Livening or Deadening the Performance Space
Ray A. Rayburn
A hall that is too live can be easily deadened by placing velour material at strategic points in the performance space. Where possible, the velour should be hung over balcony railings or freely hung away from the walls as banners. A little will often go a long way, and it is highly recommended that an acoustical consultant be engaged to oversee this temporary modification.
It is a bit more complicated to liven a space, but the technique detailed in Figure 1 produces surprising results. The effect of the 0.004 in (0.1 mm) thick vinyl material is to reflect high frequencies arriving at a fairly low grazing angle of incidence. It does not have to be stretched in any manner; rather, simply laid over the seating area. (The vinyl material in question is available at building supply stores under a number of trade names. No material less than 0.004 in. thickness should be used.)
If the walls of the hall are plaster, then increasing the humidity in the hall will reduce the acoustic absorption of the plaster, increasing the reverberation time. This technique was used for many of the RCA recordings of the Philadelphia Orchestra under Eugene Ormandy. Large steam humidifiers were run in the hall for the week before the recording session to adjust the acoustics. Because of the noise generated by the humidifiers they were then turned off for the actual recording session. As discussed in Eargle’s Microphone Book, 3e, artificial reverberation on accent microphones is an accepted practice in current classical recording; however, there is no substitute for natural reverberation to enhance the primary pickup via the main microphones and house microphones.
How Much Reverberation Is Needed in the Recording?
Music of different periods will require varying amounts of reverberation, both in level and in decay time. General recommendations for concert venues, as a function of room volume and music type, are given in Figure 2. In general, reverberation times exceeding about 2.5 seconds will sound rather unnatural on orchestral musical, whatever the period. Both classical and modern era music fare best with reverberation times on the order of 1.5 s, while romantic compositions may do with reverberation times up to 2 s. While ecclesiastical music is often heard in large spaces with up to 4 or 5 s of reverberation time, in recording it is probably better to err on the short side in order to keep the reverberation from interfering with musical details.
Kuhl (1954) developed the data shown in Figure 3. Here, monophonic recordings were played for a listening panel whose individual preferences for reverberation time are plotted. The median points are taken as target values. We need to add that these tests, if repeated today in stereo, could result in slightly longer target reverberation time values, due to the ability of stereo listening to delineate direct-to-reverberant details more accurately than in mono listening. In any event, the tendencies shown here are certainly relevant in today’s recording activities.
Once the target reverberation time has been established, the amount of reverberation introduced into the recording is critical. The sound picked up by the house microphones and introduced into the stereo mix will determine this ratio. This is an area where accurate loudspeaker monitoring conditions and experience on the part of both engineer and producer are of primary importance. In many cases, this important judgment is not made until the postproduction stage.
Maintaining a Consistent Fore-Aft Perspective
It is up to the engineer and producer to determine the effective “distance” of the pickup, relative to the stage, and make it convincing. Rather than placing microphones at Row J, we make the recording fairly close-in, and then we introduce reverberation to “zoom out” to where we want it to be. A slight increase in the amount of signal from the house microphones will result in a surprising increase in effective distance, so care should be taken in making these balances. When musical reasons justify this, it may be desirable to pan in the main flanking microphones very slightly toward the center, just to give the direct stage pickup a slightly narrower image to match the increase in apparent fore-aft distance. If this is not done, we run the risk of making a recording with conflicting spatial cues; it may sound both close-in and distant at the same time.
Above is an excerpt from Ray’s book, Eargle’s The Microphone Book, 3e.
Ray A. Rayburn is a Senior Consultant with K2 Audio LLC. He is a member of the AES Standards Working Group on Microphones, and Chair of the Standards sub-committee on Interconnections. He is also a recording engineer with a lifetime interest in microphone use, testing, and design. Ray is also the author of the recently published Eargle’s The Microphone Book, 3e. Visit the book’s companion site!