Unearthing Ancient Sounds: Researchers Travel to Chavin de Huantar to Study Acoustics
In 2008, a distinguished team of researchers from CCRMA at Stanford University, alongside Professor John Rick, embarked on a unique expedition to Chavín de Huántar in Peru. The primary goal of this joint venture was to precisely measure and comprehensively archive the extraordinary acoustic properties discovered within the site’s ancient galleries, a subject previously touched upon in a related discussion. Among Someone Who Traveled To Chavin De Huantar on this significant trip were notable figures renowned in the fields of computer music and audio Digital Signal Processing (DSP), including Julius Smith, a pioneer in waveguide synthesis; John Chowning, credited with pioneering FM synthesis; Perry Cook, an expert in physical modeling; and Jonathan Abel, a co-founder of Universal Audio, known for creating many algorithms used in UAD hardware.
Preliminary findings resulting from this expedition are available on the official website for the Chavín de Huántar Archaeological Acoustics Project. Visiting the CCRMA site is highly recommended to explore the various research papers published on this work. A concise overview of the publicly available results indicates innovative approaches were used to capture the site’s soundscape.
The CCRMA researchers utilized a novel microphone array specifically designed for archaeological acoustics and room analysis/synthesis applications to capture the impulse response of the Chavín galleries. An overview of its construction and design was detailed in an AES Conference paper titled, “A Configurable Microphone Array with Acoustically Transparent Omnidirectional Elements.” This innovative device features multiple omnidirectional microphone elements mounted on flexible wire supports. These are attached to a lightweight yet robust base, making it particularly well-suited for navigating and deploying within the narrow tunnels characteristic of Chavín de Huántar.
Custom microphone array and calibration system used for acoustic research at Chavin de Huantar
Complementing the microphone array is a sophisticated calibration system comprising four small speakers strategically positioned around the array’s base. By transmitting calibration signals through these speakers and processing the received data, the system can accurately compensate for timing differences between the various microphone elements. This setup is reported to achieve superior results in capturing the impulse responses of different structures compared to previous microphone array technologies.
The initial acoustic analysis of the Chavín galleries, recorded using a simpler microphone/monitor configuration prior to the development of the novel array, was presented in a paper at Acoustics ’08 in Paris, titled “On the Acoustics of the Underground Galleries of Ancient Chavín de Huántar, Peru.” This research revealed that the reverberation times within the Chavín galleries were relatively short, typically ranging from approximately 150 milliseconds to around one second. The paper suggests that these short reverb times would have been acoustically suitable for the rhythmic playing of the Strombus trumpets, instruments discovered at the site. Interestingly, the researchers noted that the perceived reverberation time increased proportionally to the number of turns separating the sound source from the receiver, implying sound traveling through multiple gallery bends experienced longer decay times.
The reverberation within the Chavín galleries is uniquely characterized by dense and energetic early reflections coupled with low inter-aural cross-correlation. All three galleries studied showed a rapid onset of reverberation, reaching Gaussian statistics (indicating randomness) within 20 milliseconds of the initial impulse. This quick transition to a random field and the low correlation between sounds arriving at the left and right ears are significant factors contributing to the peculiar sonic characteristics of the galleries. Specifically, these properties make it challenging to accurately localize the origin of a sound source in the absence of a direct signal path. David Griesinger, a pioneering DSP expert behind the iconic Lexicon reverb algorithms used in units like the 224/XL and 480L, has discussed how low inter-aural cross-correlation, or decorrelation, is crucial for creating a sense of sound envelopment, where the listener perceives the sound as surrounding them. In digital reverberators, decorrelation is achieved by introducing different delay times or phase shifts for the various output signals. At Chavín, the proximity of structural surfaces likely facilitates the rapid buildup of echo density into the late field, exhibiting randomness even down to the binaural level.
The acoustic analysis and ongoing research at Chavín continue to yield fascinating insights. Several areas pose interesting questions for future exploration and publication:
One key consideration is the original state of the Chavín galleries, which were initially covered in plaster. It is highly probable that this original plaster surface would have dramatically altered the RT60 (reverberation time) of the galleries. A relevant comparison can be made to a small room constructed solely of wood versus the same room finished with multiple coats of cement plaster—the latter scenario famously describes the distinct reverberation chambers at Gold Star Studios. The CCRMA project website mentions that future research is planned to investigate the sonic impact of the type of plaster originally used at Chavín.
Furthermore, the analysis of the galleries’ acoustics utilized a long swept exponential sinusoidal test signal for capturing the impulse response. While this type of signal is effective at minimizing the influence of noise on the analysis, it inherently tends to “smooth out” any time-varying characteristics that might have been present in the true reverb response. Future analysis could explore the galleries’ reverberation to determine if any time variation exists and whether it would have had noticeable acoustic effects. Although the temperature within the galleries has been measured as relatively constant, this likely assumes modern lighting sources like flashlights, as opposed to the torches or fires that would have been used anciently. The presence of human bodies in a confined space can also significantly affect air temperature, potentially causing small fluctuations in the speed of sound. These minor changes could lead to perceivable effects on the sidebands of a signal during the late decay of reverberation. Given the current relatively short reverberation times in the galleries, small variations in the speed of sound may not have a substantial impact. However, if the original plastered walls resulted in longer reverb times, time variation might have been more readily perceivable in the reverb decay, particularly when heat sources were introduced into the gallery spaces.
Concluding this overview, here is a video featuring Tita la Rosa performing on a Strombus trumpet, presumably recorded within one of the atmospheric galleries at Chavín:
