EP3291577A1 - Lautsprecher mit wärmesteuerung - Google Patents

Lautsprecher mit wärmesteuerung Download PDF

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Publication number
EP3291577A1
EP3291577A1 EP17189159.1A EP17189159A EP3291577A1 EP 3291577 A1 EP3291577 A1 EP 3291577A1 EP 17189159 A EP17189159 A EP 17189159A EP 3291577 A1 EP3291577 A1 EP 3291577A1
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EP
European Patent Office
Prior art keywords
speaker
power
model
temperature
providing
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Legal status (The legal status is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the status listed.)
Withdrawn
Application number
EP17189159.1A
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English (en)
French (fr)
Inventor
Helmut Schlaegl
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Gibson Innovations Belgium NV
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Gibson Innovations Belgium NV
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Publication of EP3291577A1 publication Critical patent/EP3291577A1/de
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    • H—ELECTRICITY
    • H04—ELECTRIC COMMUNICATION TECHNIQUE
    • H04R—LOUDSPEAKERS, MICROPHONES, GRAMOPHONE PICK-UPS OR LIKE ACOUSTIC ELECTROMECHANICAL TRANSDUCERS; DEAF-AID SETS; PUBLIC ADDRESS SYSTEMS
    • H04R3/00—Circuits for transducers, loudspeakers or microphones
    • H04R3/007—Protection circuits for transducers

Definitions

  • the present invention relates generally to speaker systems, and more particularly, to a mechanism which protects a speaker from thermal damage if driven by a high power amplifier.
  • the speaker is protected by limiting the power offered to the speaker to a predetermined 'safe' value.
  • This method ignores the current thermal status of the speaker - i.e. whether the speaker temperature is still low (and thus the speaker is still able to handle high power) or the speaker is already heated up by preceding signal (and thus can only handle a limited amount of power before exceeding critical temperature limits).
  • This protection method is therefore either too conservative (i.e. sound and loudness is limited even if unnecessary) or presents some risks (i.e. if limit is placed somewhere higher than 'safe' value, then the speaker may still get damaged).
  • the speaker impedance can be calculated and the internal speaker temperature can be calculated from the speaker impedance. If the temperature becomes too high, the power supplied to the speaker is reduced.
  • This mechanism requires extra hardware for the current measurement and has limited accuracy. Specifically, other factors such as eddy currents at high signal frequencies affect the speaker impedance and may therefore lead to wrong conclusions on the speaker temperature. Moreover, when there is no signal, the speaker impedance cannot be calculated and needs to be estimated by using other methods.
  • US 2013/0022208A discloses apparatus and methods for providing protection for audio transducers, in which the signals sent to the transducers are also coupled to a power monitoring circuit.
  • the power monitoring circuit measures and controls both the average and peak power of an amplifier output into a load.
  • the protection mechanism however does not take into account the current thermal situation of the speaker such as when heated up by preceding signals then less power can be handled safely compared to a situation where the speaker is still cold.
  • the protection mechanism allows full power to be delivered to the speaker as long as the temperature of its critical component is still within safe area.
  • the protection mechanism gradually reduces power delivered to the speaker as its temperature approaches a critical limit where damage could occur.
  • the protection mechanism acts gradually without disruptive impact to the listening experience.
  • the protection mechanism is model-based and thus no sensor hardware is needed to determine internal temperature of the speakers. There is no need for non-standard speakers with built-in sensor. Instead, standard speakers can be used.
  • an apparatus for providing speaker protection comprises a gain unit, a speaker model and a controller.
  • the gain unit is arranged for scaling an audio signal to be coupled to a speaker.
  • the speaker model is arranged for estimating the temperature behaviour of the speaker due to the scaled audio signal.
  • the controller is arranged for controlling the gain of the gain unit based on the estimated temperature behaviour to avoid overheating of the speaker.
  • the speaker model is characterized by a plurality of speaker parameters including thermal time constants of speaker elements.
  • the speaker model comprises at least one low pass filter for applying to the (RMS-converted) scaled audio signal.
  • the low pass filter(s) has time constant(s) equivalent to the thermal time constant(s) of corresponding speaker element(s).
  • the relative contribution (scale factor) of each low pass filter to the model output corresponds to the speaker element it represents.
  • the speaker parameters further include sustainable power which describes the amplifier output power which can be sustained by the speaker over extended period of time without internal temperature exceeding critical limits.
  • the speaker model comprises an equalization unit for representing the frequency dependence of actual thermal speaker behaviour. For instance, at a specific frequency the critical temperature may be reached already with lower signal level than at other frequencies - this would be represented by a proportional boost at such specific frequency to make the speaker model equally more sensitive there.
  • the speaker model is also suitable for speaker time constants which differ for heat-up and cool-down situations. This is accomplished by comparing the input to the low pass filter(s) with its output and depending on the result applying the matching parameter set. For example, if the input exceeds the current output value of the low pass filter then speaker temperature will rise over time and therefore parameters valid for such heat-up situation will be used.
  • the controller reduces the gain of the gain unit when the estimated temperature behaviour by the speaker model approaches said sustainable power of the speaker.
  • the controller may be realized by one of the known standard techniques such as PID controller or two-point controller.
  • the speaker parameters are specific for the speaker in use and are obtained from speaker characterization.
  • FIG. 1 is a system overview showing the elements that may be present in a speaker system 100 which contains a speaker protection unit 200 according to an embodiment of the invention.
  • the speaker system 100 further comprises a power amplifier 300 and speaker 400.
  • the speaker protection unit 200 comprises a gain unit 210, a speaker model 220 and a control unit 230.
  • an audio signal (A) from an audio signal source is sent to the gain unit 210 and the audio signal (A) is scaled by a factor according to a gain control signal (G) from the control unit 230.
  • the resulting signal (S) is sent to both the speaker 400 (via the power amplifier 300) and to the speaker model 220.
  • the speaker model 220 replicates the thermal behaviour of the speaker 400 according to speaker parameters stored in the memory unit 240. As this speaker model 220 is driven by the same signal (S) which is also sent to the speaker 400, the speaker model output (M) represents the temperature inside the speaker 400.
  • the model output represents a power figure which is equivalent to the temperature inside the speaker - i.e. the power which (if applied for extended period of time) would result in same temperature inside the speaker as the current temperature.
  • the controller 231 to be run entirely on level of 'Power figures' and avoid the conversion to temperature figures (which would be complex due to non-linearity introduced by thermal compression).
  • the control unit 230 compares speaker model output (M) with the critical limit (sustainable power) of the speaker 400 and calculates the gain control signal (G). As long as there is no risk of exceeding this critical limit, the gain control signal (G) assumes a neutral value so that the gain unit 210 applies unit gain to the audio signal (A). If speaker model output (M) approaches this limit (i.e. estimated temperature inside the speaker 400 comes close to critical value), gain control signal (G) is adjusted to gradually reduce the gain of the gain unit 210 to less than unity. As a result, the signal (S) driving the speaker 400 is reduced to keep internal temperature within safe boundaries.
  • M speaker model output
  • G gain control signal
  • the present invention allows the system to fully utilize the capability of the speaker up to its thermal limit at all times while avoiding the cost and complexity of actual temperature measurement via sensors inside the speaker.
  • the speaker parameters (e.g.: time constants, sustainable power, etc.) used in the speaker model 220 and in the control unit 230 are determined once at design time via a speaker characterization process which will be further described below.
  • the speaker model 220 simulates the relationship between power applied to the speaker 400 and resulting temperature rise at the most critical element within the speaker (in a conventional loudspeaker, the most critical element is usually the voice coil), such that the protection mechanism of the control unit 230 has information about the estimated internal speaker temperature without requiring a temperature sensor in the speaker itself.
  • the temperature rise depends on the specific characteristics of the speaker in use. Therefore, the speaker model 220 needs to be tailored according to the used speaker 400 via speaker characterization during product development stage.
  • FIG. 2 is an illustrative example for a typical speaker.
  • the main building blocks of the speaker model 220 include equalization unit 221, power conversion unit 222, and low pass filters 223, and phase detection unit 224:
  • Figures 3A and 3B illustrates the general relationship of the speaker impedance and input signal frequency.
  • the power absorbed by the speaker may vary depending on the frequency of the audio input signal.
  • Figure 3A illustrates this for a typical bass reflex enclosure.
  • the speaker impedance 301 exhibits a minimum, and thus a maximum of power gets absorbed by the speaker from the power source (amplifier output) at this frequency.
  • voice coil movement is minimal (due to damping by the bass port) and thus there is less heat transfer from voice coil to its surrounding due to reduced air convection. Both factors combined lead to a higher temperature rise, and the power needed to reach critical temperature 303 shows a minimum at this frequency.
  • the speaker model In order to reproduce the thermal behavior of the speaker, the speaker model needs to take such frequency dependency into account. This is achieved by applying a corresponding equalization to the input signal (S) via the equalization unit 221 - e.g. frequencies where the speaker reaches critical temperature already at low power level get boosted accordingly (resulting in signal (S') such that also the output of the speaker model (M) reaches its full value already at same low power level).
  • Figure 3B illustrates this for the case of a typical bass reflex enclosure. Highest values of the power curve 304 can be neglected as the power amplifier anyhow would not be able to deliver them. Most remaining parts of the power curve 304 can then be approximated by a straight line (odB - i.e. no equalization).
  • the power curve 304 shows a marked dip compared to all other frequencies - i.e. the critical temperature is reached already with much less power . This area therefore needs to be taken into account also in a simplified case - the speaker model needs to be made equally more sensitive for this specific frequency. This is achieved by applying a corresponding equalization 305 at the input of the speaker model.
  • the power curve 304 shows that approximately 5dB less power is required at 30 Hz to reach critical temperature - hence equalization 305 needs to introduce a 5 dB boost at this frequency such that speaker model exhibits the same behaviour.
  • the internal temperatures rises only gradually over time depending on the thermal time constants (TC) of the involved elements - e.g. depending on speaker size, the voice coil may exhibit a TC between seconds and few minutes, the magnet and frame may have a TC between few minutes and hours, etc.
  • these elements interact - e.g. heating up of the voice coil leads to transfer of thermal energy (e.g. via radiation and convection) to the surrounding magnet / speaker frame which also starts to heat up (albeit at much slower rate due to bigger TC of magnet / frame). This in turn increases the ambient temperature of the voice coil and contributes to further rise of voice coil temperature.
  • the power signal (P) is subjected to low pass filter(s) 223 which each represents a specific element of the speaker, for instance, voice coil, speaker frame, etc.
  • the time constants TC of each low pass filter 223 and relative contribution to the overall speaker model output (M) is determined in the speaker characterization process.
  • the current phase is first detected by phase detection unit 224, and the parameters will be switched-in accordingly.
  • the parameters for 'heat-up' phase containing suffix 'h' or the parameters for 'cool-down' phase marked with suffix 'c' are used in respective situations.
  • the output M(n) of the speaker model 220 represents a power level which corresponds to the current speaker temperature ('temperature-equivalent power') and is the summation of M1(n) - the speaker model output component contributed by a first speaker element having a thermal time constant TC 1x and scale factor k 1x , and M 2 (n) - the speaker model output component contributed by a second speaker element having a thermal time constant TC 2x and scale factor k 2x .
  • TC 1x , TC 2x , k 1x , k 2x are specific for the speaker in use.
  • a 1 x exp ⁇ 1 / TC 1 x
  • b 1 x 1 ⁇ a 1 x
  • a 2 x exp ⁇ 1 / TC 2 x
  • b 2 x 1 ⁇ a 2 x
  • a 1x , b 1x , a 2x , b 2x are precalculated at design time based on speaker characterization.
  • FIG. 4 presents an overview of the components in the control unit 230 according to an embodiment of the invention, which comprises a controller 231 and power conversion unit 232.
  • the controller 231 compares the output (M) of the speaker model 220 with a power level corresponding to critical temperature, Pwr_sust - the power sustainable over extended period of time without exceeding temperature limit inside the speaker.
  • the output of the controller 231 (G_pwr) is on the level of 'Power' as its input (M) is on power level. Since the gain unit 210 operates on signal levels, the output of the control unit 230 needs to be converted accordingly (square root operation). The resulting output signal (G) is fed to the gain unit 210 to control the gain applied to the audio signal (S).
  • control unit 230 determines according to the output of the speaker model 220 that speaker 400 approaches the critical temperature, then the control unit 230 generates a gain control signal (G) less than 1. This reduces the signal (S) passed to the power amplifier 300 and speaker 400. Since this equally reduces the input signal (A) to the speaker model (M), a closed control loop is formed with the result that speaker temperature is kept below critical limit at all times while still allowing the maximum amount of power to be sent to the speaker, which at current condition of the speaker is still safe.
  • G gain control signal
  • the gain may also be a limiter or compressor where the threshold level is adjusted by the control signal (G).
  • the speaker system 100 implements multi-channel or multi-band arrangements involving multiple speakers (e.g. woofer, tweeter, etc.).
  • multiple speakers e.g. woofer, tweeter, etc.
  • Gain setting may then either be done independently per speaker or in a combined manner (e.g. gain for all bands set to the same lowest value of all controllers) to keep spectral balance of sound output unchanged.
  • the speaker protection mechanism is tailored to the specific properties of the speaker in use. These properties are determined in a 'speaker characterization' step, performed once during development of the system. This results in a set of speaker parameters used by the speaker protection mechanism (ref. Figure 1 ). The process described below represents an example for a typical bass reflex speaker.
  • audio signals of different frequencies are applied to the speaker and the speaker impedance (mainly contributed by voice coil impedance) is measured.
  • FIG. 3A is a plot of speaker impedance 301 against signal frequency for a typical speaker with bass reflex enclosure. At the frequency of lowest impedance 302, a maximum of power is absorbed by the speaker when driven by an amplifier acting as voltage source in the characterization platform.
  • the frequency of lowest impedance 302 usually coincides with the frequency where voice coil excursion is lowest due to damping by port resonance. Therefore, heat transfer by air convection from voice coil to surrounding areas is less effective.
  • a stationary signal at a safe power level (e.g. 30% of rated power to avoid risk of speaker damage) is applied to the initially 'cold' speaker and the rising voice coil temperature is monitored. After temperature has reached a stable value, the signal to the speaker is removed and the monitoring of voice coil temperature continued until it has fallen close to original 'cold' value. The evolution of voice coil temperature over time during both heat-up and cool-down phase is captured.
  • Example of the measured speaker voice coil temperature data 501, 502 at respective heat-up and cool-down phases are shown in Figures 5A and 5B .
  • the measurement is performed at least for a signal using the most critical frequency (identified in the first step of the speaker characterization procedure described above) and for a signal representing the entire frequency band of interest (e.g. pink noise).
  • the key speaker parameters such as sustainable power and thermal time constants and scale factors can be derived.
  • the voice coil impedance can be determined (due to temperature coefficient of used voice material, this can change significantly at elevated temperature) and from this, the power actually absorbed by the speaker can also be determined.
  • the power required to reach the critical voice coil temperature i.e. the temperature which the voice coil can still withstand permanently without damage
  • the corresponding amplifier output level can be determined, which can be sustained by the speaker over extended period of time. This is then the parameter "sustainable power” (Pwr_sust).
  • the value of Pwr_sust obtained using pink noise serves as reference point for the controller 231 in Figure 4 .
  • the value of Pwr_sust obtained using the critical frequency 302 is used to determine the characteristics of the equalization unit 221 in the speaker model 220 in Figure 2 .
  • the process of determining Pwr_sust takes the effect of thermal compression into account. This provides the benefit that the actual speaker protection algorithm can be operated on the level of 'temperature-equivalent power' figures, instead of 'voice coil temperature' which would require more complex or iterative calculations at run time and thus complicate the implementation.
  • the time constants (TC) and scale factors for low pass filters 223 in Figure 2 can be determined from the measured voice coil temperature data points 501 and 502 in Figures 5A and 5B , e.g. by converging low pass filter parameters towards minimized difference between measured- and model-calculated data points (e.g. sum of squared error terms).
  • Scale factors indicate how much each speaker element (represented by its corresponding low pass filter in the speaker model) contributes to total speaker temperature (represented by output of model). For example, if for a given power supplied to the speaker, the voice coil temperature rises by 70 deg.
  • Figures 6A and 6B respectively show the model data points 601, 603 based on the following example model parameters set for best match to measured data 501, 502 during the heat-up and cool-down periods.
  • Error curves 602, 604 respectively correspond to the model-calculated data points 601, 603.
  • the model parameter set resulting in lowest error term is then used for the low-pass filters 223 of the speaker model 220 in Figure 2 .
  • a model based on less or more low pass filters may be appropriate to best represent the measured data.
  • a model based on two time constants as indicated in the embodiment of the invention should however be sufficient in most cases. Additional time constants may e.g. be needed in case the heat transfer through the speaker cabinet to ambient air is contributing significantly (e.g. ceramic enclosure material with large heat capacity)

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  • Physics & Mathematics (AREA)
  • Engineering & Computer Science (AREA)
  • Acoustics & Sound (AREA)
  • Signal Processing (AREA)
  • Circuit For Audible Band Transducer (AREA)
  • Amplifiers (AREA)
EP17189159.1A 2016-09-05 2017-09-04 Lautsprecher mit wärmesteuerung Withdrawn EP3291577A1 (de)

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Citations (2)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US20160119714A1 (en) * 2014-10-06 2016-04-28 Texas Instruments Incorporated Audio power limiting based on thermal modeling
GB2534949A (en) * 2015-02-02 2016-08-10 Cirrus Logic Int Semiconductor Ltd Loudspeaker protection

Patent Citations (2)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US20160119714A1 (en) * 2014-10-06 2016-04-28 Texas Instruments Incorporated Audio power limiting based on thermal modeling
GB2534949A (en) * 2015-02-02 2016-08-10 Cirrus Logic Int Semiconductor Ltd Loudspeaker protection

Non-Patent Citations (2)

* Cited by examiner, † Cited by third party
Title
AES, 60 EAST 42ND STREET, ROOM 2520 NEW YORK 10165-2520, USA, 7 October 2005 (2005-10-07), XP040372904 *
KLIPPEL ET AL: "Nonlinear Modeling of the Heat Transfer in Loudspeakers", JAES, AES, 60 EAST 42ND STREET, ROOM 2520 NEW YORK 10165-2520, USA, vol. 52, no. 1/2, 1 February 2004 (2004-02-01), pages 3 - 25, XP040507072 *

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