Measuring free air impedance with baffle?

[davidallancole]

Hello,

When measuring the impedance of a driver in Speaker Workshop to try to calculate the TS perimeters, I see there is a few different ways to set up the speaker. This link: http://vikash.info/audio/measurement_clamp/ shows some clamps that could be used to hold the speaker. The Speaker Workshop help files says to put it between 2 tables and clamp it down so that it is supported at the frame but nothing is obstructing it front or back.

Could a person use a flat baffle to mount a speaker on for measuring the free air impedance? Will this baffle affect the measurements in any way? I have a 20 lb Black Widow 15 inch driver that is hard to mount unobtrusively and though that if I build the baffle for the front of the speaker box because I know the size I want, I could then use that baffle standing up in the middle of my living room to measure the free air impedance.

Any thoughts?

 

[Dave Bullet]

I can't see any problems with that. The key with free air impedance measurements is to not increase driver Fs by any restriction to cone movement (ie. like an enclosure would do).

Personally, I've tried clamping drivers as well as just holding with my hand (magnet firmly against the flat edge of a sold table) and see little difference in the impedance and Fs. I am only using the sound card line output at about 9KHz peak to peak (ie. low volume) to do T/S measurements. Since T/S params are small signal, I figure this is an adequate enough drive level.... although I am getting weird VAS estimates via the delta mass method which I will post on separately.

Cheers,
David.

 

[davidallancole]

Hey Dave,

I have wild responses depending on whether I hold it or leave it on the bed or place it between 2 chairs. The waivering of my hand put a lot of crap into the signal, but anyways, the Fs has been anywhere from 47 to 60 Hz.

What kind of weird responses are you getting with the delta mass method? When I did mine I got around 12.5 cu ft when the manufacturer specs state 6.1 cu ft. The rest of the specs looked pretty close to manufacturer I would think. Does anything look wildly off? Its a pretty old driver with a lot of abuse and dirt and grim all over it.

I was able to prop it up on a wooden bar stool that I had around with something inline with one of the cross members on the basket holding the magnet up, so the test were done vertically.

Manufacturer
Qms 1.68
Qes 0.3
Qt 0.26
Fs 49
Vas 6.14

Measured
Qms 0.9
Qes 0.22
Qt 0.18
Fs 47.8
Vas 12.7

 

[Dave Bullet]

Hi David,

Hopefully this isn't one blind David leading another

have a look at my plots here:
http://www.diyaudio.com/forums/showthread.php?s=&threadid=114958

The first is a 10" driver with 57grms moving mass (Seas L26). The last graph is the Seas L15, a 5" driver with 8.1 gms moving mass.

I ended up with a 3 foot piece of 4x2, clamped to my heavy computer desk - sticking out like a gang plank. I then clamped the drivers to the end on a cut out point, trying to minimise any backwave interference.

The plots had hardly any noise. I used 49gms weight for the 10" 57 grm moving mass driver and various weights for the 5" driver - giving similar Vas measurements.

My problem seems to be - the bigger the driver, the worse (and smaller) the Vas gets - not bigger as in your case.

What can give a large Vas, is if you have under estimated the weight you have added to the cone. ie. if you added for example 50 grams mass to your 15" driver, but only put in 25 grams into SW foe the calculation, SW will think the driver is much more compliant than it actually is, hence making Vas much larger than it actually is.

I can't explain why Fs varies so much. In my case, Fs is pretty much consistent whether I hand hold the driver, or any amount of weight (therefore Fs change) I cause via mass.

I think it could be one of 4 things:

1. You haven't added enough mass to move the Fs down at least 25% over free air. Out of interest, how much mass are you using? What is your measured free air Fs (were the figures you provided after or before adding mass?)

2. Your sound card is not good enough to record an accurate lower impedance plot. I've read this usually occurs for drivers whose Fs is in the low 20s or 10s. If yours is at least 47Hz then this is unlikely

3. The manufacturer is wrong or the suspension has got really loose with age (you mentioned it was really old)

4. You've underestimated the actual mass you've added

Cheers,
David.

 

[AndrewT]

I don't use a baffle or anything rigid to support the test driver.

I hang the driver vertically from the lamp fitting in the middle of the room.

All noise in the house must be minimised.

 

[Vikash]

Hi David,

From the horses mouth if you like: http://www.diyaudio.com/forums/showthread.php?postid=744113#post744113

 

[Circlotron]

In my experience, the Q at resonance varies greatly depending on whether or not the speaker is held firmly or just dangling in mid air. I ended up putting the back of the magnet against a length of 100mm square wooden post, the square end against the magnet. Don't cover up any vent holes in the magnet - will completely skew results.

Some time ago I made a spreadsheet for determining t/s parameters using added mass method. Here it is if anybody is interested.

 

[Dave Bullet]

Hi David,

Do you have a largish box you could try a delta compliance test with?

In my case, I bought some old large speakers for $5 for 12" and larger driver testing.

Cheers,
David.

 

[Svante]

Putting the driver in a baffle increases the oscillating mass and the resonance frequency drops a bit.

I usually test drivers on a table, with the magnet downwards against the table. I have not given it much thought, but I can imagine that the mechanical impedance of the table could affect the parameters (very) slightly. On the other hand, so would the mechanical impedance of the loudspeaker box.

 

[davidallancole]

Hi guys,

Thanks for the replys. I don't have a large box to try the Vas calculations with.

I think I will just prop the speaker up on a stool in the middle of my room for the measurements. I found the bottom end of the curves to be more stable when proped up. If I had it sitting with the magnet flat on the table, there was a lot of noise in the low frequencies.