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Old 15th July 2014, 09:22 AM   #1
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Default Midrange power handling + baffle step question

I have some Vifa NE95W-04 'full range' drivers to use as a midrange is a 3-way system.

Firstly they look to have a nice flat response for a good range and they sound nicer than the 8" I'm using, so I want to utilise them fairly low. However they are small and I'm unsure what sort of power they'll take depending on the crossover. Should I be worried about their power handling if crossing over in the ~500Hz region? My amp will output around 70w per channel and I'm obviously not going to drive it into clipping.

Secondly regarding baffle step correction. I'm playing with the Edge software and, realising that it's just an approximation, would like to question the idea of giving the midrange drivers a very small enclosure to create a low-end 'hump' that should, again based on software (how well it models I won't really know until I try it), effectively cancel out the baffle step down until the crossover. Is this a sensible idea? Worth a try? I've never tried giving a speaker a silly little box to achieve such a response so I'm unsure of the consequencies/realities.
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Old 15th July 2014, 10:13 AM   #2
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Nothing wrong with using a small volume enclosure for the midrange. You will find that it becomes part of the crossover (a forced 2nd order high pass) so you will need to manipulate it as one of your variables in crossover design. I have done practcal lower frequency crossovers on mids with small back volume and a capacitor, for an effective 3rd order.

You shouldn't need too much step correction for a midrange and your bass step correction will usually be handled automatically by extra primary series inductance as long as you are equalizing the total free space curve.

David
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Old 15th July 2014, 10:36 AM   #3
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Sorry I don't follow the "primary series inductance". Is that based on the assumption of using a pair in series? I'm using only a single each of a tweeter, mid + woofer per channel. The woofer works out to around 4db/reference voltage more sensitive than the mid, but even then (as below) I can bi-amp if level matching becomes tricky.

As for factoring the response drop as part of the crossover design, yeah I had considered that but hadn't got to that part yet - if I can get the basic design sorted the crossover can come next.

If things start to look complicated with the crossover between woofer+mid then I think I will bi-amp it, but I'll keep my options open at this stage.
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Old 15th July 2014, 01:09 PM   #4
IG81 is offline IG81  Canada
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Quote:
Originally Posted by phreeky82 View Post
Sorry I don't follow the "primary series inductance". Is that based on the assumption of using a pair in series? I'm using only a single each of a tweeter, mid + woofer per channel. The woofer works out to around 4db/reference voltage more sensitive than the mid, but even then (as below) I can bi-amp if level matching becomes tricky.
The extra 4dB sensitivity is a good thing, possibly better than a full 6dB for use in real life rooms too. Your best bet may be to have your crossover coincide with the baffle step point.

baffle-step = c / w / 3

where:
c = speed of sound, ~343m/s
w = baffle width in meters

Simulate your driver in WinISD with a sealed box for Qtc somewhere between 0.7 and 1.0, once the 'Qa' parameter is set to ~10-20 to simulate stuffing. Add a 1st or second order high-pass at the baffle-step frequency and see how much power you can input before Xmax is broken.

An alternative to a sealed midrange enclosure is a tube of slightly larger diameter that runs to the back of the box and "vents" to the rear. Use progressively denser stuffing to the terminus, or just a whole bunch held in a pantyhose or wool sock. This will not give you as sharp an acoustic roll-off, but will nicely damp the impedance peak to facilitate a crossover.

Here's and experiment I recently did with FF85WK in a 3" diameter tube, 12" long with 24g of polyfill in a wool sock:

Click the image to open in full size.

A slightly longer tube would have been better, or one that tapers down towards the terminus so as to tune lower and move the upper Z peak further away from the Xover region.

Last edited by IG81; 15th July 2014 at 01:15 PM.
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Old 15th July 2014, 01:39 PM   #5
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Thanks for that. Yeah I specifically bought a midrange that I felt would give a nicely reduced sensitivity to the 8" drivers I already had - I'm working with a mix of existing and new components. I cannot buy all new parts as WAF seems to take both appearance and budget into consideration

I haven't heard of the tubed approach before.

Any suggestions for software (and mic) to get for measuring response? One for Linux would be handy but I think I have a laptop with Windows if needed. I feel I can trust the midrange response from Vifa as a starting point but I have very little on the woofer and tweeter, and need to measure once in the box anyway.
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Old 15th July 2014, 01:54 PM   #6
IG81 is offline IG81  Canada
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Quote:
Originally Posted by phreeky82 View Post
Thanks for that. Yeah I specifically bought a midrange that I felt would give a nicely reduced sensitivity to the 8" drivers I already had - I'm working with a mix of existing and new components. I cannot buy all new parts as WAF seems to take both appearance and budget into consideration

I haven't heard of the tubed approach before.

Any suggestions for software (and mic) to get for measuring response? One for Linux would be handy but I think I have a laptop with Windows if needed. I feel I can trust the midrange response from Vifa as a starting point but I have very little on the woofer and tweeter, and need to measure once in the box anyway.
Don't know about Linux, but ARTA can be used for free (only saving disabled) for SPL, impedance, T/S and more. I also use HolmImpulse for SPL, also free. There are a few affordable mics around. I have a Dayton UMM-6, comes with calibration file. A simple jig is required to measure impedance, this is described in the LIMP (ARTA) manual. Lots of folks seem to like REW as well, haven't tried it myself.
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Old 15th July 2014, 05:14 PM   #7
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Quote:
Originally Posted by phreeky82 View Post
Sorry I don't follow the "primary series inductance". Is that based on the assumption of using a pair in series? I'm using only a single each of a tweeter, mid + woofer per channel. The woofer works out to around 4db/reference voltage more sensitive than the mid, but even then (as below) I can bi-amp if level matching becomes tricky.
The series inductor for the woofer section. The first input leg of the woofer crossover will have a large control over general midrange level. For most designs you can get reasonably flat free field response (4 pi) by just using the right inductor level in addition to the other components that give the roll off near the tweeter crossover. Only one inductor is required.

I am saying that your "baffle step" can generally be corrected as part of the overall crossover and no additional components dedicated to baffle step are required.

David S.
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Old 15th July 2014, 05:53 PM   #8
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Using Unibox and an LR2 HP filter at 500Hz shows that xmax is not exceeded with 40W which is 100dB at 1m. And that driver is only rated at 20W anyways.

If you are also using an 8" driver, baffle width is probably going to be somewhere around 9" so your baffle diffraction point is going to be at about 500Hz. So really there is no need to boost the response below 500Hz - just use the natural bsc roll-off as part of your HP acoustic response.

By "primary series inductance" I believe Dave is just referring to your primary LP inductor that is in series with the driver. To compensate for bsc, you simply have to use a larger inductor value so the response starts rolling off earlier. Simulation software helps to make this easier to visualize. In your case, this will apply for the woofer but I don't think you need to compensate for bsc on the mid, not with a 500Hz or so xo and with that wide a baffle.
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Old 15th July 2014, 06:02 PM   #9
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Ok, Dave beat me to it.

Just to be clear, there is also a separate bsc filter consisting of a paralleled resistor and inductor inserted in series with the driver that just drops the response by 6dB, but no one really uses these anymore except in special circumstances. Not when the inductor that is used for the LP xo can do exactly the same thing.
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Old 15th July 2014, 08:20 PM   #10
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Originally Posted by jReave View Post
Just to be clear, there is also a separate bsc filter consisting of a paralleled resistor and inductor inserted in series with the driver that just drops the response by 6dB, but no one really uses these anymore except in special circumstances. Not when the inductor that is used for the LP xo can do exactly the same thing.
Especially since most drivers are inductive loads, the series inductor tends towards creating an inductive divider. That is, increasing the primary inductor doesn't really move a corner frequency up and down as much as it shelves midrange level (okay, maybe halfway between those two actions).

David
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