The Nordic Acoustic Association and the Norwegian Acoustical Society invited scientists and engineers to attend the Baltic-Nordic Acoustic Meeting for 2021 this week and despite an online set-up the program consisted of keynote lectures, invited and contributed papers in structured parallel sessions. Some of the really interesting presentations are described here but you can find the program, abstracts and papers here.
Musical rehearsal rooms
Dr. Jens Holger Rindel, Multiconsult, held a great presentation on ‘Searching the musical rehearsal room’ where he discussed how singing or playing a musical instrument in a room is greatly affected by the acoustics of the room. Rindel commented that the room itself gives support to the sound and that the room if fact behaves acoustically as an extension of the musical instrument.
Three criterias for evaluation
In his presentation he made it clear that the acoustic effect is not only a matter of reverberation time and volume, but also concerns the frequency distribution of the room modes. Rindel discussed three criterias for evaluation of the distribution of the room modes; the smoothness of the frequency response between 20 Hz and 200 Hz, the number of musical tones between 27.5 Hz and 220 Hz supported by at least one room mode, and the frequency spacing index for the 25 lowest room modes – and he concluded that:
.. rooms for music with volumes up to 300 m3 need careful consideration of the dimension ratio in order to offer a good acoustical support to the musical instruments. The frequency response at low frequencies should be as smooth as possible, which is closely connected to frequency distribution of the low-frequency room modes.
Live concert or headphone listening
Magne Skålevik from AKUTEK and Brekke&strand gave us all a brilliant presentation under the title: ‘Live concert or headphone listening? The binaural signal and the musical experience.’ Magne’s approach was very personal and he took off by ‘story telling’ about his own experience with listening to symphony orchestra concerts both live, from a ‘ghetto blaster’ and a ‘walkman’ – everything in the year of 1979.
Magne discussed if a good recording provides binaural signal qualities equivalent to those of a good live event and how localization, Source Broadening and Envelopment are among the listening aspects necessary to achieve excitement and engagement when listening to a live classical music performance.
Tchaikovsky’s 4th symphony
The work concentrated on comparisons of two live concerts with Tchaikovsky’s 4th symphony, performed by two different orchestras in two different countries, in two different years. From these Magne showed what the differences and similarities were from one concert to another. Magne also analyzed a selection of different available down-loadable recordings of the same concert (chosen from the top hits in google), one bought from Chandos, one free-version from Naxos, one top hit version from YouTube, and one version from Spotify – and he concludes that the live listening experience cannot be replaced by playback from streaming or from a record, as long as any other recording technique than dummy head or ORTF from audience position is used.
The acoustics of the recording space
Magne also discussed how the recorded sound would depend a lot on the acoustics of the recording space (and that this is not always wanted. Moving the microphone position closer to the orchestra could reduce the relative influence of the room acoustics and allow freedom to add artificial reverb on demand in the post-processing, but unfortunately the microphones would need to fly above the orchestra (!) to avoid being closer to some instruments than others!
Finally, Magne concluded that the compression issue and reduced dynamic range in radio broadcasting and streaming is well known and these media will never attract newer generations to the symphony hall since symphonic music is not a streamable experience.
Norway leads the way
In most countries, we have good room acoustics guidelines for schools – and Norway is one of the leading countries. Erlend Bolstad, COWI, held nice presentation under the title: ‘Room acoustic conditions in primary schools’ – and he discussed the relationships between activity sound levels and room acoustic conditions based on measurements from 40 lectures in 20 different classrooms in 8 different primary schools in Trondheim, Norway. Erlend chose grades five to seven, assuming that they have more stablished patterns of classroom behaviour than pupils in the lower grades.
Low reverberation times
The study revealed that the room acoustic conditions had reverberation times from 0.39 s to 0.82 (avg. 0.54 s) and that the background noise varied from 22.2 dB to 39.2 dB (avg. 33.6 dB). Erlend commented that the average reverberation time found in this study was lower than what has been found in studies in other countries (e.g. B. Shield et al, ”A survey of acoustic conditions and noise levels in secondary school classrooms in England”, J. Acoust. Soc. Am., 137, 177-188, 2015. : 0.64 s). He also mentioned that despite this, the background noise levels seemed to be more similar.
He also found that the signal-to-noise ratio, which is considered important for speech intelligibility, is also satisfying with 19.0 dB on average. Erlend discussed that the idea of lower reverberation time is very important in classroom acoustics but that it should be debated. It can lead to lower noise levels, which is positive for the general acoustic comfort and a reduced Lombard effect for the speaker but at the same time, it has been found that teachers in some cases raise their voice level due to the absence of vocal feedback from the room.
The new Munch Museum in Oslo
Jannicke Olshausen presented a paper on some of the acoustic challenges dealt with in connection with the building and construction of the new Munch museum in Oslo. The title was: Acoustic regulations and design of the multipurpose hall and exhibition halls of the new Munch museum in Oslo. The focus was on the multipurpose hall and the exhibition halls – and the complexity of the halls were clear: Music performance is to prioritized in the multipurpose hall since the museum has hosted the annual chamber music festival for years, and this tradition is planned to continue in the new building.
The acoustic design of the hall was based on the Norwegian standard for music rooms (NS 8178) which states the preferred mid-frequency reverberation time as a function of room volume depending on the type of music (amplified music, loud acoustic music and quiet acoustic music).
Strong regulations for the hearing impaired
Jannicke discussed that when it comes to museums and the exhibition halls the Norwegian regulations are strict on reverberation times. The requirements came in 2012 in order to reach the desired level of sound quality for a hearing disabled audience but the really good intentions caused some challenges for the new Munch Museum finding enough available surfaces for sound absorbing materials.
Surfaces need to be changeable
In this museum the walls will mainly be covered by paintings and the remaining wall surfaces should easily be repainted, e.g. changing the background color from one exhibition to the next. So therefore it is not possible to ‘just’ mount sound absorbing material where it would be best (acoustically). In Norway the reverberation time (in sec) should not exceed 0.2 multiplied by the ceiling heights (in m), which meant that the Munch museum should meet a maximum of 0.8 s and 1.4 s respectively for a 4 m. and 7 m. tall exhibition hall.
Odeon was used to calculate the room acoustics of the halls and it will be possible to continue the museum’s tradition of having acoustical music performances the multipurpose (a conference set-up however will require the use of electro acoustical facilities). Then it comes to the exhibition halls, they will have longer reverberation times than recommended due to lack of available surfaces for sound absorbing material, but if number of visitors are restricted (combined with the use of electro acoustical facilities), the conditions for speech will be satisfied.
The acoustic capacity (Rindel, J.H. Acoustical capacity as a means of noise control in eating establishments. Proceedings of BNAM 2012, Odense, Denmark, 18-20 June 2012 – and Rindel, J.H, Christensen, C.L, Gade, A.C. Dynamic sound source for simulating the Lombard effect in room acoustic modelling software. Proceedings of Internoise 2012, New York, USA, August 2012) for the halls of 4 m height were calculated to be approximately 110 visitors, and approximately 80 visitors in the halls of 7-8 m height. If more visitors are present than allowed by the acoustic capacity, it might still be satisfactory, if the percentage of visitors talking at the same time is low.
Speech transmission index – how to measure?
It is always a pleasure to listen to Henrik Møller, Akukon, and he had a very interesting approach to how to measure Speech Transmission Index (STI) in his presentation: A review of STI measurements.
Henrik ran through the historical steps of STI and explained how different communications channels were measured directly (by reading words or speech like sound), and listeners would write what they heard. This ‘old’ method was rather time consuming, so faster and more objective methods were needed.
One of the first measurement methods was % Articulation, defined by V.O. Knudsen and calculated from the reverberation time, the speech level and the Signal-to-Noise ratio. Later Articulation Loss of Consonants (%ALcons) was introduced by V.M.A. Peuts in 1971 and Henrik explained how the %ALcons could be estimated from statistic acoustic parameters and relatively easily be measured from the direct sound level, the reverberant sound level and the background noise level.
Directional or omni-directional speakers?
Henrik pointed attention to the different ways of measuring STI – with either a directional speaker or an omnidirectional speaker (according to the standard IEC60268-16 (2011) it is stated that the microphone should be an omnidirectional, diffuse field microphone and the loudspeaker should ideally be an artificial mouth – which means a directional loudspeaker. In both cases the sound source should be calibrated, both with respect to level and frequency to have the characteristics of human voice).
New parameter – STIgeo
Henrik stated that varieties of measurement methods in real projects will most likely give variating results in most circumstances. He therefore suggested to use a new parameter STIgeo, which should be measured with an omnidirectional microphone and an omnidirectional loudspeaker, as defined in ISO 3382-1. However, the calibration of the loudspeaker should be in accordance to the requirements of the Strength parameter, not STI. He also mentioned that the measurement should typically be done using swept sine signal.
Henrik stressed that this ‘new’ STIgeo will not, of course replace the STI measured for specific requirements, such as emergency systems, but that it could replace for instance the STI requirement is for classroom acoustics and similar.
Thank you very much for a great conference. The technical committee did a great job together will all the speakers and attendees being active in the chat during the presentations.
Author: Mai-Britt Beldam, Market Development Manager at Ecophon.