Purify SPK16 VituixCAD project

[aswave]

Hi everyone, this is my first post on ASR forums. I'm trying to learn VituixCAD and speaker crossover design. Purify released this SPK16 reference speaker design complete with all driver measurements, it is very detailed, with both horizontal and vertical planes covered and it looks like an ideal playground to learn crossovers. The first thing I don't understand is the measurement data. For example, I remove the original filter and connect only tweeter directly to source. Frequency response looks normal for a horn loaded tweeter but phase (this called 'normal phase'?) is going around 180 degrees multiple times. Is this a normal measurement? I'm trying to create a basic first order crossover with linear phase response but here even individual drivers already have this non linear phase so I do not understand where to move next.

 

[Geert]

Phase that wraps or rotates continuously as frequency increases is often the result of a time delay in the system, like the propagation delay between the speaker and the microphone

 

[aswave]

Phase that wraps or rotates continuously as frequency increases is often the result of a time delay in the system, like the propagation delay between the speaker and the microphone

Does this mean that this phase is unusable and just a measurement artifact?

 

[Geert]

Does this mean that this phase is unusable and just a measurement artifact?

Indeed. Don’t know your measurement setup, but some software allows you to compensate that delay (or ’excess phase’). Note that after that compensation the tweeter will still show a (’minimum phase”) phase shift at and below it’s resonance frequency (but not over the full bandwith, its phase should be relatively flat in its usable passband).

 

[TurtlePaul]

Does this mean that this phase is unusable and just a measurement artifact?

It is a measurement artifact. If you pull the mic a foot away from the speaker it will add about 1 ms of delay. How does that show up on a phase graph? One ms is multiple cycles of delay above 2 khz but a fraction of a cycle below 500 hz. Most measurement software has an 'unwrap phase' button which will look at the first arrival of a very high frequency signal and set that to time=0. This will result in a graph of only excess phase which is what you are looking for.

 

[Geert]

This will result in a graph of only excess phase which is what you are looking for.

Minimum phase you mean.

 

[NTK]

The time delay can be calculated and then removed from the phase plot using the method as described by J J.

...
You apparently have access to matlab and some source of head related impulse responses. Take a great big, long, FFT of that impulse response. Calculate the phase of the positive frequencies using atan2. Then "unwrap" the phase. Do a first order linear fit (polyfit) to the phase. Subtract out the part amounting to the linear fit from the phase. The slope of that linear fit, by the way, is exactly the overall time delay. Take the difference. In real data I've seen, that will not be a straight line. The variation from the straight line can be directly converted to +- time of arrival across frequency, remember phi = 2 * pi * f *t You have phi and f, now calculate 't'.

 

[Sokel]

Does this mean that this phase is unusable and just a measurement artifact?

You can unwrap phase or generate min phase in REW, just right click on the chart for the option.

 

[aswave]

You can unwrap phase or generate min phase in REW, just right click on the chart for the option.

I was hoping to use the detailed measurements provided with SPK16 kit. As far as I understand I cannot import the measurements to REW and 'unwrap' phase, is this correct?

 

[Sokel]

I was hoping to use the detailed measurements provided with SPK16 kit. As far as I understand I cannot import the measurements to REW and 'unwrap' phase, is this correct?

Depending the form, maybe you can.
Did you gave it a try?

 

[aswave]

All I have is VituixCAD project and bunch of horizontal and vertical measurement in the form of text files (like "SPK16 Tweeter FR hor 0.txt", SPK16 Tweeter FR hor 10.txt" ...), REW cannot open them. Likely I will need to make measurements myself

 

[bmc0]

I'm trying to create a basic first order crossover with linear phase response but here even individual drivers already have this non linear phase so I do not understand where to move next.

Linear phase does not mean flat (or zero) phase. It means that the unwrapped phase makes a straight line when plotted with linear frequency axis. Here's the wrapped phase of a pure 1ms delay, plotted with a log frequency axis:

Being a pure delay, it is indeed exactly linear phase.

In your example, some of the phase wraps are due to a nonzero pure delay component in the measured data. This can be removed if you're so inclined. However, the phase will still not be exactly linear because it (along with any other speaker driver) is approximately minimum phase and the magnitude is not exactly flat. As the roll-off below 2kHz is already beyond first order, you'll have a difficult time trying to make something resembling a first-order crossover passively. It's the acoustic slopes that count, not electrical.

 

[alex-z]

For the purposes of crossover design you can leave the phase data. Purifi used proper timing reference so the relative phase of the two measurements is correct despite them not being minimum phase.

Here is an example of an active crossover closely mimicking the passive version.

 

[Paul7777x]

Could this be used into a minidsp and therefore I could ignore building the x-over?

 

[aswave]

Could this be used into a minidsp and therefore I could ignore building the x-over?

If I remember correctly, in the white paper for the kit they recommend to add serial notch filter for the woofer even if using digital crossover, as this reduces distortion better than simple EQ.

 

[Dodo974]

Hi,
Here some details and pricing for this speaker kit:

PURIFI SPK16 DIY Speaker Kit for 2-Way Passive Bookshelf Speakers (Pair)

The PURIFI SPK16 is a complete DIY kit for 2-way passive bookshelf speakers developed by PURIFI to deliver high-end acoustic performance in a compact 19-liter enclosure.

www.audiophonics.fr

 

[TheLastGerman]

If I remember correctly, in the white paper for the kit they recommend to add serial notch filter for the woofer even if using digital crossover, as this reduces distortion better than simple EQ.

PTT6.5X04-NAA-08A in approx. 19l CB. Notch filter = 0.13 mH/0.06 Ω, 6.6 µF, no resistor.