Forensic Acoustics: the Gary Zerola trial

Earlier this month I was hired by Janice Bassil, of Carney & Bassil, P.C. , to conduct a series of acoustic tests to determine audibility and intelligibility between different rooms in a Beacon Hill apartment in Boston.

The apartment in question is on both the first and basement floors of a multi-unit building. The first set of requested tests was to determine if a woman’s voice in the basement bedroom could be heard by witnesses in the living room immediately above the bedroom. The bedroom was accessed by a spiral staircase in the corner of the living room. To implement the test I would need to play a woman’s speech through a loudspeaker in the bedroom at conversational level and then measure the level of the audio in the living room above.

I brought a XP-based laptop with Sonar 6.2 installed on it, a MOTU Traveler, a pair of calibrated Earthworks TC25 omni-directional small diaphragm condenser microphones, and 2 SPL meters. The first step was to calibrate the speaker to a “conversational” level (about 60-70 dB). For my source of a woman’s voice, I chose to use a woman reading Shakespeare’s “Romeo and Juliet,” played back via the D.A.W. (Sonar in this case). I lowered the output of the voice until my SPL meter averaged between 60 and 70 SPL. At this point I placed 1 TC25 about 1 meter from the speaker, off-axis and then placed its twin in the center of the living room upstairs. A partner would measure SPL in the living room while I ran the D.A.W. and measured SPL in the bedroom.
The Traveler uses digital controllers for its on-board preamps, so I was able to set both preamps to precisely the same boost in gain of 20 dB. To calibrate the whole system and to create a baseline for our further tests, I ran a series of simple sine wave tests at different frequencies and different levels. This would help to determine which frequencies in particular traveled better or worse in this acoustic space. My partner and I took SPL readings for each tone which we recorded to compare later the peak and RMS levels in the D.A.W.

Having completed our baseline tests, we began to record the voice, take readings, then decrease the output levels and repeat. The goal was to determine at what point the voice was now longer intelligible and then no longer audible. Signal to noise ratios in the living room where higher do to increased exposure to street noise due to much larger windows compared to much smaller windows in the bedroom.

After completing two additional series of similar tests, we packed up the gear and I headed home to the studio to evaluate the audio we recorded and enter the data that we compiled. I entered the data into an Excel spreadsheet and organized the audio data so that each audio recording could be listened to separately, first the audio from the bedroom and then the audio from the living room. Doing this made it very clear how much the audio dropped in level as the sound moved from the bedroom’s acoustic space into the living room’s acoustic space.

The most powerful example was to recreate the acoustic experience of the test. Using the SPL meter, I matched the reading that I took for each part of the test to the output of the speaker that I was playing the test back on. For instance, if I had taken a reading of the voice reading at 65 SPL at 1 meter from the speaker, I would set the speaker to play back the test at 65 SPL. Then I played back both the recording from the bedroom and then the recording from the living room showing the same reduction in energy that was present at the test site.

After presenting my findings to my clients, I was asked to appear on March 24th at Suffolk County Superior Court to testify in the rape trial of Gary Zerola. I answered a series of questions from both the defense and prosecution and played the audio demo of my tests as I explained previously.

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