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There are still many topics to investigate in the field of stage acoustics. Some say it has only just begun.
Anders Gade, in collaboration with AKUTEK, have launched the Stage Acoustics Data Collection (STADAC) project, published 28.05.2013. Researchers and consultants are invited to fill in their measurement data in this worksheet template (*.xlsx format) and submit it by email. Those who use the older Excel version can download the same template in *.xls format. Ready for download is also the stage acoustics questionnaire. For full package download click here. After some time, the submitted data will be shared here on the AKUTEK site. Identity of halls is important reference information in the data set, but identity will only be made available to researchers on request. On www.akutek.info, data will be published anonymously only.
Acoustics for Symphony Orchestras; status after three decades of experimental research , by A Gade 04.03.2013. This paper by Gade summarizes major contributions to the field, discuss the differences in opinion in view of the limitations associated with different experimental approaches.
Wenmaekers, Hak and van Luxemburg:
A major contribution to this field in acoustics is presented in Stage Acoustics for Symphony Orchestras in Concert Halls - the PhD thesis by JJ Dammerud.
Papers relevant to Stage Acoustics can be found in AKUTEK Paper Session.
Take part in Stage Acoustics’ research in the online Online listening test: Stage Acoustics and Hearing of Others
Stage Acoustics, support and mutual hearing in ensemble
Current view in akuTEK Research is that the following topics are strongly related: stage acoustics, rehearsal rooms, performer’s perception, perceived reverberation. Stage acoustics and rehearsal room acoustics are important in order to develop musicians, orchestras and provide for good playng conditions—to the benefit of the listeners—and must be supported by knowledge of performer’s perception in general, and perceived reverberance in particular.
Experiments (august 2011) with varying orchestra surroundings showed that introducing low frequency absorption at walls close to basses and timpani resulted in the orchestra having trouble playing in sync. This seems to confirm the old music school truth that in order to play in time it is beneficial to ‘listen to the pulse’. To stage acoustic this means that sound transmission and support from close orchestra surroundings is essential even at low frequencies (though 63Hz and 125Hz is not included in single-number ST assessment in ISO-3382).
What kind of reverberance do performers perceive? Does EDT describe reverberance close to the instrument , where EDT approaches zero? You may judge for yourself in the reverberance sound demo. More about this in the akuTEK research section Reverberation.
D Griesinger has studied soloist’s support in the running reverberation: Running reverberation RR has been suggested as a measure of the reverberance perceived during while music is being played. How loud is my Reverberation?, D Griesinger asks in an akuTEK publication. See also Objective measures of spaciousness and envelopment.
Can mutual hearing (=hearing others) be predicted and measured with simply an empty stage and assuming omni-directional source, as with the ST-parameters? Maybe not, since the sound transmission paths between musicians are sensitive to natural obstacles in an orchestra (persons, chairs, music stands,..) on stage, and by the uneven directionality of musical instruments.
A hearing related measure G50 is suggested (link1, link2). G50 is the level of the initial energy integrated from 0 to 50ms, relative to direct sound energy at 10m distance from the source. Like our ears, G50 will take more than just the direct sound into account. In stage acoustics the direct sound path is a very unreliable sound transmission channel. The significance of this is to be investigated further.
A paper on this topic was presented by Magne Skålevik at ICA2007: Sound transmission between musicians in a symphony orchestra on a concert hall stage. Presentation: PDF-version (829kB)
“Orchestra members, music stands, instruments and chairs are inherent obstacles in an orchestra, and it is shown that the presence of the musicians makes a difference to sound transmission internally on stage. This should be taken into account whenever measuring or predicting stage acoustics. The positive effect of the canopy was significantly underestimated by measurements without orchestra members on stage. Use of higher raisers may have some unwanted effects that need to be investigated further: Bass level drops as source and or receiver raises. Free direct paths will lead to stronger peak level transmission from some instruments to some listeners’ ears, but can this be evened out by ensemble effect from large instrument groups, or will it lead to unsatisfactory unevenness? The results show that inter-orchestral sound transmission is attenuated significantly from 500Hz and upwards. To compensate for this, canopy reflectors should operate effectively in this frequency range. In this same frequency range, directivity of musical instruments makes direct sound radiation unreliable. This previous conclusion that good sound transmission on stage as well as from stage rely upon diffuse surroundings providing many sound paths, maintains.”
Reflection Density and Attenuation on Stages and Rehearsal Halls, J O’Keefe, 16.09.2010
Johan Andersson and Alf Berntson concludes as follows in this paper:
The measured sound level is probably mainly determined by the sound from the own instrument and the closest surrounding instruments. The judged level is probably based on the later arrived sound level and the character of the sound.
Bath project (Barron and Dammerud)
In 2005-2008, Mike Barron lead a research project on Stage Acoustics at the University of Bath, assisted by PhD student JJ Dammerud.
PhD thesis May 2010: Stage Acoustics for Symphony Orchestras in Concert Halls (5MB). Separate chapters:
3 Musicians’ impressions of acoustic conditions
Earlier papers by Dammerud and Barron:
Stage conditions for orchestral performance, by Dammerud / Barron. Early subjective and objective studies of concert hall stage conditions for orchestral performance. Paper (488kB pdf) Presentation (616kB pdf) 21.09.2007
External source to Stage Acoustics: Stage acoustics for symphony orchestras by JJ Dammerud
Stage (Support) parameters ST early and ST late
Stage parameters were suggested by Gade in his pioneer work in the 1980’s (follow link to pdf-document, pages 24-43 ).
Among these, the support parameter ST (in a variety of versions) is the one that is mostly used, and it describes the energy foldback to the musicians on stage.
STearly The early reflected (20-100ms re direct sound) energy level relative to the initial (e.g. 0-10ms re direct sound) measured at 1.0m from an omni-directional source.
ST late The late reflected (arriving 100-1000ms re direct sound) energy level relative to the initial (e.g. 0-10ms re direct sound) measured at 1.0m from an omni-directional source.
The early support - denoted ST1 or STearly - is now commonly used to describe the degree of mutual hearing or hearing others. This is to be expected, since on most stages the early reflected energy is expected to contain sound from the whole ensemble as well as the musicians own instrument.
The late support describes the degree to which the musician hears the late reverberant sound. ST late is suggested as a descriptor of performers subjective reverberance, see also the akuTEK page Parameters. Singers often appreciates to hear their own voice filling the auditorium, and is expected to prefer high ST late values. ST late is almost solely determined by the ratio between RT and volume of the hall.
However, one should take the balance ST late - STearly into account, since if ST late is high compared to other halls, the late reverberant sound may still appear weak if also STearly is very high. If this balance is too low (say << -3dB) musicians may feel that the stage is acoustically decoupled from the auditorium. This may be the result when introducing a canopy that is too low and too dense.
In her Master thesis “Podium Acoustics for the Symphony Orchestra” (4MB pdf), Cederlöf concludes that the subjective parameter which got the highest correlation with musicians’ Overall Impression is Support.
In akuTEK research the current conclusions about ST-parameters are:
1. ST early value should be considered in relation to ST late
2. Mutual hearing conditions on a stage can not be fully measured or predicted with an omni-directional source on an empty stage
3. While ST-parameters may describe the conditions for soloists and duets rather well, they do not take the effect of obstacles and complex directivity inside a larger ensemble into account. The significance of this is yet to evaluate
More about definitions and measurement of room acoustical parameters including stage parameters can be found:
· on akuTEK page http://www.akutek.info/concert_hall_acoustics_files/parameters.htm
· on pages 9 of 10 in Room Acoustical Measurements (university pdf)
· on pages 86-89 of 107 here: link to commercial pdf (1.5MB) including recommended values.
· Example of winMLS measurement results of stage parameters including ST1 in external link to here: link to commercial HTML
Basses on stage floor
On the significance of stage floor construction, to sound radiation from instruments with endpin contact, by Guettler, Askenfelt and Buen:
It is often claimed that a compliant stage floor in contact with the end pin of a double bass will act much like a tabletop in contact with a tuning fork, and assist in radiating the low-frequent sound. On the other hand, it is also claimed with the same conviction that a compliant floor will act as an absorber of airborne sound and thus shorten the low-frequency reverberation time.
Stage acoustics for reinforced music
Also with reinforced music, the acoustic conditions can be critical for the musician’s hearing of her own instrument and hearing of others. In professional cases, the hearing is assumed to be controlled by loudspeaker monitors, implying that natural acoustics should be sufficiently damped. However, this criterion is often not met in medium to small venues where the acoustic hall radius is small and separation between musicians and separation between sound at the musicians ear and sound at the listeners ears is hard to achieve. With such under-damped conditions, musicians often complain that the sound from their own instrument is being masked by the sound from the other instruments. If the sound engineer tries improve one musician’s hearing of himself by boosting the output from this musician’s monitor, the other musicians will complain because the sound from their instruments became more masked when the first monitor was boosted. So they need a boost too. And when all monitors are being boosted they have the same problem as in the first place—only worse, since spectral masking increases with the level of the masking noise. The bold engineer would have reduced the level of all monitors instead, but then the musicians will complain that they can only hear the drummer, or the trombonist, or some other loud instrument over which the sound engineer has no control. As if these problems were not enough, if sound from stage monitors and loud instruments becomes audible in the audience area, the engineer has to regain (literally) control over the sound there by raising the main loudspeaker level. If acoustics are not sufficiently damped relative to the room volume, this will increase the masking at the musicians ear, and they will ask for even more sound from their monitors. The feedback loop is complete, and this unstable process leads to louder and louder sound, but with more and more masking, and often with the familiar howling from electro-acoustic loop on top. The sound is out of control.
The instability effect described above is detrimental to music performance. It has been tried solved by feeding the musicians with proper foldback through ear plugs, but not all musicians are comfortable with this. This leaves us with the other alternative—the architectural acoustics solution: Providing sufficient acoustic absorption in the venue volume. Now, what is sufficient? M Skålevik has suggested the Separation Index SI as a measure, and a criteria for SI. Given the common directivity of loudspeakers, there should be enough absorption in the room to satisfy the following criteria:
· The sound from the main speakers should be at least 10dB lower at the musicians ear than at the audience’ ears
· The sound from the monitors and natural sources on stage should be at least 10dB lower at the audience ears than at the musicians’ ears
· To prevent electro-acoustic feedback, the microphones should be considered like musicians’ ears in the above
· With these conditions, the Separation Index is at least SI=10dB
SI is inherently frequency dependent. Due to the fact that loudspeaker directivity and sound absorbers are generally low at low frequencies, the task of meeting the SI-criteria above at low frequencies are the most demanding. The importance, of course, is no less knowing that reinforced music is bass-heavy. In small rooms, SI will be small at low frequencies, and it will be practically impossible to separate bass sound at listeners’ ears from bass sound on stage. There will be a frequency limit below which there is no separation between stage and audience, and above which separation can be achieved with proper loudspeaker directivity and proper source-receiver distance. In larger rooms, this frequency limit decreases as the critical distance gets larger. Note that SI is expected to have some optimum value, since more separation (higher SI) than necessary is not wanted.
See also the paper On a new, variable absorption product and acceptable tolerances of T30 in halls for amplified music by Adelman-Larsen et al