In simple terms, group delay is the time it takes for an electrical input signal to become an acoustical output. It is frequency dependant and should ideally be zero seconds at all frequencies, but this is practically impossible.
In a linear-phase system the group delay is constant – this is sometimes called latency. For example, in an O 500 C set to linear-phase mode the group delay is 80 ms at all frequencies. Whilst there are perceptual benefits in having a linear-phase sound reproduction system, this latency is not practical when the sound source and the loudspeakers are located in the same room as an echo will be heard. Conversely, when working with video equipment the audio signal generally leads the video signal due to video processing and so deliberate delay on the audio is required. This called lip sync. Typically flat display screens (plasma and LCD) have a two frame video delay (80 ms for a 50 Hz frames or 66 ms for a 60 Hz frames). Delays in the region of 0…8 frames may be required in broadcast stations due to additional video effects processing.
Back to frequency-dependent delay…
The subjective effect of excessive group delay is a “loosening” of the bass or a “less dry” bass quality. Currently there is insufficient psychoacoustic research on the threshold of group delay at low frequencies. One value is known: 2.5 ms at 100 Hz. This happens to be the same as that seen in the KH 310 A and O 410. A vented cabinet with a similar low frequency performance would have about 5 ms of group delay at 100 Hz. At higher frequencies, >1 kHz, group delay should be less than 1.6 ms (55 cm or 2 ft), which it is in most loudspeaker designs. Below are some graphs showing the theoretical frequency response and group delay of different types of enclosures (sealed and bass reflex, and a vented subwoofer).
Frequency response of sealed (blue)
and vented cabinets (red)
Frequency response of a vented cabinet with a lower LF cut-off (green)
Group delay of sealed (blue)
and vented cabinets (red)
Group delay of a vented cabinet
with a lower LF cut-off (green)
It can be seen in these graphs that a vented cabinet design must have a much deeper low-frequency cut-off for it to have the same group delay as a sealed cabinet. This is a good reason for having subwoofers with a very deep low-frequency cut-off, for example 18 Hz. When this subwoofer is added to a sealed near-field loudspeaker, the group delay performance is almost unaffected.
Adding infrasonic (high pass) filters to give very low-frequency driver protection adds to the group delay shown in the above plots. For example, a sealed cabinet with a second-order infrasonic driver protection filter with a corner frequency the same as the low-frequency cut-off of the loudspeaker will have the same group delay performance as a vented cabinet with the same low-frequency cut-off and no infrasonic driver protection filter.
An additional source of group delay is bass management filtering – see graphs below:
Three crossover frequencies:
70, 80, 90 Hz
Group delay increases
with reduced crossover frequency
Adding a subwoofer to a compact sealed loudspeaker increases the group delay slightly around the crossover due to the bass management crossover filtering - see graphs below around the 100 Hz region. As mentioned above, around the low-frequency cut-off of the main loudspeaker, the group delay is about the same with or without the subwoofer due to the very deep low-frequency cut off of the subwoofer. Below this the group delay continues to rise in the subwoofer as one would expect.
Group delay of a compact sealed loudspeaker (KH 310 A)
Group delay of a compact sealed loudspeaker
with a subwoofer (KH 310 A + KH 810)
Moving onto a large vented cabinet, the effect in the group delay of adding the electronic filtering of the 80 Hz bass management crossover is shown in the right picture below. Now comparing the two left pictures (large vented and compact sealed), the group delay can be seen to be the same down to 35 Hz, and then it continues to rise down to the large vented loudspeaker’s low-frequency cut-off.
Group delay of a large vented loudspeaker (O 410)
Group delay of a large vented loudspeaker
with a subwoofer (O 410 + O 870)
So with a large vented loudspeaker we see the same group delay performance as a compact sealed loudspeaker, but with additional low-frequency extension and a higher maximum SPL. The cost is financial and a space in the room for the cabinet.