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Assessment of Loudspeaker System Short-Term Maximum
Output D. B. (Don) Keele, Jr.
DBK Associates, DBK Labs Bloomington, IN 47408
[email protected] www.DBKeele.com
(Copyright 2009, Mod: Oct. 8, 2009)
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System Maximum Output Ø The maximum level a system can generate for long or short time periods
using a prescribed test signal
Ø Is time and frequency dependent!
Ø Long Term Max Output: l Sine Wave l Broadband: EIA 426B Standard
• Shaped-spectrum pink noise with prescribed crest factor l Usually limited by thermal considerations
Ø Short Term Max Output: l The ability of a system to play loudly for a short period of time. l Usually limited by excursion-related driver non-linearity's l Also limited by available amplifier power. l How Short is Short! Usually related to the requirement to reproduce a
certain fixed number of cycles of the test frequency, i.e. for a 6.5 cycle tone burst: 20 Hz (325 ms), 200 Hz (32.5 ms, 2 kHz (3.25 ms), 20 kHz (0.325 ms).
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Why Measure Short-Term Acoustic Output?
Ø The measurement of short-term maximum peak SPL and max peak short-term input power capabilities of a loudspeaker as a function of frequency are extremely important when comparing one speaker to another.
Ø It basically tells you how loud a speaker will play for short periods of time in certain frequency bands and still sound acceptable.
Ø Unfortunately, this data is hardly ever supplied by the manufacture (either pro or consumer).
Ø All the loudspeaker reviews I did for Audio magazine (1989-2000) included a max peak input power and peak SPL graph that I measured using my 6.5-cycle tone burst (usually the last graph in the review).
Ø This knowledge is crucial when selecting crossover frequencies.
Ø It allows you to answer questions such as: Will subwoofer “A” play louder than subwoofer “B” at a specific frequency? What’s better, a stack of 2 “X” subs or a stack of 3 “Y” subs.
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Types of Short-Term Test Signals Ø Tone Bursts:
l Essentially a narrow-band signal l Pure or Unshaped (Rectangular window)
• Has relatively wide uncontrolled spectrum l Shaped or Windowed (Usually a Hann window)
• Narrow controlled spectrum l Harmonic distortion only l Can add together several bursts to create wider-band signal (essentially
a windowed sine sweep)
Ø Gaussian Noise Bursts: l Inherently a broad-band signal but can be band-limited l Can assess incoherence to determine distortion levels l Requires many signals to be averaged
Ø Multi-tone Burst: l Good measure of both harmonic and intermodulation distortion. l Bandwidth is easily controlled.
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Selected References Ø S. Linkwitz, “Shaped Tone-Burst Testing,” JAES (April
1980).
Ø D. Yong-Sheng, “A Tone Burst Method for Measuring Loudspeaker Harmonic Distortion at High Levels,” JASA (March 1985).
Ø A. Voishvillo et. al., “Measurement of Loudspeaker Large-Signal Performance – Comparison of Different Short-Term Testing Signals,” Presented at the 21st AES Conference, (June 2002). An excellent reference!
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Shaped Tone Burst Tests > Also called “Boink” Tests, Credit to Tom Holman, TMH Corp, and The Hollywood Edge, “Test and Measurement Disk Series”, > These bursts are also on available on the EIA-426-B Loudspeaker Power Rating Test CD available from ALMA, http://www.almainternational.org/EIA-426-B.html)
Ø Used to measure short-term maximum peak SPL levels in sound systems versus frequency.
Ø Used to detect the peak overload levels at various points in a sound-system signal chain (also is frequency dependent!)
Ø Etc.
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Waveform of 1-kHz 6.5-Cycle Shaped Tone Burst
(Hann Windowed)
-1.0
-0.5
0.0
0.5
1.0
AMPL
ITUD
E
10ms86420TIME - ms
Spectrum of Shaped Tone Burst (Exhibits one-third-octave wide spectrum)
-80
-70
-60
-50
-40
-30
-20
-10
0LE
VEL
- dB
2 4 6100 Hz
2 4 61kHz
2 4 610kHz
2
FREQUENCY - Hz
One-Third Octave Bandwidth
Measurement Setup for Peak Input/Output of
Loudspeakers Note: Choose a power amplifier that is more powerful than any speaker
that might be tested!
TONE BURST
SOURCE POWER
AMPLIFIER
PEAK READING VOLT METER
MIC
1m
PEAK READING SPL METER
SCOPE or FFT
Shaped Tone Bursts Peak Input/Output of Loudspeakers
Ø Procedure l 1. Use suggested test setup. l 2. Energize speaker with tone burst at low level. l 3. While monitoring the speaker’s acoustic output, raise the level
gradually until 1) the output sounds objectively distorted or 2) the acoustic output waveform (as observed on the oscilloscope) appears unacceptably distorted, which ever occurs first. Note: Distortion criteria can also be applied.
l 4. At each burst frequency, record the maximum peak input voltage and the corresponding peak acoustic sound pressure (usually at one meter on axis).
l 5. Calculate the peak input power by assuming the measured peak voltage is applied to a resistor whose value is the speaker’s rated impedance.
l 6. Plot a graph of the speaker’s maximum peak input power and
maximum peak SPL versus frequency.
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An Interesting Observation Ø After conducting many subjective boink tests on
speakers for Audio Magazine I noticed that: l At low frequencies (f < 200 Hz), my sensitivity to distortion was
quite low, i.e. a horribly distorted tone-burst waveform generated by a speaker didn’t sound all that bad!
l Conversely, at mid- and high-frequencies, a reproduced burst waveform that appeared quite clean would often sound quite bad!
l Why???
l Answer: Masking thresholds.
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Sinewave Masking Thresholds Threshold of Human Hearing
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Sinewave Masking Thresholds High-Frequency Masking: 1 kHz Tone at 80 dB SPL
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Sinewave Masking Thresholds Low-Frequency Masking: 50 Hz Tone at 80 dB SPL
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Compare Short-Term Max Output of Three Home Loudspeakers
Ø NHT Pro M-00 l A Small Self-Powered 4.5”
Woofer Two-Way Mini-Monitor System
l $500/pair Ø Focal JMLab Cobalt 820
l Floor standing 3-Way Bass-Reflex system
l $5000/Pair Ø PSB Stratus Goldi
l Floor standing 3-Way Bass-Reflex system
l $2700/Pair NHT M-00
PSB Stratus Goldi
JMLab Cobalt 820
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A Graph of Maximum Acoustic Output vs. Frequency is not
Small-Signal Frequency Response! Ø This is a very crucial distinction! Ø The long- and short-term maximum acoustic output depends only on
the available amplifier power and the power handling and non-linearity's of the loudspeaker.
Ø Any speaker can be equalized so that its small-signal frequency response is perfectly flat over any arbitrary bandwidth.
Ø All three of the measured loudspeakers had commendably flat frequency response and differed only in sensitivity and low-end extension.
NHT M-00
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JMLab Cobalt 820
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JMLab Cobalt 820
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PSB Stratus Gold
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PSB Stratus Gold
The 10,000 Watt Loudspeaker!
Above 800 Hz the system could handle ±200 Volts Peak and generate peaks of 126 dB SPL!
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System Design Goals Ø An optimum sound system design matches the measured frequency
response of the maximum peak SPL with the expected spectral content of the program material played through that system.
Ø This allows the system to play the program material at the loudest level with equal likelihood of overload in each frequency band.
Ø In addition, each intermediate stage in a properly designed sound system should reach overload at roughly the same input level to the system (Very Important!).
Ø Other considerations may come in to play such as the spectral content and level of the background noise (very important in automotive applications!).
Consumer Electronics Association CEA-2010: Standard Method of Measurement for Powered
Subwoofers
10x103
5
0
-5
-10
CO
UNT
0.25s0.200.150.100.050.00TIME
RECORDED SIGNAL
-60dB
-50
-40
-30
-20
-10
0
LEVE
L - d
B
10 Hz 100 Hz 1kHz 10kHzFREQUENCY - Hz
SIGNAL SPECTRUM
This is the Burst You Put in at 50 Hz:
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Consumer Electronics Association CEA-2010: Standard Method of Measurement for Powered
Subwoofers
-10x103
-5
0
5
CO
UNT
0.25s0.200.150.100.050.00TIME
RECORDED SIGNAL
-60dB
-50
-40
-30
-20
-10
0
LEVE
L - d
B
10 Hz 100 Hz 1kHz 10kHzFREQUENCY - Hz
SIGNAL SPECTRUM
107.4 dB Peak SPL
This is what comes out of Sub A at 50 Hz:
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Consumer Electronics Association CEA-2010: Standard Method of Measurement for Powered
Subwoofers
10x103
0
-10
CO
UNT
0.25s0.200.150.100.050.00TIME
RECORDED SIGNAL
-60dB
-50
-40
-30
-20
-10
0
LEVE
L - d
B
10 Hz 100 Hz 1kHz 10kHzFREQUENCY - Hz
SIGNAL SPECTRUM
117.3 dB Peak SPL
This is what comes out of Sub B at 50 Hz:
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CEA-2010 Measurement Results: l Sub Woofer A:
20 Hz: 50.0 dB (No usable output) 25 Hz: 50.0 dB (No usable output) 31.5 Hz: 104.9 dB
Ultra-low bass (20-31.5): 68.3 dB 40 Hz: 109.0 dB 50 Hz: 107.4 dB 63 Hz: 108.6 dB
Low bass (40-63): 108.3 dB
l Sub Woofer B: 20 Hz: 103.1 dB 25 Hz: 109.7 dB 31.5 Hz: 114.0 dB
Ultra-low bass (20-31.5): 108.9 dB 40 Hz: 111.7 dB
50 Hz: 117.4 dB
63 Hz: 119.6 dB Low bass (40-63): 116.2 dB
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That’s It!
