We move on to looking at developments in noise cancelling technology and the role it can play in achieving clarity and comfort within headsets for intercom use.
Clarity of sound is incredibly important for any member of the crew. And this is especially so when working in acoustically hostile environments such as rock concerts, car racing and sports events. The background sound isn’t always continuous and can vary enormously depending on the event.
Our human acoustic system has evolved over tens of thousands of years and is a fundamental part of our defense systems. We are very susceptible to loud and sudden noises or even disturbance in the continuum of consistent sound. Having such features of the human acoustic system (HAS) has kept us alive as any predator making the smallest noise would wake us and quickly initiate our fight or flight system.
Although we no longer usually need to fear an attacking bear in the safety of our own homes, these features of the HAS are still incredibly important to us in everyday life. For example, if we cross a seemingly safe road, we soon hear not only an unexpected car, but also the direction it is traveling from. We are able to detect the phase difference between our ears and hence approximate the direction of the sound.
When we wear a set of headphones, we are creating an artificial environment for us to listen in. If not designed correctly then this can have a detrimental effect on us due to the intricacies of the HAS. This further demonstrates that not only do the headphones need to be high-quality, but this is justification to create mechanically robust devices. If a cable becomes loose or a dry joint in the soldering develops then it can cause crackling, distortion and discontinuities in the audio.
Prolonged exposure to audio discontinuities can in itself lead to fatigue as we continuously excite our subconscious response to a perceived threat that isn’t there, resulting in physical stress. Furthermore, we need to increase our focus on trying to hear the important communication being relayed which in itself also leads to experiences of physical stress.
Maintaining Operator Comfort
Comfort for headphones splits into two areas. There’s the physical experience of wearing the devices to achieve a compromise between maintaining separation from the background audio, and keeping the headphones fixed to our heads without causing compression discomfort. And then there’s the challenge of making sure the audio replayed by the loudspeakers is consistent, continuous and predictable.
Noise cancellation splits into two approximate frequency bands. From 100hz to 1khz active electronics are used, and above 1KHz the noise blocking is achieved using foam type substances in the earmuffs.
The key is we need to block the external sounds from dominating the speech from the production crew and any clean-feeds or mix-minuses from the sound console. For the active part of the system, simplistically the audio is received on the headset microphone and played in anti-phase to the headset loudspeakers. This in effect will negate the air movement entering the headset and thus provide noise cancellation.
One of the challenges of this approach is that the background and external sounds vary enormously in frequency and amplitude. Consequently, the audio processing must have extremely small latency otherwise the effectiveness of the noise cancellation will be compromised. This is best heard at rock concerts with the high percussive dynamic sounds and with the roaring engines in car racing.
Complex Acoustical Waveforms
Vehicles travelling at high speeds with loud and powerful engines are susceptible to the Doppler effect, and reflected sounds bouncing off walls and other hard surfaces contribute to the comb filtering effect. Another audio effect in rock concerts is caused through reflections in large stadiums with long audio path delays of many milliseconds.
All these features within such acoustically hostile environments create complex waveforms with both short- and long-term components in the audio signature. If the headsets were operating in static environments, then the audio processing needed for the noise cancellation would be straight forward. However, as the sound sources, their relative environments and people within them wearing the headsets are all highly dynamic and moving, complex algorithms are needed to detect the background audio and then negate it in the headsets for the comfort of the operator.
Furthermore, we must remember that with all the noise cancellation going on and the complexity of the algorithms used to achieve it, the operator still needs to hear the director so clarity must be maintained. This is easier said than done and maintaining clarity of speech while removing unwanted background noise is a difficult and complex task to achieve and is one of the major differentiators between vendors.
Clarity And Comfort
Noise cancellation and ergonomic design leading to comfort of use has been the subject of much research in recent years, especially as complex algorithms have become available in active DSP designs. Consequently, vendors such as Bose, have optimized their design for many different industries, such as aviation and military - both of which rely on clarity of communication for the safety of their personnel. In these industries, lives are literally at risk.
This is one of the areas where broadcasters can benefit from the massive research and development investment vendors have made in other industries. Aeroplane pilots cannot run the risk of poor communication with air-traffic control, and military personnel operating in conflict zones clearly need accurate communication.
Vendors working in these industries have also had the opportunity to fine tune their products for physical durability. Although they’ve clearly achieved their objectives in relaying accurate communication in acoustically hostile environments, the need to make headsets durable and serviceable is equally important.
Learning From Other Industries
Aeroplane jet engines, aerodynamic interaction between ambient air and the flight surfaces, and cabin air-conditioning systems all contribute to the fatigue of the pilot. Noise cancelling headsets help to alleviate this. And the same is true for operating in military tanks where a combination of high transient sound sources and the resonances inside the structure of a combat tank all contribute to fatigue. Noise cancelling headphones help significantly in maintaining focus and keeping personnel alert.
Ease of use is incredibly important as it helps keep stress levels low and maintain clear communication. Aviation and military personnel wear headsets for hours at a time when working in their respective acoustically hostile environments. Broadcasters can once again benefit from the massive development in other industries so operators can wear headsets for long periods of time in comfort while maintaining clarity of communication.