Determining Acoustic Response of a Vented Box

In the absence of a gigantic anechoic chamber, where a microphone can be placed 1 meter away for 20Hz measurements to be made accurately, a more practical approach is to take measurements in Near Field. This technique dictates that the microphone be placed within the pressure field of the transducer. For a woofer, the microphone is positioned not more than 0.25" from the center of the dustcap. This proximity prevents any boundary reflections from affecting the measurements.

PL18 Near Field

The curve on the right in Figure 7 is of the PL18 measured in Near Field. Note that this is now a SPL measurement, no longer an impedance sweep. With the microphone so close to the woofer, SPL registered at about 117dB with 2.83V input.

Port Near Field

To measure the port, the microphone is re- positioned to the center of the port's mouth, on the same plane as the baffle board. The left curve in Figure 7 is the result of the sweep. The SPL is an astonishing 120dB.


(Figure 7) Near Field of Port and Vifa PL18WO09


Near Field to Far Field

Before the output of the port can be combined with the natural rolloff of the woofer, both measurements must be referenced back to 1 meter for Far Field - half space.

This is easily done in LMS by applying a Scaling Factor to the Near Field Measurements. The result (Figure 8) is equivalent to taking measurements with the microphone 1 meter away but without any boundary reflections.

Note that the port is now softer (80dB) compared to the PL18 (87dB).


(Figure 8) Scaled to 1 meter


Tail Correction & Minimum Phase

In Figure 8, it is observed that the PL18 is rolling off at less than 12dB/oct at the tail end. If left uncorrected, the accuracy of LMS computation will be affected.

Before Minimum Phase is generated, Tail Correction is applied at 25Hz to force the curves to rolloff at an asymptotic rate of 12dB/oct.

Figure 9 shows both curves with Tails corrected together with their Minimum Phase Transform.

With their Minimum Phase generated, LMS is now ready to merge the output of the port with the PL18.


(Figure 9) Tail Corrected + Minimum Phase


Final Acoustic Bass Response

The solid curve in Figure 10 is the summed response of the PL18 with port. The PL18 without port is represented by the dotted curve.

Quick inspection of the summed response looks promising. F3 and slope appear similar to simulation. More importantly, there is no peaking.

We shall now examine in detail the region below 100Hz to see how closely simulation mirrors the actual acoustic response.


(Figure 10) Summed Response of PL18WO09 & Port
 


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