1. Dynamic Range and Dynamic Range Processors

2. Relative Sound Level Graph

3. What is Dynamic Range?
Range of lowest to highest level that can be reproduced by a system
System with a high dynamic range will be quieter than one with a low dynamic range
Dynamic range is measured with the decibel (dB)

4. Dynamic Range in Digital Audio
Digital Audio at 16 bits has a dynamic range of 96dB
This is actually lower due to filter overhead
Digital Audio at 24 bits has a dynamic range of 144 dB

5. Break it down! Elements of the previous diagram:
0 dB – Unity, also called the permitted maximum level (PML)
Dynamic range – difference between permitted loudest and softest signals at the given bit depth
Headroom – the amount by which the signal is able to exceed the PML before clipping occurs
DSP noise floor – undesirable noise in the system; more on that on the next slide…
Things to notice: as bit depth increases, DR increases, headroom increases, and noise floor decreases.

6. Signal to Noise Ratio
Ratio between the loudest signal and noise that is inherent in the system (i.e. background noise, hiss, hum, etc.)
Each additional bit of resolution increases the signal-to- noise ratio 6dB!
Example: Audio CDs have about a 90dB signal to noise ratio
16-bit audio has a DR of 96 dB, 6 dB consist of the noise floor, resulting in a 90 dB signal to noise ratio

7. Signal to Noise Ratio Graph

8. Significance to Recordings

9. Dynamic Range vs. Hearing
We don’t hear the same over the entire frequency range of hearing!
The ear is more sensitive in certain frequency regions than in others

10. Fletcher-Munson Equal/Constant Loudness Curves/Contours
Sometimes called “phon” contours
Based on work at Bell labs in 1930’s
People judged when two pure tones of different frequencies were the same loudness
Most sensitive region = Hz
EXAMPLE: a sine wave at 3kHz with a certain intensity will sound LOUDER than one at either 200 or 8000 Hz with the same intensity

11. Fletcher-Munson Equal/Constant Loudness Curves/Contours
Phon=
SPL in dB at
1000Hz

12. Dynamic Range Processing
We can control dynamic range of recordings!
Bit depth tells us the total amount of dynamic range in a recording
We can further controls dynamic range with devices called dynamic range processors

13. What are dynamic range processors?
Devices that control dynamic range of a signal
4 types
Compressor
Limiter
Expander
Noise Gate

14. Compressor Like an automatic fader REDUCES dynamic range
Essentially turns DOWN louder signals, thereby allowing you to increase your amplitude
Makes softer signals seem louder and louder signals seem softer
Does not impact softer signals! Only louder signals

15. Compressed Waveform 
Compressed signal 
Original signal

16. Compressor components
Input gain
How much input goes into compressor
Threshold
User-defined level at which compressor begins to REDUCE input signal
Output gain
How much signal is sent to output

17. Compressor components (continued)
Ratio
Amount of gain reduction
Determines amount of reduction of input signal needed to cause 1 dB increase in output signal
Ratio 4:1 produces 1 dB increase in output for every 4 dB increase in input
Meaning, if the input is 4 dB over the threshold, the output will be 1 dB over the threshold
Ratio 2:1 produces 1 dB increase in output for every 2 dB increase in input

18. Compressor Diagram

19. Compressor components (continued)
Attack
Determines how fast or slowly device will turn down signals that exceed the threshold
This takes time. Sometimes can hear “pumping” or audible changes in dynamic
Slower attack = gradual addition of compressor
Fast attack = immediate addition; can hear pumping
Too slow = can hear compressor engage
Knee
Determines slope of the compression
Defined as “hard” or “soft”

20. Knee
A soft knee gradually increases compression ratio. A hard knee will be a more abrupt addition of compression.

21. Compressor Diagram 2

22. Peak vs. RMS Compression
Compressors can have “peak” or “RMS” settings for how compression is applied
Peak – responds to instantaneous level of input signal
Usually used to compress transient attacks (drums hits)
Good for transients, but can yield in abrupt gain reduction
RMS – applies root mean squared averaging function before comparing input to threshold
Often yields smoother compression results

23. Limiter Compression to the extreme
Used to keep signal peaks from reaching certain level
Most have ratios of 10:1 or 20:1 (or more)
Logic allows 30:1 compression within the compression insert
2 functions:
Prevent short term peaks from reaching their full amplitude and minimizes distortion
Prevent signal levels from increasing beyond specific point, which prevents blowing equipment; prevents distortion

24. Expander INCREASES dynamic range of a signal
Most expanders decrease the gain of a signal as the level falls below a threshold
Reducing low level signals will increase the signal’s overall dynamic range
Why?
Can also be used as noise reducers

25. Expander Same components as compressor
Input/output gain, threshold, raio, attack
Attack/release settings must be set carefully to best match material
Fast release = can get pumping effect
Slow release = can cause dynamics to return to normal state too slowly

26. Expander diagram

27. Expander Diagram 2

28. Noise Gate Expansion to the extreme
Signal above threshold passes without interference
Signal below threshold closes “gate” and mutes signal
Sidechain input = special input into noise gate
Allows external analog source to trigger gate’s output path