I am interested in measuring the vas of a speaker. I have looked at a few articles and they've said to build a box about the dimension of the. speaker as a cube. However, I feel that there is some incompleteness in the description of the process. I presume the speaker then gets screwed onto the box upside down. What doesn't seem to be explained is where exactly the measurement is made with the microphone. Maybe I'm missing something obvious. My apologies and thank you in advance for any help in this matter.
If you want to measure a change in resonance frequency you'd be better measuring impedance. No microphone needed.
https://www.sound-au.com/tsp.htm#s2
https://www.sound-au.com/tsp.htm#s2
I still don't understand. I have looked at the esp article And I still don't understand How I'm measuring Vas. I make the Cube. put the speaker over it and then? I once undertook the Added mass method. with DATS V3. The value may have been wrong and therefore I want to double check this method With a closed box method.
If you measure the frequency of resonance of the speaker sealed over the cut-out in the standard box, as well as the frequency of resonance of the speaker in free air, then you can apply the following formula to calculate Vas.
Vas = 1.15 x [(fb/fa)^2 - 1] x Vb
Where:
fb = resonance frequency in box in Hz
fa = resonance frequency in free air in Hz
Vb = volume of test box in cubic feet
(^2 means to the power of 2, i.e., squared)
Vas = 1.15 x [(fb/fa)^2 - 1] x Vb
Where:
fb = resonance frequency in box in Hz
fa = resonance frequency in free air in Hz
Vb = volume of test box in cubic feet
(^2 means to the power of 2, i.e., squared)
Last edited:
Yes, but where do I put the microphone to measure This resonant frequency of the. speaker? The microphone doesn't go in the box. so it must sit out of the box somewhere. Where does it go? Thanks.
You put a resistor in series with the speaker terminals, maybe to 20 ohms, and measure the voltage across the speaker as you sweep frequency. When the voltage is at maximum, it's at resonance.
ETA: You also measure the resonance with the speaker in free air, and use both of these frequencies in the proper places of whatever formula you use.
ETA: You also measure the resonance with the speaker in free air, and use both of these frequencies in the proper places of whatever formula you use.
Last edited:
So I need an ac Voltmeter to do this measuring a set of known frequencies and plotting the graph. It can't be registered with a microphone plotted as a frequency curve. I'm trying to get a clear picture of this process. I haven't been able to find any diagrams or videos. showing how this is exactly done.
Hi, you don't need to plot a graph nor use a microphone.
You just need to measure 2 frequencies, as posted by Galu: fb (box) and fa (free air)
Vb is the box volume that you calculate with the internal box 3 dimensions (AxBxC).
You need a signal sine wave generator (smartphone app, PC etc), an amplifier and a multimeter (AC volt meter) or an oscilloscope.
Make sure you amplifier has a flat response with no bass gain or attenuation.
If you are not sure, put the voltmeter across the amplifier output connectors and sweep the frequency with the speaker connected.
This is just a check. Voltage should be constant between 20Hz and 1kHz at least.
For each setup (speaker in free air and in the box), measure the resonance frequencies by repeating the steps below.
1. Connect your speaker to the amplifier with a resistor in series (22ohms for example).
2. Put the voltmeter/scope across speaker connectors
3. Turn on amplifier and signal generator
4. Set signal generator to 400Hz (well above normal resonance) and adjust the amplifier to provide around 200mv RMS to the speaker (just example for reference)
5. Slowly sweep down the signal generator towards the lower frequencies and observe the volt meter. The voltage will start to increase around the resonance frequency, will peak at exactly resonance frequency and then will start to decrease towards the lower frequencies.
6. Take note of the resonance frequency when the voltage peaks. Goes back and forth around this peak just to confirm.
Now you have the 3 numbers: Vb, fb and fa and are able to calculate Vas as follows:
Vas = 1.15 x [(fb/fa)^2 - 1] x Vb
Regarding the woofer installation in the box, you can do whatever you prefer. If you put the woofer facing outwards, as it is normally installed in a baffle, just discount its volume from the Vb.
ron68 has expanded the experimental method I mentioned beautifully, although the driver would be pressed face down over the cut-out in the test box so as not to take up box volume as it would if installed in the box.
The formula I gave comes from David Weems' book on Building Speaker Enclosures.
He also gives the appropriate standard box volumes for testing drivers of different frame diameters:
Up to 6": 0.2 cu ft; 8": 0.5 cu ft; 10": 1.0 cu ft; 12": 1.0 cu ft
(Obviously, the diameter of the box cut-out should be smaller than the frame diameter of the driver.)
The formula I gave comes from David Weems' book on Building Speaker Enclosures.
He also gives the appropriate standard box volumes for testing drivers of different frame diameters:
Up to 6": 0.2 cu ft; 8": 0.5 cu ft; 10": 1.0 cu ft; 12": 1.0 cu ft
(Obviously, the diameter of the box cut-out should be smaller than the frame diameter of the driver.)