Capturing Ambient Sound

A Cappella

A Cappella is Italian for “in the chapel” (a capella in latin, acapella in common use). To capture the character of the “chapel” it applies equally to the recording technique as to the vocal content. A church venue is selected for its outstanding reverberation quality which should be captured in the recording.

It is worth noting that many pieces of music are written for an intended venue, or type of venue and In 1638 Allegri based his famous Misere on Psalm 51 for exclusive performance by the Sisitine Chapel Choir for Holy Week, until Mozart memorised it in 1770 and later wrote out the score. https://www.youtube.com/watch?v=THU9zfWCUr0

Reverberation Effects

Reverberation Time

The reverberation time of a hall at any frequency, is the time taken for the sound pressure level to drop to 1/1000 (-60 dB). A study was done of 10 Churches in Rome and reported here https://www.peutz.nl/sites/peutz.nl/files/publicaties/000204.pdf

Characteristic's of the churches studied

Reverberation times

Note the dashed blue curve for the Sistine Chapel (Cappella Sisitina), CS based on Psalm 51. Note the long reverberation time of 9 seconds at bass frequencies slowly declining with frequency as dispersion increases within the auditorium.

In 1638 Allegri based his famous Misere on Psalm 51 exclusively for the Sistine Chapel Choir until Mozart memorised it in 1770 and later wrote out the score. https://www.youtube.com/watch?v=THU9zfWCUr0

Critical distance

The level of the reverberation field at any frequency was once held as constant throughout the auditorium, which would mean that wherever you sit the reverberation would sound as loud. However, the further away you sit from the source the less distinct the sound gets. The critical distance is the distance from the source (choir, singer, instrument, preacher or PA speaker) where the direct field is the same level as the reverberation field. It is therefore considered that in any setting the microphones should be situated as close as possible to the sources, and well within the critical distance to minimize the effect of the environment.

However, unlike home or small studio recordings where the reverberation field is unwanted, acapella recordings require a careful balance between the direct and reverberation fields to capture the character of the auditorium, which is best found by trial and error. However, as a rule, the reverberation time and critical distance will increase with the volume of the church, so in larger churches and cathedrals the mics may be placed further from the singers. Allegedly, when the BBC did a recording in Liverpool Cathedral the critical distance proved to be so large, with a reverberation time of 7 seconds, that they had to compensate post recording to add more reverberation. Note the mic spacing used by the BBC in Salisbury Cathedral

Whereas in home and small studio settings, multi-track recordings would be used with multiple microphones, stereo techniques may be employed for acapella performances in medium and larger churches and cathedrals.

Stereo Microphone Techniques

History

Before stereo was adopted

Edward C. Wente invented the condenser mic in 1916 working for Western Electric.

In 1928 Georg Neuman in Germany released the first condenser microphone, the CV3, which had an omnidirectional characteristic. https://www.mixonline.com/technology/1928-neumann-cmv3-condenser-microphone-383704

Dr Walter H Shottkey, best known for overcoming noise in valve amplifiers (Shot noise), invented the ribbon microphone with Dr Erwin Gerlach in 1924.

They offer a figure of 8 pattern which means they are sensitive to sounds coming in from the front and back but are insensitive to sounds coming in from the sides. In the beginning, this enabled them to offer a degree of isolation between separate parts of a band, a technique used to record swing bands.

The RCA type 44 ribbon mic was introduced in 1932 and here is the manual showing how the mic would have been placed to record a band https://coutant.org/pb31/rca44a.pdf.

Undoubtedly some cinematic license has been used in the placement of the musicians, but here we see them depicted recording the Glenn Miller band for the 1942 film "Orchestra Wives" A gold one is above the brass section, a second above the horn section with the main beam towards the rhythm section, and a third is used for the singers. Each would be relatively insensitive to the sound from the adjacent section. The black mic is unidentified.

Note the convoluted walls to the rear which disperse the low end, housing large absorbers.

The sequence can be found at the opening of the film here www.youtube.com/watch?v=qtJggZeec_0

Advent of Stereo

In 1931, Alan Blumlein invented stereophonic sound, which he demonstrated at Abbey Road studios using crossed figure of 8 ribbon microphones. Covered by UK patent 394325, published in 1933.

8 years later, Shure released the famous Unidyne type 55, the first dynamic mic to use a technique developed by Ben Bauer for producing a cardioid (heart-shaped) pattern. In 1958 Alan Blumlein's 1931 patent was revised by adding the crossed cardioid technique used today.

Today, the ribbon mic has largely been surpassed by mics using a number of condenser capsules offering a range of switchable characteristics

In 1953, AKG produced their C12 condenser microphone which led to the C414XLII. A modernisation of the capsule led to the C414 XLS and the C314 used by us (Acapella-Recording).

This has led to two common methods of recording stereo, crossed cardioid patterns, or mid-side.

Alan Blumlein's patented microphone configurations

Taken from wikipedia page here en.wikipedia.org/wiki/United_Kingdom_patent_394325#CITEREFEargle2012

Above are the three configurations devised by Blumlein. The MS and XY are shown exactly as in Blumlein's 1933 patent 394325; the splayed XY is shown in its later, American 1958 implementation with cardioid microphones. This refers to the fact that the nulls to the rear of the pattern are splayed 120° apart. The left and right signals were derived from the mid (M) and side (S) components using a transformer (his "shuffler") which produced a very wide stereo field. Today this has been replaced by VST plugins such as the Zoom MS decoder. shown below.

Blumlein's MS (mid side) technique was modified by the BBC in 1961 when they converted their mono FM system to stereo. The mid component, derived by adding the left and right channels (L+R) to replace the existing mono channel, and the side, derived by subtracting them (L-R) to produce a difference channel that was added to the signal on a subcarrier at the top of the frequency spectrum. This would be ignored by mono receivers or used by stereo receivers to recover the left and right channels in the same way as the MS decoder VST plugins.

Today's techniques

Here you can see how Alan Blumlein's ideas have taken shape today.