How to time-align speaker drivers
In this application note, we show you how to "time align" the drivers in your active DSP loudspeaker using the delay function of the miniDSP platforms.
In a multi-way loudspeaker, the sound from different drivers will take slightly different amounts of time to reach your ears. The figure below illustrates a typical scenario: since the woofer cone is "deeper" than the tweeter, the acoustic center of the woofer is further away from the listener's ear than the tweeter. The sound from the woofer will therefore arrive at the listener's ear slightly later than the sound from the tweeter. This can have a detrimental effect on the speaker's response around the crossover.
One approach to this issue is to build a slanted or stepped baffle. In passive crossover design, the designer may use specific techniques to shift the phase of one driver or another to compensate. In a miniDSP crossover, we use the Delay parameter in the output channels to delay the signal from the tweeter by a small amount of time. The result is that the acoustic waveform from both drivers will arrive at the listener's ear at the same time.
How much time delay?
Sounds travels at 343 meters per second, or 1126 feet per second. If you can measure approximately the distance between the tweeter and the woofer cone as indicated above, you can calculate the time delay with one of these formulae:
- Delay in milliseconds = distance in cm / 34.3
- Delay in milliseconds = distance in inches / 13.5
Alternatively, if you have an acoustic measurement program, you can calculate the delay by measuring the arrival time of the signal from each driver, and calculating the difference. The result is the time delay needed. The arrival time measurement can be done in Room EQ Wizard (REW) with the use of a two-channel soundcard - see the REW documentation for Use Loopback as Timing Reference.
Another method is to run a measurement sweep on both drivers at the same time. If one driver is significantly delayed, you will be able to measure the time difference between the impulse response peaks.
How to set time delay
Almost all miniDSP DSP platforms will support time alignment, but for the sake of clarity, we'll use the miniDSP 2x4. In the interface to any of the miniDSP 2x4 plugins (Stereo 2-way PEQ, 4-way PEQ, and so on), click on the Delay/Gain/RMS block. Set the time delay of the closest driver as indicated in the screenshot below.
In the plugins with eight output channels (4x10, 10x10, miniSharc), all delays appear on the Output tab. With these plugins, you can implement three-way or four-way speakers and use time delays to time-align all drivers. Just remember to calculate delays relative to the furthest-away driver. This screenshot shows four channels of one of these plugins:
Once you have delays set correctly for all drivers, your crossovers will be easier to design. Don't forget to ask on the miniDSP forum if you have further questions!
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