Material Study

By: IRINEO JAIMES

TEAM

Nick Christian

Frank Schabold

Erich Pfister

Acoustical Testing 1 Dr. Lauren Ronsse, Dr. Dominique Chéenne

10/31/2014

Table of Contents

Abstract……………………………………….……………………………3

Introduction……………………………………..………………………..3

Measurement set up and Equipment……….…………..…...4

Test Method…………………………..…………….….…………………5

Results……………………………..………..…………………………….11

Conclusion…………………………………………………………………12

Abstract

The purpose for this experiment was to find the signal chain flow of the black box with

only two inputs and two outputs. Inside the black box there could be various possibilities for

how the signal chain flow was set up, using available equipment the team will have to figure

out the components that make up the signal chain flow.

Introduction

Presented to the team was a black box with two inputs, input 1 and input 2, and two outputs,

output A and output B.

Figure 1: the black box

The black box was set up by the previous student team, having succeeded in finding the signal

chain flow. Using Arta, analog signal tone generator, voltage meter, and auditory hearing the

team had to investigate what parameters could create the data the team was gathering. Based

on results the team had to conclude what processors made up the signal chain flow, with a

deadline of 3 weeks.

Measurement set up and Equipment

Testing equipment involved

- Voltage Meter - Signal Tone Generator - US-Pre amp - A loudspeaker, for playback purposes - ARTA

Figure 2: us-pre amp Figure 3: voltage meter (left), Signal Tone Generator (right)

Testing Method

A simple signal chain flow diagraph was created to get a visual interior representation of

the black box.

Figure 4: signal chain flow diagram of unknown processors

First test method involved playing “Short Skirt, Long Jacket” by Cake. The team concluded the

most effective way to understand what processing was happening through the signal chain flow

was to play a song through the black box vs through a loudspeaker, and using auditory cues

compare signals. First signal chain tested was input 1 with output A going to the loudspeaker.

Instantly the team noticed two audible signal, one from the loudspeaker and a faint signal

coming from inside the black box. Shutting of the Loudspeaker and increasing the gain on the

pre-amp it was clear input 1 had to be a speaker. To further conform input 1 was a loudspeaker

a sine sweep was played through input 1. The sine sweep test proved two pieces of

information, one being a loudspeaker was present and two being based on the strong presents

of the low end of the sine sweep an amplifier had to be present. Disconnecting the power

supply proved the existence of an amplifier.

For experimentation purposes, with the supply power still disconnected, the testing

loudspeaker was turned on. Interestingly the exterior noise of the black box seemed to be

amplified through the loudspeaker. To the teams knowledge only a few pieces of equipment

could produce a signal with their own power supply. In order to prove a dynamic microphone

was present a simple tap and a vocal recording test were created. The tap test involved tapping

the top of the black box and capturing the response of the tap using Arta. For the vocal

recording test a team member spoke in close proximity to the black box, then playback of the

recorded signal through the loudspeaker was done. Using these two methods, the team

concluded output A was a dynamic microphone.

Next the signal chain of input 2 output B were tested using the loudspeaker. Only

notable change to the way “Shirt Skirt, Long Jacket” sounded was a decrease in audio volume

and an increase of a hissing sound. Very similar to the way a recording with a low signal to

noise ratio sounds. With various possibilities to contribute to a hissing sound being audible.

The signal was instead connected into the pre-amp, feeding back into the computer, to have a

visual representation of the frequency spectrum. Figure 5 shows the frequency content of the

black box in input 2, output B.

Figure 5: spectrum analyzer, using ARTA, of black box

It was difficult to determine if the noise was being generated from input 2 or output B, since

input 1 would have yielded a signal coming from the black box and would not have interacted

with output B, and output A, being a dynamic microphone would only output exterior noise.

Only option was to disconnect input 2 and compare before and after results, and upon doing so

nothing changed. With these results, the team established output B as the generator of the

hissing sound. A sine sweep was done 10 times and the average is illustrated below.

Figure 6: Sine Sweep test sending signal through black

box, sine sweep provided by using ARTA.

The results seem to be suggesting the use of an equalizer processor. Zooming into the peaks in

the high frequency domain, the center frequency of both peaks seem to be around 8,500 Hz

and 20,000 Hz, commonly altered frequencies in an equalizer.

Figure 7: Zoom of higher frequency content, Frequency peaks at 8,500 and 19,500

The team decided on a parametric equalizer. To eliminate false evidence of an equalizer the

power source of the black box was disconnected once again, and the peaks disappeared

meaning it was also an active parametric equalizer.

Input 2 still unknown proved to be the most difficult to resolve. An assumption was

made of a possible compressor/gate being present in input. This assumption arose from the

initial volume decrease noticed when the team first tested the signal chain. Running a drum

loop through the black box yielded Figure 8 below.

Figure 8: Top: drum loop sample, Bottom: drum loop run through black box

There was what looks to be compression on being of the tail of the drum loop. However, this is

not enough to information to determine input 2 as a compressor. To test a compressor/gate, a

signal generated tone was split and sent into the black box and the other split signal was

unaltered, both split signals then were connected to independent channels on the voltage

meter. The results would be a output vs input graph, represented by Figure 9.

Figure 9: Compression measured at 1 kHz, input vs output

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Compression measured at 1kHz

What was derived was a graph very similar to what a compressor with a compression ratio

would yield. The team came to an agreement input 2 was a compressor/limiter.

Results

The final day of testing the team outlined a singal chain flow diagram to present to AT1

instructor. The teams final conclusion was input 1 was a speaker with no connected output, its

acoustical output would be picked up by the microphone in output A. Input 2 is a

compressor/limiter with an output into a active parametric equalizer. When the black box was

opened it was close with the exception of missing two components of the black box, an

additional equalizer and a pad. Additional equalizer before the speaker, and the pad before the

compressor in signal flow. Coming fairly close the project was determined a succes and the

AT1 instructors assigned our team to assemble a new black box for the preceding student

group.

Figure 10: Front of black box Figure 11: Back of black box

Conclusion

Developing test to looking into specific features of the black box enabled the team to

figure out what processors were inside. Arta, a signal generator, voltage meter were great

tools to investigate the black box assigned to the team. The final signal chain flow of the team

vs the real signal chain flow of the black box were very similar, with the exception of an

equalizer before the speaker and a pad before the compressor in signal flow. The pad was the

contribution to the unusually high compression ratio. Predicting a compressor/limiter was still

an acceptable prediction since compression was in fact occurring. The equalizer that came

before the speaker in the signal chain proved difficult to test, and a point could be made that it

could be nearly impossible without having a straight output form the speaker inside the black

box. Overall the processors in the black box were close to the team’s predicted results.