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In the Forum: Playback Listening
In the Thread: Remedies the Beauty
Post Subject: Be careful with passive radiatorsPosted by rowuk on: 10/28/2022
The passive radiator is simply put a closed box Helmholtz resonator with low "q". What does that mean? It means that the passive radiator only resonates at its tuned frequency and its multiples - regardless of the frequency radiated by the driven woofer.
Let us say that the passive is tuned to 30 Hz. Our recording of Also Sprach Zarathustra starts with a pipe organ low C. This is 32 Hz. The Tannoy driver moves at 32 Hz, the passive radiator can only move at its tuned frequency - 30 Hz. As it adds additional pressure, we believe that it is a "louder" 32 Hz. If we measure the actual frequencies, we find 32(Tannoy)/30(PR) and the sum (30+32) 62 and difference (30-32) 2 Hz coming out of the speaker. The actual amount of additional pressure depends on the Q of the resonance.
The reason that we believe differently is because most frequency response sweeps are made with the assumption that the pressure picked up by the microphone correlates to the given input frequency. It does not. Measure and weep.
I noticed this effect when tuning a piano once. A detuned string still got my tuning fork to resonate.
https://www.hunecke.de/en/knowledge/absorbers/helmholtz-resonators.html
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