A paper cone speaker unit has no voice coil and is not connected to a power source, so it can only be measured indirectly. This method is similar to the way a passive radiator is tested.
When the paper cone speaker unit reaches its resonant frequency, the vibration amplitude of the unit itself becomes the largest. This is the same concept as a speaker reaching its maximum vibration amplitude at its resonant frequency.
Therefore, we place an active speaker nearby as the signal source. When the sound reaches the paper cone and causes it to resonate, we can observe the vibration amplitude with our eyes or with measuring instruments.
When the vibration amplitude reaches its maximum, the frequency at that moment is the resonant frequency. To avoid affecting the accuracy of the test, the frequency response of the sound source should be flat, without large peaks or dips.
At the beginning, we should start testing from the lowest frequency and then gradually increase the frequency to find the resonant frequency.
This process follows the low- and mid-frequency sections of the frequency response curve and the impedance curve.
To improve test accuracy and reduce eye fatigue during observation, a high-sensitivity microphone, or a vibration sensor, can be placed on the paper cone being tested and connected to an amplifier.
When the paper cone under test resonates, the microphone will receive the highest sound pressure level, and the amplifier will show its maximum reading.
A laser instrument can also be used to measure the resonant vibration amplitude. This method is called laser holography.
Holography is a two-step imaging method that uses light interference to record an object and then uses diffraction to reproduce the object.
When this imaging method is used to measure vibration amplitude, it can show the overall two-dimensional vibration pattern. It can measure very small vibration amplitudes at the scale of light wavelengths and can also measure the diaphragm without physical contact.
There are several interferometric measurement methods used in holography, and the most common one is the time-average method.
This method uses continuous exposure for a period much longer than the vibration cycle of the object. Although it can only measure steady-state vibration and cannot measure phase, it can measure the absolute value of the vibration amplitude.
