Is the Human Body a Sound Absorber?

MFAC / MSAC

The Relationship Between Sound and Humans in Concert Halls and Anechoic Chambers

In the field of acoustic design, the human body is considered an element that influences the acoustic environment. This article focuses on the acoustic properties of the human body, particularly its sound absorption characteristics, and explains how these are taken into account in the design of concert halls and anechoic chambers.

The Human Body Absorbs Mid-to-High Frequencies Well

The human body generally absorbs sound effectively in the frequency range of 500 Hz to 5,000 Hz. This is because the body is largely composed of soft tissues rich in water content, which readily convert sound wave energy into heat.

Additionally, clothing significantly contributes to sound absorption. Materials like wool, fleece, and cotton are particularly effective at diffusing and absorbing high-frequency sounds. Therefore, in concert halls, the combination of the human body and clothing functions collectively as a sound-absorbing material.

This mid-to-high frequency range encompasses the frequencies of human speech and many musical instruments, making it crucial in acoustic spaces. Consequently, the presence or absence of people in a space can cause noticeable changes in sound reverberation and sound pressure levels.

Concert Halls Are Designed Assuming Human Presence

Acoustic designs of concert halls are based on the assumption that the hall is fully occupied. When the audience is seated, mid-to-high frequency sounds are absorbed, altering the acoustic characteristics of the space. While the impact of a single person may be minimal, a large number of people can significantly affect acoustics and audibility.

To compensate for this, seating in concert halls is made from materials with sound absorption properties equivalent to the human body, ensuring consistent acoustic performance even when seats are unoccupied. During the design phase, simulations and head-and-torso mannequins are used to replicate the acoustic properties of the human body and predict changes in sound diffusion and reverberation.

In Anechoic Chambers, Human Presence Causes Measurement Errors

In contrast, anechoic chambers aim to eliminate the influence of human presence during design and operation, as it can lead to measurement errors.

Particularly in anechoic chambers used for evaluating acoustic equipment or vehicle development (e.g., VSAC or EAAC), precise free-field conditions adhering to ISO standards (ISO 3745, ISO 3744, etc.) are required. Under these conditions, the presence of a person in the chamber can reduce sound pressure levels by approximately 0.5 to 1.5 dB, affecting measurement results.

Therefore, acoustic measurements in anechoic chambers are typically conducted without anyone present inside. Measurement equipment is set up outside the chamber, with microphones placed inside to capture sound.

At High Frequencies, Reflection Becomes Dominant

While the human body absorbs mid-to-high frequency sounds, at even higher frequencies (such as ultrasonic ranges), sound waves are more likely to be reflected by the skin or clothing. As a result, evaluations involving ultrasonic sensors consider the reflective properties of the human body, often employing dummy bodies or conducting assessments in the absence of people.

Conclusion: Utilizing Human Acoustic Properties in Design

Concert halls are designed with the premise that the acoustic space is completed by the presence of people. Conversely, anechoic chambers are constructed to provide an accurate acoustic evaluation environment by eliminating human presence.

Even within the same acoustic space, the approach to human acoustic properties (EQ characteristics) significantly influences design strategies. In the field of acoustic evaluation, designs that account for these human characteristics are essential, enabling the space itself to function as an optimized “acoustic system” tailored to its intended purpose.