Fathom f112 Measurements

Measurements were performed using the Sample Champion program developed by Paolo Guidorzi of Purebits. The program is a Maximum Length Sequence (MLS) program that can generate an impulse response from this pseudorandom signal. The Fast Fourier Transformation is applied to the impulse response to generate the system frequency response. The operating principle is the same as that of the well known MLSSA program.

Test samples were taken using 16 bit resolution with a 48 kHz sampling frequency. Fast Fourier Transformations were performed at 64K allowing for a frequency resolution of .73 Hz, which is more refined than even 1/24^th octave resolution at the lowest octave; the A0 key on the piano has a fundamental frequency of 27.5 Hz.

Figure 1: Impulse response.

Figure 1 is the subwoofer’s impulse response. The basic shape of the curve is typical of low frequency driver; here we see a fairly smooth curve that decays within a few cycles indicating good damping characteristics.

Figure 2: Frequency response.

Figure 2 shows the nearfield frequency response for a sound pressure level of 80 dB when measured at 1 meter. Significant roll off starts to occur around 21 to 22 Hz based on this measurement, which is within reasonable measurement tolerance of the published specifications.

Figure 3: Power compression curves.

Figure 3 shows frequency response at various SPL levels at 1 meter. Measurements were taken nearfield with peak sound pressures of 90, 100, 105, and 109 dB when measured at 1 meter from the sub. Analysis of these curves must include some understanding of measurement and instrumentation limitations. Due to environmental conditions prevailing in a Chicago winter, ground plane measurements were precluded as a possibility and due to the high output capabilities of this sub, nearfield measurement of max SPL caused clipping of the microphone preamp as volume is increased.

Tests I performed indicate that the sub can get between 105 and 109 dB at 1 meter before compression using a wide band frequency input such as an MLS signal. Consideration of measurement accuracy at high SPL must be tempered with understanding that MLS testing is sensitive to any nonlinearity in the measurement, including from test gear. The microphone preamp indicated clipping at 109 dB under the maximum gain adjustment I could make, so it is very likely that the f112 can actually get higher than indicated by these measurements.

Figure 4: Maximum in room SPL at 2 meters by frequency.

Figure 4 was provided by JL Audio with a reported maximum rms SPL output of over 117 dB for a single frequency, a 62 Hz sine wave, and 112 dB averaged over a range between 25 and 62 Hz with THD between 5.2 to 9.9% for various frequencies, in room at 2 meters.

For real world input signals, such as music, the frequency content will be somewhere between these two limits and correspondingly, the SPL output will also be between these two bounds. Either way, SPL that exceeds 109 dB is a substantial amount of output.