Sarah Kaddatz

Matt McQuaid, Chris Hulik, TJ Ohler

The Loudspeaker Study

October 24th, 2012

Attn: Dr. Dominique J. Chéenne, Dr. Lauren Ronsse

Abstract:

In The Loudspeaker Study, the frequency response, the crossover’s cut-off point, and the

polar directivity of a Tannoy System 8 loudspeaker were analyzed using the Columbia College

Chicago anechoic chamber and TEF 20 equipment. It was determined to have an almost linear

frequency response from approximately 100 Hz to possibly exceeding 20 kHz, a single crossover

cut-off point at approximately 1634 Hz, and as expected, the polar pattern varied by frequency.

It was omnidirectional up to 200 Hz, less sensitive on the back of speaker and a supercardioid at

400 Hz, and more directional as the frequency increased.

Introduction:

The Loudspeaker Study was completed as a requirement for Acoustical Testing I at

Columbia College Chicago in room LL1 of the 33 E Congress Building. This report was

completed by Sarah Kaddatz. The study itself was completed with group members: Matt

McQuaid, Chris Hulik, and TJ Ohler. In The Loudspeaker Study, the frequency response, the

crossover’s cut-off point, and the polar directivity of a Tannoy System 8 loudspeaker were

analyzed using the Columbia College Chicago anechoic chamber and TEF 20 equipment.

Signal Flow of the Test Equipment:

The TEF 20 unit was connected directly to the Behringer EP1500 power amplifier for the

Tannoy System 8 loudspeaker and to the APEX Tubessence preamplifier for the Behringer

ECM8000 microphone for measurement. To avoid a ground loop in the current patch system

that connects the classroom area to the anechoic chamber, 2 XLR cables were run directly into

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the chamber through a hole in the wall between both spaces (Fig. A-1 and Fig. A-2 in Appendix

A contain pictures and a more detailed description). The wall is depicted with a grey box in

Fig.1 which denotes the signal flow.

Wall >

Fig. 1: Equipment on the left side of the wall was in the classroom. Equipment on the right side was inside the anechoic chamber. This diagram was created by Matt McQuaid and is used with his consent with a few minor alterations. These changes include a representation of the wall to show what equipment was inside the chamber and more arrows to depict signal flow through the entire system.

The Tannoy System 8 loudspeaker was placed on an Outline ET2-ST2 automated turntable (to be

used for polar pattern measurements later) and this revolving unit was placed on a table with the

speaker on top of it (Fig. A-3 in Appendix A contains a picture of this portion of the setup). The

base of the speaker was 33.25” from the floor of the chamber. The Behringer microphone was

placed in a microphone stand approximately 5 feet away from the loudspeaker. The chamber

was 10’ wide by 9’4” high by 13’7” long. Pictures of the set-up of the experiment, along with

more descriptions can be found in Appendix A.

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Frequency Response:

To calibrate the microphone, pink noise was generated through the TEF 20 unit and the

Tannoy System 8 loudspeaker. A Quest 2900 Sound Level Meter was used to measure the dBA-

SPL of the sound emanating from it. A TEF TDS measurement was then taken and compared to

the measurement registered by the Quest SPL meter. If the dB levels were not within a

reasonable tolerance of 1 dB, the microphone preamplifier was adjusted and the TDS

measurement was conducted again. When this was achieved the Quest meter measured 80 dBA -

SPL and the TEF TDS measured 80.73 dB. The TEF TDS measurement was run a second time

as an overlay to the first measurement to demonstrate consistency in the measurement. The

second measurement was recorded at 80.44 dB. A graph of this verification, along with a brief

discussion of time resolution versus frequency resolution can be found in Fig 2.

Fig. 2: This graph depicts both measurements which calibrated the microphone and verified TEF’s reliably. The 2nd measurement is overlayed on top of the 1st with impressive similarity.Note: The direct sound is the point where the cursor drew lines to both the y and x axis on the graph. Everything after this point represents reflective surfaces in the room such as the pipe near the ceiling of the room, the table the speaker is siting on, patches of the walls lacking sufficient absorptive material, and other sources for reflection. All reflections after the direct sound are at least 20 dB lower in amplitude. This allows for better frequency resolution by sacrificing the time resolution when making measurements. (Time resolution and frequency resolution are inversely related. Time resolution is correlated to room size and reflections in the aforementioned room. A higher Hz time resolution leads to getting more reflections from farther away surfaces. Since the measurements were taken in an anechoic chamber, this can be sacrificed for better frequency resolution.)

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This measurement gave the needed delay for the frequency analysis at 4.41 ms; the time

it took for the sound to travel the 5 feet from the speaker to the microphone. There were two

methods to complete the frequency resolution measurement. Since an anechoic chamber was

used to test the speaker sensitivity, time resolution could be sacrificed and the task could be

completed in one long (duration of time) sweep from 20 Hz to 20kHz. Fig. 2 shows an ETC to

demonstrate this point. While the chamber may not be completely anechoic (low frequencies

protruded the chamber walls during later directivity tests and reflections exist in the room), Fig.

2 demonstrates that all reflections were a minimum of 20 dB quieter than the direct sound.

The other method was to break apart the frequencies into sections, make several

measurements and overlay them to create a whole picture. The second method is more desirable

for non-anechoic environments where long sweeps can compromise measurements due to room

reflections. The first method will be discussed here, but both were completed and a discussion of

the second method can be found in Appendix B.

The first method was completed with 8192 samples in 199.7 seconds from 20-20,000 Hz

and successfully achieved 10.0 Hz frequency resolution. The graph of the first method is shown

in Fig. 3 on the next page:

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Fig. 3: Frequency response measurement from 20-20.000 Hz completed in one sine wave sweep over 199.7 seconds.

An almost linear response exists from 100 Hz up to the 20kHz upper threshold of human

hearing. The irregularities (peaks and dips) found between 300 Hz and 2kHz could be attributed

to the design of the speaker (such as the size of the woofer and frequencies that resonate within

the cabinet the speaker is housed in).

Crossover Frequency:

To find the crossover frequency of the speaker between the tweeter and the woofer, the

phase of the speaker was examined from the frequency resolution measurement taken from

20-20,000 Hz (see Appendix D for this graph). From this graph, it was determined that the

crossover frequency was around 1635 Hz because this is where the phase shift of the speaker

peaks close to 90 degrees and then settles down as the tweeter takes control of the sound above

this point. Another measurement was taken, restricted to 1250-2000 Hz to allow for better

frequency resolution and a closer measurement to the actual crossover frequency. 8192 samples

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were taken at 6 Hz/ second over 124.587 seconds to achieve 2.5 Hz frequency resolution. The

graph resulting from this is presented in Fig. 4:

Frequency Sensitivity Phase Line Area of Interest

Fig. 4: Frequency response measurement with phase from 1250-2000 Hz completed in one sine wave sweep over 124.587 seconds. The area of interest is the approximation of the crossover point.

From this graph, it was determined that the closest the speaker came to a 90° phase shift

was 82.8° at 1633.9 Hz. This drastic change in phase is due to the different acoustic centers of

the tweeter and woofer which can cause cancellations in the signal. It is 82.8° phase shift here

instead of 90° because 90° would be the measured electrical phase shift at the crossover point.

The coaxial speaker has a different acoustic center for each of its two drivers (the woofer and the

tweeter). Due to this, sound cancellations occur and a less-degree phase shift is observed.

Polar Pattern:

The exact parameters used during the directivity pattern can be found in Appendix E.

The revolving unit that rotates the speaker in-between TEF sweep measurements was set to 12.5

degree intervals. 30 TEF sweeps were made total (one at each 12.5 degree interval). These

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sweeps were then compiled to the traditional polar pattern. Fig. 5 is a sample of some of the data

we collected at 63, 500, 4000, and 16000 Hz. A complete polar pattern response for every octave

and 1/3 octave we measured can be seen in Appendix F.

63 Hz, 500 Hz, 4000 Hz, 16,000 Hz

Fig. 5: Sample of data collected at 63, 500, 4000, and 16000 Hz, spread out and displayed from an angle to allow better discrepancy between the frequencies.

As expected, as the frequency increases, so does the directivity of the speaker. Below 40 Hz,

there appears to be interference with the measurement as sound waves below this should be

omnidirectional through the speaker. Instead, it looks like a gear from a clock or watch

mechanism. This could be due to external noise occurring in room LL1 that is seeping into the

anechoic chamber and keeping it from its full potential. Between 40-400 Hz, the speaker is

omnidirectional due to the size of these frequencies being much larger than the speaker itself.

The waves can simply diffract around the speaker without a problem. From 400-500 Hz, the

speaker starts to have more of a cardioid directivity as the size of the sound wave approaches the

size of the speaker and as the frequencies continue to increase, the response reflects the increase

in directivity.

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Conclusion:

The frequency response, crossover frequency, and directivity of the speaker were found

using TEF. The Tannoy System 8 speaker had an excellent linear response from 100 Hz to

excess of 20 kHz. The anechoic chamber was unfinished at the time of this study and could have

contributed to errors in our measurements. If the experiment were to be repeated, it may be

beneficial to test above 20 kHz to see when the frequency response drops off the high end like it

does on the lower end. It may not be perceived by the human hear, but it could most likely still

be felt in some way psychoacoustically. It may also be beneficial to explore what in the room

corresponded to what reflections in the TDS measurements by using absorption to cover up the

table, the pipe hanging from the ceiling, etc.

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Appendices

A: Pictures of the Setup

Fig. A-1: The hole in the wall where the 2 XLR cables were strung through from the classroom to the anechoic chamber. Picture is taken to the right and towards the ceiling in comparison to the entrance of the anechoic chamber (Fig A-2).

Fig. A-2: The entrance to the anechoic chamber where the tests were conducted.

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Fig. A-3: Set-up inside the anechoic chamber. The speaker (left) was placed on the Revolving unit (circled in white) and both were placed on a table. The Behringer microphone was placed on a stand and hooked up to the TEF 20 unit via one of the XLR cables that was strung through the hole in the wall (Fig. A-1).

Fig. A-4: The gain control setting on the APEX Tubessence microphone pre-amp is shown above (left). The phantom power (right) was also engaged so the condenser microphone could be utilized. Images were cropped instead of showing the whole front in order to show more detail of the gain control.

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Fig. A-5: The XLR cable connecting the microphone pre-amp to the TEF 20 unit (TEF 20 is what is shown) and the BNC to XLR cable connecting the TEF 20 unit to the speaker amplifier inside the anechoic chamber.

Fig. A-6: The Behringer EP1500 speaker amplifier used between the TEF 20 unit and the Tannoy System 8 Loudspeaker.

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B: Composite Frequency Response

Fig. B: Measurements were completed in octave bands starting with 20-40 Hz, then proceeding with 40-80Hz, etc. Exact settings for each measurement are provided in Appendix C. Measurements were completed with approximately 3-4 Hz frequency resolution. Each consecutive measurement starts 1 Hz below where the previous measurement ended so when the measurements are overlayed, the dB at 40 Hz (for instance) is the same for both measurements. This is due to the speaker needing to “warm up” and break its inertia and give a consistent reading. It should be noted that this method is almost identical to the first method presented previously, except there is more accuracy below 100 Hz and overall the frequency resolution is improved. These circled dips can be ignored when analyzing the measurement.

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C: Settings for Each Octave of the Composite Frequency Response

Fig. C: The settings for each octave of the composite frequency response.

D: Frequency Response Graph with Phase Line

Frequency Sensitivity Phase Line Area of Interest

Fig. D: The area of interest is circled as a possible crossover point due to the high degree of phase shift and this degree gradually decreasing after this point.Note: The degree of phase shift increased from 81.4 degrees to 82.8 degrees between this measurement and the one between 1250 and 2000Hz by taking more samples over a smaller frequency interval, allowing for a greater accuracy.

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E. Settings for Polar Response Graphs

Fig. E: Above are the settings TEF used to take the polar response measurements. The 12.5 degree step was matched by the blue dial on the patch bay that operates the revolving unit to rotate the speaker between measurements.

F. Full Polar Measurements for Every 1/3 and Whole Octave

Fig. F: This shows every measurement the computer to made while measuring the polar patterns.

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Sarah Kaddatz

Matt McQuaid, Chris Hulik, TJ Ohley

Extra Credit Microphone Comparison

Purpose of Study:

This study was conducted as an extra credit exercise alongside The Loudspeaker Study

for Acoustical Testing 1 in the anechoic chamber at Columbia College Chicago in room LL1 of

the 33 E Congress Building. Its purpose is to compare a Shure dynamic microphone to the

Behringer ECM8000 condenser microphone in terms of frequency sensitivity.

Procedure:

The same basic procedure was followed as the frequency response testing of the Tannoy

System 8 loudspeaker. However, a QSC loudspeaker was used for this measurement. The Quest

2900 SPL meter gave a reading of 84.1 dBA with pink noise sent through the system and the

TEF 20 TDS measurements’ direct sound was recorded at 84.03 dB twice consecutively using

the Behringer microphone. This microphone was then replaced with the dynamic Shure

microphone with much effort to not move the placement of the stand.

The microphone pre-amp had its phantom power shut off for the Shure microphone since

it was not needed or recommended to use phantom power with dynamic microphones.

Condenser microphones are more sensitive than dynamic ones, in part, due to the lighter mass on

the diaphragm. This does not make the comparison between the Shure and Behringer

microphones ideal, but the frequency response can be viewed as similar, just with less sensitivity.

This may help account for the approximate 15 dB loss in sensitivity the Shure has in comparison

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to the Behringer. Note how the frequency response (Fig. 6) and ETC (Fig. 7) measurements are

almost identical if this dB loss due to comparing a dynamic and condenser microphone.

Behringer Shure

Fig. 6: The frequency resolution measurement of the Behringer and Shure microphones.

Behringer Shure

Fig. 7: The ETC measurement of the Behringer and Shure microphones to verify that the microphone was placed in very close to the same position relative to the QSC loudspeaker.

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