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2019 Top 20 | #20: Working In Tandem: Combining Near And Far Field Measurements

Insights into overcoming the inaccuracy of quasi-anechoic techniques at low frequencies.

Editor’s Note: Part 1 and Part 2 of this series provided the essentials of loudspeaker measurement. Here the author builds upon that discussion with insights regarding combining near and far field measurements.

One approach to overcoming the inaccuracy of quasi-anechoic techniques at low frequencies involves combining them with near field measurements. Keele [1] demonstrated that the far field frequency response of a loudspeaker at low frequencies can be estimated by measurements in the near field.

This relationship is valid at low frequencies, at which the driver behaves like a rigid piston. In practice, the measurement microphone must be placed very close to the dust cap of the driver. Theoretically, to be within 1 dB of the true near field pressure, the microphone must be within 0.11r of the dust cap, where r is the driver radius.

For example, for the 127 mm (5 in) diameter driver listed above, the microphone would need to be less than 7 mm (0.28 in) from the dust cap. At this close distance, reflections and noise are essentially eliminated.

Struck and Temme [2] presented a method whereby a loudspeaker’s full-range frequency response is derived by combining its low-frequency response as measured by the near field technique with its high-frequency response as measured using a quasi-anechoic technique. This is illustrated in Figure 1.

Figure 1. Graphic illustrating the technique of combining near field and far field measurements to obtain full-range frequency response (excluding diffraction effects).

The far field quasi-anechoic measurement is conducted at some distance greater than 3M, where M is the most significant dimension of the loudspeaker enclosure. This measurement provides data that is useful at frequencies greater than 1/T, where T is the length of the time window applied to the impulse response to eliminate room reflections.

The near field measurement is conducted with the microphone very close to the driver. This measurement is valid at frequencies below f=c/πM, where c is the speed of sound and M is the significant cabinet dimension. The near field measurement results in a frequency response curve that is much higher in level than the far field response.

The curves are combined by shifting the near field response curve down in level such that it matches the far field curve at some point in the overlap region and splicing the curves together. The overlap region is the frequency range between 1/T and c/πM.

For the near field/far field splice technique to work, the room must be large enough relative to the size of the loudspeaker for there to be an overlap frequency range. In addition, if the loudspeaker system has one or more ports or multiple drivers, the technique becomes more complicated: A near field measurement of each driver and port must be made and these must be summed in the complex domain (i.e., with regard to magnitude and phase) after first scaling the component responses to account for their different radiating surface areas.

Diffraction Effects

A disadvantage of using a near field measurement to estimate the low-frequency response of a loudspeaker enclosure in the far field is that it does not include the effects of diffraction from the enclosure edges, sometimes referred to as baffle step diffraction. This effect causes an apparent “loss” at low frequencies and ripples in the frequency response at higher frequencies which are not present when the driver is mounted in an infinite baffle.

In simple terms, the baffle diffraction effect can be explained as a transition from 4π space to 2π space radiation as the wavelength of sound decreases with increasing frequency. At low frequencies, where the wavelength is long compared to the baffle dimensions, the baffle is acoustically transparent, and the sound radiates into 4π space. At high frequencies, where the wavelength is short compared to the baffle dimensions, a driver radiates into a half-space (or 2π space). As a result, the overall mean sound pressure is twice the level (or 6 dB higher) at high frequencies.

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