Hearing is believing - but Testing Helps


We live in the age of measurement and all too often the results are prejudiced by the methods employed and the ease of digitally processing data. As Per Bruel, the founder of B&K, arguably the foremost measurement company in audio history put so succinctly,
The development of standards is not always based on true science but more often a consensus of minds. The result is that all over world we use not only an incorrect frequency weighting, but also a wrong scale for noise levels, for which it was not intended in the first place.’
Noise levels are used as a design tool in many ways that do not always conform with the way we hear. Steady state noise is usually associated with ventilation systems and is rightly the basis for comparison with Noise Rating (NR) curves. Intermittent noise, however, is often masked by the steady kind, because of ‘Critical Bandwidths’ and short duration low frequency sounds are not heard as loudly because our hearing works on a ‘sample and hold’ basis that requires a longer exposure to register bass. For example a quickly passing tube train may register at NR20 with a fast reading analyser and yet be inaudible to anyone in the studio! A good maxim is to use a slow reading meter setting or a one second averaging (Leq, 1s) for good subjective comparisons between test reports and audibility.
The importance of meaningful definitions is more relevant in sound isolation than almost any other aspect of acoustic design in sound studios and auditoria. It is much easier to correct a reverberation problem than modify several tonnes of studio shell. Every time a recording is interrupted or a performance is compromised by noise intrusion there is an implication of blame. Any ambiguity in either the definition or execution of the studio design or in the measurement of intrusive noise is an invitation for legal and commercial dispute that can far outweigh any initial cost in building the studio to the required (and agreed) standard in the first place. On the other hand, new technology and a more relaxed attitude to the audio delivery process, coupled to a challenging economic climate conspire to place very real constraints on the designer, to deliver a product that is good enough for the job but no better than necessary. This trend has polarised the audio production industry in two basic directions- one towards the cheap and cheerful, studio in a box approach and the other, an engineering methodology based on the careful matching of performance to need with a detailed specification and ‘employer’s requirement’.
There is often a question of budget constraint placed by ‘design and build’ contracts but in almost every dispute that has been referred to me there is a woeful lack of basic understanding of the expectations of the client and the intentions of the provider.
Of course, what really matters about a sound studio is what comes out at the end of the day and that depends a lot on the accuracy of the monitoring environment. Traditionally, everything is judged on reverberation times and the spectral balance of the speaker system at the mixing location. This is still true in the case of film theatres but more and more mixes are made on small speakers in rooms that are often not purpose designed control rooms. The results can be truly awful or, with a little help, surprisingly good.
Even in small rooms with ‘near-fields’ the bass response can be uneven. This is because 99% of the bass radiates into the room and 1% directly to your ear. Fortunately, inverse square law reduces the room energy rapidly but standing waves and discrete reflections can cause peak and troughs in the spectrum. Such things can now be measured accurately using FFT and MLS (maximum length sequence-don’t ask) analysis.
In my experience, simple foam wedges and lightweight panels do little to improve room modes (standing waves). The best solution is measurement of source, determination of the problem and the placement of tuned, high Q resonating absorbers, that actually change the phase of the offending wave by impedance and diffusion.
A few hour measurement can save an awful lot of time and money wasted on smoke and mirror acoustic products, even if they do come in purple.

Tags: munro acoustics | audio | testing | noise levels | N/A
Contributing Author N/A

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