Optimal REW settings for measuring amplifiers

reallystate · Sep 23, 2025

Hello everyone,

I'm starting to use REW to measure the performance of my headphone amplifiers and I have several questions about the optimal configuration. I've done some reading, but I would appreciate some expert advice to ensure my measurements are accurate. My main questions are:

Should I measure at the maximum possible sample rate, or should I limit it to 48 kHz?
Should I use low-pass and high-pass filters? If so, at what frequency should I set them?
What FFT value and window type should I choose? Should these values be changed for different types of tests (THD vs level, IMD vs level, etc.)?
What sweep length value should I choose when measuring the frequency response of a device, since this choice affects the phase measurement result?
What buffer size should I set in the sound card settings section?
Should I use the Overlap option?

Thank you in advance for your guidance.

John Mulcahy · Sep 24, 2025

reallystate said:
Should I measure at the maximum possible sample rate, or should I limit it to 48 kHz?

Generally best measuring at the lowest rate that covers the bandwidth you are interested in.

reallystate said:
Should I use low-pass and high-pass filters?

Can't think why you would.

reallystate said:
What FFT value and window type should I choose?

For RTA measurements longer FFTs would give greater frequency resolution and lower noise floor. 64k is generally sufficient for many purposes. Blackman-Harris 7 is a good window choice for most measurements, though you could use a Rectangular window if your input and output are on the same interface.

reallystate said:
What sweep length value should I choose when measuring the frequency response of a device, since this choice affects the phase measurement result?

No, sweep length doesn't affect phase. Longer sweeps give greater signal-to-noise and a lower distortion noise floor.

reallystate said:
What buffer size should I set in the sound card settings section?

16k is usually sufficient.

reallystate said:
Should I use the Overlap option?

Using maximum overlap will produce faster RTA display updates, each update needs to wait for sufficient samples to be acquired.

reallystate · Sep 29, 2025

Thank you for your responses.

John Mulcahy said:
Can't think why you would.

When i use low-pass and high-pass filters, the noise is lower

John Mulcahy said:
Generally best measuring at the lowest rate that covers the bandwidth you are interested in.

I don't know what sampling frequency is worth looking at for measuring amplifiers. At a sampling frequency of 48 kHz, the noise is lower than at 192 kHz. And if you also enable filters at 192 kHz, the noise will be even lower. On the other hand, the use of filters can negatively affect the calculation of the distortion level.

John Mulcahy said:
No, sweep length doesn't affect phase

Is this a bug or a feature?

John Mulcahy · Sep 30, 2025

The distortion limits for most published specifications are 20 Hz to 20 kHz. Sample rate will have an effect if your limits exceed half the sample rate.

For sweep length differences attach the mdat file with the measurements rather than images.

reallystate · Sep 30, 2025

John Mulcahy said:
For sweep length differences attach the mdat file with the measurements rather than images.

reallystate · Sep 30, 2025

John Mulcahy said:
Sample rate will have an effect if your limits exceed half the sample rate.

When I measure a certain device at -20 dBFS using 192 kHz with 20-20k Hz filters, the noise level is -81.5 dBr, but when I use 48 kHz with 20-20k Hz filters, the noise level is -75.9 dBr. This can be seen in the screenshots above. What sampling rate should I use in this case?

John Mulcahy · Sep 30, 2025

Looks like an issue with clock rate differences between the input and output devices. If you add a loopback connection as a timing reference REW can use it to correct the clock difference.

John Mulcahy · Sep 30, 2025

Interface behaviour can also change with sample rate, best check that. For headphone amps 48k should be fine.

reallystate · Oct 20, 2025

When conducting THD vs level and THD vs frequency, the noise floor level is measured and the indicators differ in both cases. Can we assume that the results of the second test better reflect the real situation? What frequency is Noise floor measured at when performing a THD vs level test? According to the generator readings, it is at 1 Hz. But judging by the readings on the RTA screen, it is measured at different frequencies.

Letshuoer DT01 THD vs level 1 kHz Noise floor 192 kHz 20-20k Hz.jpg

Letshuoer DT01 THD vs frequency Noise floor 192 kHz.jpg

John Mulcahy · Oct 20, 2025

The noise floor figure for stepped level THD measurements is the maximum noise level within a one octave span centred on the the stimulus frequency. THD vs frequency shows the noise floor spectrum.

reallystate · Oct 23, 2025

Can you increase the amount of space allocated for the measurement name display below the graph? There's still plenty of space to the right of the graph names in the exported image, but the names still don't fit.

John Mulcahy · Oct 23, 2025

It's a little difficult to manage. All the legend entries for the various graphs need to keep the same width. I can adapt the width to the measurement names, but it would have to be to suit the longest name of the currently loaded measurements otherwise the legend height could change when moving between individual measurements. I'll look at that for the next build.

Optimal REW settings for measuring amplifiers

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