jwilzewske: SR: Chamber Musicians Plugged and Unplugged

Ahnert & Steffen book, Berlin concerthall

O ur string quartet is planning to acquire some sound reinforcement (SR) gear, to enable us to better control the sound levels and acoustics in some of the venues where we perform. I feel like the sales guys are genuinely trying to help us, but the fact that we’re classical musicians leads them to try to recommend all sorts of exotic things that they think would please us, like really low total harmonic distortion (THD) specs and so on. Invariably, these seem to us to be over-spec’ed and over-priced. We’re a classical string quartet and, yes, of course the SR requirements are different from what they’d sell to a rock band or to a symphony. But this is after all just a commercial sound system, right? Aren’t there engineering equations that determine what’s really necessary? I feel like the fact that we are acoustic musicians and classically trained and somewhat finicky ... brings out the worst ad hocness in the salesmen’s nature.”
— Anonymous.

I wrap a piece of masking tape around the top three strings (most violinists don’t seem to mind) and clip the mic with a windscreen on the high side between the bridge and tailpiece to that tape.”
— Brian Frost, 15-MAR-2007, SR Forum.

W here precisely do you wrap that tape? I have one young charge who is the keeper of an incalculably expensive Guarneri. I may not touch it.”
— Tony Tissot, 15-MAR-2007, SR Forum.

I f you have four condenser mikes, just position one each on a tall boom above each violin and the viola and another in front of the cello on a short stand with a boom. If the quartet has decent instruments you do not need to have the mikes right on top of the instruments. A distance instrument-to-mike of 25 to 50 cm usually works for me. If you don’t have four condensers, use a dynamic on the cello. If you don’t have three but do have two, try one over the mid-point between the two violins, and one on the viola. "Open up the gain" on the violin pair 3 to 6 dB more than the viola mic.”
— Lee Brenkman, 24-FEB-2007, SR Forum.

It does seem that there’s a shortage of reliable guidance for sound engineering for small classical ensembles. Yes, there’s plenty of information on SR for symphonies, and there are quite a few acoustics engineering and architect firms out there that take on those large and well-paying civic projects. But there are not many resources that have detailed information or services for string quartets or other small groups. Some links provided below may be of interest, though.

Ahnert & Steffen book, Fig. 3.14, required/available peak SPL, as a function of ratio of room volume to reverb time; “symphony” is approx. 10 W curve [conductor’s podium], “string quartet” is approx. 0.1 W curve

It is true that commercial sound work for a full symphony orchestra in an air-conditioned hall imposes fewer demands on an SR system than a commercial sound system for a small ensemble. After all, the peak sound pressure levels (SPL) for an orchestral fortissimo at the conductor’s podium are equivalent to 10 W of power or higher. A single violin’s peak SPL is less than 1/100th of that.

Carnegie Hall, Stern Auditorium, Yamaha PM1D mixer

Let’s assume that your SR loudspeakers can deliver a level L_p = 110 dB at the ears of listener who is at the furthest distance from the stage—equivalent to the fortissimo for a full orchestra at the conductor’s podium. If the diffuse ambient noise level from a typical concert hall’s air-conditioning system is 32 dB at such a listener’s seat, then L_pdiff = L_p2 – L_p1 = (110 – 32) = 78 dB. If you add 6 dB to avoid undesired addition of levels, you get 84 dB. So you calculate that the THD target for the hall and those peak SPLs is 100 x 10 ^{( -84 / 20 )} = 0.006%. You can confer with an acoustics engineer or a sound designer at an architect firm for a more expert and precise answer than the crude suggestion I’m making here, but you can imagine from the calculation above that you probably won’t be needing amplifier gear whose THD specs are better than about 0.01%.

If the air-handling HVAC is turned off, though, and if the audience is really attentive and really quiet and if there’s zero street noise coming in through the concert hall walls and roof, then the diffuse ambient noise level will typically be closer to about 18 dB instead of 32 dB, in which case L_pdiff = 92 dB, or, with an extra margin of 6 dB, make that 98 dB. With that you get is 100 x 10 ^{( -98 / 20 )} = 0.001% THD, about an order of magnitude better than the figure we calculated before. So you naturally wonder whether a sound system could be that good, to take advantage of such a quiet space to provide those dynamics, with a sound so clean that it’s indistinguishable from unreinforced acoustics.

Part of the answer to this question is that you need to think about every element in the audio chain—from the mics, to mixer, to amplifiers, to loudspeakers. The loudspeakers that are available for practical SR systems are, at best, not better than about 1% THD. And, in general, carefully thought-out designs for contemporary commercial sound work (including symphonic venues) have THD around 0.8%. And a THD of 0.8% represents 20 log₁₀ (0.8 / 100) = –42 dB in terms of signal-to-noise ratio (SNR).

For a harmonic to equal this amplitude, you’d have 42 + 32 = 74 dB with the air conditioning “on”, or 42 + 18 = 60 dB with the air conditioning “off”.

Stark book, Fig 3.3, typical Sound Pressure Levels (SPL), in decibels

In other words, if you’re buying SR gear with fabulously better THD specs (lower than, say, 0.8%) then you wouldn’t actually be deriving any benefit from those specs unless (a) your venues are tremendously quiet and (b) the musical passages include delicate pianissimos below 70 dB SPL.

Incidentally, one of the venues where I’ve witnessed heavy-duty SR and mixing gear deployed pretty unsuccessfully was the Oude Muziek Festival in Utrecht in 2007. The Sampson-Bezuidenhout concert was held in the CentralStudios facility, in Studio-1. There were about 3,000 persons packed into that cavernous 1,000 m² (volume = 10,000 m³) hall, which is about twice as deep as it is wide. The miking of the performers was scanty. The reverb from front-of-house to rear was far too large, and the delays and phasing of the reinforcement loudspeaker arrays were wrong. Much of the ensemble’s sound was never captured by the mics and what was captured was piped and amplified in such a way that confusing delayed reflections arrived at your ears hundreds of milliseconds after the reinforced sound. The vocal parts tolerated this somewhat better than Kristian Bezuidenhout’s fortepiano.

Ahnert & Steffen book, Fig. 3.14, required loudspeaker power, as a function of level difference L - Lk in dB and distance from source to listener in m; design for ‘L – Lk = 0 dB’ curve

According to Ahnert’s and Frank’s book (p. 285) the required amp power in Watts is a function of the ratio of the volume of the room and the mean reverberation time:

P = 0.10 x V / T,

where V is in m³ and T is in sec. (See also top graph, above, the one at the very top of this post.)

Ahnert & Steffen book, Fig. 3.15, reverb required (in seconds, referenced to 500 Hz) by volume of room (in m3); “chamber music” is curve 3, curve 2 is “symphony”, curve 1 is “organ”; CentralStudios should’ve been SR-tuned to 2.0 sec according to this …

CentralStudios’ equipment has the requisite power. But on this occasion the power available was under-utilized. Possibly the sound-check with the empty hall was not subsequently amended when the hall filled with people.

Central Studios, Sampson & Bezuidenhout concert, Oude Muziek Utrecht 2007

Even in Row 11 where I sat, the sound was weak—it was difficult to hear the direct sound from the performers on-stage, and it was difficult to make sense of the disparate SR and reflected sounds impinging on us from further to the rear.

Central Studios, Utrecht, Studio 2, setup for refreshments at interval

jwilzewske

Saturday, March 29, 2008

SR: Chamber Musicians Plugged and Unplugged

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