diffraction effects at the baffle edge to almost nothing. The guide actually shapes the diffraction to produce the response plot you see. Distortion at the low end (where stress is highest on a tweeter) is greatly reduced since to hit the same SPL at 1500Hz, you only need a small fraction of the power you need without a guide. You'll be able to drive the tweeter to much higher levels because of this. And the guide acts to more closely match the off axis characteristics of a larger woofer, improving power response.

87dB should be about right based on the TS parameters of the woofer, with perhaps 3dB of baffle step compensation. It may go higher or lower after listening to the overall balance for a while.

[bkeane1259]

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

Ah ha!! …..said the blind man to his deaf friend.

I see what you did there.

I wasn't factoring in the huge impact of the wave guide. Now it makes sense. See, this is what happens to shmucks like me who only get to fiddle around with normal drivers. You just don't often see that type of (raw) response curve in the PCD plots. I can safely say that I’ve never had the need to throttle anything down that much and that’s what was throwing me off initially. Thanks for the explanation.

[[email protected]]

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

Bryan needs to try a waveguide. The low end boost is really nice.

[mzisserson]

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

Bryan needs to try a waveguide. The low end boost is really nice.

Are the tradeoffs worth it?

You're referring to the raw tweeter response at 1500Hz being reduced so much? That's a consequence of EQ'ing for the effects of the waveguide. System sensitivity is set by the average level I see back at 200-300Hz on the woofer, where baffle step compensation needs to take place. That just happens to fall near the 80dB line on the plot. After identifying the system sensitivity, I shape the responses to follow that line.

The fact that the waveguide "boosts" the tweeter output so much allows for the use of a low crossover point since the amount of signal you need to hit the baseline sensitivity is now 15dB less power. Using the guide has multiple benefits over simply flush mounting on a flat baffle. The smooth curvature of this guide act like a huge roundover and reduce diffraction effects at the baffle edge to almost nothing. The guide actually shapes the diffraction to produce the response plot you see. Distortion at the low end (where stress is highest on a tweeter) is greatly reduced since to hit the same SPL at 1500Hz, you only need a small fraction of the power you need without a guide. You'll be able to drive the tweeter to much higher levels because of this. And the guide acts to more closely match the off axis characteristics of a larger woofer, improving power response.

87dB should be about right based on the TS parameters of the woofer, with perhaps 3dB of baffle step compensation. It may go higher or lower after listening to the overall balance for a while.

So these are active and passive? Why would you need an EQ while in teh design phase, I don't get it. I thought the goal was to design a speakers that inherently on its own DOSEN't need EQ. If I have to EQ the speaker, then the WG just ain't worth it, sorry.

[jonasz]

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

I think he's eq'ing the response down with the passive filter.

An EQ doesnt do the same for the offaxis as a WG does.

[Pete Schumacher]

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

Are the tradeoffs worth it?



So these are active and passive? Why would you need an EQ while in teh design phase, I don't get it. I thought the goal was to design a speakers that inherently on its own DOSEN't need EQ. If I have to EQ the speaker, then the WG just ain't worth it, sorry.

I think he's eq'ing the response down with the passive filter.

An EQ doesnt do the same for the offaxis as a WG does.

Mike, what Jonasz said.

Every passive crossover involves EQ. It's there to smooth the frequency response of the system, including variances in driver response as well as enclosure induced diffraction.

In this case, crossing a 28mm dome tweeter to an 8" woofer, it's definitely worth it. The improved power handling, the better directivity match to the large woofer, the smooth off axis performance, closer acoustic alignment between woofer and tweeter, lack of baffle diffraction on the tweeter response, all add up to offset whatever shortcomings might result.

You really should try out a design using a guide on a dome tweeter Mike. It doesn't have to be as deep as this one which gives a 15dB boost at 1500Hz. A shallow one can offer a much more uniform off axis response while greatly reducing diffraction artifacts. Time alignment is a nice feature too.

[mzisserson]

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

I think he's eq'ing the response down with the passive filter.

An EQ doesnt do the same for the offaxis as a WG does.

I have never heard a crossover called an EQ before, he also talks about them seperately.

You mean kill the power responce?

[mzisserson]

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

Every passive crossover involves EQ. .

I am dense.....

Are we now calling passive crossovers "EQ's"

Edit: Pete, I still would like to see evidence, or some real study if in a "normal" home environement waveguiding is worth it because I can count on one finger how many WG systems I think are remotely tolerable in a living room. As the space grows I believe it becomes far more necessary, but I believe, and have yet to test, that within the confines of a home (short of a 30x30 livingroom with vaulted ceilings) the advantages of stringer off-axis responce and better power responce is subjectivly superior.

Everyone talks about WG's like they are the second coming and I have yet to see any evidence but a very rolled off top end off axis, and boosting the low end of a tweeter, in which case save yourself teh hassle and just pick the right damn tweeter in the first place! AGain, I thik there is a point where it begins to make sense. Just not sitting 5-15' from a speaker in a room.

[Pete Schumacher]

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

I am dense.....

Are we now calling passive crossovers "EQ's"

Edit: Pete, I still would like to see evidence, or some real study if in a "normal" home environement waveguiding is worth it because I can count on one finger how many WG systems I think are remotely tolerable in a living room. As the space grows I believe it becomes far more necessary, but I believe, and have yet to test, that within the confines of a home (short of a 30x30 livingroom with vaulted ceilings) the advantages of stringer off-axis responce and better power responce is subjectivly superior.

Everyone talks about WG's like they are the second coming and I have yet to see any evidence but a very rolled off top end off axis, and boosting the low end of a tweeter, in which case save yourself teh hassle and just pick the right damn tweeter in the first place! AGain, I thik there is a point where it begins to make sense. Just not sitting 5-15' from a speaker in a room.

Fair enough Mike. Not having used them yourself, I can see why you'd not see any evidence that they offer some advantages.

Acoustic offset. That's a benefit right there.

Better diffraction control.

Top end does not roll off as you move off axis when the guide matches the tweeter well. I've posted numerous times the off axis response of the 8" WG with the RS28F tweeter, but I'll post it again to answer your doubts and misconceptions.

On axis:


20 - 40 - 60 degrees off axis.



This is a far more uniform off axis response than a non waveguide tweeter matched to an 8" woofer.

And not every waveguide needs to provide a massive low end boost to be useful. This one happens to provide the boost due to the need to get the acoustic centers aligned. The 2" deep guide results in the change at the tweeter's low end. But a shallower guide will produce a lot less gain without losing the other benefits, especially diffraction mitigation.

A crossover provides 2 functions. One is to seamlessly hand off power from one driver to the other. I know you know that. The other thing it does is provide EQ to keep response flat. For example, that EQ can take the form of a notch filter in the pass band to take care of a diffraction bump at 800Hz. It's all part of the crossover. Sorry if my use of the term EQ threw you off.

[mzisserson]

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

A crossover provides 2 functions. One is to seamlessly hand off power from one driver to the other. I know you know that. The other thing it does is provide EQ to keep response flat. For example, that EQ can take the form of a notch filter in the pass band to take care of a diffraction bump at 800Hz. It's all part of the crossover. Sorry if my use of the term EQ threw you off.

Yes Pete, I understand what a crossover does. Just never heard anyone refer to what it is doing as EQ-ing. With all the razzle-dazzle DSP's and active becoming more popular, I was unsure exactly what you were getting at.

Again, there is this approach like a waveguide is the end-all be-all and the best thing to ever happen to a tweeter. Everyone talks of the obvious advantages, no one ever speaks openly about the disadvantages because as you know when you get them wrong, they can be VERY wrong. It is not ALL upside to a waveguide. It would be nice to see some fair and balanced design around one instead of response measurements and "ta-dah" this is the best. I'll get off it now, this still looks very interesting. Good luck.