Ported Loudspeaker Splice Test Sequence
Measures the anechoic response of a loudspeaker in an ordinary room by “splicing” near field and time-windowed, far field measurements
The purpose of this sequence is to measure the anechoic response of a loudspeaker in an ordinary room using both a near field and time-windowed, far field measurement “spliced” together to cover the full bandwidth of the loudspeaker’s response from 20 to 40 kHz.
First, the near field frequency response is measured using a 1/12th octave stepped sine sweep by placing the microphone very close to the low frequency driver and the port(s) if any. Then the far field frequency response is measured using a continuous log sweep with the Time Selective Response analysis algorithm.
The near field measurement is not affected by room reflections but at high frequencies, it does not represent the free field response. The far field, time-windowed measurement is not affected by room reflections but at low frequencies, it is affected by the room size which limits the width of the time window and corresponding frequency resolution. The goal is to find an overlap range of the two measurements where they are both valid and to choose a frequency at which to splice the two halves of the measurements together and obtain the full range free field response of the loudspeaker. This is done in several Post-processing steps including the inverse FFT of the final frequency response back to the time domain to calculate the impulse response.
There are two subsequences for both sealed and ported loudspeakers. The Near Field Ported loudspeaker sequence measures both the port and woofer. Then it asks for the surface area of both the woofer and port (or ports combined). It uses this information to scale the port level to the woofer level before complex summing their responses (see the references at the end of this application note for specifics).
The difference in level between the near and far field (both amplitude and phase) at the splice frequency is calculated and displayed in the table along with the phase correction and microphone distance from the
loudspeaker.
The sequence can be run with stored data as well and experimentation with the time window can be performed without having to remeasure data. The curve data and time waveforms (e.g. impulse response) can be exported for further analysis in programs such as our SoundMap™ time-frequency analysis software.
