The Importance of Time Alignment

TIME DELAY

 

Time alignment is often overlooked in loudspeakers. But what is “Time Alignment”, you may ask.

The majority of loudspeakers consist of multiple drivers to cover as much of the audio spectrum as possible. For example, a 2-way bookshelf speaker will have a tweeter for the treble and a midwoofer for the midrange and bass.

With both of them are mounted onto the front panel, the acoustic centers of the two drivers are not on the same vertical plane. When sound is emitted, sound¬† from the driver with the acoustic center that’s nearer to the listener will arrive first, followed by the one that’s farther back. This difference in arrival time results in destructive interference along the crossover region.

The ideal situation is for the two sounds to arrive at the same time. This, in short, is what time alignment is all about. The following example shows a typical 2-way horn loaded speaker and the associated problems.

Speaker consists of a Dayton RS180S, a 7″ aluminum cone midwoofer and a Pyle PH612 horn with a Peavey RX14 compression driver screwed on.

Both drivers were crossed electronically at 2kHz(24dB/oct) with the Pioneer FH-X731BT.

The Blue trace in Fig 1 is the RS180S and the Red trace is the RX14 with the PH612 horn.

rs180_rx14_2k_24db_crop

(Fig 1) Low Pass of Dayton RS180S and High Pass of Peavey RX14 with PH612 horn. (No Smoothing, mic at 1 meter tweeter axis, 5 msec gating)

Not Time Aligned Summed Response

Fig 2 are the summed responses without Time Alignment. 
The Blue trace is the summed response of the RS180S and the RX14 wired in Reversed Phase.The Black trace is with the RX14 normally wired (ie RED to RED).

rs180s_rx14_2k_24db_in_out_phase_hp_lp_crop

(Fig 2) Summed Responses with no Time Alignment

Time Aligned Summed Response

In Fig 3, the RS180S was delayed by 2 inches with the Pioneer FH-X731BT.

The Blue trace is the summed response of the RS180S and the RX14 wired normally (RED to RED).

The Black trace is with the RX14 wired in Reversed Phase.

ta_rs180s_rx14_2k_24db_in_out_phase_hp_lp_crop

(Fig 3) Summed Responses with Time Alignment

Comparing the Black traces in Fig 2 and 3, it is clear that when the RS180S is delayed by 2 inches, the notch is sharper and deeper (about 20dB). The tip is also closer to the crossover frequency.
Summed Response Overlay without Time Alignment

Fig 4 is a close-up of the crossover and the summed response (Violet trace). The cancellation before the crossover point is clearly visible. This is destructive.

2k_24db_no_ta

(Fig 4) Close-up of summed response with no Time Alignment

Summed Response Overlay with Time Alignment

In Fig 5, the close-up of the summed response with Time Alignment (Violet trace) shows no cancellations on either side of the crossover point. This shows that sounds from the RS180S and the RX14 arrive at the listener’s position at the same time.

2k_24db_ta

(Fig 5) Close-up of summed response with Time Alignment

In conclusion, a properly designed loudspeaker is not simply the beauty of the loudspeaker box, the quality of the drivers and crossover components. All sounds from individual drivers must reach the listener at the same time. At the very least, that’s what a loudspeaker should do.

More information on Time-Alignment

Loudspeaker Time Alignment Wiki
miniDSP Time-Alignment
Lenard Audio Time-Alignment
Time-Alignment in Loudspeakers
Wavelet Analysis
Testing Loudspeakers

JBL 4200 Series
UREI_813b
SB Acoustics Time-Alignment