Voltage on speaker terminals

[Mfinzi]

Greetings, when carrying frequency response measurements in REW whats the best option to measure the speaker terminal voltages: multimeter or oscilloscope, l need to measure 2.83 volts and does that represent an rms value for the voltage. Thanks M finzi

 

[rayma]

Most DVMs do not have an accurate AC response, so a scope with properly adjusted
attenuator probes would be better. Check the response directly on the source to
verify the scope and probe accuracy. Power uses Vrms for calculation purposes.

The 2.83Vrms represents 1W into 8R; so (2.83Vrms x 2.83Vrms) / 8 ohms = 1W

 

[Mfinzi]

Thanks for such a prompt response. I really appreciate

 

[rayma]

One more thing, make sure you connect the scope probe ground clip to the ground terminal
of the amplifier output on each channel. Never connect the probe ground clip to the hot output
terminal of the amplifier, since the amplifier would be shorted.

A bridged amplifier, including most digital amplifiers, should be measured with a differential probe.
It is possible to use two regular probes per channel, with the ground clips unconnected.
Set the scope to A-B mode.

 

[jan.didden]

Reasoning test:

How accurate is a scope vertical amp? How accurate can you read that scope? How accurate is your multimeter?

Jan

 

[bentoronto]

Most DVMs do not have an accurate AC response, so a scope with properly adjusted
attenuator probes would be better. Check the response directly on the source to
verify the scope and probe accuracy. Power uses Vrms for calculation purposes.

The 2.83Vrms represents 1W into 8R; so (2.83Vrms x 2.83Vrms) / 8 ohms = 1W

jan.didden is right.

There's more error in trying to guess the RMS by eyeballing a scope reticulum peak voltage than by any error in even a cheap meter.

Once again misapplied accuracy, if you are within half a volt of 2.83XXXX, you're good to go in the logarithmic world of audio.

B.

 

[rayma]

There's more error in trying to guess the RMS by eyeballing a scope reticulum peak
voltage than by any error in even a cheap meter.

Cheap meters are not true rms, and mostly aren't even rated above 1kHz, some only to 60Hz.

A calibration can be done with the source, and it's easy to use a scope to visually read
the p-p output to within a half division, just over 1%. I did this often years ago, before
getting better T&M equipment. Most scopes made in the last couple of decades
don't have parallax error since they're LCD, and many have built-in voltage readout,
so no visual reading needed (like a $100 PicoScope).