Decibel Loss Report - User Guide

Anthony Conte

October 27, 2025 14:02

FireCAD dB Loss Report - User Guide

Overview

The dB Loss Report is a lump sum-style aggregated report designed specifically for 25V and 70V audio speaker circuits. This report is the audio circuit equivalent of the Lump Sum Report, combining wattage-based calculations with decibel (dB) loss analysis. It groups speakers by part number while showing acoustic signal attenuation, making it ideal for speaker circuit material takeoffs with audio performance verification.

What is a dB Loss Report?

A dB Loss Report provides an aggregated view of speakers on an audio circuit, showing:

Speaker quantities grouped by part number
Wattage consumption per speaker and totals
Decibel (dB) loss from voltage drop
Circuit capacity vs. usage (watts-based)
Voltage drop summary with audio impact

Key Distinction: This report uses the same aggregation method as Lump Sum Reports, but for audio speaker circuits with wattage calculations and dB loss conversion.

When to Use This Report

Use the dB Loss report when:

Creating material takeoffs for 25V/70V speaker circuits
Verifying audio performance meets requirements
Quickly checking circuit wattage capacity
Generating simplified speaker circuit summaries
Performing cost estimates for speaker installations
Comparing speaker quantities across multiple circuits
Need both speaker counts AND audio performance verification

Report Structure

Each circuit generates a separate worksheet containing:

1. Circuit Header

Project name
Panel and circuit identification
Report title (e.g., "PANEL-1 SPKR-1 DB LOSS REPORT")

2. Wattage Summary Section

Field	Description
Max. Circuit Watts	Maximum wattage limit for the audio circuit
Total Circuit Watts	Sum of all speaker wattages
Spare Circuit Watts	Remaining wattage capacity
Spare Circuit Watts %	Percentage of unused capacity
Max. Card Watts (optional)	Maximum wattage for the amplifier card
Total Card Watts (optional)	Total watts from all circuits on amplifier
Spare Card Watts (optional)	Remaining amplifier capacity
Spare Card Watts % (optional)	Percentage of unused amplifier capacity

Color Coding:

🟢 Green: Within limits (good)
🔴 Red: Exceeds limits or below -1.5 dB (bad)
⚪ Gray: Calculated values

3. Power Summary Section

Field	Description
Starting Calc. Voltage	Voltage at circuit origin (typically 25V or 70V)
Max. dB Loss	Total decibel loss from start to end-of-line
Voltage Drop %	Percentage of voltage lost
Min. Operational Voltage	Minimum voltage required for speakers
End Of Line Voltage	Actual voltage at the last speaker
Wire Resistance (Ω/kFt)	Resistance per 1000 feet for selected wire
Total Circuit Length	Total wire length including overage
Total Circuit Resistance	Total wire resistance for round-trip circuit

4. Device Summary Table

The report includes 6 default columns:

Default Columns:

1. Symbol: Speaker symbol (for drawing placement)

2. Quantity: Number of identical speakers

3. Part No: Manufacturer part number (sorted alphabetically)

4. Description: Speaker description

5. Watts: Power consumption per speaker (tap setting)

6. Total Watts: Quantity × Watts

Calculation Methods Explained

1. Watts To Amps Conversion

Why This Conversion is Necessary

Voltage drop calculations require current (Amperes) because of Ohm's Law: V = I × R. However, speakers are rated by power consumption (Watts), not current. This conversion bridges the gap between speaker specifications and electrical circuit analysis.

The fundamental relationship:

Power (Watts) = Voltage (Volts) × Current (Amps)
P = V × I

Therefore:
Current (Amps) = Power (Watts) / Voltage (Volts)
I = P / V

Understanding Constant Voltage Audio Systems

25V and 70V audio systems are constant voltage distribution systems where the amplifier maintains a fixed RMS voltage on the circuit, regardless of the number of speakers connected (within the amplifier's power capacity).

Key Characteristics:

25V systems: Amplifier outputs 25V RMS continuously
70V systems: Amplifier outputs 70V RMS continuously
Speakers connect in parallel across the constant voltage line
Each speaker draws current based on its wattage tap setting
Total current = sum of all individual speaker currents

This is fundamentally different from traditional audio systems where voltage varies with signal level.

Speaker Tap Settings

Most commercial speakers for fire alarm voice evacuation have multiple tap settings - internal transformers that allow you to select the power level:

Common 25V Speaker Taps:

0.25W, 0.5W, 1W, 2W, 4W, 8W

Common 70V Speaker Taps:

0.5W, 1W, 2W, 4W, 8W, 15W, 30W

Each tap setting draws a different current:

For 25V speakers:

0.25W tap: I = 0.25 / 25 = 0.010 A (10 mA)
0.5W tap:  I = 0.5 / 25  = 0.020 A (20 mA)
1.0W tap:  I = 1.0 / 25  = 0.040 A (40 mA)
2.0W tap:  I = 2.0 / 25  = 0.080 A (80 mA)
4.0W tap:  I = 4.0 / 25  = 0.160 A (160 mA)

For 70V speakers:

0.5W tap:  I = 0.5 / 70  = 0.007 A (7 mA)
1.0W tap:  I = 1.0 / 70  = 0.014 A (14 mA)
2.0W tap:  I = 2.0 / 70  = 0.029 A (29 mA)
4.0W tap:  I = 4.0 / 70  = 0.057 A (57 mA)
8.0W tap:  I = 8.0 / 70  = 0.114 A (114 mA)

Notice: 70V systems draw less current for the same wattage, which allows smaller wire gauges or longer circuit runs.

Practical Implications for Circuit Design

Higher wattage taps = More current = More voltage drop:

Example Circuit: 10 speakers, 400 feet of 18 AWG wire (6.5 Ω/kFt)

Scenario 1: All speakers at 1W tap (25V)

Total Watts: 10 × 1.0W = 10.0W
Total Current: 10.0W / 25V = 0.40A
Wire Resistance: 6.5 × 2 × 400 / 1000 = 5.2Ω
Voltage Drop: 5.2Ω × 0.40A = 2.08V
End Voltage: 25V - 2.08V = 22.92V ✅ (Good!)
dB Loss: 20 × Log₁₀(22.92 / 25) = -0.75 dB ✅

Scenario 2: All speakers at 2W tap (25V)

Total Watts: 10 × 2.0W = 20.0W
Total Current: 20.0W / 25V = 0.80A
Wire Resistance: 6.5 × 2 × 400 / 1000 = 5.2Ω
Voltage Drop: 5.2Ω × 0.80A = 4.16V
End Voltage: 25V - 4.16V = 20.84V ⚠️ (Marginal)
dB Loss: 20 × Log₁₀(20.84 / 25) = -1.58 dB ❌ (Exceeds -1.5 dB)

Scenario 3: All speakers at 4W tap (25V)

Total Watts: 10 × 4.0W = 40.0W
Total Current: 40.0W / 25V = 1.60A
Wire Resistance: 6.5 × 2 × 400 / 1000 = 5.2Ω
Voltage Drop: 5.2Ω × 1.60A = 8.32V
End Voltage: 25V - 8.32V = 16.68V ❌ (Too low!)
dB Loss: 20 × Log₁₀(16.68 / 25) = -3.53 dB ❌ (Far exceeds limit)

Key Lesson: Selecting appropriate speaker taps is critical. Use the lowest tap setting that meets sound pressure level (SPL) requirements to minimize current draw, voltage drop, and dB loss.

Advantages of 70V Systems

Same circuit with 70V instead of 25V:

70V System - All speakers at 2W tap:

Total Watts: 10 × 2.0W = 20.0W (same total power)
Total Current: 20.0W / 70V = 0.286A (much less current!)
Wire Resistance: 6.5 × 2 × 400 / 1000 = 5.2Ω
Voltage Drop: 5.2Ω × 0.286A = 1.49V
End Voltage: 70V - 1.49V = 68.51V ✅
dB Loss: 20 × Log₁₀(68.51 / 70) = -0.19 dB ✅ (Excellent!)

This demonstrates why 70V systems are preferred for long wire runs or high-power installations.

Why Not Just Use Watts Directly?

You might wonder: "Why convert to amps at all? Why not calculate voltage drop directly from watts?"

Answer: Voltage drop is governed by Ohm's Law (V = I × R), which requires current and resistance. Wire resistance doesn't directly interact with power - it interacts with current flow. The conversion is a necessary step in the calculation chain:

Step 1: Total Watts → Total Amps (using circuit voltage)
Step 2: Amps + Total Wire Resistance → Total Voltage Drop (using Ohm's Law)
Step 3: Voltage Drop → dB Loss (using logarithmic formula)

Formula Summary

Current (A) = Total Circuit Watts / Starting Voltage

For dB Loss reports:

Converts total circuit wattage to equivalent current
Used to calculate total voltage drop across entire circuit
Aggregated calculation (lump sum approach)

Example:

Total Circuit Watts: 53.0W
Starting Voltage: 25V
Equivalent Current: 53.0 ÷ 25 = 2.12A

2. Total Resistance Calculation

Total Resistance (Ω) = Wire Resistance (Ω/kFt) × 2 × Total Circuit Length (Ft) / 1000

The ×2 factor accounts for round-trip current flow.

Example:

Wire: 18 AWG (6.5 Ω/kFt)
Total Circuit Length: 460 feet
Total Resistance: 6.5 × 2 × 460 / 1000 = 5.98Ω

3. Total Voltage Drop Calculation

Total Voltage Drop (V) = Total Resistance (Ω) × Equivalent Current (A)

Calculates worst-case voltage loss across entire circuit.

Example:

Total Resistance: 5.98Ω
Equivalent Current: 1.2A
Total Voltage Drop: 5.98 × 1.2 = 7.18V

4. End Of Line Voltage Calculation

End Of Line Voltage = Starting Voltage - Total Voltage Drop

Example:

Starting Voltage: 25V
Total Voltage Drop: 7.18V
End Of Line Voltage: 25 - 7.18 = 17.82V

5. Max. dB Loss Calculation

Max. dB Loss = 20 × Log₁₀(End Of Line Voltage / Starting Voltage)

Calculates total acoustic signal attenuation.

Example:

End Of Line Voltage: 17.82V
Starting Voltage: 25V
Max. dB Loss: 20 × Log₁₀(17.82 / 25) = -2.51 dB ❌ (Exceeds -1.5 dB limit)

Complete Example - 25V Speaker Circuit

Circuit Configuration:

Amplifier: Audio Amplifier Panel
Circuit: SPKR-2
Starting Voltage: 25V (constant voltage audio system)
Wire: 18 AWG (6.5 Ω/kFt)
Total Circuit Length: 460 feet (includes 15% overage)
Max Circuit Watts: 100W
Min Operational Voltage: 19V

Devices on Circuit (All Speakers):

15× SP-C25 (Ceiling Speaker 25V) @ 2.0W tap each
10× SP-W15 (Wall Speaker 25V) @ 1.5W tap each
8× SP-C10 (Ceiling Speaker 25V) @ 1.0W tap each
1× EOL-R (End of Line) @ 0W

Calculations:

Total Circuit Watts:

= (15 × 2.0) + (10 × 1.5) + (8 × 1.0) + (1 × 0)
= 30.0 + 15.0 + 8.0 + 0
= 53.0 W

Watts To Amps Conversion:

= 53.0 / 25
= 2.12 A

Total Resistance:

= 6.5 × 2 × 460 / 1000
= 5.98 Ω

Total Voltage Drop:

= 5.98 × 2.12
= 12.68 V

End Of Line Voltage:

= 25 - 12.68
= 12.32 V ❌ (Below 19V minimum)

Max. dB Loss:

= 20 × Log₁₀(12.32 / 25)
= 20 × Log₁₀(0.493)
= 20 × (-0.307)
= -6.14 dB ❌ (Far exceeds -1.5 dB threshold)

dB Loss Report Table:

Symbol	Qty	Part No	Description	Watts	Total Watts
[SYM]	1	EOL-R	End of Line	0.0	0.0
[SYM]	8	SP-C10	Ceiling Speaker 25V	1.0	8.0
[SYM]	15	SP-C25	Ceiling Speaker 25V	2.0	30.0
[SYM]	10	SP-W15	Wall Speaker 25V	1.5	15.0

Summary Results:

✅ Total Circuit Watts: 53.0W (within 100W limit)
❌ End Of Line Voltage: 12.32V (far below 19V minimum)
❌ Max. dB Loss: -6.14 dB (far exceeds -1.5 dB maximum)
❌ Voltage Drop: 50.7% (unacceptable - typically max 10-15%)

Action Required: Circuit design is not viable. Must increase wire gauge significantly (14 AWG or 12 AWG) or split into multiple circuits.

Corrected Example with Larger Wire

Same Configuration, Better Wire:

Wire: 14 AWG (2.5 Ω/kFt) instead of 18 AWG

New Calculations:

Total Resistance:

= 2.5 × 2 × 460 / 1000
= 2.30 Ω

Total Voltage Drop:

= 2.30 × 2.12
= 4.88 V

End Of Line Voltage:

= 25 - 4.88
= 20.12 V ✅ (Above 19V minimum)

Max. dB Loss:

= 20 × Log₁₀(20.12 / 25)
= -1.87 dB ⚠️ (Still exceeds -1.5 dB, but much better)

Using 12 AWG (1.6 Ω/kFt):

Total Resistance: 1.6 × 2 × 460 / 1000 = 1.47 Ω
Total Voltage Drop: 1.47 × 2.12 = 3.12 V
End Of Line Voltage: 25 - 3.12 = 21.88 V ✅
Max. dB Loss: 20 × Log₁₀(21.88 / 25) = -1.18 dB ✅ (Good!)
Voltage Drop: 12.5% (Acceptable)

Understanding Report Types

dB Loss Report (This Report)

Format: Lump sum (aggregated by part number)
Circuit Type: 25V/70V audio speaker circuits
Focus: Wattage and dB loss
Best For: Material takeoffs with audio verification
Shows: Quantities, watts, total dB loss
Equivalent To: Lump Sum Report (for NAC circuits)

Speaker Schedule Report

Format: Point-to-point (sequential)
Circuit Type: 25V/70V audio speaker circuits
Focus: Device-by-device voltage and dB loss
Best For: Detailed design verification
Shows: Every speaker location, incremental dB loss
Equivalent To: Point-to-Point Report (for NAC circuits)

Lump Sum Report

Format: Lump sum (aggregated by part number)
Circuit Type: 24V DC notification circuits (NAC)
Focus: Current (Amps)
Best For: Non-speaker notification device takeoffs
Shows: Quantities, current, voltage drop (no dB)

Point-to-Point Report

Format: Point-to-point (sequential)
Circuit Type: 24V DC notification circuits (NAC)
Focus: Current (Amps)
Best For: Detailed NAC design verification
Shows: Every device, voltage drop (no dB)

Key Relationships:

Speaker Schedule = Point-to-Point + dB conversion (for audio)
dB Loss = Lump Sum + dB conversion (for audio)

Frequently Asked Questions

Q1: What's the difference between dB Loss and Speaker Schedule?

dB Loss Report:

Aggregated by part number (lump sum)
Shows quantities and totals
Single dB loss value for entire circuit
Fast overview format
Audio equivalent of Lump Sum Report

Speaker Schedule:

Shows every speaker individually
Speaker locations and sequential analysis
Incremental dB loss at each speaker
Detailed troubleshooting format
Audio equivalent of Point-to-Point Report

Both calculate the same total dB loss, but show it differently.

Q2: Can I use dB Loss reports for horn/strobe circuits?

No. dB Loss reports are specifically for 25V/70V audio speaker circuits. Use:

dB Loss: Audio speakers on 25V/70V circuits only
Lump Sum: Horn/strobe/notification appliances on 24V DC NAC circuits

Q3: What starting voltage should I use?

25V Systems: Typical for most fire alarm voice evacuation

Starting voltage: 25V RMS
Common in smaller to medium buildings
Standard speaker tap settings: 0.25W, 0.5W, 1W, 2W, 4W

70V Systems: For large buildings or long wire runs

Starting voltage: 70V RMS
Allows smaller wire gauge for same wattage
Common speaker tap settings: 0.5W, 1W, 2W, 4W, 8W, 15W, 30W

Q4: Why is the dB loss so high in the first example?

The example intentionally shows a failed design to demonstrate:

18 AWG wire is too small for 53W over 460 feet
High current (2.12A) causes excessive voltage drop (50.7%)
Voltage drop directly causes dB loss (-6.14 dB)
Solution: Use 12 AWG wire to reduce dB loss to -1.18 dB

This teaches the importance of proper wire sizing for audio circuits.

Q5: What is an acceptable dB loss for speaker circuits?

Typical Limits:

Voice Evacuation Systems: -1.0 dB maximum
Standard Speaker Circuits: -1.5 dB maximum
General Audio/Paging: -2.0 dB may be acceptable

Best Practice: Target -1.0 dB or better to allow safety margin.

Best Practices

1. Wire Sizing for Audio Circuits

Target: -1.0 dB or better
Maximum: -1.5 dB for most applications
Use larger wire for long runs or high-power speakers
Don't rely on minimum acceptable values

2. 25V vs. 70V Selection

Choose 25V when:

Shorter wire runs (<500>
Lower power speakers (<5w>
Standard office/commercial buildings

Choose 70V when:

Long wire runs (500 ft)
Higher power speakers (5W each)
Large buildings, warehouses, parking structures
Want to use smaller wire gauge

3. Speaker Tap Selection

Use lowest tap setting that meets SPL requirements
Higher tap = more wattage = more voltage drop = more dB loss
Many speakers have multiple tap options (0.5W / 1W / 2W / 4W)
Select appropriate tap during design phase

Technical Notes

Relationship to Lump Sum Reports

The dB Loss Report uses identical aggregation and calculation methods to Lump Sum Reports:

1. Group devices by part number: GROUP BY PartNo, Watts

2. Sum quantities and wattages

3. Calculate total resistance from circuit length

4. Calculate voltage drop from total current

The key difference: dB Loss adds one more step:

5. Convert voltage drop to dB loss: dB = 20 × Log₁₀(V₁ / V₂)

This is necessary because audio circuit performance is measured in decibels.

Wire Resistance Values for Audio Circuits

Common wire resistances (Ω per 1000 ft):

Wire Gauge	Resistance (Ω/kFt)	Typical Use
12 AWG	1.6	Long runs, high power
14 AWG	2.5	Medium runs, standard
16 AWG	4.0	Short runs, low power
18 AWG	6.5	Very short runs only

For 25V speaker circuits: 14 AWG or 12 AWG recommended for most installations.

Related Reports

Speaker Schedule: Point-to-point analysis for audio speaker circuits
Lump Sum Reports: Aggregated current-based for standard NAC circuits (no dB conversion)
Point-to-Point Reports: Sequential voltage analysis for NAC circuits (no dB conversion)
Battery Calculation: Backup power requirements for all circuits

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