The Loudspeaker Study

Chris Nottoli

Melissa King, Joshua Roberts, Javier Forero, Frank Minella

Columbia College Chicago

Acoustical Testing I

Dr. Dominique Chéenne, Dr. Lauren Ronsse

October 16th 2013

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Table of Contents

Abstract: ................................................................................................................................................... 3

Introduction: ........................................................................................................................................... 3

Materials: ................................................................................................................................................. 4

Frequency Response: ........................................................................................................................... 4

Crossover: ................................................................................................................................................ 8

Polar Directivity: ................................................................................................................................... 9

Conclusions: .......................................................................................................................................... 10

Additional Tests: ................................................................................................................................. 11

Introduction: ................................................................................................................................................... 11

Damping Results: ........................................................................................................................................... 11

Comparison Results: ..................................................................................................................................... 12

Appendix A: ........................................................................................................................................... 14

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Abstract: The frequency response, crossover frequency, and polar directivity of an Event

TR8XL speaker were analyzed inside Columbia College Chicago’s anechoic chamber using

TEF 20. The frequency response of the speaker was also analyzed in a non-anechoic

environment. The response was relatively linear, showing many irregularities throughout

the frequency spectrum. When compared with the test conducted outside of the anechoic

chamber, the irregularities were still present. The crossover point occurred at 1898 Hz in

contradiction to where the magnitudes of the drivers crossed at 2059 Hz. The polar

directivity varied throughout the frequencies where at lower frequencies the speaker was

omnidirectional and became directional at higher frequencies.

Introduction: The loudspeaker analysis was conducted at Columbia College Chicago, using the

anechoic chamber located in LL01 with TEF 6.0 software. Josh Roberts, Melissa King, Javier

Forero, Mike Minella, and the author of this report collaborated on this study. The

objective was to analyze the frequency response both inside and outside of the anechoic

chamber, the crossover frequency, and polar directivity. Two additional tests were

conducted to determine the frequency response as mass was added to the subwoofer and a

comparison with a Shure SM-63 microphone.

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Materials:

Speaker: Event TR8XL; QSC-K8

Microphone: Electro-Voice RE-55; Shure SM-63

TEF 6.0

Outline electronic turntable

Computer with TEF 20 and Outline interface

Anechoic Chamber 13’ 10” X 10’ 2” X 8’ 2”

Frequency Response: This initial speaker used in this analysis was a QSC-K8. Preliminary steps were

taken before the frequency tests were conducted. The Electro-Voice microphone was

calibrated with TEF to .00099 V/Pascal. A test to determine if the microphone was

receiving signal from the speaker was then conducted. As seen in Appendix A-1, a

magnitude of less than 0 decibels was being obtained before the direct sound had reached

the microphone. Upon observing the frequency response in Appendix A-2, there was no

valid signal received by the microphone. Fig. 1 shows the signal flow followed to correct the

absence of a proper signal. The equipment rack patch bay had been altered since last used

and was corrected. This correction can be found in Appendix A-3 along with all other

patched connections used within these tests.

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Fig. 1: The output of the signal flow from TEF is displayed on the right. The input signal is displayed on the left.

A time response, found in Appendix A-7, was conducted to again determine if the

microphone obtained proper signal. Tested at 2 feet, the direct sound arrived at 3.81ms.

The arrival of direct sound was expected at 2ms. The 1.8ms of late arrival of the acquired to

the expected direct sound explains the speaker change. This late arrival time occurs

because the QSC speaker stays on standby until a signal is present to determine if the signal

can potentially destroy the speaker by DC power. A time response was conducted on an

Event TR8XL at 2 feet. The signal reached the microphone after 1.65ms ensuring that the

results being received were accurate as seen in Fig. 2.

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Fig. 2: Time response on the Event TR8XL speaker at 2 feet. Direct sound reached the microphone at 1.65ms.

In obtaining the frequency response, the microphone was repositioned 1 meter in

front of the speaker as seen in Appendix A-8. Time resolution was sacrificed since the test

was conducted inside an anechoic chamber allowing for a 15 minutes sweep from 20–

20,000 Hz. The graph of the frequency response can be found in Fig. 3. The response of the

speaker was relatively linear and had many irregularities especially at 225 Hz, 360 Hz, and

550 Hz. This could be due to the size of the speaker and the tendency of the low frequency

driver to produce an inaccurate response. The speaker also rolls off at 17,000 Hz indicating

it cannot accurately reproduce the full frequency spectrum. There was also a noticeable

drop in magnitude between 2700 Hz and 5500 Hz occurring after the crossover point. The

differences between phases of the two drivers may be the cause of this phenomenon.

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Fig. 3: The frequency response swept 20Hz- 20,000Hz over a 15-minute period. The frequency resolution used in the test was 11.0 Hz with a 2Hz bandwidth.

The frequency response of this speaker was also tested in a non-anechoic

environment, found in Appendix A-9. Time resolution could not be sacrificed so that

reflections off the nearest objects would not be captured in the response. The microphone

was placed 1 foot from the speaker so the time resolution will only allow the direct sound

from the speaker to reach the microphone with minimum reflections from surrounding

objects. Therefore, ten tests were conducted in octaves from 20Hz-40Hz, 39Hz -80Hz, etc.

to maximize the best possible time resolution. The graph in Fig. 4 shows the frequency

response obtained. Since time resolution could not be sacrificed, frequency resolution was

very difficult to achieve. However, many characteristics of Fig. 3 were still present.

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Fig. 4: Frequency response of an Event TR8XL in a non-anechoic room. Breaks in response are due to the start/stop of each test. An overlap of 1Hz at the start of each test was used to overlay from the previous test.

The inaccuracies of the low frequency driver were still present. The crossover

frequency can be pointed through a dip in the magnitude. The poor response of the high

frequency driver is also evident as it rolls off at 7,000Hz. The discontinuities that occur

throughout the graph are due to the 10 individual tests. It can be concluded that Fig. 4 was

an excellent representation to how the speaker reproduced sound in a non-perfect space.

Crossover:

The initial crossover point was hypothesized to occur between 2,000-3,000 Hz

where the magnitude in Fig. 3 drops roughly 3dB. Tested in the anechoic chamber, the low

frequency driver was covered with absorptive material. The microphone was then placed

directly in front of the high frequency driver. Another test was conducted covering the high

frequency driver and placing the microphone directly in front of the low frequency driver.

Results could be found in Appendix A-11. An additional test was conducted from 1,000-

4,000 Hz, placing the microphone directly between the high and low frequency drivers due

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to the lack of interactions of phase information obtained from Appendix A-12. As seen in

Fig. 5, the crossover occurs at 1898 Hz where the phase is 88 degrees out of phase.

Fig. 5: The phase test was conducted between 1,000 Hz and 4,000 Hz for 600s with a bandwidth of 2.2 Hz. Note the 30 dB drop in magnitude at 1898 Hz. As the phase is 88 degrees out of phase cancelation occurs, which caused the significant drop on magnitude.

Additionally, the acoustic and electrical phases were not at the same frequency. The

high frequency driver crossed at 2059 Hz. This may also have occurred because the sound

was bleeding through the absorptive material, skewing the data received.

Polar Directivity: The expected directivity of the loudspeaker was hypothesized to be omnidirectional

at lower frequencies and become more directional as frequency increases. Measured at 1

meter from the microphone in the anechoic chamber, the loudspeaker was centered on an

Outline electronic turntable. The Outline interface was set to turn every 5 degrees with

relation to the parameters in Appendix A-13 along with the speaker’s polar directivity in A-

14.

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A 3.5-minute sweep time with small interval changes were used to capture as much

detail about the directivity as possible. Below 32 Hz, the directivity was rugged looking,

which can be explained either by poor low frequency reproduction, any resonance of the

speaker itself, or both. The speaker was in fact omnidirectional between 32-50 HZ meaning

the wavelength of these frequencies diffracted around the back of the speaker housing. The

speaker then became subcardioid between 56-250 Hz. At 500 Hz, the frequencies become

smaller than the speaker where directional characteristics began to appear.

Conclusions: The frequency response of the TR8XL loudspeaker showed a relatively linear

response from 700 Hz- 2,700 Hz. With a roll off before 100 Hz and after 17,000 Hz, the

speaker cannot accurately reproduce these frequencies. However, most of those

frequencies are inaudible to most humans and can be overlooked. The response in a non-

anechoic room showed similar characteristics to the test conducted in the anechoic

chamber, but showed how it actually reproduced in a room it would most likely be used in.

Further testing of the frequency response in a studio control room would give better

insight as to how it may affect the outcome of a recording.

The crossover was found to occur at 1898 Hz where the drivers were 88 degrees out

of phase. This crossover point contradicts where the two magnitudes crossed when testing

individual drivers. Additional tests could be taken to determine how the speaker’s port

hole affects the outcome of all tests in this report. Also, using more absorptive material and

eliminating any bleed through of sound could be tested to determine if the crossover in

magnitude of each driver shifts closer to the electrical crossover.

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Finally, the polar directivity was found to be omnidirectional below 56 Hz,

subcardioid between 56-250 Hz, and an increase in directivity as frequency increased.

Further testing into the single hypercardioid response at 280 Hz could be investigated to

determine if a possible resonance of the table or speaker caused this to occur.

Additional Tests:

Introduction: As an extra credit study, the frequency response of the Event TR8XL was tested

adding eight coins, more specifically quarters, to the low frequency speaker to evaluate the

effects of damping. Each quarter was taped around the speaker evenly to ensure that extra

mass affected the whole speaker as seen in Appendix A-15. The frequency response of the

speaker with and without quarters would be compared. In addition, the Shure SM-63

microphone was compared with the Electro-Voice RE-55 expecting to obtain similar results

since both are dynamic. Quarters were removed from this test, since damping was not the

concern.

Damping Results: The results of damping were as expected seeing a 3-6dB decrease in magnitude up

until 700 Hz. By adding more mass, the speaker, seen in Appendix A-15, would have had to

exert more force in order to produce the same frequencies at the same magnitude

explaining this occurrence. As the frequency increased, the effects of damping were not as

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noticeable since higher frequencies stopped relaying on the size of the cone taking less

force to produce the frequencies as seen in Fig. 6.

Fig. 6: Damping with Quarter. This test foucused in between 20 Hz and 1,000Hz with a two minute sweep. Note that the peaks at 60hz, 120 Hz, 180 Hz, etc. are due to the vibrations of the quarters on the cone. As the crossover nears and the high frequency driver begins, these peaks become less prominent.

Comparison Results:

The frequency response of the Electro-Voice was used as a reference so that a

comparison with the Shure SM-63 can be analyzed. Many similar characteristics were

found between the two microphones; for example, the drop in magnitude between 2,700-

5,500 Hz was still present, as was the roll off at 17,000 Hz. The differences in the frequency

response can be due to the sensitivity of the microphones, the size of the diaphragm, and

calibration. Overall, the Electro-Voice continued to produce more accurate results that can

be easily interpreted.

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Fig. 7: The comparison between Electro-Voice and Shure SM-63. Sweep time of 60 s. and a 167 Hz bandwidth were used to get a general sense of the differences.

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Appendix A:

Fig. A-1: Time response test conduced at 2’2”. The decibel level before the direct sound had obtained decibels below 0 dB. In addition, the direct sound was captured at 3.66ms. This “late” arrival of direct sound was to be investigated.

Fig. A-2: Frequency response related to Fig. A-1, showing no true signal being received.

Fig. A-3: Patch Bay connection at equipment rack. The signal from TEF goes through station two output 3 to anechoic input 3, which in return gets set to the speaker. Anechoic 1 is the signal received from the microphone, which gets sent to station 2 output 1 to TEF (mic A).

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Fig. A-4: TEF 20 connection; BNC line out to station two XLR input 3(Fig. A-5) and mic A input from station two output 1.

Fig. A-5: Station two patch bay mentioned in Fig. A-4.

Fig. A-6: Chamber snake. Channel 1 is receives signal from the microphone. Channel 3 is the output signal to the speaker.

Fig. A-7: Time response after patch bay correction. Direct sound reaches the microphone at 3.81ms.

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Fig. A-8: General set up for loudspeaker tests in the anechoic chamber.

Fig. 9: Set up for analyzing the frequency response of the loudspeaker in a non-anechoic environment. Microphone was placed 1ft away from loudspeaker.

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Fig. A-10: Parameters for frequency response in a non-anechoic room. A 0.88ms delay was included in each test to account for the 1 foot distance between the speaker and the microphone.

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Fig. A-11: Crossover with absorption covering each driver. The expected crossover was thought to occur where both magnitude cross. Note that the high frequency driver appears to be producing low frequency tones. This is due to the bleed through of the low frequency driver, which had an effect to where the magnitudes crossed.

Fig. A-12: Crossover phase related to A-11.

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Fig. A-13: Polar directivity parameters.

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Fig. A-14: Polar Directivity graph.

Fig. A-15: Eight quarters were added to the speaker to determine the effect of damping as mass is added.