When measuring subwoofers with Room EQ Wizard (REW), users sometimes see impulse response (IR) delays that appear much higher than expected from the subwoofer’s internal DSP latency alone. This article explains why this happens and how to interpret those measurements correctly.
Key point upfront
The delay shown in REW is not only the subwoofer’s internal processing delay. It is the total acoustic and system delay seen at the microphone.
What the REW delay actually represents
The impulse response delay in REW includes several components combined:
Acoustic travel time from the subwoofer to the microphone
Any latency added by electronics in the signal chain
Internal DSP processing delay inside the subwoofer
Timing offsets introduced by the measurement method itself
Because all of these are included, the measured value is often much higher than the subwoofer’s DSP latency specification.
Distance and speed of sound
Sound travels at roughly 343 m/s, which corresponds to about:
2.9 ms per meter
0.9 ms per foot
Even modest differences in placement can add significant delay. For example:
3 m of extra distance adds ~9 ms
5 m of extra distance adds ~14–15 ms
If two subwoofers are measured from different physical locations, or microphone placements in the room, their measured delays will naturally differ, even if their electronics are identical.
Measurement timing reference and audio buffering
REW needs a timing reference to determine when the impulse starts. If no proper reference is used, the displayed delay can include additional, unrelated latency.
Common causes:
Measuring without a loopback or acoustic timing reference
Audio driver buffering in the operating system
Different output paths (USB, HDMI, different sound devices)
Wireless or Bluetooth audio paths (can add very large latency)
If two subwoofers are measured using different signal paths or timing references, their delays are not directly comparable.
AVR and processor signal delays
When measuring through an AV receiver or surround processor, additional delay may be introduced by:
Bass management and crossover filters
Room correction systems (Dirac, Audyssey, YPAO, etc.)
Upmixing or surround processing modes
Different distance or delay settings per channel
Subwoofer channels often receive different processing than main speakers, which can add several milliseconds of latency.
Filtering and group delay at low frequencies
Subwoofers typically operate with steep low-pass filters and EQ. These filters can introduce group delay, especially at low frequencies.
Factors that increase apparent delay:
Steep crossover slopes
Linear-phase or FIR filtering
Heavy EQ at very low frequencies
External DSP units in the signal chain
REW may lock onto a later portion of the impulse response when low-frequency energy dominates, making the delay appear larger than the actual DSP pipeline latency.
Room effects and phase interactions
Room acoustics can influence where REW detects the “main” impulse peak:
Strong reflections arriving shortly after the direct sound
Partial cancellation at the listening position
Phase interaction between subwoofer and main speakers
In these cases, the first arrival may be reduced, and a later arrival can be interpreted as the main impulse, increasing the reported delay.
Different REW delay metrics
REW displays several timing-related values, which are sometimes confused:
IR delay or estimated IR delay
Group delay
Excess group delay
These metrics describe different things. Group delay values, especially at low frequencies, can be much larger than the actual signal latency and should not be confused with DSP delay.
How to get meaningful and comparable measurements
To compare subwoofer timing correctly:
Measure all subwoofers from the same physical location if possible
Use identical microphone placement
Use the same REW settings, output device, and signal path
Use an acoustic timing reference or loopback timing reference
Disable room correction and unnecessary processing during measurement
Compare subwoofer delay relative to a main speaker, not as an absolute number
Measuring relative delay (sub vs main speaker) removes most of the uncertainty caused by PC, driver, and system latency.
Summary
A large delay value in REW does not mean a subwoofer has excessive internal latency. In most cases, the measured delay is dominated by:
Distance and acoustic travel time
Measurement method and timing reference
System and AVR processing
Low-frequency filtering and room interaction
Understanding these factors helps ensure impulse response measurements are interpreted correctly and avoids incorrect conclusions about subwoofer performance.
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