This document discusses simulating a loudness control in LTspice using .wav audio files. It begins with background on equal-loudness contours and how loudness controls can restore low and high frequencies at lower listening levels. The document shows how the frequency response of a simulated loudness control matches equal-loudness contours. It compares the response of a loudness control circuit to a flat volume control and demonstrates using .wav files as inputs and outputs of the LTspice simulation.

1. LTSPICE AUDIO SIMULATION:
A LOUDNESS CONTROL
Bee Technologies Inc.
20AUG2012
1

2. EQUAL-LOUDNESS CONTOUR
 Equal-loudness contour is a measure of sound pressure (dB SPL), over
the frequency spectrum, for which the human ear perceive the same
loudness when presented with pure steady tones (unit is the phon).
120
INTENSITY LEVEL (dB)
100
80
100
90
60 80
70
60 Loudness level (phon)
40
10 100 1000 10000
FREQUENCY (Hz)
Contours of equal loudness (from the Radiotron Designers Handbook).
For more info . . .
http://www.sfu.ca/sonic-studio/handbook/Equal_Loudness_Contours.html
2

3. A LOUDNESS CONTROL
 A loudness control is used as an attempt to restore extreme low and high
frequencies. As the sound level decreases, the ear becomes less sensitive
to both extreme low and high frequencies. Thus, when listening at reduced
PARAMETERS:
RVOL = 500k
levels, if the frequency response of your system is maintained flat, you will
VOL = 0.0245
not hear those extremes as well.
PARAMETERS:
RVOL1 = {(1-VOL+1u)*RVOL}
RVOL2 = {(VOL+1u)*RVOL}
R1 20
Input
V1 200k
1Vac RVOL-1 C1
0Vdc {RVOL1} 76.9739p R2
2.2119meg
Output
0
0 RVOL-2
{RVOL2}
R3
330 -20
R4 C2
2MEG 58.7883n R5
1 -40
4.6695k
L1
20m
C3
3.9244n
-60
2 20Hz 100Hz 1.0KHz 10KHz 50KHz
DB(V(OUTPUT))
0 Frequency
A loudness control and its frequency response (example)
3

4. THE LOUDNESS CONTROL VS. EQUAL-LOUDNESS CONTOUR
20
0
INTENSITY LEVEL (dB)
-20
100
100(Sim)
90
90(Sim)
80
-40
80(Sim)
70
70(Sim)
60
60(Sim)
-60
10 100 1000 10000
FREQUENCY (Hz)
Frequency response of the loudness control (simulation) is compared to the equal-loudness contour.
4

5. Loudness Control vs. Flat Volume
The High-Pass
PARAMETERS: PARAMETERS: circuit is
RVOL = 500k RVOL = 500k disconnected, the
VOL = 0.0245 VOL = 0.0245
Low-Pass circuit
PARAMETERS: PARAMETERS: is short to ground
RVOL1 = {(1-VOL+1u)*RVOL} RVOL1 = {(1-VOL+1u)*RVOL}
RVOL2 = {(VOL+1u)*RVOL} RVOL2 = {(VOL+1u)*RVOL}
R1 R1
Input Input
V1 200k V1 200k
1Vac RVOL-1 C1 1Vac RVOL-1 C1
0Vdc {RVOL1} 76.9739p R2 0Vdc {RVOL1} 76.9739p R2
2.2119meg 2.2119meg
Output Output
0 RVOL-2 0 RVOL-2
{RVOL2} {RVOL2}
R3 R3
330 330
R4 C2 R4 C2
2MEG 58.7883n R5 2MEG 58.7883n R5
1 1
4.6695k 4.6695k
L1 L1
20m 20m
C3 C3
3.9244n 3.9244n
2 2
0 0
LOUDNESS CONTROL FLAT VOLUME
5

6. Loudness Control vs. Flat Volume
20 20
0 0
-20 -20
-40 -40
-60 -60
20Hz 100Hz 1.0KHz 10KHz 50KHz 20Hz 100Hz 1.0KHz 10KHz 50KHz
DB(V(OUTPUT)) DB(V(OUTPUT))
Frequency Frequency
LOUDNESS CONTROL RESPONSE FLAT VOLUME RESPONSE
6

7. LTspice Simulation: .WAV IN/OUT Loudness Control
Low to high swept audible signal (Input)
-30 dB level down with the loudness control (Output)
Loudness Control with .WAV input and output Simulation result: input and output signal
7

8. LTspice Simulation: .WAV IN/OUT Flat Volume
Low to high swept audible signal (Input)
-30 dB level down with the flat volume (Output)
Flat Volume with .WAV input and output Simulation result: input and output signal
8

9. .WAV FILES LOCATION
An input file Output files created
by LTSpice
9

10. USING WAVE FILE (.WAV) AS INPUT
Using audio signal (WAVE file) as an input voltage for LTSpice could be done
by replace the voltage expression of the voltage source with a WAVE-file
expression: wavefile=“file location" chan=0 (as seen in the example
below)
“chan” refers to the respective channel in the WAVE file used for the
simulation, can be a number between 1 and 65535  although usually
channel 0 refer to the left channel and channel 1 refers to the right channel
A loudness control and its frequency response (example)
10

11. USING WAVE FILE (.WAV) AS OUTPUT
Exporting an output signal as an audio signal (WAVE file) could be done by
adding SPICE directive as: .wave=“file location" 16 44100 Output (as
seen in the example below)
“16” refers to the bitrate, 44100 is the sampling frequency and Output is
referring to a label in circuit.
A loudness control and its frequency response (example)
11

12. LTspice Simulation: music01.WAV IN/OUT Loudness Control
music01.wav audio signal (Input)
-30 dB level down with the loudness control (Output)
Loudness Control with music01.WAV input and output Simulation result: input and output signal
12

13. LTspice Simulation: music01.WAV IN/OUT Flat Volume
Low to high swept audible signal (Input)
-30 dB level down with the flat volume (Output)
Flat Volume with music01.WAV input and output Simulation result: input and output signal
13

14. .WAV FILES LOCATION
Output files created
by LTSpice
An input file
Output files created
by LTSpice
14

15. VIDEO DEMONSTRATION
15