Making of M2

By Man-Sun Huh
21 Feb 2000

4th step: Measurement

Modern computer optimized speaker design method requires measurement data of each unit(W/O X-over network) installed in cabinet. With measured data, LEAP(or Clio, calsod...) can simulate network.

So, good measurement of each unit installed in cabinet is very important. One have to measure both Impedance and Sound pressure level. Measurement of Impedance is quite simple for measurement program such as LMS, Clio, MLSSA... Fig.3 and Fig.4 shows measurement result of woofer and tweeter Fig. 3. Impedance measurement of woofer unit Fig. 4. Impedance measurement of tweeter unit.

Impedance can show us many information about enclosure and unit. It's like X-ray for man.

In Fig.3 we can see that duct tuning frequency is 38Hz (dip point between two peak at low frequency). Besides duct tuning frequency, one can see defeats of system. We can see some small ripples at 212Hz, 732Hz, 1367Hz, and 3960Hz. After some other experiments, I found the reason of each ripple.

212Hz ripple:

It's a resonance frequency of woofer frame. It doesn't happen when I measured a unit alone. It happens when installed on cabinet and It even can be felt by touching fingers to the frame. Excel woofer have only 4 screw holes on frame and frame is not well damped. I think SEAS have to consider to make more holes on the frame. To dampen that resonance, I attached damping materials to the back frame(See strips of Dynamat in Fig. 5. Treatment for woofer frame resonance) After that treatment it disappeared clearly.

732Hz ripple:

It is the cavity resonance of 230mm(depth of cabinet) and clearly disappeared after I stuffed sound absorbing material(Dacron). Almost any inner cavity resonance can be eliminated with sound absorbing material.

1,367Hz ripple:

The cause of this ripple is not clear but also disappeared after I stuffed sound absorbing material.

3,960Hz ripple:

It's the cone-break up resonance frequency of woofer diaphragm. If one want to use metal diaphragm unit, this always happens. It's because metal has low damping. To dampen that resonance I designed parallel LRC filter in Woofer crossover network.

Measurement of sound pressure level is not so easy. One have to measure in anechoic condition. I guess few of person have seen anechoic room. It's quite expensive room. Furthermore all anechoic room have limitation at low frequency because of the size of wedge.(Around 50Hz)

Then how do I measure anechoic response? I have a perfect anechoic room with free of charge. It's a large automobile parking lot near my house. And I use ground plane method for measurement. Fig.6 shows the method Fig. 6. Ground plane method. It measures 6dB higher because sound projected area is half the sphere. So, you can simply subtract -6dB to get exact value. But to measure outdoor measurement, you should have battery powered measurement system. I used notebook and battery powered amplifier to drive loudspeaker.

Fig. 7. SPL of woofer (W/O X-over) and Fig. 8. SPL of tweeter (W/O X-over) show sound pressure level of woofer and tweeter. In Woofer response, you can see so called 6dB loss at low frequency(around 300Hz). It happens because speaker baffle has limited area. Low frequency sound wave project to 4PI area while high frequency sound wave project to 2PI area. We have to consider this effect when designing a low pass crossover network and this is why almost every textbook formula of low pass filter doesn't work in real loudspeaker design. So, If you want to make real hi-fi loudspeaker, you have to use program which can import measured sound pressure and Impedance data (LEAP, CLIO, CALSOD, ...).

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