ANSI EIA 426-B-1998 pdf download Loudspeakers, Optimum Amplifier Power
7.2.5 CAUTION- WEAR EAR PROTECTION! High sound levels of annoying signals are produced which can cause hearing damage with prolonged exposure. The operator is advised to wear appropriate protection such as ear plugs, preferably balanced attenuation devices which will still permit the operator to listen critially for distortion or other signs of distress in the device under test.
7.2.6 Test sequence Power compression is the least stressful test and should be run first, followed by the distortion test, with the accelerated life test performed last.
7.3 Section A一power compression This procedure tests the degree to which acoustic output of the speaker is compressed at the optimum amplifier power. 7.3.1 Procedure Refer to Annex A.1 and A.3, and to the equipment block diagram in Annex B (nomative).
7.3.1 Place the microphone in a close-mic. position (refer to Keele paper cited in section 5.3), on axis with its geometric center, and connect the microphone output to the input of the 1/3- octave real time analyzer. Using the switched attenuator, apply the variable rate swept sine wave test signal to the speaker at an amplitude 20 dB below the optimum amplifier power, and record the reference spectrum of the speaker output.
7.3.2 Reduce the drive level by 6 dB. Using the switched attenuator, simultaneously raise the amplitude of the test signal by 20 dB and attenuate the output of the microphone amplifier by 20 dB; this applies considerably less than full power to the loudspeaker. The spectrum produced should track the original curve, but should be about 6 dB lower in amplitude, indicating lttle or no compression taking place. At this time, the operator should listen for distortion; there should be minimal distortion at this reduced power level. If severe distortion occurs, the operator should stop the test, noting the frequency at which the distortion was worst, and should proceed immediately to the distortion test procedure described in section 7.4 If only minimal distortion is heard, the operator should slowly restore the drive level to the maximum value, or until power compression is observed, whichever occurs first. To avoid overheating high frequency drive units, the bursts of test sweeps should be kept brief – only three or four cycles before a pause一when near full power.
7.3.3 Repeat the test sweep for one full minute, and then record the output spectrum. If no compression takes place, the displayed amplitude of the measured spectrum will be identical to that of the previously recorded signal at all frequencies. The speaker shall have passed the test if each 1/3 octave band of the displayed spectrum of the full power signal is reduced in amplitude by no more than 6 dB compared to the reference level established in section 7.3. 1 from 40 Hz to 10 kHz or through the bandwidth specified by the manufacturer.