Driver Measurements

lordy101

Member
2018-12-12 2:36 am
Looking for help with modelling enclosures for the SB Acoustics SB15NRXC30-4. This is a five inch woofer which I will be paring with the SB29RDC-4 tweeter. I have two woofers per enclosure so a 2.5 way is the plan in a 25 litre enclosure according to WINISD. WINISD suggests a 3"x7.5" port which seems big for such small woofers. This will fit in the box but does this seem right?
I have used DATS V2 and the measurements are different from manufacturer specs.. SB give the VAS as 16.7L but DATS says around 10.78L. Fs is also 41hz according to SB but DATS gives me around 50hz for three of the drivers and 57hz for one of them!
I am just a bit concerned to cut a three inch hole in a box but if its normal to be so far off manufacturer specs then I will go ahead.
WIN ISD plots below and any advice from someone who's used them would be appreciated.
 

Attachments

  • Frequency response.png
    Frequency response.png
    33.3 KB · Views: 194
  • Cone excursion.png
    Cone excursion.png
    31.7 KB · Views: 194
  • Port velocity.png
    Port velocity.png
    35 KB · Views: 193
Hi,

Drivers will probably need some loosening up when brand new.
I guess some break in using a low frequency signal to exercise the cone, suspension, and voice coil wouldn't go amiss. Does DATS have a low frequency signal generator that can be used to drive an amp to help exercise these new drivers? A couple of hours running should do it, be careful with the amps volume control you probably only want 3mm of excursion at 50Hz or so to help loosen them up

Hopefully after the break in period the speaker parameters that will have changed from your original readings. I also spotted you haven't input data for Le which you may want to input as SB went to all the trouble of getting a nice low figure for your midwoofer. I don't think it will effect the outcome for port dimension though.

One question, in your software examples did you have both woofers connected, and will the software allow you to run them in series, I wouldn't expect two 4ohm drivers in parallel unless you have a big and powerful amp? you could check this by looking at the impedance graph with one or two woofers selected.
 
I am new to this as well (building my 3rd set of speakers now) so take this all with a grain of salt.

First, the measurements from manufactures are often optimistic, so the FS being a bit higher is likely correct. I would be a little concerned with the 57hz one. After a bit of run in they should settle in and sound better and be a little closer to spec but none of my drivers ever got quite to what was published. I usually just play pink noise at low/mid volume (out of phase so it is quieter.)

what I saw the first time I did it was significant changes in the first ~6 hours and then it settled in to a constant value. I went ahead and played it overnight and saw only a small change after that. If that oddball driver still is that far away from the others I might consider exchanging it if that is easy to do. (did it pass the rub and buzz test?)

Second the issues with the VAS, this is the most error prone value (I am assuming you are using the added weight method) I am not sure what to advise you on that front. Idea 1 is use another method to see which one correlates the best.
Idea 2 is go closer to the larger value if sound quality is your priority because there is
less downside to overshooting than undershooting.
Idea 3 is make two test boxes (even out of cardboard if you have to) and measure them and see what looks better.

finally about the port, I think Winisd just spits out whatever values keeps the port velocity down below 13m/s at full blast (assuming you entered the max wattage of the woofer as the signal.). So I would treat this as a maximum size rather then the ideal size. especially if you can flair the ends you can get away with higher port velocity without audible chuffing. Secondly realistically how often are you driving the speaker to its max while critically listening. I would enter about half the actual rms rated value for the woofer and see what that spits out and use the nearest round number. In the end I don't think it matters that much as long as it fits in the box with enough clearance (1x the diameter of the port). I may be wrong but I don't think there is a downside to an oversized port.

good luck and have fun!
 

lordy101

Member
2018-12-12 2:36 am
Thank you both for taking the time to reply to my post. I have done some running in but perhaps some more would be of benefit so I will try and test again to see if there is any difference. I think both DATS and REW have signal generators to help so will look into this as well. To this point I have played music through them only at moderate levels. The drivers will be run 2.5 rather than 2 way, that's the plan anyway. If I do end up going 2 way then a series connection to make then 8 ohms rather than 2.
I am also concerned about the 57hz driver and I have contacted the supplier so will try to get this replaced. For info all drivers including the 57hz one passed the rub and buzz test but it just seems to far out. I used the added mass method to test VAS with the smallest weight I could find (14g). The port will have 4" clearance so should be fine for depth. I have just modelled the manufacturer specs and overlayed them with mine and whilst its not exactly the same its not a whole lot different. Cone excursion, port velocity and response are all pretty close. I may be making more of this than I should.
 

Attachments

  • Measurements Manu vs mine.jpg
    Measurements Manu vs mine.jpg
    197.9 KB · Views: 93
I wouldn't worry about the specs being off if there's nothing obviously wrong with the driver. The cone assembly can't physically be any lighter, so the only thing that can raise Fs is a stiffer suspension. If there's nothing overtly wrong with the spider or surround, I'd say your grouping of 50-57Hz is a reasonable one and the difference from the quoted 41Hz can be chalked up to differences in measurement technique/equipment. I'll bet that the differences once mounted in an enclosure will be inaudible.
 

TMM

Member
2007-09-01 8:37 am
Australia
With the measured Fs being significantly higher than the datasheet suggests, this can be for a several reasons.

Some drivers require some 'break-in' to loosen up the spider before they perform as intended. Some people run the drivers with a low frequency test tone at high excursion for hours to speed up this process. If you had no requirement to measure the speakers (building to an existing design) then you'd just put them in and let them 'naturally' break in by listening to music.
I myself have found it just as effective to push the cone to full excursion either way with my fingers a few times. The best way I can describe it is that some spiders feel like cardboard out of the box, and after exercising them they feel more like stiff fabric and Fs drops by 5-10Hz.

If the ambient temperature is low, the Fs will be higher as rubber surrounds become stiffer as the temperature drops. Measurements should be conducted at around 20-25*C to match the datasheet.

The Fs may be dependant on excursion. The DATS puts out a relatively weak signal so the measurement of Fs can be significantly higher compared to a system which measures Fs at full excursion. Usually this affects cheaper drivers with less linear motors. An SB driver I wouldn't expect to be affected much at all by this.

If none of the above help your case, it could be that you have some drivers which don't meet their spec. Good luck getting a driver replaced due to not measuring as per the datasheet - I have bought drivers which measured 65Hz when the datasheet said 45Hz, even after trying all the tricks above. When I complained about it to a fairly reputable US distributor starting with P and ending with "press", I was essentially told to get lost. A difference like that will rob you of a couple dB of bass amplitude below Fs and in my case turned a driver which would otherwise be light on bass but acceptable without a subwoofer, into a driver which was bass-anaemic and needed a sub.
 
Last edited: