Anechoic Chamber Free Loudspeaker Test System ---- Type 1600F

Technical knowledge

Comparison methods for testing frequency response of loudspeaker under anechoic chamber free condition.

In principle, the frequency response of a loudspeaker should be measured in an anechoic chamber. The anechoic chamber can (1) isolating noise from outside; and (2) eliminating reflection waves.

Because to construct an anechoic chamber is very expensive, the following three anechoic chamber-free methods have been developed :

(1) TDS method; (2) pulse FFT method; and (3) Special test box method (developed by SUNLIGHT).

Amount them, the TDS method is obsolete. The principle will not be described here any more because now this method is seldom used. Here the methods (2) and (3), which are common used nowadays, are compared.

All other electro-acoustic measuring instruments use the pulse FFT method. It claims that anechoic chamber may not be used. However, if measurement is made under free anechoic chamber condition, a lot of disadvantages comes :

A.   The pure test space should be so large than not practical.

B.   Operator should have deeper technical knowledge. He is able to make accurate calculation on the reflecting time. These are the basis for correct operation.

C.   The frequency intervals at low frequencies are too large. For midrange and woofer loudspeakers, the intervals are larger than practical requirement. It is only acceptable for the measurement of tweeter .

D.   Distortion can not be measured under free anechoic chamber condition.

E.   Results are not stable because it is easy to be influenced by the environment noise. Results may be different even under very low background noise.

However, SUNLIGHT 1600F has not the above shortcomings. The special design of 1600F is originated from SUNLIGHT. This one of the inventions from SUNLIGHT.

The following description explains why the pulse FFT method has above five shortcomings.

Pulse FFT method :

In this method, a very short voltage pulse is fed to the loudspeaker by the instrument. The propagation time of the direct wave of the pulsed sound from Points P to Q is about 2.9 ms (0.0029 second). The propagation time of the reflected wave of the pulsed sound from Points P to M to Q is about 9.2 ms. In order to avoid the effect of the reflected wave, the sampling time of the A/D converter must be chosen as 6.3 ms (see lower figure). Then FFT calculations are made to derive the frequency response curve of the loudspeaker.

According to the FFT theory, the frequency interval should be the reciprocal of the sampling time 6.3 ms, i.e. 160 Hz. It means that the measured points which form the curve have equal frequency interval of 160 Hz. What we know is only limited to the frequencies of 160 Hz, 320 Hz, 480 Hz, 640 Hz, 800 Hz, 960 Hz... Nobody knows the response below 160 Hz, and at frequencies between the neighboring points. Exactly to say, the given curve is a smoothed segment.

To reduce the frequency interval, the distance of the shortest reflection path (P to M to Q) should be increased.  However, this distance is limited by the real size of the room, so that it is not practical in most cases. In other words, the interval is very difficult to be smaller than 160 Hz.

We can conclude that this method is suitable only for the measurement of frequent response at higher frequencies (e.g. > 1 kHz).

Easy to be influenced by noise : When the pulse FFT method is used, the pulse fed to the loudspeaker is very narrow, that means the “Pulse Energy” is very small or the “Pulse sound energy” generated by the loudspeaker is also very low. This is why the measured frequency response curve is not accurate under noisy environment.

In SUNLIGHT 1600F, entirely different principle is used to reach the free anechoic chamber  measurement. There is no above shortcomings. A special designed test box is used in 1600F. The box isolates environment noise considerably. With the addition of the “Tracking Filter”, the influence of environment noise is further eliminated. These measures ensure that 1600F can be used in noisy production line.

The Test Box 143B in 1600F has special construction. Together with the software corresponding to the construction of the box, the free anechoic chamber  test system is formed. Since the characteristics of individual Test Box could not be exactly the same (e.g. slight difference in the density of the sound absorbing material), each 143B must be calibrated and corrected against a large anechoic chamber individually. The calibrated result is stored in the software so that the disk of one 143B can not be used in other 143B. In other words, each 143B must use its own attached software. It is true even for two 143Bs manufactured in the same batch.

SUNLIGHT has a large anechoic chamber with a height of two floors ( see the About Us ).  Each Test Box 143B was calibrated carefully against the anechoic chamber. It makes sure that the measured frequency response of the loudspeakers is correct.

The frequency response (including sound pressure sensitivity) of a loudspeaker measured by SUNLIGHT 1600F is the same (or very close) to that measured in a large anechoic chamber according to CNS 4784 or JIS 5531 National Standard.