Radio Shack Sound Level Meter Uživatelský manuál stáhnout pdf (Strana 4)

4

audioXpress

2007 www.audioXpress .com

relative accuracy. Using the new or

digital meter may not produce ideal

results, because its low-frequency ex-

tension is -3dB at 41Hz and 32Hz,

respectively.

• Low-frequency room-mode analysis.

This is probably OK for small to

medium rooms for the old RS SPL

meter, but may not be suitable for the

new or digital meter, because room

modes may appear reduced in ampli-

tude. Attenuation of low-frequency

room mode peaks with a parametric

EQ may result in under-compensa-

tion if what is measured is deficient

in output.

Of the four domestic rooms in

which I performed low-frequency

room-mode analysis, 40Hz was the

lowest room mode encountered in

a particular room with the approxi-

mate dimensions of 13´ W × 13´ D ×

8´ H (4m × 4m × 2.4m). Obviously,

larger rooms will have lower room

modes, and for these cases, none of

the RS SPL meters should be used

for accurate low-frequency room-

mode analysis.

• Measuring loudspeakers. With the

C-weighted frequency characteris-

tic and deviating frequency response

above 550Hz, it is probably not

a good idea to use any of the RS

SPL meters to measure loudspeak-

ers accurately, unless the meter can

be calibrated to compensate for the

low- and high-frequency response

deviations. Even then, I’m unsure of

the implications of using a device,

such as the digital RS SPL meter,

for precision acoustics measurement,

which requires up to ±10.4dB of am-

plitude correction. For example, a

slightly off-axis high-frequency mea-

surement with copious amounts of

compensation may exhibit a highly

erroneous result.

CONCLUSION

The absolute dB SPL measurements for

all four units were surprisingly accurate.

It is unfortunate that the older-model

sound level meters (which were the only

meters that mostly met the manufac-

turer’s specifications for frequency re-

sponse) are no longer in production.

Neither the new meter nor the digi-

tal meter seems to demonstrate fidelity

similar to the older models. However, if

you are aware of its frequency-response

limitations, even the new or digital

meter can be a highly useful and afford-

able acoustic tool, especially for home-

theater speaker level matching and some

subwoofer level matching. The old RS

SPL meter should be suitable for low-

frequency room mode analysis in all but

the largest domestic rooms.

For the new or digital meter, you

could perform low-frequency room-

mode analysis in perhaps smaller rooms,

although accurate loudspeaker measure-

ments are best avoided for all meters.

In any case, the new Analog-Display

Radio Shack Sound Level Meter can

be had for $44.99 (US) and the Digital-

Display Sound Level Meter for $49.99

(US), which are bargains considering its

capabilities and features. Anyone curious

about measuring sound should own one.

I would like to thank HSU Research

for allowing me to use their measure-

ment facilities. Additional thanks for

Dr. Poh Ser Hsu for his insight, and Mr.

Lanny Dalton for the loan of the digital

sound level meter.

aX

REFERENCES

1. S. Müller and P. Massarani, “Transfer-

Function Measurement with Sweeps,”

J. Audio Eng. Soc., Vol. 49, pp. 443-471

(2001 Jun.).

2. J. D’Appolito, Testing Loudspeakers

(Old Colony Sound Lab, 888-924-

9465, www.audioXpress.com).

3. LMS 4.1 User Manual (LinearX Sys-

tems Inc., Tualatin, 2000).

4. M. Gander, “Ground-Plane Acoustic

Measurement of Loudspeaker Sys-

tems,” J. Audio Eng. Soc., Vol. 30, pp.

723-731 (1982 Oct.).

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