Historical and Technical Aspects of the Single Microphone Bluegrass Technique

By Andy Highfield

When considering the role of the microphone in traditional bluegrass bands, it is very important to understand both the context in which sound reinforcement was gradually introduced, and also the technical factors that applied at the time.

The introduction of amplified music was a gradual process, and was intended to overcome the demands of playing for large audiences that were increasingly used to listening at higher volumes than previous generations. This trend was also mirrored in the purely acoustic instruments of the first quarter of the 20^th Century: the development of large bodied steel string guitars, Gibson arch-top mandolins and the Mastertone banjo, for example. The demand of players and listening public alike was for volume, in order to compete with the sound of the popular jazz, big band and swing music of the day. Even the loudest acoustic instruments have limits, however, especially when faced with large audiences, tent shows, and outdoor performances and so the need to reinforce the sound became a matter of urgency.

Prior to the late 1920’s all music heard in public performances was purely acoustic. This only changed as vacuum tube electronic amplification became a practical reality. One factor often overlooked is that at that time concert halls, theatres, churches and similar venues were specifically designed to provide an acoustic environment in which relatively quiet (by today’s standards) music and speech could be clearly heard. George Gruhn (1) has commented upon hearing purely acoustic un-amplified music in the Ryman Auditorium, built in 1990, and home of the Grand Old Opry. Even in a venue seating 1800 people the instruments could be heard with great clarity. The volume levels attained, would, however, be a long way from those modern audiences have come to expect.

Churches in the US were one of the early wide adopters of sound reinforcement, and it is likely that this is where many traditional musicians first encountered working with a PA. The big names at that time were Western Electric and RCA.

This is a typical PA amplifier, by RCA, from 1949. It features a 10-Watt output and has the ability to mix two microphone inputs. There is a single ‘tone’ control, a master volume, and an on-off switch.

The early and very popular RCA 44A had a figure-of-8 pickup pattern and a very smooth, silky sound with a pronounced low frequency emphasis when used to close mic vocals (this property made it a great favourite of the ‘crooners’ of the day, who exploited these properties extensively both in live performance and recordings).

The 44A was replaced later in the 1940’s by the variable pattern (figure-of-8 through omni-directional) model 77A. While still a ribbon microphone, this is a very different design, with completely different properties. This microphone does not exhibit the same low frequency boost when used close-up in omni-directional mode, and the omni-directional pick-up option made it a natural for miking musical groups. It still has that classic ‘smooth’ ribbon sound, however. The frequency response of these microphones is in the order of 30-15,000 Hz, with a quoted sensitivity of -55dB. The 77A also featured a ‘bass roll off’ adjustment, marked M (music), V1 and V2 (voice).

These are the two microphones most often seen with the early Bill Monroe, Stanley Brothers, Flatt and Scruggs and other classic bluegrass outfits. Both the 77A and 44A were used routinely on WSM, the Grand Old Opry, and feature in many classic images from the period. They can also be heard on the majority of recordings from the same era. It is interesting to speculate just how aware the bands were of the differences between these two microphones, as their different properties would have had a huge impact upon the ‘choreography’ of taking breaks, etc.  The positions and distance required to obtain the best ‘mix’ with a 77A in omni-directional mode would be entirely different from that of a 44A with its fixed figure-of-8 pattern.

It is extremely important to realise that both of these microphones pick up sound from the front and rear equally (in the case of the 44A) and from the front, rear and sides equally in the case of the 77A in omni-directional mode. From a sound reinforcement perspective, this is a veritable nightmare, as avoiding feedback at anything other than very low sound levels will prove all but impossible. This is, however, what those classic bands used, and only serves to reinforce the point that modern audience expectations and listening levels are entirely different from those of 50 or more years ago.

Another microphone occasionally seen in band photographs from this era includes the Electrovoice V2, which was EV’s answer to the popularity of the RCA 44A (though with a much reduced HF response of around 35-11,000 Hz and a reduced sensitivity of  –64dB).

Although initially introduced in 1941 in the form of the Western Electric 639A, it was not until the early 1950’s that quality microphones specifically designed for PA use with more selective, directional pick-up patterns became generally available. This need for a directional response, with good rejection of rear and side pickup was first identified by Marshall and Harry of Bell Laboratories (2) who noted that due to the characteristics of the sound systems then in use, directivity needed to be supplied by the microphone in order to produce a more natural balance of direct-to-reverberant sound. They further stated, "This (feedback in a reinforcement system) is merely a special case of extraneous noise, and its effect can generally be reduced by directivity in the microphone."

This particular microphone, although designed for public address use, was also widely adopted by broadcasters. The famous Louisiana Hayride, for example, was broadcast using a 55S.

Unlike the RCA 44A and 77A, the 55S is not a ribbon microphone, but is a standard moving coil dynamic design. Although it offered much improved durability over the extremely fragile ribbon designs, many complained that it did not sound as ‘smooth’, and preferred to continue using their RCA’s, both for recording and for live sound applications. Even into the 1960’s, bands such as the Country Gentlemen were recording and performing with RCA ribbon microphones.

Many current bluegrass bands attempt to emulate the classic look and sound achieved by these pioneer bands. Unfortunately, very few seem to understand the differences between the equipment used originally, and the tools they are using to try to replicate this sound. In addition, it should never be forgotten that early bands used one microphone not out of choice, but because they had to. It was usually the only option. Multi-microphone systems and mixers simply did not exist.  Furthermore, as already mentioned, the sound levels that audiences expected in the 1940’s and 1950’s were very much lower than that demanded today.  These are factors that need careful consideration when deciding if this method is viable in the first place, or how to achieve the best results if you do decide to give it a try.

That Old-Time Look

Most current ‘emulators’ of the one-microphone method employ a large diaphragm condenser microphone (the Audio Technica  4033 is one popular choice, the Shure KSM-44 is another). The first thing to be aware of is that although a modern large diaphragm condenser microphone might look superficially like the microphones used in the 1940’s and 50’s, in reality, they are completely different in design, sound, sensitivity (3), and overall performance. A modern LD condenser microphone sounds absolutely nothing like a ribbon microphone (the average LD condenser microphone will have a pronounced HF peak vs. a gentle HF roll off typical of a ribbon), and will also deliver a much greater output and increased sensitivity. This combination of properties renders them highly susceptible to feedback, despite their advantage of either a fixed or selectable cardioid pickup pattern. If the room acoustics are problematic, or on-stage monitors are being employed, obtaining satisfactory volume levels with such a microphone will prove a real challenge. Such microphones are also extremely sensitive to stage/floor noise, and are also easily damaged by wind, moisture and shock. If using one of these microphones on stage, always use a good shock mount and high quality ‘pop’ screen for vocal use.

To give a direct specification comparison, one popular modern microphone used by bluegrass bands seeking to emulate the “one-mic” method is the AT 4033. This microphone has a frequency response of 30-20,000 Hz compared to the typical 30-15,000 Hz frequency response of a high quality velocity ribbon microphone, and a sensitivity of -32 dB compared to –64 db of the RCA 44A. What that means in practice is that the modern microphone is not only more capable of translating very high frequencies, but that it is vastly more sensitive to very quiet sounds across the entire spectrum. In a public address context, especially when taking into account today’s higher volume expectations, this is likely to prove seriously problematic.

To put this into plain English: one reason why the one-microphone method worked well in the 1930’s, 40’s and 50’s was that the microphones used were relatively insensitive, volume expectations were extremely low compared to what the post rock and roll generations demanded, the venues involved would typically have been engineered to provide ‘good acoustics’ for performances, and the frequency response of the microphones used tended to mitigate against feedback, despite their non-existent rejection of sound from the rear.

A further complicating factor is that the transient response (the ability of the diaphragm to move rapidly) of a LD condenser microphone is very poorly suited to instruments such as banjo or mandolin, and is only moderately acceptable for guitar. They are, however, very well suited to vocals or fiddle. In the studio, a small diaphragm microphone will almost invariably be selected for banjo, mandolin, guitar or percussion because its transient response is far superior, and it will prove better able to reproduce these instruments accurately.  For example, if miking a banjo, I would invariably select my Neumann KM-84 over my U-87; whereas the reverse would be apply for vocals. The transient response of good ribbon microphones is excellent, incidentally, far better than typical LD condensers, due to their very low mass. Maybe those old-timers knew something…

Solutions

It is still possible to obtain good quality ribbon microphones, though be warned – they are notoriously fragile and they will not tolerate abuse. Another caveat: never apply phantom power to a ribbon microphone without first checking that it will safely accept it. Most will not and will be destroyed immediately. Some modern examples are described as “active” ribbons and are designed to work with phantom power, but make sure first. If you have never experienced a ribbon microphone, you may be surprised at how good they can sound. It remains the case, however, that the typical figure-of-8 pickup pattern is far from ideal in terms of feedback suppression; so achievable volume levels may not be what you (or your audience) will find acceptable.

A possibly better solution, one increasingly being used by bands who wish to retain that old-time, classic look and ‘lead break choreography’, but still achieve sound levels suited to modern venues, is to use a single microphone stand, but mount separate small diaphragm, highly directional (tight cardioid or hyper-cardioid) instrument microphones positioned below the main LD or ribbon vocal microphone and fed separately into the mixer.  This type of ‘microphone tree’ permits the classic ‘look and feel’ of those 40’s and 50’s performances but delivers vastly better clarity and gain before feedback (the microphones are all at the right height for instruments, for one thing, and each can be ‘focussed’ on a particular instrument). It also allows individual microphones to be selected to suit specific instruments rather than struggling with a one-fits-all compromise. A slight variation on the above is to use a second pair of LD condensers mounted at a lower level on separate stands. Both approaches will deliver much increased gain before feedback compared to a single microphone and will also permit delivery of a much better overall sonic balance between instruments and voice at today’s typical listening levels. Del McCoury and Doyle Lawson and Quicksilver use this method very successfully (both bands typically use between two and three microphones, plus a separate channel for the bass).

In all cases, obtaining adequate levels for string bass can be particularly problematic, and the best approach is usually to mic this separately or even – sacrilege to many – use a bridge pickup. This is the approach adopted by several of the most traditionally oriented outfits, including the Del McCoury band.

Performance tips:

• If using a single microphone, be prepared to reduce volume levels drastically, and do without stage monitors. Rely instead on direct sound. That is the way they did it 50 years ago. Do not expect to achieve modern volume levels without severe feedback problems.

• In-ear monitor systems (IEM’s) can be used with a single mic system, but many bands may find the high initial investment and extended, relatively complex set-up time prohibitive. It is probably best to simply keep levels down and rely upon hearing each other directly.

• If you cannot provide a sound engineer experienced in acoustic music to constantly monitor and control your live sound, you may like to consider an automatic ‘feedback suppressor’ between your desk and power amplifier. These will automatically identify feedback as it starts, and using an ultra-narrow automatically assigned notch filter, suppress the problem frequency in a matter of milliseconds. Most are relatively, though not entirely, transparent.

• Most large diaphragm condenser microphones are not designed for stage use; they are primarily recording tools. Do not expect to be able to treat them like an SM-58, or that famous Electrovoice that you could hammer nails in with. They are fragile and need to be treated with care. Rare exceptions are the Shure KSM-44 and KSM-32 (both have been used regularly by Ricky Skaggs). These have the capsule mounted in an internal shock mount and an internal ‘pop’ screen is also fitted (though additional shock and pop filtering is highly recommended).

• Experiment with a ribbon microphone for that smooth, silky vocal sound. Some surprisingly decent sounding models are now available at relatively low cost from Chinese manufacturers.  I would not recommend using a high-end, expensive ribbon microphone on stage or on the road. They are just too fragile. Mistakes can become seriously expensive. They cost at least $1,000 each and can be destroyed by a single blast of wind, relatively mild shock, or misapplied +48V phantom power.

• One frequent complaint by those using a single microphone is that outlying instruments or vocals cannot be heard. That is especially so where a cardioid model is employed. Instead, use a multi-pattern microphone and switch it to an omni-directional pick-up pattern. Do be prepared for feedback problems, however, and watch that volume level....

• Try the ‘microphone tree’ method. It may well deliver a significant improvement in gain before feedback and overall mix quality while still preserving the ‘choreography’ around what appears to be a single microphone. Most of your audience will not notice anyway. Or care. What they really care about is being able to hear the vocals and breaks clearly and not being assaulted by continuous feedback.

• Some venue acoustics are highly problematic. Low ceilings. Reflective walls. Resonant spaces. In such environments you soon realise why modern, multi-mic and close miking techniques are so popular. If using a single mic in such a venue, all you can do is keep the volume level right down and hope you have a very attentive, quiet audience.

(1) Gruhn, George. The consequences of amplification for acoustic and electric instruments. www.gruhn.com/newsletter/newsltr18.html

(3) Marshall, R.N and Harry, W.R., 1941.  A new microphone providing uniform directivity over an extended frequency range: The Journal of the Acoustical Society of America -Volume 12, Issue 4, pp. 481-498

(3) A microphone sensitivity specification indicates how much electrical output (in thousandths of a volt or "millivolts") a microphone produces for a certain sound pressure input (in dB SPL). If two microphones are subject to the same sound pressure level and one puts out a stronger signal (higher voltage), that microphone is said to have higher sensitivity. Generally the greater the pressure sensitivity the more sensitive the microphone is to quieter sounds.  Also the greater the signal will be produced relative to noise in cables, etc. This is beneficial in recording applications, but in PA situations, greater sensitivity will also make the microphone more susceptible to feedback.