# Presentation: 129189

## Advanced Audio TutorialPart 1: Motivation & Basics

- Robert Olson
- May 30, 2002
- ANL/MCS Futures Laboratory

## Agenda

| 8:30 – 10:00  | Welcome |
| --- | --- |
|  | Introduction |
|  | Acoustics |
|  | Microphones |
|  | Cabling |
|  | Units of measure |
| 10:00 – 10:30  | Break/Refreshments  |
| 10:30 – 11:45  | Echo Cancellation & Mixing |
| 11:45 – 12:30 | Lunch |

| 12:30 – 2:30 | Speakers |
| --- | --- |
|  | Audio codecs |
|  | Audio Computer Hardware |
|  | Configuration and Troubleshooting |
| 2:30 – 3:00 | Break / Refreshments |
| 3:00 – 5:00 | Demonstrations |
|  | Multicast Tutorial |

## Requirements

- Natural high quality sound
- Full Duplex
- Hands free – microphones open

## Motivation 

- Audio is critical to the collaborative experience
- It is hard to get right
- Sensitive to individual installations
- Hence, no cookie-cutter configurations
- Many adjustment points
- People are different

## Some configurations to think about

- Small office
- Conference room
- Show floor
- Auditorium
- Hotel meeting room

## Major issues of concern

- Echo
- Background noise
- Microphone choice & placement
- Microphone coverage
- Speaker choice & placement (house speakers?)
- Intelligibility of received audio
- Other interference

## Quality Challenges

- Echo Canceller

- Echo Canceller

- Echo Canceller

- Mixer

- Network Audio
- Codec / Computer

## What is a decibel?

- Logarithmic scale
- Doubling of intensity is a 3dB increase

## Challenge: Quality audio at the Microphone

- Want to pick up voices, not fans
- Inverse-square law:
- Audio levels drop by the square of the distance
- For each doubling of distance, lose 6 dB
- Guidelines
- Keep voices closer to microphones than competing sounds
- Optimize for 2-3 feet between voice and microphone
- Too close results in bad inverse square effects – mic tuned for 2” will be nearly 16 dB down at 1’

## Predicting quality wrt Ambient Noise

- Measure SPL (Sound Pressure Level) at microphone location
- Measure distance D from furthest talker to microphone location
- Compute SPL of speaker at mic location:
- 68 dB – 20 log10 D
- Subtract background SPL from speaker SPL at mic
- Need at least 20dB difference for good audio

## Speech vs. Ambient Noise

*Handbook For Sound Engineers*, p1285

## Direct and Reflected Audio

- Sound source

- Direct sound

- Listener

- Reflected sound

## Critical Distance DC

- Distance at which direct & reflected sound are at the **same intensity**
- Microphone placed at DC or further from talker will have **poor quality**
- “echoey”, “reverberant”, “bottom of the barrel”
- Speech hard to understand due to blurring of direct and reflected speech

## Estimating DC

- Place white noise source at location of talker
- Place sound level meter 1’ from source
- Adjust level for 90 dB reading
- Move meter to 2’, reading will drop 4-6 dB
- Continue doubling distance until meter remains steady
- This is DC

## Microphone placement wrt DC

- For optimal audio, place microphones no further than 30% DC for omnidirectional, 50% DC for directional.
- What if I can’t?
- Install acoustic dampening on surfaces
- Accept poor audio
- Physics doesn’t give you any other solution

## Reverberation Time

- The time it takes for a decaying sound to drop 60 dB below its initial level
- Can be _[calculated](http://www.yrbe.edu.on.ca/~mdhs/music/oac_proj97/music/reverbcalc.html)__:_
- Where Se is the “effective absorption area” in square meters

## Reverberation Time (2)

- Or you can measure it

## Target Reverberation Times

- From http://www.ada-noisesolutions.com/ansi.htm

## Effects of Excessive Reverberation

- Late reflections interfere with direct speech, muddying sound
- Plain speech:
- Speech with 600ms reverb:
- 1.3s reverb:
- 2s reverb:
- 5s reverb:

## Frequency Response

- (Still talking about quality at the mic)
- Optimal frequency response
- 80 Hz (deep male voice) – 10Khz (consonants)
- Acceptable frequency response (telco)
- 300 Hz – 3000 Hz

## Frequency Response: specified in product literature

- From the Crown PCC-160 spec sheet.

## Directionality

- Sensitivity of sound relative to the direction from which it arrives
- Three basic types
- Omnidirectional
- Unidirectional
- Bidirectional

## Omnidirectional Microphone

- Picks up from all directions equally
- Not necessarily useful for conferencing

- Crown CM-150

## Unidirectional Microphone

- Uses acoustic delay channel behind microphone pickup capsule to induce destructive interference to sound received by sides & back
- Cardioid
- Supercardioid
- Hypercardioid

## Cardioid

- Null behind the microphone
- (Crown CM-200)

## Supercardiod

- Two nulls, at angle to rear of mic
- Shure WCM16

## Hypercardiod

## Phasing effects

- Direct sound and reflect sound combine and interfere
- Generates comb filtering effect, deterimental to audio
- Also shows up with multiple microphones

## Phasing & Multiple Microphones

- If multiple mics on at the same time, use the 3:1 rule:
- Separate mics at 3x the distance from talkers
- Comb effects aren’t noticable
- (With automixing, this shouldn’t be an issue)

## Good use of phase: Boundary Microphones

- If a microphone is placed very close to a surface, any comb filtering takes place at frequencies too high to hear
- But everything else is in-phase and reinforced
- Pressure Zone Microphones
- Phase-Coherent Cardioid Microphones

## PZM Microphones

- Omnidirectional microphone capsule pointed down at a surface
- Results in hemispherical pickup pattern
- Works best on large flat surface (Middle of conference table)

## PCC Microphone

- Cardioid microphone capsule placed close to a boundary
- Results in vertical cardioid pickup pattern

## Microphone Placement Guidelines

- Remember DC: place microphones as close as possible to talkers
- For tables, PCC microphones work great
- One mic per 1-2 people
- Remember they are directional!

## Conference table placement

- PZM microphone in center
- Beware environmental noise

- No more than 3’ from talkers

## Lecterns

- PCC works great on a lectern
- Don’t hide it inside!

## Audience microphones

- Hanging microphones useful
- (Upstairs node has Audio-Technica AT853-a)
- Need to carefully lay out for good coverage

## Questioner microphones

- Directional mic on microphone stand
- (Bring lots of cable and gaff tape)
- If you can convince people to come up to them...
- Good for nailing questioners down to get them on camera
- If levels are right, can leave open even when not in use

## Wireless microphones

- Useful for roving presenters
- Not necessarily as useful for collaboration
- Lavaliere microphones good for low visual impact
- Headworn good for noisy situations (demo at Supercomputing)
- Hassles: frequency choices, batteries, rustling wires, presenters playing with the buttons on the transmitter

## Handheld wireless

- Good for pass-around Q&A
- Intrusive
- Risk of losing control of the floor
- Batteries, rustling, etc. problems