Not Everything You Need to Know About Automotive System & Speaker Specs........

News Blog2011

Nov 11

Why These Parameters?

We have a unique perspective on loudspeaker design and specifications, having designed loudspeakers for over 10 years and performed research in that area as well during those years and afterward. We also spent 15 years after designing loudspeakers designing, tuning, and researching the systems in cars. That experience ranges from entry level, non-branded design, to branded, and to Luxury branded systems. That experience also extends into creating innovations in design, which ultimately became new standards for both the luxury brand and the standard brand.

The major factors in perceived sound quality from Olive [1,2] and Klippel [3,4] and from our practical experience are:

Frequency Response Smoothness On and Off-Axis, Perceived Bass Extension, and the term “Naturalness”. Naturalness can be defined in spatial terms as “the feeling of space”, i.e., the correct sense of ambience, or the appropriate levels of direct and reflected information. And it can be defined as a lack of colorization in the sound, which is to say a lack of Distortion and Dynamic Compression. The contributing parameters for loudspeakers for those major factors are:

Frequency Response Sensitivity (+/- dB) at a standard level, Bandwidth and Dynamic Capability (Distortion and Power Compression)

Frequency Response Smoothness goals can be simply understood, and its Sensitivity can be directly derived.

Bandwidth defines the upper and lower useable limits of a loudspeaker. Those upper limits are defined by off-axis measurements, and the lower limits are defined by a frequency point below which distortion is considered perceptible. Both upper and lower frequency limits are also defined by the effects of power compression, which can narrow the useful bandwidth of a given loudspeaker if the compression causes the upper or lower bands to fall outside of the defined bandwidth limits.

Harmonic Distortion is a measure of the cleanliness of a sound being produced. Power Compression measures the loudspeaker’s dynamic capability.

Meaning and Background Theory of Each Spec.

Frequency Response: Sensitivity and Smoothness (± x dB)

This is voltage Sensitivity, dBSPL measured with 2.83V @ 1 meter. 2.83V is 1W for 8W for a standardized voltage sensitivity test.

To set a target for a component SPL, the system level targets are used.

The relationship of rated acoustic power to cutoff frequency and driver displacement… From Small [5]

Start with Volume Displacement. This is the amount of air a given loudspeaker can move. From Small [5], to obtain the maximum rated SPL, the Volume Displacement (VD) can be set and the design parameters – surface area (Sd) and peak excursion (xp) – can be determined to meet the goals, and match the amplifier voltage capability (peak voltage, Vp).

Example SPL levels are determined from existing vehicles, and future “reach” targets are set.

[Picture 3 - Existing Branded Audio Vol Disp]

Example of existing (branded) systems’ SPL and Volume Displacement and Speaker Architectures

Frequency Response: Bandwidth

Distortion: Near Resonance (Fo)

• Acceptable limits on THD levels in the low frequency range of a loudspeaker (near resonance Fo) generally set, AES2-1984 [6]

• 20% for Woofers and Subwoofers

• 10% for midranges and tweeters.

• Acceptance of these levels of THD is related to the phenomenon of low frequency masking that occurs in human hearing.

• Greater masking occurs for lower frequencies. Toole[7]

• Harmonic Distortion: Harmonics that are generated above the fundamental frequency being reproduced.

(a) A simple view simultaneous masking of a tone at 2kHz by another, louder one at 500Hz. It shows that the masking effects spreads substantially upward in frequency and only slightly downward

Lower frequency fundamentals mask the higher frequency harmonics.
The lower the frequency of the fundamental, the more masking of distortion that will occur.

(b) The substantial masking effects of very low frequencies, especially at high sound levels. (a) & (b) From Fielder, Figure 1.60 in Talbot-Smith, 1999.[8]

Distortion: Above Resonance (Fo)

The same reasoning as for distortion near resonance applies for distortion Above Resonance
Also from experiments performed by myself [9,10]and House [11]
The upper Harmonic Distortion limit for just audible mid-band distortion is set to 5% at maximum power.
Capability of branded and luxury branded components have set the limits lower in distortion for the mid-band.

This leads to less audible and inaudible distortion levels

[Picture 8 - Distortion above resonance]

Caveats for Distortion Near and Above Resonace (Fo):
While THD levels help quantify the potential sound quality, it is readily accepted that other forms of nonlinear measurements could provide greater insight into audible distortion in loudspeakers, and could be more directly related to human perception. (Klippel, Kaizer, Small, Cunningham, Reed and Hawksford, IEC 60268-5, Keele, Zwicker, Tan and Zacharov, Cibelli, Voishvillo, Toole [12-26, 7] )

Power: Max RMS Power From System Amplifier

Power specification comes from Measured System Sound Levels and deriving Wattage via Volume Displacement (reverse from before)

Known system examples of power amplifiers:

Non-branded: H/U powered, some external amps
Branded & Luxury: existing level of amps and SPL
Voltage levels come from amp design, which is driven by cost and ingenuity (i.e. step up w/o cost
up)

RMS Max Voltage drives Speaker Power Rating (W= V²rms/ Imp)

Power Compression: Burst and Thermal

Power Compression is not a standard specification for OEM audio. It is however a good measure of how dynamic and linear a system will be,
- Will the system be as un-colored at high levels as it is at nominal levels
- Will transients, such as a kick drum and dynamic vocals and instruments, sound natural
- or Will they sound dull and strained by too much compression
It is a better measure of Power Handling capability. Gander[27], Toole [7]
It leads a designer toward designs and analysis of designs that are better matched to their application. [27]
- This leads us to the notion of how to specify a speaker’s ability to handle higher power levels, and how efficiently they do that.
- Specifying a speaker’s peak voltage handling rather than simple power handling. This is a topic outside of this information.
Power Compression is measured as a dB drop from the SPL levels for a speaker measured at nominal voltage levels. The speaker’s maximum voltage capability is first determined by incrementally increasing the input until mechanical or thermal limits are reached.
The speaker is cooled down and the Burst power compression is run at the maximum voltage possible for a very short duration
The Speaker is cooled and the long-term Thermal compression is measured. Impedance changes for loudspeakers with higher current through the coil, causing temperature increases, therefore compression occurs.
Ideal Thermal Power Compression has been stated to be 1.25dB [27]
Practical limits for each brand level were set for the level of system quality variation expected for each brand level

What Happens When the Specification Isn’t Met?

System Issues:

Sensitivity:

If a single speaker is 1dB below the target Sensitivity, and there are 4 speakers in the system, the system will be 4dB below its overall targets. That is more than half as loud as it potentially could be. (A change of 3dB in SPL is a doubling in perceived loudness.)

Bandwidth/System Response:

Loss of basic content in the region of crossover, a miss-match due to deviation from specification -- will cause lack of clarity, lack of linearity.

Distortion:

Any values of THD above the specified values are audible and will cause coloration and clarity issues.

Power Handling:

A 10% drop in Wattage capability for a speaker is a 1% drop in Volume Displacement. That is a 0.5 dB SPL drop in the speaker’s output.
If this is a 16cm woofer, and there are 4 of them, there would be a 2dB drop below targets for the system. Any loss in any other woofer in the system would add to that.
So, in example, for a 50% drop in Wattage capability (e.g., 16W down from 32W), that equates to a 5% drop in Volume Displacement, and a 2.5dB SPL drop in speaker output. If there are 4 of those speakers, the overall system loss is 10dB.
A loss in power handling capability means the system is unable to satisfy the end- users expectation of “loud enough”, and unable to mask background noise enough. This keeps the system from meeting its SPL targets – it could meet the levels if the gain into the speaker is increased beyond an acceptable range, which means it meets the SPL targets but adds distortion to the system.

Power Compression:

There will be loss of dynamics and linearity. The system will seem colored, less clear, less natural, and generally lacking in fidelity with transients, such as drums or any dynamic instrument or voice.

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References

[1] Olive, S. “A Multiple Regression Model for Predicting Loudspeaker Preference Using Objective Measurements: Part 1-Listening Test Results,” 116th Convention, Audio Eng. Soc., Preprint 6113. (2004).

[2] Olive, S. “A Multiple Regression Model for Predicting Loudspeaker Preference Using Objective Measurements: Part 2-Development of the Model,” 117th Convention, Audio Eng. Soc., Preprint 6190. (2004).

[3] Klippel, W. “Multidimensional Relationship between Subjective Listening Impression and Objective Loudspeaker Parameters,” Acustica, 70, pp. 45-54. (1990).

[4] Klippel, W. “Assessing the Subjectively Perceived Loudspeaker Quality on the Basis of Objective Parameters,” 88th Convention, Audio Eng. Soc., Preprint 2929. (1990).

[5] R. H. Small, “Closed-Box Loudspeaker Systems, Part I: Analysis,” J. Audio Eng. Soc., vol. 20, pp. 798-808 (1972 Dec.).

[6] AES2-1984 (r. 2003), “AES Recommended Practice—Specification of Loudspeaker Components Used in Professional Audio and Sound Reinforcement,” Audio Engineering Society, New York (2003).

[7] F. E. Toole, Sound Reproduction (Focal Press, Elsevier, Amsterdam, The Netherlands, 2008).

[8] Talbot-Smith, M., editor (1999). Audio Engineer’s Reference Book, Focal Press, Oxford.

[9] R. Shively, N. House, “Perceived Boundary Effects in an Automotive Interior”, 100th Convention Audio Eng. Soc. Preprint 4245 (1996).

[10] R. Shively, “Subjective Evaluation of Reproduced Sounds in Automotive Spaces”, 15th Intl. Conf. Audio Eng. Soc. (1998).

[11] N. House, “Aspects of the Vehicle Listening Environment”, 87th Convention Audio Eng. Soc. Preprint 2873 (1989).

[12] W. Klippel, “Dynamic Measurement and Interpretation of the Nonlinear Parameters of Electrodynamic Loudspeakers,” J. Audio Eng. Soc., vol 38, pp. 944-955 (1990 December).

[13] A. J. M. Kaizer, “Modeling of the Nonlinear Response of an Electrodynamic Loudspeaker by a Volterra Series Expansion,” J. Audio Eng. Soc., vol. 35, pp. 421-433 (1987 June).

[14] R. H. Small, “Vented-Box Loudspeaker Systems, Part II: Large-Signal Analysis,” J. Audio Eng. Soc., vol. 21, pp. 438-444 (1973 July/Aug.).

[15] W. J. Cunningham, “Non-Linear Distortion in Dynamic Loudspeakers Due to Magnetic Effects,” J. Acoust. Soc. Am., vol. 21, pp. 202-207 (1949).

[16] M. J. Reed and M. O. Hawksford, “Practical Modeling of Nonlinear Audio Systems Using the Volterra Series,” presented at the 100th Convention of the Audio Engineering Society, J. Audio Eng. Soc. (Abstracts), vol. 44, pp. 649–650 (1996 July/Aug.), preprint 4264.

[17] IEC 60268-5, “Sound System Equipment—Part 5 Loudspeakers,” International Electrotechnical Commission, Geneva, Switzerland (2000).

[18] D. Keele, “Method to Measure Intermodulation Distortion in Loudspeakers,” proposals for the working group SC-04-03-C, Audio Engineering Society, New York (2000).

[19] D. B. Keele, “Development of Test Signals for the EIA-426-B Loudspeaker Power-Rating Compact Disk,” presented at the 111th Convention of the Audio Engineering Society, J. Audio Eng. Soc. (Abstracts), vol. 49, p.1224 (2001 Dec.), convention paper 5451.

[20] E. Zwicker, H. Fastl, and H. Frater, Psychoacoustics: Facts and Models, Springer ser. in Information Sciences, 2nd updated ed. (Springer, New York, 1999).

[21] C. T. Tan, B. C. J. Moore, and N. Zacharov, “The Effect of Nonlinear Distortion on the Perceived Quality of Music and Speech Signals,” J. Audio Eng. Soc., vol. 51, pp. 1012–1031 (2003 Nov.).

[22] M. J. Reed and M. O. J. Hawksford, “Comparison of Audio System Nonlinear Performance in Volterra Space,” presented at the 103rd Convention of the Audio Engineering Society, J. Audio Eng. Soc. (Abstracts), vol. 45, p. 1026 (1997 Nov.), preprint 4606.

[23] G. Cibelli, E. Ugolotti, and A. Bellini, “Dynamic Measurements of Low-Frequency Loudspeakers Modeled by Volterra Series,” presented at the 106th Convention of the Audio Engineering Society, J. Audio Eng. Soc. (Abstracts), vol. 47, p. 534 (1999 June), preprint 4968.

[24] Voishvillo, A. “Assessment of Nonlinearity in Transducers and Sound Systems-From THD to Perceptual Models,” 121st Convention, Audio Eng. Soc., Preprint 6910. (2006).

[25] R. H. Small, “Direct-Radiator Loudspeaker System Analysis,” J.Audio Eng. Soc. , Vol 20, pp. 383-395 (1972 June).

[26] A. Voishvillo “Graphing, Interpretation, and Comparison of Results of Loudspeaker Nonlinear Distortion Measurements” J. Audio Eng. Soc., vol. 52,pp. 332–357 (2004 April).

[27] M. R. Gander, “Dynamic Linearity and Power Compression in Moving-Coil Loudspeakers, “J. Audio Eng. Soc., vol. 34, pp. 627-646 (1986 Sept.).

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Roger Shively https://www.linkedin.com/in/roger-shively/