A B737 takeoff warning horn alert volume was so low the crew did not identify it until takeoff thrus...

Narrative:

We complied with all procedures and checklists including throttle bursts during the before takeoff checklist. We got no takeoff configuration warnings at any time. I was the pilot flying. I advanced the throttles to about 40-50 percent N1; so they were matched before I set takeoff power. No horn. When I set takeoff power we got a muffled horn sound which we did not immediately recognize as the takeoff configuration warning horn. The sound was muffled and weak almost like it was powered by a dying battery. The horn did not sound normal. There was no associated warning light with the muffled horn. We rejected the takeoff at about 40-60 knots; notified tower and exited the runway. We checked our configuration and everything was set appropriately; including the flaps set at 1 per the performance data. We still got the warning horn during throttle bursts. We reran all checklists to see if there was anything else we missed and still got the warning horn. We taxied back to the gate and made a maintenance write up.

Original NASA ASRS Text

Title: A B737 Takeoff Warning Horn alert volume was so low the crew did not identify it until takeoff thrust was set. When the warning was finally identified at about 50 knots; the takeoff was rejected and the aircraft returned to the gate.

Narrative: We complied with all procedures and checklists including throttle bursts during the Before Takeoff Checklist. We got no Takeoff Configuration Warnings at any time. I was the Pilot Flying. I advanced the throttles to about 40-50 percent N1; so they were matched before I set takeoff power. No horn. When I set takeoff power we got a muffled horn sound which we did not immediately recognize as the Takeoff Configuration Warning horn. The sound was muffled and weak almost like it was powered by a dying battery. The horn did not sound normal. There was no associated warning light with the muffled horn. We rejected the takeoff at about 40-60 knots; notified Tower and exited the runway. We checked our configuration and everything was set appropriately; including the flaps set at 1 per the performance data. We still got the warning horn during throttle bursts. We reran all checklists to see if there was anything else we missed and still got the warning horn. We taxied back to the gate and made a Maintenance write up.

Data retrieved from NASA's ASRS site and automatically converted to unabbreviated mixed upper/lowercase text. This report is for informational purposes with no guarantee of accuracy. See NASA's ASRS site for official report.

Attributes
ACN	1303788
Time
Date	201510
Local Time Of Day	1201-1800
Place
Locale Reference	ZZZ.Airport
State Reference	US
Environment
Flight Conditions	VMC
Light	Daylight
Aircraft 1
Make Model Name	B737 Undifferentiated or Other Model
Operating Under FAR Part	Part 121
Flight Phase	Takeoff
Flight Plan	IFR
Component
Aircraft Component	Indicating and Warning - Flight & Navigation Systems
Person 1
Function	First Officer Pilot Flying
Events
Anomaly	Aircraft Equipment Problem Less Severe

A B737 takeoff warning horn alert volume was so low the crew did not identify it until takeoff thrust was set. When the warning was finally identified at about 50 knots; the takeoff was rejected and the aircraft returned to the gate.