Cumulative Spectral Decay is the rate at which a speaker stops moving (and making sound) after the input signal is cut instantly. So, a broadband pulse goes into the speaker and a measurement system captures the resulting sound coming out of the speaker and plots it over time and spread across a frequency range.
Stereophile is pretty good in presenting these plots and they are consistent how they set the parameters of the measurement. As such, here is a Cumulative Spectral Decay plot for a very high end speaker:
The vertical axis (up and down) is the amplitude of the signal being measured. Stereophile almost always uses a 24dB window, in this case is it +/- 12dB SPL.
The horizontal X-axis (left to right) is frequency - just like we always see in frequency response charts, but limited to 400Hz to 20,000Hz.
The horizontal Z-axis(front to back) represents time. In this case each line is 0.06mS later than the previous one which is behind it. So, starting at the back of the chart, each subsequent line moving forward represents what sound energy was still being measured 0.06mS after the previous line.
So, the very first line, at the back, should directly copy the measured frequency response of the speaker being tested. The next line is what the speaker is still generating 0.06mS after the pulse ended. The third line in front of that is what sound the speaker was making 0.12mS after the pulse ended, and so on until the end of the graph at nearly 4.00mS.
Why does this matter and what does it tell you?
If the goal of a speaker is to accurately follow the voltage of the incoming signal and convert it accurately into acoustic sound we can hear, it should start moving immediately when the voltage rises, but when the voltage stops, so should the speaker cones, domes, horns, diaphragms, etc. If the speaker is moving about after the voltage stops, then when the next voltage swing goes in it will have to recover and start moving again, potentially being forced to attempt to go from instantly moving outward to inward because the new voltage is counter the "resonating" of the system.
Also, if you want to experience true, accurate dynamics - as many do - a speaker's ability to stop moving after the signal stops is critical in order to reproduce the silent spots between peaks in the sound. Dynamics is all about the difference between high peaks and quiet valleys in the sound. That ratio of high SPL to low SPL is a measure of a dynamic system. But, if the speaker is continuing to make sound when the signal says it should be silent, then the range between high and low will be less and thus the dynamic range is less and the sound will be less accurately dynamic.
The chart representing Cumulative Spectral Decay is one way to show how well controlled a speaker is. It reflects the combined dynamic decay performance of the entire speaker system, from the quality of the drivers to the construction of the cabinet to the type of crossover used.
It also shows significant resonance problems in speakers, such as in the chart above there is clearly a tweeter resonance at about 14kHz and again at 19kHz leading me to believe the tweeter has a metal dome since many metal dome tweeters have resonances like this in their upper registers (and one reason I am not a fan of them).
That chart is for a high end speaker, and it is pretty good. You may notice a huge drop off in the SPL just past 6 samples, or 0.36mS after the voltage pulse ended. That's not bad, but there is a long resonance at 1kHz which isn't good, suggesting the tweeter is better at dynamics than the midrange - another reason (to me at least) that the sound of the tweeter should match the sound of the midrange/woofer. I believe most would notice that the sound in the treble is not extremely well mated to the sound in the midrange. But, some like that sound, so more power to them.
That's a start. Please wait to reply until I post one more time with less stellar Cumulative Spectral Decay chart.
Stereophile is pretty good in presenting these plots and they are consistent how they set the parameters of the measurement. As such, here is a Cumulative Spectral Decay plot for a very high end speaker:
The vertical axis (up and down) is the amplitude of the signal being measured. Stereophile almost always uses a 24dB window, in this case is it +/- 12dB SPL.
The horizontal X-axis (left to right) is frequency - just like we always see in frequency response charts, but limited to 400Hz to 20,000Hz.
The horizontal Z-axis(front to back) represents time. In this case each line is 0.06mS later than the previous one which is behind it. So, starting at the back of the chart, each subsequent line moving forward represents what sound energy was still being measured 0.06mS after the previous line.
So, the very first line, at the back, should directly copy the measured frequency response of the speaker being tested. The next line is what the speaker is still generating 0.06mS after the pulse ended. The third line in front of that is what sound the speaker was making 0.12mS after the pulse ended, and so on until the end of the graph at nearly 4.00mS.
Why does this matter and what does it tell you?
If the goal of a speaker is to accurately follow the voltage of the incoming signal and convert it accurately into acoustic sound we can hear, it should start moving immediately when the voltage rises, but when the voltage stops, so should the speaker cones, domes, horns, diaphragms, etc. If the speaker is moving about after the voltage stops, then when the next voltage swing goes in it will have to recover and start moving again, potentially being forced to attempt to go from instantly moving outward to inward because the new voltage is counter the "resonating" of the system.
Also, if you want to experience true, accurate dynamics - as many do - a speaker's ability to stop moving after the signal stops is critical in order to reproduce the silent spots between peaks in the sound. Dynamics is all about the difference between high peaks and quiet valleys in the sound. That ratio of high SPL to low SPL is a measure of a dynamic system. But, if the speaker is continuing to make sound when the signal says it should be silent, then the range between high and low will be less and thus the dynamic range is less and the sound will be less accurately dynamic.
The chart representing Cumulative Spectral Decay is one way to show how well controlled a speaker is. It reflects the combined dynamic decay performance of the entire speaker system, from the quality of the drivers to the construction of the cabinet to the type of crossover used.
It also shows significant resonance problems in speakers, such as in the chart above there is clearly a tweeter resonance at about 14kHz and again at 19kHz leading me to believe the tweeter has a metal dome since many metal dome tweeters have resonances like this in their upper registers (and one reason I am not a fan of them).
That chart is for a high end speaker, and it is pretty good. You may notice a huge drop off in the SPL just past 6 samples, or 0.36mS after the voltage pulse ended. That's not bad, but there is a long resonance at 1kHz which isn't good, suggesting the tweeter is better at dynamics than the midrange - another reason (to me at least) that the sound of the tweeter should match the sound of the midrange/woofer. I believe most would notice that the sound in the treble is not extremely well mated to the sound in the midrange. But, some like that sound, so more power to them.
That's a start. Please wait to reply until I post one more time with less stellar Cumulative Spectral Decay chart.
