What is Echo and Reverberation?

An echo is, by definition, a reflection of sound. If so many reflections are observed that you can't distinguish between them, the proper term is reverberation, which is what we experience in most enclosed spaces, or rooms. The most current understanding of acoustic science tells us reverberation in a room is caused by sound waves bouncing back and forth between parallel wall surfaces. Our research, however, has shown us that this over-simplification is highly improbable.

Sound travels very fast, at 340 meters per second. Based on the rate at which sound waves physically decay, even in an idealized environment, audible sound would decay by 60 decibels in mere tenths of a second. However, in most rooms, a measured decay time of one second or more is common. No amount of passive reinforcement would be able to provide enough sustained increase in the sound pressure level to lengthen the decay time by an order of magnitude. Some form of acoustic amplification is therefore necessary to add the amount of energy required to increase the decay time.

One example of acoustic amplification that we see in enclosed spaces is room modes. Certain frequencies are reinforced by the physical dimensions of the room when the size of the wavelength corresponds with the size of the room. This results in an amplification of the frequency, which is often observed as a boomy bass note that is much louder and sustains longer than frequencies above and below it.

The focus of our research has been analyzing the discrepancy between the rate at which sound should theoretically decay in a room and the much longer observed decay time that we see in practice. Some form of acoustic amplification is required to produce such a difference in the decay time, and we have discovered that the corners of a room act as a horn, one of the original acoustic amplifiers. When sound energy travels into a corner, the sound energy folds into this angled structure and creates a phase distortion. This distorted signal is then reflected back into the room and amplified by the shape of the corner. This causes an increase in the sound pressure level and explains the increased decay time observed in enclosed spaces. In addition to the undesired lengthening of the decay, the distorted signal also further decreases the fidelity of the sound in the room. This distortion is the cause of most of the frequency response aberrations that we see in every room.


How do the Eighth Nerve products work?

The Eighth Nerve products are unique among acoustic devices. They are designed to allow sound energy to travel past the products into the corner, and then trap the distorted, amplified return energy by cycling the reflections back into the absorptive material until they are fully attenuated. By eliminating this distorted and amplified return wave, we reduce the reverberation time (RT-60) and typical room frequency anomalies, which results in a flattening of the in-room frequency response.

Another benefit of this approach is that we do not over-attenuate the higher frequencies present in the room. All other acoustic products have an unintended side-effect of over-attenuating high frequency energy. This is due to the fact that all acoustic absorptive materials are more effective at attenuating high frequency energy, with its smaller wavelengths, than lower frequency waves, which have much larger wavelengths that are often physically larger than the absorptive material itself. Adding this type of material can result in a perceived improvement in the sound due to the reduction of higher frequency energy: because the high frequency energy is reduced, many of the effects of reverberation are masked, but this does not reduce the amount of distortion present, and adversely affects the accuracy of the in-room frequency response further. By only affecting the sound returning from the corners, the Eighth Nerve products retain a balanced high frequency response.

The phase anomalies caused by corner distortion are responsible for peaks and nulls in the lower frequencies as well. Although the size of the products would not indicate an ability to affect low frequency response, eliminating these phase anomalies before they propagate into the room results in a significant improvement in low frequency response by reducing the peaks and reducing the severity of the nulls.

In addition to the technical improvements achieved by flattening the in-room frequency response, many qualitative benefits are realized. Reducing the overall distortion level in the room produces a sound that is much more natural, and listener fatigue is all but eliminated. The soundstage and imaging of the system are vastly improved in definition and scale. Dynamic contrasts and ambient information are more apparent. And because the products are small and inconspicuous, there is hardly any visual compromise to your listening or performing space.