The Loudspeaker Study

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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.

Transcript of The Loudspeaker Study

  • Sarah Kaddatz

    Matt McQuaid, Chris Hulik, TJ Ohler

    The Loudspeaker Study

    October 24th, 2012

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

  • Abstract:

    In The Loudspeaker Study, the frequency response, the crossovers 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

    crossovers 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 94 high by 137 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 TEFs 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 wave