Electro-Acoustics Project
Hadi Sumoro
Contents
1. Summary 2
2. Overview of the Measurement Project 3
3. Measurement 4
4. Data and Analysis 5
A. Measurement Data 5
B. Analysis and Comments 18
5. Documentation 20
I. Summary
This measurement project is intended to determine and compare speaker’s coverage angle based on its physical size. The speakers used in this project are Tannoy Reveal 6P and JBL Creature II Multimedia speaker. Tannoy Reveal 6P uses a 6” multi fibre paper pulp cone and a 1” titanium dome and JBL uses a 1” diaphragm.
II. Overview of the Measurement Project
The measurement was done in a living room with a small chamber built from blankets. The living room is carpeted, the blankets which cover the sides are thicker than the ceiling blanket and there are foams at the back of the speaker to reduce reflections when the speakers are rotated 180 degree[1].
The speakers are placed on a round chair[2] that can be rotated 360 degrees inside the small chamber one meter away from the SPL meter. The SPL meter used is Radioshack digital type with C weighting slow response.
The measurement was done with a 1/3 octave filtered pink noise at 250Hz, 500Hz, 1KHz, 2KHz, 4KHz and 8KHz, 44Khz 16 bit resolution. The Tannoy Reveal 6P was connected to ART SLA2 power amplifier. The JBL creature is self-powered and connected directly to the Benchmark DAC-1 DA converter and the soundcard used was an M-audio Firewire 410.
The
JBL creature is a 2.1 multimedia system, the crossover built in the system is
set at 180Hz and the subwoofer will be assumed to be off for 250Hz and 500Hz
measurement.
III. The Measurement
The small chamber is firstly tested to make sure that high frequency above 1 KHz is being absorbed as much as possible. This test is done by playing 1 KHz, moving the chair and aiming the speaker so that the image clearly comes from the speaker being tested only. The problem was that the ceiling cover is too thin and some reflections can be heard from above. This is solved by putting some other materials on the ceiling’s chamber (towels – not documented).
The 0 degree reference point is taken around 75dB (for lower frequency) – 85dB (above 1 KHz). The background noise (low rumbles) is about 55 dB on the living room and about 53 dB inside the small chamber.
The first measurement was done to the JBL speaker. Elevation is needed to avoid excessive reflection from the chair[3] due to the small physical size of the speaker. The horizontal measurement is done by placing the speaker on axis and rotates the chair 180 degree in 5 degrees increment to its right. Assuming an acoustic reciprocal response on the other side of the speaker, the response of the speaker can be plotted fully 360 degree.
Vertical coverage is measured by putting the speaker 90 degree to the side and plotted 45 degree above and below the 0 degree axis in 5 degree increment. The back vertical radiation of the speaker is not plotted in this measurement.
The second measurement is done for the Tannoy Reveal 6P using a same way. No elevation is used due to the physical size of the speaker which is covering almost the whole surface area of the chair.
IV. Data and Analysis
A. Measurement Data
The 3dB and 6 dB down point of the speakers are plotted below.
|
Frequency (Hz) |
Tannoy Reveal 6P |
JBL Creature II |
||||||
|
Horizontal Angle |
Vertical Angle |
Horizontal Angle |
Vertical Angle |
|||||
|
-3dB |
-6dB |
-3dB |
-6dB |
-3dB |
-6dB |
-3dB |
-6dB |
|
|
250 |
-- |
-- |
+45 |
-- |
-- |
-- |
-- |
-- |
|
500 |
60 |
105 |
-45 |
-- |
-- |
-- |
-- |
-- |
|
1000 |
75 |
105 |
+60 |
-- |
-- |
-- |
-- |
-- |
|
2000 |
45 |
75 |
+60 |
-70 |
135 |
-- |
-60 |
-- |
|
4000 |
70 |
65 |
+5 |
+15 |
60 |
90 |
+25 |
-85 |
|
8000 |
30 |
55 |
+45 |
+55 |
70 |
80 |
-45 |
-65 |
Note: Some values are rounded up and
measured only on the other side.
The next couple pages are the detailed result of the measurement
B.
Analysis and Comments
The Tannoy Reveal 6P is intended for a studio use. Normally, most people expect to have a pretty wide horizontal coverage and a narrow vertical coverage. From the measurement, we can conclude that the Tannoy has a wide horizontal coverage even in the 1 KHz-2 KHz frequency range. The vertical angle has a pretty wide coverage and some lobes happen in the higher frequency (above 2 KHz). For a studio use, this wide vertical coverage angle can result a severe comb filtering due to the desk/console area if the speaker isn’t placed correctly.
The JBL at the other hand, it’s very small and the radiation, either vertical/horizontal direction is very wide and considered omni up to midhigh frequencies. On the vertical coverage measurement, notice that the vertical coverage angle on the higher frequency tends to have down point at the bottom front of the speaker. This is caused by the “frame” in front of the driver. On the documentation page picture 3, notice that the bottom front of the diaphragm has a longer frame. I assume the outer box design is more important in this JBL speaker since it’s marketed as a multimedia speaker, mostly for laptops. However, it might be a small advantage to reduce the higher frequency comb filtering if it’s placed on the table, below the ear position.
For a studio use, the Tannoy speaker has a very good imaging and a wide coverage radiation area, the placement is a very important thing to be considered. On the other hand, the JBL has a very omni directional characteristic, placing this speaker on a table below the ear position will give a good result because of the hemispherical radiation due to the table area which reinforces the midlow to mid frequencies. The JBL speaker is not intended to be placed on the same or above the ear level because the diaphragm itself is facing slightly upward and will give a slight darker quality of sound. Since it’s very small and doesn’t produce a very good midlow frequency, placing it on the ear level height can be another advantage. As a user of this JBL Creature II multimedia speaker, placing it on the height of the ear position is preferable to me.
V. Documentation
Picture 1
Picture 2
Picture 3