Vas measurements by added-mass method in Speaker Workshop

[tcpip]

I am trying to measure Vas for a few mid-woofers that I have. I have already measured the other parameters and have got believable results.

For Vas measurements, I haven't yet gotten around to sticking the drivers in a sealed box, so I wanted to try the quicker added-mass method. Is this method much less accurate than the sealed-box method? I have official data sheets for the driver, and the Sd given there matches with the Sd I'm measuring from middle-of-surround to middle-of-surround. (Everyone tells me that added-mass Vas depends heavily on an accurate Sd.)

I'm placing two five-rupee coins (exactly 10g each) on the cone, and taking the added-mass impedance measurements. They give me nice clean graphs, sometimes with twin-peaks. And they have given me Vas figures of 24L, 27L, 29L and 30L. The published figures in the datasheet are higher: 36.8L. The data sheet is here:

Peerless India 6.5" Kevlar cone woofers

I repeated the tests with three coins instead of two (ie. 30g weight), and the Vas measured was just 5-7% away from the 20g measurements. I believe this is how it should be, within a reasonable range of weights?

The other parameters I've measured tally quite closely with the data sheets. I'm getting 0.39 as measured Qts for three out of the four pieces, and Fs in the range of 42-44Hz (published is 39Hz).

My question basically is: can I trust my added-mass Vas, or should I necessarily do the sealed box measurements too?

[jomor]

i thought Peerless where made in China.....


i m not sure which method is more accurate, but i would trust the measurement more than the manufacturer's data. make sure your drivers are runned in before measuring them. If you havent done so, create a .wav file ( google for a sine generator ) of 10 Hz, in stereo, with reversed phase on the left (or right channel), and burn it into a cd. 10Hz are not audible (reversing the phase in one channel helps too) and you can play this for hours to run in the woofers before making any measurements. Fs for example will be quite higher if the driver is brand new..

[jbateman]

I can't say how accurate the added mass method is...I just don't think these issues are all that critical, myself.
But one thing you could do is move on to the next phase, where you select RESOURCE/NEW ENCLOSURE. You can specify box dimensions, insert your driver, and model it's response. Then you could vary the VAS parameter and see how it affects the frequency response. It may not make much difference....a fraction of a dB or something like that.

Actually, I think there's a way to let the SW calculate the optimal box for flattest response. I think after you create new enclosure, you select CALCULATE/ then pick sealed or vented, and the program creates a new box and/or port. You could then change the VAS of your driver, then repeat the process and see what difference SW chooses for your box size. (This is certainly less time consuming than building a physical box just to do the SEALED BOX method)

[JMB]

Measurements are most accurate as you get further away from DC (0 Hz). With the added mass approach, you are decreasing Fs and thereby decreasing accuracy if the frequencies are too low. As long as you are not using frequencies much lower than 20 Hz, you should be fine. Also remember that with the added mass method, you want to decrease the Fs by about 25%.

The closed box method raises the Fs as it's comparitor and is therefore a better choice if the frequencies involved are very low.

The accuracy of each depends upon how accurate you are with your measurements including the weight and how well sealed the box is for closed box measurements.

HTH

Jay

[Zaph]

I've measured hundreds of woofers using both methods. Delta compliance Vas is generally much more accurate than delta mass. I've determined that the mass inaccuracy does not come from a bad weight measurement of the mass itself, but the way it is affixed to the cone. Any adhesive method using a soft compound effectively adds damping to the mass, greatly reducing accuracy. For example, the "blue-tack with coins" method I used years ago. So, if you have some test boxes handy, delta compliance is the way to go. But if not, there's a better way to do delta mass:

It requires a selection of small and medium size neodymium magnets and some medium density closed cell foam. The magnets must be a known weight. Get them accurately measured if you need to. Basically, to use this method, you place neo magnets opposite each other on both sides of the cone and they grip the cone strongly due to their attraction to the magnet on the other side of the cone. If multiple weights are required, space them evenly arond the cone.

I mention the foam requirement because sometimes the magnets are so strong that they may be difficult to remove or the pressure may cause a mark on the cone if it has a textured surface. A thin strip of foam may make the magnet easier to remove, and reduce the potential for marking the cone. Find a few various thicknesses of cardboard to practice on before trying a real cone.

There is no more accurate way to do delta mass measurements.

Aside from that, don't be surprised if you vas measurements are way off spec anyway. Manufacturer specifications are rarely accurate.

[JMB]

Hi Zaph,

Thanks for your clarification. I have tried both methods (Magnet and Blue Tac) and I agree that the magnet method is more accurate if you can evenly distribute the weight. In my experience it is best to get a bunch of lighter magnets so that you can evenly distribute the weight. I have seen some inconsistent VAS measurements with unevenly placed magnets. These inconsistencies seem to be able to cause even more inaccuracy than the blue tac method, at times. Have you had the same experience?

The blue tac method tends to be fairly evenly distributed in weight if done correctly but I can see your point about the damping effect where the effective weight is influenced by the acceleration. I had just empirically noted that my results seemed better with the magnets, again evenly distributed.

Regarding the box method, I agree that it is generally more accurate assuming you can get enough of a change in VAS for your calculations.

Jay

[tcpip]

Thanks, all of you. You've given me all my answers.

I'll do a sensitivity analysis... try calculating QB3 box size with my measured Vas, and then with the manufacturer's published Vas, and see how much the box size differs. I'll also see what happens to the response curve if the real Vas is 36L and I build a box as per what would be the size for QB3 for 30L.

And then I guess I'll try to see if I can do the sealed-box measurement anyway.

Incidentally, I didn't apply either magnets or Blu-tak. I just placed the coins on the cone, just like that. And about the percentage decrease in Fs, I got about a 50% decrease with 30g.

Thanks again.

[tcpip]

Originally posted by jomor
i thought Peerless where made in China.....

The Peerless that most non-Indian DIYers know is a Danish company. Their drivers are made in China today, some other country tomorrow, based on economics. But there's another "Peerless", which is Peerless India, whose Website carries the datasheet I'd pointed to in my earlier post. This is an OEM speaker driver manufacturer. They were born with a tie-up with Peerless of Denmark, to get technology from Denmark and make drivers using cheap labour in India. It seems the Danish tie-up is now gone, but the name remains. Peerless India continues to build drivers in bulk under OEM contracts for a lot of big names in the world. Incidentally, the GR Research M130 series of 5" midbass drivers are made by Peerless India, I believe, and are slightly customised versions of their own S13NG-8 driver or a very close cousin, it is believed. This GR-Research driver is a well respected one in DIY circles: Murphyblaster.com has used it in many projects.

Some of the excess lots of these Peerless India drivers are released in a very unpredictable, unstructured and informal way into the Indian market, and are superior to any other Indian-made drivers available in the market here. The Kevlar drivers I'm working with are made by Peerless India and have been made available in the local market in the same way. They cost the INR equivalent of about $30 each. With this kind of supply channels, you never know when a particular model will disappear from the market's inventory, and whether it'll ever re-appear. Make speakers while they last!

i m not sure which method is more accurate, but i would trust the measurement more than the manufacturer's data. make sure your drivers are runned in before measuring them.

Can you give me some data of exactly what percentage change of Fs, Qts and Vas you have seen before and after breakin? I've heard data from some diyers that break-in is a myth when it comes to impact on measured parameters... hardly any measurable change is seen. I just wanted to verify this fact.

[jomor]

thanks for the info

[wintermute]

Hi TCPIP,

one thing to be aware of is that as the fs increases the VAS will decrease, therefore since your FS measurements are higher than published specs it is normal that your vas measurements are lower... whether they are in proportion or not I don't know, but I wouldn't worry too much

I used delta mass, using a concave washer stuck to the center of the dust cap with some magic tape (the scotch stuff that doesn't leave residue when you peel it off). I did measurements a number of times taking the washer off and putting it back on and got consistent results. I'd previously used bluetack and plasticine, but found that they marked the cone and gave inconsistent results.

I think if the weights were sitting loose they would have been vibrating like crazy and affecting your results... Also if the wheight is significant you might not get accurate results if the cone is horizontal, you probably should mount it vertically.

Tony.