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Putting acoustics to the test

Test and measurement has never been the most glamorous part of audio but it has always been necessary. As Kevin Hilton discovers, the need for better acoustic spaces and more efficient devices to operate in them, measurement technology is not only helping produce better systems but is also being pushed to the limits itself

Acoustics is a science, one that is deeply rooted in physics with a whole array of measuring devices and technologies at its disposal to create the best possible sounding spaces. There is also a more subjective, intuitive aspect to this kind of work, where the experience and ‘golden ears’ of the acoustician come into play as much as technology.

Today, there is also an increasing demand for better sound in more personalised situations such as gaming and listening to music. This, comments Jim Weir, applications specialist in acoustic, audio and environmental noise at Brüel & Kjær Sound and Vibration Measurement, is having a positive influence on AV installations. “The personal use of higher quality audio in headphones and in-ear buds that general consumers have these days and the opportunity to use those in a wide range of environments, like meeting scenarios, has at least challenged the corporate world to improve their audio quality,” he says.

Weir continues that, previously, there was not an emphasis on work facilities to address the acoustic environment. This is changing post-pandemic, calling for a change in approach from companies. “While they’re trying hard to move away from open office plans, they [still] want corporate areas that are open,” he observes. “We – and some of our major consultant customers – are seeing a lot more emphasis on speech privacy. This is because they have to have areas where meetings are more isolated. When people come back to the office they want to have a decent experience. With some of the basic measurements consultants are doing, they are seeing a desire to improve quality.”

As with any technical consideration, the level of acoustic treatment – and the calibration involved – depends on the purpose of a space. Weir explains that in a basic conference room, the key is keeping the noise levels down, which is a “by-product” of providing better isolation. “They want reasonable intelligibility,” he says. “Testing for speech intelligibility is something more of a secondary nature until you get to the larger corporate facilities. We’re also seeing organisations, including ourselves, creating specific rooms for recording podcasts and trying to improve the quality of those spaces.”

In doing this, Weir adds, the starting point is always background noise level. “People speak at a normal voice level of 55dB, but there are many home environments that struggle to get 30dB of dynamic range,” he says. “They have a hard time getting the signal to noise ratio up to 30dB, which means you need to have a background noise level well below that in a room. You can avoid that if you’re going to post-process, say for a podcast, because most of the time the background noise is consistent. Usually it’s things like a laptop, which can be moved further away or placed in a cabinet. Another aspect of noise is HVAC [heating, ventilation and air conditioning]. Again that can be removed in post-processing but during live events there is nothing worse than having an air conditioning vent going.”

In agreeing the acoustic noise floor from HVAC and sources outside a room should be low compared to normal speech levels, Roger Schwenke, senior scientist at Meyer Sound, points out that the reverberation time should be appropriate to the cubic volume of the room. For determining this, he says ITU-R BS 116-1 (Methods for the Subjective Assessment of Small Impairments in Audio Systems) is a “good, but maybe conservatively low, guide”.

In terms of the equipment used in corporate spaces, Schwenke outlines that microphones and loudspeakers should have a dynamic range able to accommodate the loudest anticipated sounds but not increase the noise floor. “Some consideration should be made to the path between the loudspeakers and the mics,” he says. “If loudspeakers can be arranged to be off-axis of the mics – or vice versa – then conference system echo-cancellation algorithms will be less stressed. Projects now are more willing to put effort into getting the HVAC noise floor low and to isolate rooms from external noises. Once the noise floor is low and the reverberation is controlled, the quality of the audio system is more apparent.”

A recently published acoustics standard is AES75, which covers the measurement of loudspeaker maximum sound levels using noise, specifically the M-Noise test signal. Existing standards also specify noise-based test signals and acknowledge that typical programme material has a diminishing RMS (root mean square – measured in Watts, and referring to the continuous power handling of a loudspeaker) level with increasing frequency. While M-Noise also does this, it features a relatively constant peak level as a function of frequency. This means the peak level (RMS level or crest factor) increases with frequency, which is regarded as an important additional characteristic.

As Schwenke points out, AES75 calls for a measurement analyser has the ability to not only analyse every sample at every frequency, but also produces overlapping time records. This contrasts with older technologies, which capture a time record and then check how the signal altered from input to output. AES75 was published in 2022 and the AES is always reviewing and considering other possible standards for acoustics. Among these is the Reverberation Slope Ratio (T60SR6), a proposed standard that would define a metric for scoring and grading the proportional relationship between the longest and shortest of six reverberation values, applied to fully-enclosed venues using a sound reinforcement system.

Chris Adair, of Adair Acoustic, considers this should be incorporated into new standards for the audio performance of venues and rooms employing amplified reproduction. “Today’s audience expectations are higher than ever and easily met with a good choice of modern equipment,” he says. “However, one key reason the qualitative expectations aren’t easily met is poor room acoustics; specifically excessive low frequency and very low frequency reverberation. The fact is that modern equipment can only add reverberation, not remove it. The venues themselves need a good passive acoustic response.”

The main target of any future standard featuring the Reverberation Slope Ratio would be concert and other entertainment venues. But it would still have relevance for the corporate AV sector, particularly presentation events and, possibly, larger meeting rooms. Michele Noselli of loudspeaker manufacturer Outline observes that the study and implementation of optimal acoustics is “of the utmost importance”. When it comes to creating acoustics and acoustic spaces for corporate AV, Noselli says it is crucial to consider he following: reverberation, with the decay time in a room being critical for clear speech and good quality sound; sound absorption to prevent echoes and reflections; isolation to stop sound leaking in from adjacent areas; background noise, which should be kept to a minimum; a well designed sound reinforcement system for clear and intelligible audio; and the flexibility a room to be used for different types of presentation.

Outline produces the ET Series turntable for polar measurements and GSR omni-directional architectural acoustics system, which Noselli says are used in a variety of industry sectors. “The accuracy of measurement necessary for the design of audio equipment, particularly loudspeakers and microphones, has had to improve significantly to meet the growing demands for high quality sound,” he says. “With the advancement in technology, people’s expectations for sound quality have increased and, as a result, manufacturers are under greater pressure to design equipment that meets these expectations.”

There is now a similar imperative for designing meeting rooms, which, post-pandemic, have taken on even greater importance due to the continuing reliance on hybrid working. Manufacturers such as Shure, which has targeted the meeting/conference room market with its MXA920 ceiling array microphone, see a need for at least minimum acoustic treatment using specialised tiles above the participants and carpeting beneath them. Beyond this, proper microphone and loudspeaker placement plays a part in optimising the performance of a space.

When it comes to the practicalities of measurement, Genelec’s R&D director Aki Mäkivirta comments that modern computers – and laptops in particular – have “removed any computational limitations” and enabled “truly mobile” measuring approaches, with wireless applications for larger venues. “Computers can also combine measurement with optimisation to automatically adjust and tune audio in spaces to requirements,” he says.

As for the future of this vital but still relatively arcane discipline, Mäkivirta speculates there may be increased application of directionally sensitive microphone technology, which could enable a more complete understanding of how audio behaves in specific spaces, leading to improved system optimisation. Artificial intelligence could also play a role, reducing the need for operators to fully understand or interpret measured data.

The demands of remote and hybrid meeting systems are, concludes Jim Weir at Brüel & Kjær, pushing the boundaries of acoustic measurement technologies. “What we’re seeing is not just one or two-channel measurements, but up to eight channels of input and a couple of channels of output so we can simulate more complicated scenarios involving direction finding and beamforming,” he says. “Audio sometimes drives a lot of technology in general, such as creating artificial environments for gaming and virtual reality. It could be an Atmos system in a movie theatre, or a home cinema sound bar with 17 drivers for beaming and directionally controlling the sound. DSP technology and our ability to implement it in ways that enhance our acoustic environments and experiences is what is driving the test and measurement market as well.”

The result of this will be ever more sophisticated measuring systems, which in turn will produce both better products and better acoustic spaces. Which can only be a good for audio in corporate AV and sound in general.